coderdaily.github.io/concurrent.html at master · coderdaily/coderdaily.github.io · GitHub

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  <li>
    <a href="#并发">并发</a>
    <ul>
      <li>
        <a href="#并发在一段时间内多个程序一起执行">并发：在一段时间内，多个程序一起执行</a>
        <ul>
          <li>
            <a href="#实现并发的思路">实现并发的思路：</a>
          </li>
        </ul>
      </li>
      <li>
        <a href="#并行在一个时刻多个程序同时执行">并行：在一个时刻，多个程序同时执行</a>
      </li>
      <li>
        <a href="#区别">区别：</a>
      </li>
    </ul>
  </li>
  <li>
    <a href="#为什么通常采用多线程而非多进程">为什么通常采用多线程而非多进程？</a>
  </li>
  <li>
    <a href="#线程是怎么实现的">线程是怎么实现的？</a>
    <ul>
      <li>
        <a href="#内核线程kernel-level-threadklt实现">内核线程（Kernel-Level Thread,KLT）实现</a>
      </li>
      <li>
        <a href="#用户线程实现"><s>用户线程实现</s></a>
      </li>
      <li>
        <a href="#用户线程加轻量级进程混合实现">用户线程加轻量级进程混合实现</a>
      </li>
    </ul>
  </li>
  <li>
    <a href="#线程是如何调度的">线程是如何调度的？</a>
    <ul>
      <li>
        <a href="#协同式调度">协同式调度</a>
      </li>
      <li>
        <a href="#抢占式调度">抢占式调度</a>
      </li>
    </ul>
  </li>
  <li>
    <a href="#java-线程的优先级">Java 线程的优先级</a>
  </li>
  <li>
    <a href="#java-线程的状态">Java 线程的状态</a>
    <ul>
      <li>
        <a href="#状态的定义与流转">状态的定义与流转：</a>
      </li>
      <li>
        <a href="#blocked-状态可能的原因">BLOCKED 状态可能的原因：</a>
      </li>
      <li>
        <a href="#blocked-与-waiting-的区别">BLOCKED 与 WAITING 的区别</a>
      </li>
    </ul>
  </li>
  <li>
    <a href="#java线程的其他知识">Java线程的其他知识</a>
    <ul>
      <li>
        <ul>
          <li>
            <a href="#runable-thread">Runable、Thread</a>
          </li>
          <li>
            <a href="#线程基本操作">线程基本操作</a>
            <ul>
              <li>
                <a href="#新建线程-threadstart">新建线程-Thread.start()</a>
              </li>
              <li>
                <a href="#终止线程-threadstop">终止线程-<s>Thread.stop()</s></a>
              </li>
              <li>
                <a href="#线程中断-threadinterrupt">线程中断-Thread.interrupt</a>
              </li>
              <li>
                <a href="#等待与通知waitnotify">等待与通知——wait()/notify()</a>
              </li>
              <li>
                <a href="#线程挂起-threadsuspend">线程挂起-<s>Thread.suspend()</s></a>
              </li>
              <li>
                <a href="#join和yeild">join和yeild</a>
              </li>
              <li>
                <a href="#守护线程-threadsetdeamontrue">守护线程-Thread.setDeamon(true)</a>
              </li>
            </ul>
          </li>
          <li>
            <a href="#线程安全-与-线程不安全">线程安全 与 线程不安全</a>
          </li>
          <li>
            <a href="#进程间通信">进程间通信</a>
            <ul>
              <li>
                <a href="#管道-pipe">管道( pipe )</a>
              </li>
              <li>
                <a href="#信号量-semophore">信号量( semophore )</a>
              </li>
              <li>
                <a href="#消息队列-message-queue">消息队列( message queue )</a>
              </li>
              <li>
                <a href="#信号-sinal">信号 ( sinal )</a>
              </li>
              <li>
                <a href="#共享内存-shared-memory">共享内存( shared memory )</a>
              </li>
              <li>
                <a href="#套接字-socket">套接字( socket )</a>
              </li>
            </ul>
          </li>
          <li>
            <a href="#线程间通信">线程间通信</a>
            <ul>
              <li>
                <a href="#1-通过共享对象通信">1、通过共享对象通信</a>
              </li>
              <li>
                <a href="#2-忙等待">2、忙等待</a>
              </li>
              <li>
                <a href="#3-waitnotify和notifyall">3、wait(),notify()和notifyAll()</a>
                <ul>
                  <li>
                    <a href="#例-1通知其他线程">例 1：通知其他线程</a>
                  </li>
                  <li>
                    <a href="#例-2使用wait-notify通知">例 2：使用wait()、notify()通知</a>
                  </li>
                  <li>
                    <a href="#例-3利用countdownlatch类实现">例 3：利用CountDownLatch类实现</a>
                  </li>
                </ul>
              </li>
              <li>
                <a href="#4-丢失的信号">4、丢失的信号</a>
              </li>
              <li>
                <a href="#5-假唤醒">5、假唤醒</a>
              </li>
              <li>
                <a href="#6-多线程等待相同信号">6、多线程等待相同信号</a>
              </li>
              <li>
                <a href="#7-不要对常量字符串或全局对象调用wait">7、不要对常量字符串或全局对象调用wait()</a>
              </li>
            </ul>
          </li>
        </ul>
      </li>
      <li>
        <a href="#同步和异步">同步和异步：</a>
      </li>
      <li>
        <a href="#临界区">临界区：</a>
      </li>
      <li>
        <a href="#死锁-饥饿-活锁">死锁、饥饿、活锁：</a>
        <ul>
          <li>
            <a href="#死锁">死锁</a>
            <ul>
              <li>
                <a href="#死锁发生的条件">死锁发生的条件？</a>
              </li>
              <li>
                <a href="#解决死锁问题">解决死锁问题</a>
              </li>
            </ul>
          </li>
        </ul>
      </li>
      <li>
        <a href="#多线程的特性">多线程的特性：</a>
        <ul>
          <li>
            <a href="#原子性synchronized">原子性：synchronized</a>
          </li>
          <li>
            <a href="#可见性volatile">可见性：volatile</a>
          </li>
          <li>
            <a href="#有序性">有序性：</a>
          </li>
        </ul>
      </li>
    </ul>
  </li></ul></p><p data-source-line="3">参考：</p><ul data-source-line="4"><li><a href="http://www.ruanyifeng.com/blog/2013/04/processes_and_threads.html" target="_blank">进程与线程</a></li><li><a href="https://segmentfault.com/a/1190000000398256#shared_object" target="_blank">JAVA线程间通信简介</a></li><li><a href="https://my.oschina.net/hosee/blog/599000" target="_blank">[高并发Java 二] 多线程基础</a></li><li><a href="https://my.oschina.net/hosee/blog/597934" target="_blank">[高并发Java 一] 前言</a></li></ul><h1 data-source-line="9" id="并发"><a class="markdownIt-Anchor" href="#并发"></a>并发</h1><h2 data-source-line="11" id="并发在一段时间内多个程序一起执行"><a class="markdownIt-Anchor" href="#并发在一段时间内多个程序一起执行"></a>并发：在一段时间内，多个程序一起执行</h2><p data-source-line="13">当有多个线程在操作时,如果系统只有一个CPU,则它根本不可能真正同时进行一个以上的线程，它只能把CPU运行时间划分成若干个时间段,再将时间 段分配给各个线程执行，在一个时间段的线程代码运行时，其它线程处于挂起状。这种方式我们称之为并发(Concurrent)。</p><p data-source-line="15">偏重于多个任务交替执行，而多个任务之间有可能还是串行的，而并行是真正意义上的“同时执行”。</p><p data-source-line="17">并发通常是指在单处理器上提高程序的性能。
如果没有<mark>阻塞</mark>，那么在单处理器机器上使用并发就没有什么意义了。</p><h3 data-source-line="20" id="实现并发的思路"><a class="markdownIt-Anchor" href="#实现并发的思路"></a>实现并发的思路：</h3><ol data-source-line="22"><li>每个任务作为进程在其自己的地址空间中执行。也就是实现并发并不一定只有多线程这一个思路，还可以多进程去实现。</li><li>在执行程序表示的单一进程中创建任务。</li></ol><h2 data-source-line="25" id="并行在一个时刻多个程序同时执行"><a class="markdownIt-Anchor" href="#并行在一个时刻多个程序同时执行"></a>并行：在一个时刻，多个程序同时执行</h2><p data-source-line="27">当系统有一个以上CPU时,则线程的操作有可能非并发。当一个CPU执行一个线程时，另一个CPU可以执行另一个线程，两个线程互不抢占CPU资源，可以同时进行，这种方式我们称之为并行(Parallel)。</p><h2 data-source-line="29" id="区别"><a class="markdownIt-Anchor" href="#区别"></a>区别：</h2><p data-source-line="31">并发和并行是即相似又有区别的两个概念，并行是指两个或者多个事件在同一时刻发生；而并发是指两个或多个事件在同一时间间隔内发生。在多道程序环境下，并发性是指在一段时间内宏观上有多个程序在同时运行，但在单处理机系统中，每一时刻却仅能有一道程序执行，故微观上这些程序只能是分时地交替执行。倘若在计算机系统中有多个处理机，则这些可以并发执行的程序便可被分配到多个处理机上，实现并行执行，即利用每个处理机来处理一个可并发执行的程序，这样，多个程序便可以同时执行。</p><h1 data-source-line="33" id="为什么通常采用多线程而非多进程"><a class="markdownIt-Anchor" href="#为什么通常采用多线程而非多进程"></a>为什么通常采用多线程而非多进程？</h1><p data-source-line="35">因为多进程比多线程更消耗资源。进程的切换是非常重量级的操作，非常消耗资源。各个线程既可以共享进程资源（内存地址、文件 IO 等），又可以独立调度（线程是 CPU 调度的基本单位），所以线程会较为广泛的用于并发设计。</p><p data-source-line="37">另外，操作系统的设计，可以归结为三点：</p><ul data-source-line="38"><li>以多进程形式，允许多个任务同时运行；</li><li>以多线程形式，允许单个任务分成不同的部分运行；</li><li>提供协调机制，一方面防止进程之间和线程之间产生冲突，另一方面允许进程之间和线程之间共享资源。</li></ul><h1 data-source-line="42" id="线程是怎么实现的"><a class="markdownIt-Anchor" href="#线程是怎么实现的"></a>线程是怎么实现的？</h1><p data-source-line="44">目前主流的操作系统都提供了线程实现，java则提供的线程实现方法都是native的，因为不同的硬件和操作系统提供线程调度方式并不尽相同，所以java没用采用和平台无关的统一手段来实现。</p><h2 data-source-line="46" id="内核线程kernel-level-threadklt实现"><a class="markdownIt-Anchor" href="#内核线程kernel-level-threadklt实现"></a>内核线程（Kernel-Level Thread,KLT）实现</h2><p data-source-line="48">就是直接由操作系统内核支持的线程，这种线程由内核来完成线程切换。</p><p data-source-line="50">但是程序一般不会直接使用内核线程，而是去使用内核线程的一种高级接口—轻量级进程（Light Weight Process,LWP），轻量级进程就是我们所讲的线程，这种轻量级进程与内核线程之间1：1的对应关系。</p><p data-source-line="52"><img data-src="http%3A%2F%2Fnote.youdao.com%2Fyws%2Fres%2F42161%2FC15E04DF175A4F7E9FD16C495BAD656A" alt="image" src="http://note.youdao.com/yws/res/42161/C15E04DF175A4F7E9FD16C495BAD656A?ynotemdtimestamp=1562307580979" data-processed="http%3A%2F%2Fnote.youdao.com%2Fyws%2Fres%2F42161%2FC15E04DF175A4F7E9FD16C495BAD656A"></p><p data-source-line="54">Strength：</p><ul data-source-line="55"><li>内核直接支持，由操作系统内核创建和撤销。内核维护进程及线程的上下文信息以及线程切换。一个内核线程由于I/O操作而阻塞，不会影响其它线程的运行。</li></ul><p data-source-line="57">Weaknesses:</p><ul data-source-line="58"><li>线程的操作、创建、同步等都需要系统调用，而系统调用代价比较高，需要在用户态和内核态中来回切换。</li><li>每个轻量级的进程都需要一个内核线程来支持，需要消耗一定的内核资源，因此 LWP 是有限的。</li></ul><h2 data-source-line="61" id="用户线程实现"><a class="markdownIt-Anchor" href="#用户线程实现"></a><s>用户线程实现</s></h2><p data-source-line="63">用户线程指不需要内核支持而在用户程序中实现的线程，其不依赖于操作系统核心，应用进程利用线程库提供创建、同步、调度和管理线程的函数来控制用户线程。</p><p data-source-line="65"><img data-src="http%3A%2F%2Fnote.youdao.com%2Fyws%2Fres%2F42184%2F65F24BF661D0425A9219F9FF803D3296" alt="image" src="http://note.youdao.com/yws/res/42184/65F24BF661D0425A9219F9FF803D3296?ynotemdtimestamp=1562307580979" data-processed="http%3A%2F%2Fnote.youdao.com%2Fyws%2Fres%2F42184%2F65F24BF661D0425A9219F9FF803D3296"></p><p data-source-line="67">S:</p><ul data-source-line="68"><li>切换由用户态程序自己控制内核切换,不需要内核干涉，少了进出内核态的消耗。</li></ul><p data-source-line="70">W:</p><ul data-source-line="71"><li>没有系统内核的支援，所有的线程操作都需要自己处理，java曾经用过，不过最后还是放弃了。</li></ul><h2 data-source-line="73" id="用户线程加轻量级进程混合实现"><a class="markdownIt-Anchor" href="#用户线程加轻量级进程混合实现"></a>用户线程加轻量级进程混合实现</h2><p data-source-line="75">S:</p><ul data-source-line="76"><li>这种混合模式下，既存在用户线程，也存在轻量级进程。用户线程还是完全建立在用户空间中，因此用户线程的创建、切换、析构等依然廉价，可以支持大规模的用户线程并发。</li><li>操作系统提供支持的轻量进程作为用户线程和内核线程之间的桥梁，用户线程的系统调用要通过轻量级线程来完成，大大降低了进程阻塞的风险。用户线程和轻量级进程比是N:M多对对的关系。</li></ul><p data-source-line="79"><img data-src="http%3A%2F%2Fnote.youdao.com%2Fyws%2Fres%2F42178%2F2D3B728F85824EE5B1EC39D9E5C95BAC" alt="image" src="http://note.youdao.com/yws/res/42178/2D3B728F85824EE5B1EC39D9E5C95BAC?ynotemdtimestamp=1562307580979" data-processed="http%3A%2F%2Fnote.youdao.com%2Fyws%2Fres%2F42178%2F2D3B728F85824EE5B1EC39D9E5C95BAC"></p><h1 data-source-line="81" id="线程是如何调度的"><a class="markdownIt-Anchor" href="#线程是如何调度的"></a>线程是如何调度的？</h1><p data-source-line="83">线程调度主要是指系统为线程分配处理器使用权的过程，主要分为：协同式线程调度和抢占式线程调度。</p><h2 data-source-line="85" id="协同式调度"><a class="markdownIt-Anchor" href="#协同式调度"></a>协同式调度</h2><p data-source-line="87">线程的执行时间由线程本身来控制，线程把自己的工作执行完成以后，主动通知系统切换到另一个线程上。像lua的“协同历程”就是如此实现的。</p><p data-source-line="89">S:</p><ul data-source-line="90"><li>实现简单，线程把自己的事情干完后进行线程切换，切换操作对线程自己是可知的。无同步问题</li></ul><p data-source-line="92">W:</p><ul data-source-line="93"><li>线程执行时间不可控制，如果某个线程出现问题阻塞，会造成程序阻塞。</li><li>这几乎可以认为是一个扯淡的调度方式，执行时间不可控是肯定不能接受的，如果要是使用这种方式肯定还得需要其他方面的限制。存在即合理吧~</li></ul><h2 data-source-line="96" id="抢占式调度"><a class="markdownIt-Anchor" href="#抢占式调度"></a>抢占式调度</h2><p data-source-line="98">抢占式线程调度中每个线程<strong>由系统来分配执行时间</strong>，线程的切换不由线程本身来决定。所以不像协同式调度那样，出现单个线程阻塞造成整个进程阻塞的问题。Java就是采用这种方式。</p><p data-source-line="100">但是既然由系统分配时间，如果某线程想要更多的执行时间怎么办？答案就是设置优先级。</p><h1 data-source-line="102" id="java-线程的优先级"><a class="markdownIt-Anchor" href="#java-线程的优先级"></a>Java 线程的优先级</h1><p data-source-line="104">优先级代表了抢占CPU机会的大小。</p><p data-source-line="106">Thread类中有3个变量定义了线程优先级。（总共有0-10个优先级，级别为0的是分配给JVM的，应用程序不能设置该级别）</p><pre data-source-line="108"><code class="hljs"><span class="hljs-keyword">public</span> <span class="hljs-keyword">final</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">int</span> MIN_PRIORITY = <span class="hljs-number">1</span>;
<span class="hljs-keyword">public</span> <span class="hljs-keyword">final</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">int</span> NORM_PRIORITY = <span class="hljs-number">5</span>;
<span class="hljs-keyword">public</span> <span class="hljs-keyword">final</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">int</span> MAX_PRIORITY = <span class="hljs-number">10</span>;

</code></pre><p data-source-line="114">不同的操作系统有着不同的优先级，例如Linux有140个优先级，Windows有7个，Java又是跨平台的，所以JAVA相当于用着10个优先级做了一个<strong>粗略的映射</strong>（所以可以认为是有点稍微不靠谱的）。</p><pre data-source-line="116"><code class="hljs"><span class="hljs-keyword">package</span> test;

<span class="hljs-keyword">public</span> <span class="hljs-keyword">class</span> Test
{
	<span class="hljs-keyword">public</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">class</span> High <span class="hljs-keyword">extends</span> Thread
	{
		<span class="hljs-keyword">static</span> <span class="hljs-keyword">int</span> <span class="hljs-keyword">count</span> = <span class="hljs-number">0</span>;
		@Override
		<span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> run()
		{
			<span class="hljs-keyword">while</span> (<span class="hljs-keyword">true</span>)
			{
				<span class="hljs-keyword">synchronized</span> (Test.<span class="hljs-keyword">class</span>)
				{
					<span class="hljs-keyword">count</span>++;
					<span class="hljs-keyword">if</span> (<span class="hljs-keyword">count</span> &gt; <span class="hljs-number">10000000</span>)
					{
						System.out.<span class="hljs-keyword">println</span>(<span class="hljs-string">"High"</span>);
						<span class="hljs-keyword">break</span>;
					}
				}
			}
		}
	}
	<span class="hljs-keyword">public</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">class</span> Low <span class="hljs-keyword">extends</span> Thread
	{
		<span class="hljs-keyword">static</span> <span class="hljs-keyword">int</span> <span class="hljs-keyword">count</span> = <span class="hljs-number">0</span>;
		@Override
		<span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> run()
		{
			<span class="hljs-keyword">while</span> (<span class="hljs-keyword">true</span>)
			{
				<span class="hljs-keyword">synchronized</span> (Test.<span class="hljs-keyword">class</span>)
				{
					<span class="hljs-keyword">count</span>++;
					<span class="hljs-keyword">if</span> (<span class="hljs-keyword">count</span> &gt; <span class="hljs-number">10000000</span>)
					{
						System.out.<span class="hljs-keyword">println</span>(<span class="hljs-string">"Low"</span>);
						<span class="hljs-keyword">break</span>;
					}
				}
			}
		}
	}

	<span class="hljs-keyword">public</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">void</span> main(String[] args) <span class="hljs-keyword">throws</span> InterruptedException
	{
		High high = <span class="hljs-keyword">new</span> High();
		Low low = <span class="hljs-keyword">new</span> Low();
		high.setPriority(Thread.MAX_PRIORITY);
		low.setPriority(Thread.MIN_PRIORITY);
		low.start();
		high.start();
	}
}
</code></pre><p data-source-line="173">让一个高优先级的线程和低优先级的线程同时争夺一个锁，看看哪个最先完成。
当然并不一定是高优先级一定先完成。再多次运行后发现，高优先级完成的<mark>概率比较大</mark>，但是低优先级还是有可能先完成的。</p><p data-source-line="176">既然都已经有机会执行线程了，那么接下来肯定是如何更好的利用线程了，所以 JAVA 又针对线程定义了一些状态和状态之间的流转</p><h1 data-source-line="178" id="java-线程的状态"><a class="markdownIt-Anchor" href="#java-线程的状态"></a>Java 线程的状态</h1><h2 data-source-line="180" id="状态的定义与流转"><a class="markdownIt-Anchor" href="#状态的定义与流转"></a>状态的定义与流转：</h2><p data-source-line="182"><img data-src="http%3A%2F%2Fnote.youdao.com%2Fyws%2Fres%2F42223%2F3259B40DE0D0411984701E672AEB9942" alt="image" src="http://note.youdao.com/yws/res/42223/3259B40DE0D0411984701E672AEB9942?ynotemdtimestamp=1562307580979" data-processed="http%3A%2F%2Fnote.youdao.com%2Fyws%2Fres%2F42223%2F3259B40DE0D0411984701E672AEB9942"></p><p data-source-line="184">解析：</p><p data-source-line="186">当new出一个线程时，其实线程并没有工作。它只是生成了一个实体，当你调用这个实例的start方法时，线程才真正地被启动。启动后到Runnable状态，Runnable表示该线程的资源等等已经被准备好，已经可以执行了，但是并不表示一定在执行状态，由于时间片轮转，该线程也可能此时并没有在执行。对于我们来说，该线程可以认为已经被执行了，但是是否真实执行，还得看物理cpu的调度。当线程任务执行结束后，线程就到了Terminated状态。</p><p data-source-line="188">有时候在线程的执行当中，不可避免的会申请某些锁或某个对象的监视器，当无法获取时，这个线程会被阻塞住，会被挂起，到了Blocked状态。如果这个线程调用了wait方法，它就处于一个Waiting状态。进入Waiting状态的线程会等待其他线程给它notify，通知到之后由Waiting状态又切换到Runnable状态继续执行。当然等待状态有两种，一种是无限期等待，直到被notify。一直则是有限期等待，比如等待10秒还是没有被notify，则自动切换到Runnable状态。</p><p data-source-line="190">Thread的类中定义的线程状态枚举类：</p><pre data-source-line="192"><code class="hljs">public enum State {
        /**
         * Thread <span class="hljs-keyword">state</span> <span class="hljs-keyword">for</span> a thread which has not yet started.
         */
        NEW,

        /**
         * Thread <span class="hljs-keyword">state</span> <span class="hljs-keyword">for</span> a runnable thread.  A thread <span class="hljs-keyword">in</span> the runnable
         * <span class="hljs-keyword">state</span> is executing <span class="hljs-keyword">in</span> the Java virtual machine but it may
         * be waiting <span class="hljs-keyword">for</span> other resources <span class="hljs-keyword">from</span> the operating system
         * such as processor.
         */
        RUNNABLE,

        /**
         * Thread <span class="hljs-keyword">state</span> <span class="hljs-keyword">for</span> a thread blocked waiting <span class="hljs-keyword">for</span> a monitor lock.
         * A thread <span class="hljs-keyword">in</span> the blocked <span class="hljs-keyword">state</span> is waiting <span class="hljs-keyword">for</span> a monitor lock
         * <span class="hljs-keyword">to</span> enter a synchronized <span class="hljs-built_in">block</span>/method or
         * reenter a synchronized <span class="hljs-built_in">block</span>/method after calling
         * {@link Object<span class="hljs-comment">#wait() Object.wait}.</span>
         */
        BLOCKED,

        /**
         * Thread <span class="hljs-keyword">state</span> <span class="hljs-keyword">for</span> a waiting thread.
         * A thread is <span class="hljs-keyword">in</span> the waiting <span class="hljs-keyword">state</span> due <span class="hljs-keyword">to</span> calling one of the
         * following methods:
         * <span class="hljs-variable">&lt;ul&gt;</span>
         *   <span class="hljs-variable">&lt;li&gt;</span>{@link Object<span class="hljs-comment">#wait() Object.wait} with no timeout&lt;/li&gt;</span>
         *   <span class="hljs-variable">&lt;li&gt;</span>{@link <span class="hljs-comment">#join() Thread.join} with no timeout&lt;/li&gt;</span>
         *   <span class="hljs-variable">&lt;li&gt;</span>{@link LockSupport<span class="hljs-comment">#park() LockSupport.park}&lt;/li&gt;</span>
         * &lt;/ul&gt;
         *
         * <span class="hljs-variable">&lt;p&gt;</span>A thread <span class="hljs-keyword">in</span> the waiting <span class="hljs-keyword">state</span> is waiting <span class="hljs-keyword">for</span> another thread <span class="hljs-keyword">to</span>
         * perform a particular action.
         *
         * For example, a thread that has called <span class="hljs-variable">&lt;tt&gt;</span>Object.wait()&lt;/tt&gt;
         * <span class="hljs-keyword">on</span> an object is waiting <span class="hljs-keyword">for</span> another thread <span class="hljs-keyword">to</span> call
         * <span class="hljs-variable">&lt;tt&gt;</span>Object.notify()&lt;/tt&gt; or <span class="hljs-variable">&lt;tt&gt;</span>Object.notifyAll()&lt;/tt&gt; <span class="hljs-keyword">on</span>
         * that object. A thread that has called <span class="hljs-variable">&lt;tt&gt;</span>Thread.join()&lt;/tt&gt;
         * is waiting <span class="hljs-keyword">for</span> a specified thread <span class="hljs-keyword">to</span> terminate.
         */
        WAITING,

        /**
         * Thread <span class="hljs-keyword">state</span> <span class="hljs-keyword">for</span> a waiting thread with a specified waiting time.
         * A thread is <span class="hljs-keyword">in</span> the timed waiting <span class="hljs-keyword">state</span> due <span class="hljs-keyword">to</span> calling one of
         * the following methods with a specified positive waiting time:
         * <span class="hljs-variable">&lt;ul&gt;</span>
         *   <span class="hljs-variable">&lt;li&gt;</span>{@link <span class="hljs-comment">#sleep Thread.sleep}&lt;/li&gt;</span>
         *   <span class="hljs-variable">&lt;li&gt;</span>{@link Object<span class="hljs-comment">#wait(long) Object.wait} with timeout&lt;/li&gt;</span>
         *   <span class="hljs-variable">&lt;li&gt;</span>{@link <span class="hljs-comment">#join(long) Thread.join} with timeout&lt;/li&gt;</span>
         *   <span class="hljs-variable">&lt;li&gt;</span>{@link LockSupport<span class="hljs-comment">#parkNanos LockSupport.parkNanos}&lt;/li&gt;</span>
         *   <span class="hljs-variable">&lt;li&gt;</span>{@link LockSupport<span class="hljs-comment">#parkUntil LockSupport.parkUntil}&lt;/li&gt;</span>
         * &lt;/ul&gt;
         */
        TIMED_WAITING,

        /**
         * Thread <span class="hljs-keyword">state</span> <span class="hljs-keyword">for</span> a terminated thread.
         * The thread has completed execution.
         */
        TERMINATED;
    }
</code></pre><h2 data-source-line="259" id="blocked-状态可能的原因"><a class="markdownIt-Anchor" href="#blocked-状态可能的原因"></a>BLOCKED 状态可能的原因：</h2><ol data-source-line="261"><li>通过调用sleep(milliseconds)。</li><li>通过调用wait().</li><li>任务在等待某个任务的输入或输出完成。</li><li>任务试图在<mark>某个对象上</mark>调用其同步控制方法，但是另一个任务已经获得该锁时。</li></ol><h2 data-source-line="266" id="blocked-与-waiting-的区别"><a class="markdownIt-Anchor" href="#blocked-与-waiting-的区别"></a>BLOCKED 与 WAITING 的区别</h2><ul data-source-line="268"><li>BLOCKED 是指线程正在等待获取锁；</li><li>WAITING 是指线程正在等待其他线程发来的通知（notify），收到通知后，可能会顺序向后执行（RUNNABLE），也可能会再次获取锁，进而被阻塞住（BLOCKED）。</li></ul><h1 data-source-line="271" id="java线程的其他知识"><a class="markdownIt-Anchor" href="#java线程的其他知识"></a>Java线程的其他知识</h1><h3 data-source-line="273" id="runable-thread"><a class="markdownIt-Anchor" href="#runable-thread"></a>Runable、Thread</h3><ul data-source-line="275"><li>Runable是一个接口，相当于对任务的一种描述（在run()内描述任务）。</li><li>Thread是一个类。将任务附着到线程上，从而实现线程的行为。</li></ul><h3 data-source-line="279" id="线程基本操作"><a class="markdownIt-Anchor" href="#线程基本操作"></a>线程基本操作</h3><h4 data-source-line="282" id="新建线程-threadstart"><a class="markdownIt-Anchor" href="#新建线程-threadstart"></a>新建线程-Thread.start()</h4><p data-source-line="283">start方法其实是在一个新的操作系统线程上面去调用run方法。换句话说，<mark>直接调用run方法而不是调用start方法</mark>的话，它并不会开启新的线程，而是在调用run的当前的线程当中执行你的操作。</p><pre data-source-line="286"><code class="hljs"><span class="hljs-comment">//<span class="zh-hans">输出</span>t1</span>

<span class="hljs-keyword">Thread</span> <span class="hljs-keyword">thread</span> = <span class="hljs-literal">new</span> <span class="hljs-keyword">Thread</span>(<span class="hljs-string">"t1"</span>)
{
	@Override
	<span class="hljs-keyword">public</span> <span class="hljs-literal">void</span> run()
	{
		<span class="hljs-comment">// TODO Auto-generated method stub</span>
		System.out.println(<span class="hljs-keyword">Thread</span>.currentThread().getName());
	}
};
<span class="hljs-keyword">thread</span>.start();
</code></pre><pre data-source-line="301"><code class="hljs"><span class="hljs-comment">//<span class="zh-hans">输出</span>main<span class="zh-hans">，直接调用</span>run<span class="zh-hans">其实就是一个普通的函数调用而已，并没有达到多线程的作用</span></span>

<span class="hljs-keyword">Thread</span> <span class="hljs-keyword">thread</span> = <span class="hljs-literal">new</span> <span class="hljs-keyword">Thread</span>(<span class="hljs-string">"t1"</span>)
{
	@Override
	<span class="hljs-keyword">public</span> <span class="hljs-literal">void</span> run()
	{
		<span class="hljs-comment">// TODO Auto-generated method stub</span>
		System.out.println(<span class="hljs-keyword">Thread</span>.currentThread().getName());
	}
};
<span class="hljs-keyword">thread</span>.run();
</code></pre><p data-source-line="315">run方法的实现有两种方式。
其一就是上面的方法，还有一种是</p><pre data-source-line="318"><code class="hljs"><span class="hljs-keyword">Thread</span> t1=<span class="hljs-keyword">new</span> <span class="hljs-keyword">Thread</span>(<span class="hljs-keyword">new</span> CreateThread3())<span class="hljs-comment">;</span>
</code></pre><p data-source-line="321">此种方法显然显得更加面向对象。</p><h4 data-source-line="323" id="终止线程-threadstop"><a class="markdownIt-Anchor" href="#终止线程-threadstop"></a>终止线程-<s>Thread.stop()</s></h4><blockquote data-source-line="325"><p>Thread.stop() 不推荐使用。它会释放所有monitor</p></blockquote><p data-source-line="327">原因在于stop方法太过"暴力"了，无论线程执行到哪里，它将会立即停止掉线程。</p><p data-source-line="329"><img data-src="http%3A%2F%2Fstatic.oschina.net%2Fuploads%2Fspace%2F2016%2F0107%2F113702_iwlY_2243330.jpg" alt="link" src="http://static.oschina.net/uploads/space/2016/0107/113702_iwlY_2243330.jpg?ynotemdtimestamp=1562307580979" data-processed="http%3A%2F%2Fstatic.oschina.net%2Fuploads%2Fspace%2F2016%2F0107%2F113702_iwlY_2243330.jpg"></p><p data-source-line="331">当写线程得到锁以后开始写入数据，写完id = 1，在准备将name = 1时被stop,释放锁。读线程获得锁进行读操作，读到的id为1，而name还是0，导致了数据不一致。</p><p data-source-line="333"><mark>最重要的是这种错误不会抛出异常，将很难被发现。</mark></p><p data-source-line="335">既然stop()会破坏原子逻辑，那么我们可以自定义一个volatile变量，自己去实现一个终止线程的方法。</p><pre data-source-line="337"><code class="hljs"><span class="hljs-keyword">package</span> <span class="zh-hans">线程基础</span>;

<span class="hljs-comment">/**
 * Description:volatile<span class="zh-hans">具备可见性</span>
 * &lt;p&gt;
 * 1. <span class="zh-hans">在设置成</span>false<span class="zh-hans">之后，线程依然没有关闭，原因是只改变了本线程的</span>isRunning<span class="zh-hans">变量的副本，而没有改变全局变量的值</span>.&lt;br&gt;
 * 2. <span class="zh-hans">可以加上</span>volatile<span class="zh-hans">关键字设置该变量为其他线程可见。</span>
 *
 * <span class="hljs-doctag">@author</span> SunYukun
 * <span class="hljs-doctag">@date</span> 2017/7/2
 */</span>
<span class="hljs-keyword">public</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Volatile_01</span> <span class="hljs-keyword">extends</span> <span class="hljs-title">Thread</span> </span>{
    <span class="hljs-keyword">private</span> <span class="hljs-keyword">volatile</span> <span class="hljs-keyword">boolean</span> isRunning = <span class="hljs-keyword">true</span>;

    <span class="hljs-function"><span class="hljs-keyword">public</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">void</span> <span class="hljs-title">main</span><span class="hljs-params">(String[] args)</span> <span class="hljs-keyword">throws</span> Exception </span>{
        Volatile_01 thread = <span class="hljs-keyword">new</span> Volatile_01();
        thread.start();

        Thread.sleep(<span class="hljs-number">1000</span>);
        thread.setIsRunning(<span class="hljs-keyword">false</span>);
        System.out.println(<span class="hljs-string">"<span class="zh-hans">设置成</span>false"</span>);
    }

    <span class="hljs-function"><span class="hljs-keyword">private</span> <span class="hljs-keyword">void</span> <span class="hljs-title">setIsRunning</span><span class="hljs-params">(<span class="hljs-keyword">boolean</span> flag)</span> </span>{
        isRunning = flag;
    }

    <span class="hljs-meta">@Override</span>
    <span class="hljs-function"><span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">run</span><span class="hljs-params">()</span> </span>{
        <span class="hljs-keyword">while</span> (isRunning) {
            System.out.println(<span class="hljs-string">"running ...."</span>);
        }
        System.out.println(<span class="hljs-string">"stop"</span>);
    }
}

</code></pre><h4 data-source-line="379" id="线程中断-threadinterrupt"><a class="markdownIt-Anchor" href="#线程中断-threadinterrupt"></a>线程中断-Thread.interrupt</h4><p data-source-line="381">Java的中断是一种协作机制。也就是说调用线程对象的interrupt方法并不一定就中断了正在运行的线程，它只是要求线程自己在<mark>合适的时机中断自己</mark>。</p><p data-source-line="383">每个线程都有一个boolean的中断状态（<mark>不一定就是对象的属性，事实上，该状态也确实不是Thread的字段</mark>），interrupt方法仅仅只是将该状态置为true。对于非阻塞中的线程, 只是改变了中断状态, 即Thread.isInterrupted()将返回true，<mark>并不会使程序停止</mark>;</p><pre data-source-line="385"><code class="hljs"><span class="hljs-function"><span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">run</span>(<span class="hljs-params"></span>)</span>{<span class="hljs-comment">//<span class="zh-hans">线程</span>t1</span>
   <span class="hljs-keyword">while</span>(<span class="hljs-literal">true</span>){
      Thread.<span class="hljs-keyword">yield</span>();
   }
}
t1.interrupt();
</code></pre><p data-source-line="393">这样使线程t1中断，是不会有效果的，只是更改了中断状态位。</p><p data-source-line="395">如果希望非常优雅地终止这个线程，就该这样做：</p><pre data-source-line="397"><code class="hljs"><span class="hljs-function"><span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">run</span>(<span class="hljs-params"></span>)</span>{
    <span class="hljs-keyword">while</span>(<span class="hljs-literal">true</span>)
    {
        <span class="hljs-keyword">if</span>(Thread.currentThread().isInterrupted())
        {
           System.<span class="hljs-keyword">out</span>.println(<span class="hljs-string">"Interruted!"</span>);
           <span class="hljs-keyword">break</span>;
        }
        Thread.<span class="hljs-keyword">yield</span>();
    }
}
</code></pre><p data-source-line="410"><ins>使用中断，就对数据一致性有了一定的保证。</ins></p><p data-source-line="412">对于可取消的阻塞状态中的线程, 比如等待在这些函数上的线程, Thread.sleep(), Object.wait(), Thread.join(), 这个线程收到中断信号后, 会<mark>抛出InterruptedException</mark>, 同时会<mark>把中断状态置回为false.</mark></p><p data-source-line="414">对于取消阻塞状态中的线程，可以这样书写代码：</p><pre data-source-line="416"><code class="hljs"><span class="hljs-function"><span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">run</span>(<span class="hljs-params"></span>)</span>{
    <span class="hljs-keyword">while</span>(<span class="hljs-literal">true</span>){
        <span class="hljs-keyword">if</span>(Thread.currentThread().isInterrupted()){
            System.<span class="hljs-keyword">out</span>.println(<span class="hljs-string">"Interrupted!"</span>);
            <span class="hljs-keyword">break</span>;
        }
        <span class="hljs-keyword">try</span> {
           Thread.sleep(<span class="hljs-number">2000</span>);
        } <span class="hljs-keyword">catch</span> (InterruptedException e) {
           System.<span class="hljs-keyword">out</span>.println(<span class="hljs-string">"Interruted When Sleep"</span>);
           <span class="hljs-comment">//<span class="zh-hans">设置中断状态，抛出异常后会清除中断标记位</span></span>
           Thread.currentThread().interrupt();
        }
        Thread.<span class="hljs-keyword">yield</span>();
    }
}
</code></pre><h4 data-source-line="435" id="等待与通知waitnotify"><a class="markdownIt-Anchor" href="#等待与通知waitnotify"></a>等待与通知——wait()/notify()</h4><ol data-source-line="437"><li>属于Object类的方法；</li><li>wait()是释放锁的；</li><li><mark>notify()不释放锁；</mark></li><li>需要配合synchronized关键字使用。</li></ol><pre data-source-line="442"><code class="hljs">   <span class="hljs-keyword">public</span> <span class="hljs-keyword">final</span> <span class="hljs-function"><span class="hljs-keyword">void</span> <span class="hljs-title">wait</span><span class="hljs-params">()</span> <span class="hljs-keyword">throws</span> InterruptedException </span>{
        wait(<span class="hljs-number">0</span>);
    }
</code></pre><pre data-source-line="448"><code class="hljs"><span class="hljs-keyword">public</span> <span class="hljs-keyword">final</span> <span class="hljs-keyword">native</span> <span class="hljs-function"><span class="hljs-keyword">void</span> <span class="hljs-title">notify</span><span class="hljs-params">()</span></span>;
</code></pre><p data-source-line="452">如果在一个线程上调用一个对象的object.wait()，那么这个线程就会进入object对象的等待队列。</p><p data-source-line="454">调用notify()会随机的唤醒一个等待队列中的线程。</p><h4 data-source-line="459" id="线程挂起-threadsuspend"><a class="markdownIt-Anchor" href="#线程挂起-threadsuspend"></a>线程挂起-<s>Thread.suspend()</s></h4><p data-source-line="460">挂起（suspend）和继续执行（resume）线程</p><p data-source-line="462">suspend()<mark>不会释放锁</mark>
如果加锁发生在resume()之前 ，则死锁发生
这两个方法都是Deprecated方法，不推荐使用。</p><p data-source-line="466">原因在于，suspend不释放锁，因此没有线程可以访问被它锁住的临界区资源，直到被其他线程resume。</p><p data-source-line="468">因为无法控制线程运行的先后顺序，如果其他线程的resume方法先被运行，那则后运行的suspend，将一直占有这把锁，造成死锁发生。</p><p data-source-line="470">用以下代码来模拟这个场景</p><pre data-source-line="473"><code class="hljs"><span class="hljs-keyword">package</span> test;

<span class="hljs-keyword">public</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Test</span>
</span>{
	<span class="hljs-keyword">static</span> Object u = <span class="hljs-keyword">new</span> Object();
	<span class="hljs-keyword">static</span> TestSuspendThread t1 = <span class="hljs-keyword">new</span> TestSuspendThread(<span class="hljs-string">"t1"</span>);
	<span class="hljs-keyword">static</span> TestSuspendThread t2 = <span class="hljs-keyword">new</span> TestSuspendThread(<span class="hljs-string">"t2"</span>);

	<span class="hljs-keyword">public</span> <span class="hljs-keyword">static</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">TestSuspendThread</span> <span class="hljs-keyword">extends</span> <span class="hljs-title">Thread</span>
	</span>{
		<span class="hljs-function"><span class="hljs-keyword">public</span> <span class="hljs-title">TestSuspendThread</span><span class="hljs-params">(String name)</span>
		</span>{
			setName(name);
		}

		<span class="hljs-meta">@Override</span>
		<span class="hljs-function"><span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">run</span><span class="hljs-params">()</span>
		</span>{
			<span class="hljs-keyword">synchronized</span> (u)
			{
				System.out.println(<span class="hljs-string">"in "</span> + getName());
				Thread.currentThread().suspend();
			}
		}
	}

	<span class="hljs-function"><span class="hljs-keyword">public</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">void</span> <span class="hljs-title">main</span><span class="hljs-params">(String[] args)</span> <span class="hljs-keyword">throws</span> InterruptedException
	</span>{
		t1.start();
		Thread.sleep(<span class="hljs-number">100</span>);
		t2.start();
		t1.resume();
		t2.resume();
		t1.join();
		t2.join();
	}
}
</code></pre><p data-source-line="512">让t1,t2同时争夺一把锁，争夺到的线程suspend，然后再resume，按理来说，应该某个线程争夺后被resume释放了锁，然后另一个线程争夺掉锁，再被resume。
结果输出是：</p><pre data-source-line="515"><code class="hljs"><span class="hljs-keyword">in</span> t1
<span class="hljs-keyword">in</span> t2
</code></pre><p data-source-line="519">说明两个线程都争夺到了锁，但是控制台的红灯还是亮着的，说明t1,t2一定有线程没有执行完。我们dump出堆来看看
<img data-src="http%3A%2F%2Fstatic.oschina.net%2Fuploads%2Fspace%2F2016%2F0107%2F150701_t6gX_2243330.jpg" alt="image" src="http://static.oschina.net/uploads/space/2016/0107/150701_t6gX_2243330.jpg?ynotemdtimestamp=1562307580979" data-processed="http%3A%2F%2Fstatic.oschina.net%2Fuploads%2Fspace%2F2016%2F0107%2F150701_t6gX_2243330.jpg"></p><h4 data-source-line="522" id="join和yeild"><a class="markdownIt-Anchor" href="#join和yeild"></a>join和yeild</h4><p data-source-line="524">yeild是个native静态方法，这个方法是想把自己占有的cpu时间<mark>释放掉，然后和其他线程一起竞争</mark>(注意yeild的线程还是有可能争夺到cpu，注意与sleep区别)。在javadoc中也说明了，yeild是个基本不会用到的方法，一般在debug和test中使用。</p><p data-source-line="526">join方法的意思是<mark>等待其他线程结束，就如suspend那节的代码，想让主线程等待t1,t2结束以后再结束。没有结束的话，主线程就一直阻塞在那里</mark>。按我的理解就是，调用此方法，就是给t1,t2加入到该主线程中，也就是主线程得等到他们都结束了，才能结束自己的任务。</p><pre data-source-line="529"><code class="hljs"><span class="hljs-keyword">package</span> test;

<span class="hljs-keyword">public</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Test</span>
</span>{
	<span class="hljs-keyword">public</span> <span class="hljs-keyword">volatile</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">int</span> i = <span class="hljs-number">0</span>;

	<span class="hljs-keyword">public</span> <span class="hljs-keyword">static</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">AddThread</span> <span class="hljs-keyword">extends</span> <span class="hljs-title">Thread</span>
	</span>{
		<span class="hljs-meta">@Override</span>
		<span class="hljs-function"><span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">run</span><span class="hljs-params">()</span>
		</span>{
			<span class="hljs-keyword">for</span> (i = <span class="hljs-number">0</span>; i &lt; <span class="hljs-number">10000000</span>; i++)
				;
		}
	}

	<span class="hljs-function"><span class="hljs-keyword">public</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">void</span> <span class="hljs-title">main</span><span class="hljs-params">(String[] args)</span> <span class="hljs-keyword">throws</span> InterruptedException
	</span>{
		AddThread at = <span class="hljs-keyword">new</span> AddThread();
		at.start();
		at.join();
		System.out.println(i);
	}
}
</code></pre><p data-source-line="555">如果把上述代码的at.join去掉，则主线程会直接运行结束，i的值会很小。如果有join,打印出的i的值一定是10000000。
那么join是怎么实现的呢？</p><p data-source-line="558">join的本质 ：</p><pre data-source-line="561"><code class="hljs"><span class="hljs-keyword">while</span> <span class="hljs-comment">(isAlive()</span>) {<span class="hljs-comment">//<span class="zh-hans">意为无限期的等待</span></span>
    wait<span class="hljs-comment">(0)</span>;
}
</code></pre><p data-source-line="566">join()方法也可以传递一个时间，意为有限期地等待，超过了这个时间就自动唤醒。</p><p data-source-line="568">这样就有一个问题，谁来notify这个线程呢，在thread类中没有地方调用了notify？</p><p data-source-line="570">在javadoc中，找到了相关解释。当一个线程运行完成终止后，将会<mark>调用notifyAll方法去唤醒等待在当前线程实例上的所有线程,这个操作是jvm自己完成的</mark>。</p><p data-source-line="572">所以javadoc中还给了我们一个建议，<mark>不要使用wait和notify/notifyall在线程实例上</mark>。因为jvm会自己调用，有可能与你调用期望的结果不同。</p><h4 data-source-line="574" id="守护线程-threadsetdeamontrue"><a class="markdownIt-Anchor" href="#守护线程-threadsetdeamontrue"></a>守护线程-Thread.setDeamon(true)</h4><blockquote data-source-line="576"><p>Daemon 中的 ae 为一个音，重音在 /di:/ 上，好象是指希腊守护神的意思吧。在计算机专业英语中是守护线程的意思。<mark>原以为</mark>如果一个线程被光荣地定义为守护线程，一定会直到进程运行到最后一刻，但真错了，如果一个线程是守护线程，那么，主线程运行结束时，如果没有任何非守护线程在运行，守护线程就会自尽了。</p></blockquote><blockquote data-source-line="578"><p>setDaemon方法是Thread中的方法，默认为false状态，可将该线程标记为守护线程或用户线程，该方法必须在启动线程前调用，具有最低的优先级，让系统资源优先调用其他线程。当正在运行的线程都是守护线程时，Java 虚拟机退出。</p></blockquote><ol data-source-line="580"><li>在后台默默地完成一些系统性的服务，比如垃圾回收线程、JIT线程就可以理解为守护线程。</li><li><strong>当一个Java应用内，所有非守护进程都结束时，Java虚拟机就会自然退出。</strong></li><li>setDaemon需要在start方法调用之前使用。</li><li>主线程结束后：
1） 用户线程将会继续运行
2） 如果没有用户线程，都是后台进程的话，那么jvm结束。</li></ol><p data-source-line="588">这样就开启了一个守护线程：</p><pre data-source-line="590"><code class="hljs"><span class="hljs-type">Thread</span> t=<span class="hljs-function"><span class="hljs-keyword">new</span> <span class="hljs-title">DaemonT</span>();
<span class="hljs-title">t</span>.<span class="hljs-title">setDaemon</span>(true);
<span class="hljs-title">t</span>.<span class="hljs-title">start</span>();

</span></code></pre><pre data-source-line="597"><code class="hljs"><span class="hljs-keyword">package</span> test;

<span class="hljs-keyword">public</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Test</span>
</span>{
	<span class="hljs-keyword">public</span> <span class="hljs-keyword">static</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">DaemonThread</span> <span class="hljs-keyword">extends</span> <span class="hljs-title">Thread</span>
	</span>{
		<span class="hljs-meta">@Override</span>
		<span class="hljs-function"><span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">run</span><span class="hljs-params">()</span>
		</span>{
			<span class="hljs-keyword">for</span> (<span class="hljs-keyword">int</span> i = <span class="hljs-number">0</span>; i &lt; <span class="hljs-number">10000000</span>; i++)
			{
				System.out.println(<span class="hljs-string">"hi"</span>);
			}
		}
	}

	<span class="hljs-function"><span class="hljs-keyword">public</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">void</span> <span class="hljs-title">main</span><span class="hljs-params">(String[] args)</span> <span class="hljs-keyword">throws</span> InterruptedException
	</span>{
		DaemonThread dt = <span class="hljs-keyword">new</span> DaemonThread();
		dt.start();
	}
}
</code></pre><p data-source-line="621">当线程dt不是一个守护线程时，在运行后，我们能看到控制台输出hi
当在start之前加入</p><pre data-source-line="624"><code class="hljs">dt.setDaemon(true)<span class="hljs-comment">;</span>
</code></pre><p data-source-line="627">控制台就直接退出了，并没有输出。</p><h3 data-source-line="637" id="线程安全-与-线程不安全"><a class="markdownIt-Anchor" href="#线程安全-与-线程不安全"></a>线程安全 与 线程不安全</h3><p data-source-line="638">线程安全</p><blockquote data-source-line="640"><p>就是多线程访问时，采用了加锁机制，当一个线程访问该类的某个数据时，进行保护，其他线程不能进行访问直到该线程读取完，其他线程才可使用。不会出现数据不一致或者数据污染。</p></blockquote><p data-source-line="642">线程不安全</p><blockquote data-source-line="643"><p>就是不提供数据访问保护，有可能出现多个线程先后更改数据造成所得到的数据是脏数据</p></blockquote><h3 data-source-line="645" id="进程间通信"><a class="markdownIt-Anchor" href="#进程间通信"></a>进程间通信</h3><h4 data-source-line="647" id="管道-pipe"><a class="markdownIt-Anchor" href="#管道-pipe"></a>管道( pipe )</h4><p data-source-line="649">管道是一种半双工的通信方式，数据只能单向流动，而且只能在具有亲缘关系的进程间使用。进程的亲缘关系通常是指父子进程关系。</p><p data-source-line="651">有名管道也是半双工的通信方式，但是它允许无亲缘关系进程间的通信。</p><h4 data-source-line="653" id="信号量-semophore"><a class="markdownIt-Anchor" href="#信号量-semophore"></a>信号量( semophore )</h4><p data-source-line="654">信号量是一个计数器，可以用来控制多个进程对共享资源的访问。它常作为一种锁机制，防止某进程正在访问共享资源时，其他进程也访问该资源。因此，主要作为进程间以及同一进程内不同线程之间的同步手段。</p><h4 data-source-line="657" id="消息队列-message-queue"><a class="markdownIt-Anchor" href="#消息队列-message-queue"></a>消息队列( message queue )</h4><p data-source-line="658">消息队列是由消息的链表，存放在内核中并由消息队列标识符标识。消息队列克服了信号传递信息少、管道只能承载无格式字节流以及缓冲区大小受限等缺点。</p><h4 data-source-line="661" id="信号-sinal"><a class="markdownIt-Anchor" href="#信号-sinal"></a>信号 ( sinal )</h4><p data-source-line="662">信号是一种比较复杂的通信方式，用于通知接收进程某个事件已经发生。</p><h4 data-source-line="664" id="共享内存-shared-memory"><a class="markdownIt-Anchor" href="#共享内存-shared-memory"></a>共享内存( shared memory )</h4><p data-source-line="665">共享内存就是映射一段能被其他进程所访问的内存，这段共享内存由一个进程创建，但多个进程都可以访问。共享内存是最快的 IPC 方式，它是针对其他进程间通信方式运行效率低而专门设计的。它往往与其他通信机制，如信号两，配合使用，来实现进程间的同步和通信。</p><h4 data-source-line="667" id="套接字-socket"><a class="markdownIt-Anchor" href="#套接字-socket"></a>套接字( socket )</h4><p data-source-line="668">套解口也是一种进程间通信机制，与其他通信机制不同的是，它可用于不同及其间的进程通信。</p><h3 data-source-line="671" id="线程间通信"><a class="markdownIt-Anchor" href="#线程间通信"></a>线程间通信</h3><p data-source-line="673">线程间的相互作用：线程之间需要一些协调通信，来共同完成一件任务。</p><p data-source-line="675">Object类中相关的方法有两个notify方法和三个wait方法：</p><p data-source-line="677"><a href="http://docs.oracle.com/javase/7/docs/api/java/lang/Object.html" target="_blank">http://docs.oracle.com/javase/7/docs/api/java/lang/Object.html</a></p><p data-source-line="679">因为wait和notify方法定义在Object类中，因此会被所有的类所继承。</p><p data-source-line="681"><mark>这些方法都是final的</mark>，即它们都是不能被重写的，不能通过子类覆写去改变它们的行为。</p><h4 data-source-line="683" id="1-通过共享对象通信"><a class="markdownIt-Anchor" href="#1-通过共享对象通信"></a>1、通过共享对象通信</h4><p data-source-line="685">线程间发送信号的一个简单方式是在共享对象的变量里设置信号值。线程A在一个同步块里设置boolean型成员变量hasDataToProcess为true，线程B也在同步块里读取hasDataToProcess这个成员变量。这个简单的例子使用了一个持有信号的对象，并提供了set和check方法:</p><pre data-source-line="687"><code class="hljs"><span class="hljs-keyword">public</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">MySignal</span></span>{

  <span class="hljs-keyword">protected</span> <span class="hljs-keyword">boolean</span> hasDataToProcess = <span class="hljs-keyword">false</span>;

  <span class="hljs-function"><span class="hljs-keyword">public</span> <span class="hljs-keyword">synchronized</span> <span class="hljs-keyword">boolean</span> <span class="hljs-title">hasDataToProcess</span><span class="hljs-params">()</span></span>{
    <span class="hljs-keyword">return</span> <span class="hljs-keyword">this</span>.hasDataToProcess;
  }

  <span class="hljs-function"><span class="hljs-keyword">public</span> <span class="hljs-keyword">synchronized</span> <span class="hljs-keyword">void</span> <span class="hljs-title">setHasDataToProcess</span><span class="hljs-params">(<span class="hljs-keyword">boolean</span> hasData)</span></span>{
    <span class="hljs-keyword">this</span>.hasDataToProcess = hasData;
  }

}
</code></pre><p data-source-line="703">线程A和B必须获得指向一个MySignal共享实例的引用，以便进行通信。如果它们持有的引用指向不同的MySingal实例，那么彼此将不能检测到对方的信号。需要处理的数据可以存放在一个共享缓存区里，它和MySignal实例是分开存放的。</p><h4 data-source-line="704" id="2-忙等待"><a class="markdownIt-Anchor" href="#2-忙等待"></a>2、忙等待</h4><h4 data-source-line="706" id="3-waitnotify和notifyall"><a class="markdownIt-Anchor" href="#3-waitnotify和notifyall"></a>3、wait(),notify()和notifyAll()</h4><p data-source-line="708">在一个对象上调用wait()和notify()时，这个object对象也就成了多个线程之间的一个通信手段。</p><h5 data-source-line="710" id="例-1通知其他线程"><a class="markdownIt-Anchor" href="#例-1通知其他线程"></a>例 1：通知其他线程</h5><ul data-source-line="712"><li><ol><li>t1线程去添加list，希望加够5个之后，让t2线程感知到。</li></ol></li><li><ol start="2"><li>但是不好的地方是，t2线程是一直启动着的,一直在<mark>轮询</mark>。</li></ol></li></ul><pre data-source-line="715"><code class="hljs"><span class="hljs-keyword">public</span> class MyThread_01 {
            <span class="hljs-keyword">private</span> <span class="hljs-keyword">volatile</span> <span class="hljs-keyword">static</span> List&lt;<span class="hljs-keyword">String</span>&gt; list = <span class="hljs-keyword">new</span> ArrayList&lt;<span class="hljs-keyword">String</span>&gt;();

            <span class="hljs-keyword">public</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">void</span> main(<span class="hljs-keyword">String</span>[] args) {

                <span class="hljs-keyword">final</span> MyThread_02 t = <span class="hljs-keyword">new</span> MyThread_02();
                <span class="hljs-comment">//<span class="zh-hans">线程</span>1 <span class="zh-hans">往</span>list<span class="zh-hans">中加数据</span></span>
                Thread t1 = <span class="hljs-keyword">new</span> Thread(<span class="hljs-keyword">new</span> Runnable() {
                    <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> run() {
                        <span class="hljs-keyword">try</span> {
                            <span class="hljs-keyword">for</span> (<span class="hljs-built_in">int</span> i = <span class="hljs-number">0</span>; i &lt; <span class="hljs-number">10</span>; i++) {
                                t.<span class="hljs-built_in">add</span>();
                                System.out.<span class="hljs-built_in">println</span>(<span class="hljs-string">"<span class="zh-hans">当前线程</span>"</span> + Thread.currentThread().getName() + <span class="hljs-string">":<span class="zh-hans">添加了一个数据。。</span>"</span>);
                                Thread.sleep(<span class="hljs-number">500</span>);
                            }
                        } <span class="hljs-keyword">catch</span> (InterruptedException e) {
                            e.printStackTrace();
                        }

                    }
                }, <span class="hljs-string">"t1"</span>);

                <span class="hljs-comment">//<span class="zh-hans">线程</span>2 <span class="zh-hans">当</span>list<span class="zh-hans">数组中有</span>5<span class="zh-hans">个元素时</span>,<span class="zh-hans">就打印一句话，表示自己收到了通知</span></span>
                Thread t2 = <span class="hljs-keyword">new</span> Thread(<span class="hljs-keyword">new</span> Runnable() {
                    <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> run() {
                        <span class="hljs-keyword">while</span> (<span class="hljs-keyword">true</span>) {
                            <span class="hljs-keyword">if</span> (t.<span class="hljs-built_in">size</span>() == <span class="hljs-number">5</span>) {
                                System.out.<span class="hljs-built_in">println</span>(<span class="hljs-string">"<span class="zh-hans">当前线程</span>"</span> + Thread.currentThread().getName() + <span class="hljs-string">":<span class="zh-hans">接收到通知。。并停止线程</span>"</span>);
                                <span class="hljs-keyword">throw</span> <span class="hljs-keyword">new</span> RuntimeException();
                            }
                        }
                        <span class="hljs-comment">//<span class="zh-hans">打印一句话</span></span>
                    }
                }, <span class="hljs-string">"t2"</span>);

                t1.start();
                t2.start();
            }

            <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-built_in">add</span>() {
                list.<span class="hljs-built_in">add</span>(<span class="hljs-string">"1"</span>);
            }

            <span class="hljs-keyword">public</span> <span class="hljs-built_in">int</span> <span class="hljs-built_in">size</span>() {
                <span class="hljs-keyword">return</span> list.<span class="hljs-built_in">size</span>();
    }
}
</code></pre><h5 data-source-line="764" id="例-2使用wait-notify通知"><a class="markdownIt-Anchor" href="#例-2使用wait-notify通知"></a>例 2：使用wait()、notify()通知</h5><ul data-source-line="766"><li><ol><li>并未实时通知对方；因为t1拿到线程之后不释放锁，所以非得等到它执行完以后，才有机会执行t2，到那时也就不等于5了。</li></ol></li><li><ol start="2"><li>wait()会阻塞。</li></ol></li></ul><pre data-source-line="769"><code class="hljs"><span class="hljs-keyword">public</span> class MyThread_02 {
    <span class="hljs-comment">//<span class="zh-hans">声明一个</span> <span class="zh-hans">实例变量</span></span>
    <span class="hljs-keyword">private</span> <span class="hljs-keyword">volatile</span> <span class="hljs-keyword">static</span> List&lt;<span class="hljs-keyword">String</span>&gt; list = <span class="hljs-keyword">new</span> ArrayList&lt;<span class="hljs-keyword">String</span>&gt;();

    <span class="hljs-keyword">public</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">void</span> main(<span class="hljs-keyword">String</span>[] args) {

        <span class="hljs-keyword">final</span> MyThread_02 t = <span class="hljs-keyword">new</span> MyThread_02();
        <span class="hljs-comment">//<span class="zh-hans">全局对象锁</span></span>
        <span class="hljs-keyword">final</span> <span class="hljs-keyword">Object</span> lock = <span class="hljs-keyword">new</span> <span class="hljs-keyword">Object</span>();
        <span class="hljs-comment">//<span class="zh-hans">线程</span>1 <span class="zh-hans">往</span>list<span class="zh-hans">中加数据</span></span>
        Thread t1 = <span class="hljs-keyword">new</span> Thread(<span class="hljs-keyword">new</span> Runnable() {
            <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> run() {
                <span class="hljs-keyword">try</span> {
                    <span class="hljs-keyword">synchronized</span> (lock) {<span class="hljs-comment">//<span class="zh-hans">说明会对整个这块代码块加上锁。</span></span>
                        <span class="hljs-keyword">for</span> (<span class="hljs-built_in">int</span> i = <span class="hljs-number">0</span>; i &lt; <span class="hljs-number">10</span>; i++) {
                            t.<span class="hljs-built_in">add</span>();
                            System.out.<span class="hljs-built_in">println</span>(<span class="hljs-string">"<span class="zh-hans">当前线程</span>"</span> + Thread.currentThread().getName() + <span class="hljs-string">":<span class="zh-hans">添加了一个数据。。</span>"</span>);
                            Thread.sleep(<span class="hljs-number">500</span>);
                            <span class="hljs-keyword">if</span> (list.<span class="hljs-built_in">size</span>() == <span class="hljs-number">5</span>) {
                                <span class="hljs-comment">//<span class="zh-hans">发送通知</span> <span class="zh-hans">但是</span>notifi<span class="zh-hans">方法不释放锁</span>   <span class="zh-hans">锁被释放</span> <span class="zh-hans">是</span>10<span class="zh-hans">次循环结束后释放</span></span>
                                System.out.<span class="hljs-built_in">println</span>(<span class="hljs-string">"<span class="zh-hans">已经发出通知。。</span>"</span>);
                                lock.notify();
                            }
                        }
                    }
                } <span class="hljs-keyword">catch</span> (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }, <span class="hljs-string">"t1"</span>);

        <span class="hljs-comment">//<span class="zh-hans">线程</span>2 <span class="zh-hans">当</span>list<span class="zh-hans">数组中有</span>5<span class="zh-hans">个元素时</span>,<span class="zh-hans">就打印一句话，表示自己收到了通知</span></span>
        Thread t2 = <span class="hljs-keyword">new</span> Thread(<span class="hljs-keyword">new</span> Runnable() {

            <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> run() {
                <span class="hljs-keyword">synchronized</span> (lock) {
                    <span class="hljs-keyword">try</span> {
                        <span class="hljs-keyword">if</span> (t.<span class="hljs-built_in">size</span>() != <span class="hljs-number">5</span>) {
                            <span class="hljs-comment">//wait<span class="zh-hans">释放锁</span> <span class="zh-hans">此时的线程就在这儿阻塞</span> <span class="zh-hans">所以下面的</span>print<span class="zh-hans">语句没有打印</span> <span class="zh-hans">相反的</span>t1<span class="zh-hans">线程获得了锁</span> <span class="zh-hans">也发出了通知</span></span>
                            <span class="hljs-comment">//<span class="zh-hans">这时</span> <span class="zh-hans">线程</span>2<span class="zh-hans">就从阻塞状态中到</span>active<span class="zh-hans">状态</span> <span class="zh-hans">于是就有了下面的打印</span> <span class="zh-hans">线程阻塞时</span> <span class="zh-hans">就让出了</span>cpu</span>
                            lock.wait();
                        }
                    } <span class="hljs-keyword">catch</span> (InterruptedException e) {
                        e.printStackTrace();
                    }

                    <span class="hljs-comment">//<span class="zh-hans">打印一句话</span></span>
                    System.out.<span class="hljs-built_in">println</span>(<span class="hljs-string">"<span class="zh-hans">当前线程</span>"</span> + Thread.currentThread().getName() + <span class="hljs-string">":<span class="zh-hans">接收到通知。。</span>"</span>);
                    <span class="hljs-keyword">throw</span> <span class="hljs-keyword">new</span> RuntimeException();
                }
            }
        }, <span class="hljs-string">"t2"</span>);

        <span class="hljs-comment">//<span class="zh-hans">注意肯定</span>t2<span class="zh-hans">先启动，若是后启动的话，因为</span>notify<span class="zh-hans">不释放锁，所以会造成</span>t2<span class="zh-hans">不会执行。</span></span>
        t2.start();
        t1.start();
    }

    <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-built_in">add</span>() {
        list.<span class="hljs-built_in">add</span>(<span class="hljs-string">"1"</span>);
    }

    <span class="hljs-keyword">public</span> <span class="hljs-built_in">int</span> <span class="hljs-built_in">size</span>() {
        <span class="hljs-keyword">return</span> list.<span class="hljs-built_in">size</span>();
    }
}

</code></pre><h5 data-source-line="839" id="例-3利用countdownlatch类实现"><a class="markdownIt-Anchor" href="#例-3利用countdownlatch类实现"></a>例 3：利用CountDownLatch类实现</h5><ol data-source-line="841"><li>实时通知。</li></ol><pre data-source-line="843"><code class="hljs"><span class="hljs-keyword">public</span> class MyThread_03 {
    <span class="hljs-comment">//<span class="zh-hans">声明一个</span> <span class="zh-hans">实例变量</span></span>
    <span class="hljs-keyword">private</span> <span class="hljs-keyword">volatile</span> <span class="hljs-keyword">static</span> List&lt;<span class="hljs-keyword">String</span>&gt; list = <span class="hljs-keyword">new</span> ArrayList&lt;<span class="hljs-keyword">String</span>&gt;();

    <span class="hljs-keyword">public</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">void</span> main(<span class="hljs-keyword">String</span>[] args) {

        <span class="hljs-keyword">final</span> MyThread_03 t = <span class="hljs-keyword">new</span> MyThread_03();

        <span class="hljs-comment">//<span class="zh-hans">只需要执行</span>1<span class="zh-hans">次</span>c.countDown();</span>
        <span class="hljs-keyword">final</span> CountDownLatch c = <span class="hljs-keyword">new</span> CountDownLatch(<span class="hljs-number">1</span>);

        <span class="hljs-comment">//<span class="zh-hans">线程</span>1 <span class="zh-hans">往</span>list<span class="zh-hans">中加数据</span></span>
        Thread t1 = <span class="hljs-keyword">new</span> Thread(<span class="hljs-keyword">new</span> Runnable() {
            <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> run() {
                <span class="hljs-keyword">try</span> {
                    <span class="hljs-keyword">for</span> (<span class="hljs-built_in">int</span> i = <span class="hljs-number">0</span>; i &lt; <span class="hljs-number">10</span>; i++) {
                        t.<span class="hljs-built_in">add</span>();
                        System.out.<span class="hljs-built_in">println</span>(<span class="hljs-string">"<span class="zh-hans">当前线程</span>"</span> + Thread.currentThread().getName() + <span class="hljs-string">":<span class="zh-hans">添加了一个数据。。</span>"</span>);
                        Thread.sleep(<span class="hljs-number">500</span>);
                        <span class="hljs-keyword">if</span> (list.<span class="hljs-built_in">size</span>() == <span class="hljs-number">5</span>) {
                            <span class="hljs-comment">//<span class="zh-hans">发送通知</span> <span class="zh-hans">但是</span>notifi<span class="zh-hans">方法不释放锁</span>   <span class="zh-hans">锁被释放是在</span>10<span class="zh-hans">次循环结束后释放</span>(<span class="zh-hans">即</span>synchronized<span class="zh-hans">块结束后</span>)</span>
                            System.out.<span class="hljs-built_in">println</span>(<span class="hljs-string">"<span class="zh-hans">已经发出通知。。</span>"</span>);
                            c.countDown();
                        }
                    }
                } <span class="hljs-keyword">catch</span> (InterruptedException e) {
                    e.printStackTrace();
                }

            }
        }, <span class="hljs-string">"t1"</span>);

        <span class="hljs-comment">//<span class="zh-hans">线程</span>2 <span class="zh-hans">当</span>list<span class="zh-hans">数组中有</span>5<span class="zh-hans">个元素时</span> <span class="zh-hans">就打印一句话</span></span>
        Thread t2 = <span class="hljs-keyword">new</span> Thread(<span class="hljs-keyword">new</span> Runnable() {

            <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> run() {
                <span class="hljs-keyword">try</span> {
                    <span class="hljs-keyword">if</span> (t.<span class="hljs-built_in">size</span>() != <span class="hljs-number">5</span>) {
                        c.await();
                    }
                } <span class="hljs-keyword">catch</span> (InterruptedException e) {
                    e.printStackTrace();
                }
                <span class="hljs-comment">//<span class="zh-hans">打印一句话</span></span>
                System.out.<span class="hljs-built_in">println</span>(<span class="hljs-string">"<span class="zh-hans">当前线程</span>"</span> + Thread.currentThread().getName() + <span class="hljs-string">":<span class="zh-hans">接收到通知。。</span>"</span>);
                <span class="hljs-keyword">throw</span> <span class="hljs-keyword">new</span> RuntimeException();
            }
            <span class="hljs-comment">//}</span>
        }, <span class="hljs-string">"t2"</span>);

        t2.start();
        t1.start();
    }

    <span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-built_in">add</span>() {
        list.<span class="hljs-built_in">add</span>(<span class="hljs-string">"1"</span>);
    }

    <span class="hljs-keyword">public</span> <span class="hljs-built_in">int</span> <span class="hljs-built_in">size</span>() {
        <span class="hljs-keyword">return</span> list.<span class="hljs-built_in">size</span>();
    }

}

</code></pre><h4 data-source-line="916" id="4-丢失的信号"><a class="markdownIt-Anchor" href="#4-丢失的信号"></a>4、丢失的信号</h4><h4 data-source-line="918" id="5-假唤醒"><a class="markdownIt-Anchor" href="#5-假唤醒"></a>5、假唤醒</h4><h4 data-source-line="920" id="6-多线程等待相同信号"><a class="markdownIt-Anchor" href="#6-多线程等待相同信号"></a>6、多线程等待相同信号</h4><h4 data-source-line="922" id="7-不要对常量字符串或全局对象调用wait"><a class="markdownIt-Anchor" href="#7-不要对常量字符串或全局对象调用wait"></a>7、不要对常量字符串或全局对象调用wait()</h4><h2 data-source-line="930" id="同步和异步"><a class="markdownIt-Anchor" href="#同步和异步"></a>同步和异步：</h2><h2 data-source-line="932" id="临界区"><a class="markdownIt-Anchor" href="#临界区"></a>临界区：</h2><p data-source-line="934">临界区表示一种公共资源，在并行程序中，临界区资源时需要保护的。</p><h2 data-source-line="946" id="死锁-饥饿-活锁"><a class="markdownIt-Anchor" href="#死锁-饥饿-活锁"></a>死锁、饥饿、活锁：</h2><h3 data-source-line="948" id="死锁"><a class="markdownIt-Anchor" href="#死锁"></a>死锁</h3><p data-source-line="951">有个著名的例子——“哲学家进餐”问题，可以形象的表述死锁问题。</p><blockquote data-source-line="952"><p>5个哲学家围坐在一个圆桌上，每两个哲学家之间都有一只筷子，也就是只有5支筷子，哲学家平时进行思考，只有当他们饥饿时，才拿起筷子吃饭。</p></blockquote><p data-source-line="954">代码实现：</p><pre data-source-line="956"><code class="hljs"><span class="hljs-keyword">package</span> <span class="zh-hans">线程基础</span>;

<span class="hljs-keyword">import</span> java.util.Random;
<span class="hljs-keyword">import</span> java.util.concurrent.ExecutorService;
<span class="hljs-keyword">import</span> java.util.concurrent.Executors;
<span class="hljs-keyword">import</span> java.util.concurrent.TimeUnit;

<span class="hljs-comment">/**
 * Author: sun.yukun@foxmail.com
 * Time: 2017/9/17 <span class="zh-hans">下午</span>8:33
 * Blog: coderdaily.net
 * &lt;p&gt;
 * <span class="zh-hans">哲学家进餐问题</span>-<span class="zh-hans">模拟死锁</span>
 */</span>
<span class="hljs-keyword">public</span> <span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Philosopher</span> <span class="hljs-keyword">implements</span> <span class="hljs-title">Runnable</span> </span>{
    <span class="hljs-comment">//    <span class="zh-hans">左边的筷子</span></span>
    <span class="hljs-keyword">private</span> Chopstick left;
    <span class="hljs-comment">//    <span class="zh-hans">右边的筷子</span></span>
    <span class="hljs-keyword">private</span> Chopstick right;

    <span class="hljs-keyword">private</span> <span class="hljs-keyword">final</span> <span class="hljs-keyword">int</span> id;
    <span class="hljs-keyword">private</span> <span class="hljs-keyword">final</span> <span class="hljs-keyword">int</span> ponderFactor;
    <span class="hljs-keyword">private</span> Random rand = <span class="hljs-keyword">new</span> Random(<span class="hljs-number">47</span>);

    <span class="hljs-function"><span class="hljs-keyword">public</span> <span class="hljs-title">Philosopher</span><span class="hljs-params">(Chopstick left, Chopstick right, <span class="hljs-keyword">int</span> id, <span class="hljs-keyword">int</span> ponder)</span> </span>{
        <span class="hljs-keyword">this</span>.left = left;
        <span class="hljs-keyword">this</span>.right = right;
        <span class="hljs-keyword">this</span>.id = id;
        <span class="hljs-keyword">this</span>.ponderFactor = ponder;
    }

    <span class="hljs-function"><span class="hljs-keyword">private</span> <span class="hljs-keyword">void</span> <span class="hljs-title">pause</span><span class="hljs-params">()</span> <span class="hljs-keyword">throws</span> InterruptedException </span>{
        <span class="hljs-keyword">if</span> (ponderFactor == <span class="hljs-number">0</span>) {
            <span class="hljs-keyword">return</span>;
        }
        TimeUnit.MICROSECONDS.sleep(rand.nextInt(ponderFactor * <span class="hljs-number">250</span>));
    }

    <span class="hljs-function"><span class="hljs-keyword">public</span> <span class="hljs-keyword">void</span> <span class="hljs-title">run</span><span class="hljs-params">()</span> </span>{
        <span class="hljs-keyword">try</span> {
            <span class="hljs-keyword">while</span> (!Thread.interrupted()) {
                System.out.println(<span class="hljs-keyword">this</span> + <span class="hljs-string">"   thinking..."</span>);
                pause();

                System.out.println(<span class="hljs-keyword">this</span> + <span class="hljs-string">"   grabbing right"</span>);
                right.take();

                System.out.println(<span class="hljs-keyword">this</span> + <span class="hljs-string">"   grabbing left"</span>);
                left.take();

                System.out.println(<span class="hljs-keyword">this</span> + <span class="hljs-string">"   eating....."</span>);
                pause();

                right.drop();
                left.drop();
            }
        } <span class="hljs-keyword">catch</span> (Exception e) {
            System.out.println(<span class="hljs-keyword">this</span> + <span class="hljs-string">"   exiting via interrupt....."</span>);
            e.printStackTrace();
        }
    }

    <span class="hljs-meta">@Override</span>
    <span class="hljs-function"><span class="hljs-keyword">public</span> String <span class="hljs-title">toString</span><span class="hljs-params">()</span> </span>{
        <span class="hljs-keyword">return</span> <span class="hljs-string">"Philosopher{"</span> +
                <span class="hljs-string">"id="</span> + id +
                <span class="hljs-string">'}'</span>;
    }
}

<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">Chopstick</span> </span>{
    <span class="hljs-comment">//    <span class="zh-hans">筷子是否被使用</span></span>
    <span class="hljs-keyword">private</span> <span class="hljs-keyword">boolean</span> taken = <span class="hljs-keyword">false</span>;

    <span class="hljs-comment">//<span class="zh-hans">拿筷子</span></span>
    <span class="hljs-function"><span class="hljs-keyword">public</span> <span class="hljs-keyword">synchronized</span> <span class="hljs-keyword">void</span> <span class="hljs-title">take</span><span class="hljs-params">()</span> <span class="hljs-keyword">throws</span> InterruptedException </span>{
        <span class="hljs-keyword">while</span> (taken) {
            wait();
            taken = <span class="hljs-keyword">true</span>;
        }
    }

    <span class="hljs-comment">//<span class="zh-hans">放下筷子</span></span>
    <span class="hljs-function"><span class="hljs-keyword">public</span> <span class="hljs-keyword">synchronized</span> <span class="hljs-keyword">void</span> <span class="hljs-title">drop</span><span class="hljs-params">()</span> </span>{
        taken = <span class="hljs-keyword">false</span>;
        notifyAll();
    }
}

<span class="hljs-class"><span class="hljs-keyword">class</span> <span class="hljs-title">DeadLockingDining</span> </span>{
    <span class="hljs-function"><span class="hljs-keyword">public</span> <span class="hljs-keyword">static</span> <span class="hljs-keyword">void</span> <span class="hljs-title">main</span><span class="hljs-params">(String[] args)</span> </span>{
        Chopstick[] chopsticks = <span class="hljs-keyword">new</span> Chopstick[<span class="hljs-number">5</span>];
        ExecutorService exec = Executors.newCachedThreadPool();

        <span class="hljs-keyword">for</span> (<span class="hljs-keyword">int</span> i = <span class="hljs-number">0</span>; i &lt; <span class="hljs-number">5</span>; i++) {
            chopsticks[i] = <span class="hljs-keyword">new</span> Chopstick();
        }
        <span class="hljs-keyword">for</span> (<span class="hljs-keyword">int</span> i = <span class="hljs-number">0</span>; i &lt; <span class="hljs-number">5</span>; i++) {
<span class="hljs-comment">//            <span class="zh-hans">把思考时间减少至</span>0<span class="zh-hans">，即</span>ponder = 0<span class="zh-hans">，会更容易发生死锁。</span></span>
            exec.execute(<span class="hljs-keyword">new</span> Philosopher(chopsticks[i], chopsticks[(i + <span class="hljs-number">1</span>) % <span class="hljs-number">5</span>], i, <span class="hljs-number">5</span>));
        }
        exec.shutdown();
    }
}
</code></pre><h4 data-source-line="1063" id="死锁发生的条件"><a class="markdownIt-Anchor" href="#死锁发生的条件"></a>死锁发生的条件？</h4><p data-source-line="1065">四者缺一不可！</p><ul data-source-line="1067"><li>至少有一个资源不能共享。</li><li>至少有一个任务必须持有一个资源，且正在等待一个被别的任务占用的资源。</li><li>资源不能被抢占。我的理解是永久占有。</li><li>task1等待task2的资源...然后task N等待task1资源，也就是得形成一个环。</li></ul><h4 data-source-line="1072" id="解决死锁问题"><a class="markdownIt-Anchor" href="#解决死锁问题"></a>解决死锁问题</h4><p data-source-line="1074">既然知道了死锁发生的条件，就可以按照这个思路去解决上面的哲学家问题，四者破其一即可。</p><p data-source-line="1076">例如，打破原来哲学家们统一先拿右面的筷子，然后再拿左边的筷子的顺序，可以让最后一个哲学家顺序正好相反，从而打破这个循环等待条件。</p><h2 data-source-line="1078" id="多线程的特性"><a class="markdownIt-Anchor" href="#多线程的特性"></a>多线程的特性：</h2><h3 data-source-line="1080" id="原子性synchronized"><a class="markdownIt-Anchor" href="#原子性synchronized"></a>原子性：synchronized</h3><p data-source-line="1081">是指即使在多个线程一起执行的时候，一个操作一旦开始，就不会被其他线程干扰。</p><h3 data-source-line="1083" id="可见性volatile"><a class="markdownIt-Anchor" href="#可见性volatile"></a>可见性：volatile</h3><p data-source-line="1085">是指当一个线程修改了某一个变量的值，其他线程是否是否立即知道这个修改。</p><h3 data-source-line="1087" id="有序性"><a class="markdownIt-Anchor" href="#有序性"></a>有序性：</h3><p data-source-line="1089">我们只是习惯地认为代码的执行是依次执行的，但是在程序执行时可能会发生指令重排等问题。</p></div></article><body></body></html>