javaevolved.github.io/compact-object-headers.html at 54439b3c08bf93a7dcd94a9df2ab9d45dc8f3ece · javaevolved/javaevolved.github.io · GitHub

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      <span>Compact object headers</span>
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        <span class="badge tooling">Tooling</span>
        <span class="badge advanced">advanced</span>
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      <h1>Compact object headers</h1>
      <p>Cut object header size in half for better memory density and cache usage.</p>
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    <section class="compare-section">
      <div class="section-label">Code Comparison</div>
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            <span class="compare-tag old">✕ Java 8</span>
            <button class="copy-btn" data-code="old">Copy</button>
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            <pre class="code-text">// Default: 128-bit object header
// = 16 bytes overhead per object
// A boolean field object = 32 bytes!
// Mark word (64) + Klass pointer (64)</pre>
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        <div class="compare-panel modern-panel">
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            <span class="compare-tag modern">✓ Java 25</span>
            <button class="copy-btn" data-code="modern">Copy</button>
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          <div class="compare-code">
            <pre class="code-text">// -XX:+UseCompactObjectHeaders
// 64-bit object header
// = 8 bytes overhead per object
// 50% less header memory
// More objects fit in cache</pre>
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      <div class="section-label">Why the modern way wins</div>
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          <div class="why-icon">📦</div>
          <h3>50% smaller headers</h3>
          <p>8 bytes instead of 16 per object.</p>
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        <div class="why-card">
          <div class="why-icon">⚡</div>
          <h3>Better cache usage</h3>
          <p>More objects fit in CPU cache lines.</p>
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          <div class="why-icon">📊</div>
          <h3>Higher density</h3>
          <p>Fit more objects in the same heap size.</p>
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        <div class="info-label">Old Approach</div>
        <div class="info-value red">128-bit Headers</div>
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      <div class="info-card">
        <div class="info-label">Modern Approach</div>
        <div class="info-value green">64-bit Headers</div>
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        <div class="info-label">Since JDK</div>
        <div class="info-value accent">25</div>
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        <div class="info-label">Difficulty</div>
        <div class="info-value blue">advanced</div>
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        <div class="bs-feature-name">Compact object headers</div>
        <span class="baseline-badge widely">Available</span>
        <p class="bs-desc">Finalized in JDK 25 LTS (JEP 519, Sept 2025).</p>
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    <section class="explanation">
      <h2>How it works</h2>
      <p>Compact object headers reduce the per-object overhead from 128 bits to 64 bits on 64-bit platforms. This saves memory and improves cache utilization, especially for applications with many small objects.</p>
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// 2. javac Test.java
// 3. java Test
// Just to test one expression!</pre>
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              <pre class="code-text">$ jshell
jshell&gt; &quot;hello&quot;.chars().count()
$1 ==&gt; 5
jshell&gt; List.of(1,2,3).reversed()
$2 ==&gt; [3, 2, 1]</pre>
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$ java HelloWorld
// Two steps every time</pre>
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// Also works with shebangs:
#!/usr/bin/java --source 25</pre>
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