Reconsider whether to generally allow bean overriding by name [SPR-10808]

[spring-projects-issues]

Emerson Farrugia opened SPR-10808 and commented

I've hit an issue where a single component scan was finding two @Beans with the same name. The beans weren't intentionally named the same, it was just an oversight. The container initialised one bean and ignored the other, logging at info instead of warn. The logging level is the first problem, since logging at info makes it much more likely the message is missed. The second problem is that the ignored bean was explicitly marked as @Primary. I've attached files with an example.

As far as I can tell, there's no way to control which bean gets picked in this situation. @Order on the @Configuration doesn't help.

In terms of fixes, I suggest the info message gets changed to warning. I'd personally even go one further. Given the functionality of @Profile, @Primary, and that autowiring by type is encouraged, does bean overriding still make sense in 3.1+.x? @Profile and @Primary give us a deterministic way of controlling autowire candidates. In contrast, quiet non-deterministic overrides by name seem too fragile to fit.

---

Affects: 3.2.3

Attachments:

• Bar.java (101 bytes)
• BarConfiguration.java (446 bytes)
• Foo.java (101 bytes)
• FooConfiguration.java (378 bytes)
• Launcher.java (712 bytes)
• SomeInterface.java (90 bytes)

Issue Links:

• Allow @Bean methods to override definitions in XML [SPR-7028] #11690 Allow @Bean methods to override definitions in XML
• When a bean definition is overridden, detect whether the definition has changed and log accordingly [SPR-7607] #12263 When a bean definition is overridden, detect whether the definition has changed and log accordingly
• Allow @Bean to override a dynamically registered bean definition [SPR-10607] #15236 Allow @Bean to override a dynamically registered bean definition
• @Primary annotation is not working for beans with the same name [SPR-13980] #18552 @Primary annotation is not working for beans with the same name
• Regression: @Primary annotation not working when overriding bean definition with classpath scanning [SPR-12934] #17527 Regression: @Primary annotation not working when overriding bean definition with classpath scanning
• Auto-resolve bean name conflicts for scanned classes [SPR-14665] #19229 Auto-resolve bean name conflicts for scanned classes

2 votes, 7 watchers

[spring-projects-issues]

Juergen Hoeller commented

I'm afraid that bean definition overriding is being used by intention quite frequently, which is why we're logging at info level there... Warn level would be inappropriate for an intended override.

Effectively, even if it may be surprising when they're happening by accident, overrides themselves are deterministic. The non-deterministic twist comes in via component scanning where the order of registration is up to the classpath structure or even specific ClassLoader behavior. As an alternative, if you register your configuration classes explicitly (through XML bean class entries or AnnotationConfigApplicationContext register calls), they have a clear order and therefore also well-defined overriding behavior.

@Primary is semantically only meant to apply for by-type matching, not for bean definition overriding. We could extend its definition to overriding but I'm not sure we'd do ourselves a favor with it.

All of that said, I personally tend to agree that overriding should be avoided wherever possible these days, preferring other means to express special registration arrangements. Note that you may call setAllowBeanDefinitionOverriding(false) on DefaultListableBeanFactory and also on the common ApplicationContext implementation classes...

Juergen

[spring-projects-issues]

Emerson Farrugia commented

Thanks for the explanation. I think it's worth exploring whether overriding is intentionally being used because it offers value over @Order, @Profile, and @Primary, or whether it's simply a carryover from when those annotations weren't available.

Are there any other circumstances where XML bean definition order matters in Spring, or is that only a side effect of overrides? If the latter, that's quite a cost for a feature whose benefit may be outdated.

Allow me to outline the problem I hit, to see if the weirdness is as jarring as I'm making it out to be. Assume you have a system which uses Hiberate, JDBC, or a mixture of the two. You organised your code into two packages com.foo.persistence.hibernate and com.foo.persistence.jdbc, containing configuration classes

HibernateTransactionManagerConfiguration (defines HibernateTransactionManager as transactionManager)
HibernateConfiguration (defines SessionFactory)

and

DataSourceTransactionManagerConfiguration (defines DataSourceTransactionManager as transactionManager)
JdbcConfiguration (defines JdbcTemplate)

respectively. Both *TransactionManagerConfiguration define a bean transactionManager, which must be named that way because other infrastructure classes wire it in by name. The idea is that an application that needs JDBC will component scan the jdbc package, one that needs Hibernate will scan the hibernate package, and one that needs both scans both.

Assume your code needs to use both Hibernate and JDBC, and therefore must use the HibernateTransactionManager. A simple component scan like

<context:component-scan base-package="com.foo.persistence.hibernate"/>
<context:component-scan base-package="com.foo.persistence.jdbc"/>

fails when the JDBC transaction manager is picked up because it appears last, but if defined as

<context:component-scan base-package="com.foo.persistence.jdbc"/>
<context:component-scan base-package="com.foo.persistence.hibernate"/>

it works. That's not obvious by any stretch, and the problem only manifests itself as a runtime exception when a Hibernate session isn't found, unless using setAllowBeanDefinitionOverriding (not straightforward on web application contexts).

IMO, a @Primary annotation on the HibernateTransactionManager, or an @Order annotation to have more flexibility are better ways to disambiguate.

[spring-projects-issues]

Modestas Kažinauskas commented

Hi, I think I found the same issue in 1.4.0.BUILD-SNAPSHOT

Bean name is taken as init method name

Please see my code below

@Configuration
public class ConfigA {
    @Bean
    @Primary
    public MyObject init() {
        return new MyObject("A");
    }
}

@Configuration
public class ConfigB {
    @Bean
    public MyObject init() {
        return new MyObject("B");
    }
}

public class MyObject {
    private String name;

    public MyObject(String name) {
        this.name = name;
    }
    ...
}

@Configuration
public class Runner {
    @Autowired
    private MyObject myObject;

    @PostConstruct
    void init() {
        System.out.println(myObject.getName()); //Output is B, but primary bean is A
    }
}

2016-02-25 08:39:49.351 INFO 1196 --- [ main] o.s.b.f.s.DefaultListableBeanFactory : Overriding bean definition for bean 'init' with a different definition: replacing
[Root bean: class [null]; scope=; abstract=false; lazyInit=false; autowireMode=3; dependencyCheck=0; autowireCandidate=true; primary=true; factoryBeanName=configA; factoryMethodName=init; initMethodName=null; destroyMethodName=(inferred); defined in class path resource [com/modzo/ConfigA.class]]
with
[Root bean: class [null]; scope=; abstract=false; lazyInit=false; autowireMode=3; dependencyCheck=0; autowireCandidate=true; primary=false; factoryBeanName=configB; factoryMethodName=init; initMethodName=null; destroyMethodName=(inferred); defined in class path resource [com/modzo/ConfigB.class]]

[bclozel]

See spring-projects/spring-boot#13609 - bean overriding has been disabled by default in Spring Boot 2.1.

[jhoeller]

Superseded by #31288.