Spring creating multiple instances of a singleton?

问题

I have a graph of Spring beans which autowire each other. Heavily simplified illustration:

<context:annotation-config/>
<bean class="Foo"/>
<bean class="Bar"/>
<bean class="Baz"/>

...

public class Foo {
   @Autowired Bar bar;
   @Autowired Baz baz;
}

public class Bar {
   @Autowired Foo foo;
}

public class Baz {
   @Autowired Foo foo;
}

All of these beans don't have scope specified which imply they are singletons (making them explicit singletons doesn't change anything, I've tried).

The problem is that after the instantiation of a single application context, instances of Bar and Baz contain different instances of Foo. How could this happen?

I have tried to create public no args constructor for Foo and debugging has confirmed Foo is created more than once. The stack trace for all of these creations is here.

I have also tried to enable debug logging for Spring, and among all other lines, got the following:

DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'Foo'
DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'Foo'
DEBUG org.springframework.beans.factory.support.DefaultListableBeanFactory - Creating shared instance of singleton bean 'Foo'

I understand that my beans are cross-referencing each other, but I would expect Spring framework to respect singleton scope and initialize a singleton bean once, and then autowire it to whoever wants it.

The interesting fact that if I use old school private constructor with public static Foo getInstance accessor, this works just fine - no exceptions are thrown during the context setup.

FWIW, I am using Spring version 3.0.5 (also tried with 3.1.2, same results) with o.s.c.s.ClassPathXmlApplicationContext(String ...configLocations) constructor.

I can easily convert my code to use static initializer but I want to understand why would Spring behave this way. Is this a bug?

EDIT: Some additional investigation showed that

After the application context is initialized, all subsequent requests to context.getBean(Foo.class) always return the same instance of Foo.
Replacing @Autowired with setters (about 20 usages of this bean) still results multiple constructions of this object, but all dependencies are injected with the same reference.

To me above suggests that this is a Spring bug pertaining to @Autowired implementation. I am going to post to Spring community forums and post back here if I manage to obtain anything useful.

回答1:

Child context(s) can reinstantiate the same singleton beans if you are not careful with context:component-scan annotations (there are other Spring context scan annotations as well such as MVC ones and others). This is a common problem when using Spring servlets in web applications, see Why DispatcherServlet creates another application context?

Make sure you are not re-scanning your components in child contexts, or you are scanning only specific packages/annotations and excluding said packages/annotations from root context component scan.

回答2:

For some reason we are getting this popping up randomly in integration tests and services as well (spring version 4.1.4, java 1.8).

Looks like there might be more than one culprit - Autowiring appeared to be causing this at first.

However, we have resolved the most consistent failures by ensuring we give each impacted bean an 'id' field.

回答3:

Try using setter injection instead of constructor way and see if it works.In the spring bean xml specify Bean A ref to Bean B and vice versa.

回答4:

My Spring configuration was like follows:

<context:annotation-config/>

<bean class="Bar" />
<bean class="Foo" />
<bean class="Baz" />

Classes are identical to yours

Test app like follows:

import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;

public class SpringTest {

    /**
     * @param args
     */
    public static void main(String[] args) {
        ApplicationContext ctx = new ClassPathXmlApplicationContext("META-INF/spring/testctx.xml");

        Foo foo = ctx.getBean(Foo.class);
        Baz baz = ctx.getBean(Baz.class);
        Bar bar = ctx.getBean(Bar.class);

        System.out.println(foo.equals(baz.foo));
        System.out.println(foo.equals(bar.foo));
        System.out.println(baz.equals(foo.baz));

        System.out.println(foo.baz.toString());
        System.out.println(baz.toString());
        System.out.println(foo.bar.toString());
        System.out.println(bar.toString());

    }

}

Output from test app like follows:

true
true
true
Baz@8aef2b
Baz@8aef2b
Bar@215bf054
Bar@215bf054

Using 3.0.6 it works perfectly fine (singleton beans are indeed singletons). There might be something else you did not illustrate here messing up your configuration. Of course, as a side note, using default package may cause some misterious magic to happen ;-)

来源：https://stackoverflow.com/questions/11547240/spring-creating-multiple-instances-of-a-singleton

标签

java

Spring

singleton

autowired

applicationcontext