Saturday, August 25, 2012

Spring Scoped Proxy

Consider two Spring beans defined this way:

@Component
class SingletonScopedBean{
 @Autowired private PrototypeScopedBean prototypeScopedBean;
 
 public String getState(){
  return this.prototypeScopedBean.getState();
 }
}

@Component
@Scope(value="prototype")
class PrototypeScopedBean{
 private final String state;
 
 public PrototypeScopedBean(){
  this.state = UUID.randomUUID().toString();
 }

 public String getState() {
  return state;
 }
}

Here a prototype scoped bean is injected into a Singleton scoped bean.

Now, consider this test using these beans:

@RunWith(SpringJUnit4ClassRunner.class)
@ContextConfiguration
public class ScopedProxyTest {
 
 @Autowired private SingletonScopedBean singletonScopedBean;
 
 @Test
 public void testScopedProxy() {
  assertThat(singletonScopedBean.getState(), not(equalTo(singletonScopedBean.getState())));
 }
 
 @Configuration
 @ComponentScan("org.bk.samples.scopedproxy")
 public static class SpringContext{}

}

The point to note is that there is only 1 instance of PrototypeScopedBean that is created here - and that 1 instance is injected into the SingletonScopedBean, so the above test which actually expects a new instance of PrototypeScopedBean with each invocation of getState() method will fail.

If a new instance is desired with every request to PrototypeScopedBean (and in general if a bean with longer scope has a bean with shorter scope as a dependency, and the shorter scope needs to be respected), then there are a few solutions:

1. Lookup method injection - which can be read about here
2. A better solution is using Scoped proxies -

A scoped proxy can be specified this way using @Configuration:
@Component
@Scope(value="prototype", proxyMode=ScopedProxyMode.TARGET_CLASS)
class PrototypeScopedBean{
 private final String state;
 
 public PrototypeScopedBean(){
  this.state = UUID.randomUUID().toString();
 }

 public String getState() {
  return state;
 }

}

With this change, the bean injected into the SingletonScopedBean is not the PrototypeScopedBean itself, but a proxy to the bean (created using CGLIB or Dynamic proxies) and this proxy understands the scope and returns instances based on the requirements of the scope, the test should now work as expected.

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