Injections and generic types in Spring (2.5)

[patb]

Hello,

I just struggled with Spring and understanding why it can't determine a generic type during injection. Please, have a look at this simple code:

public class GenericInjectTest {
	private Ship< Integer > ship = new Ship< Integer >();

	private void beanInit() {
		System.out.println( ship.getData() );
		System.out.println( ship.getData().getClass() );
	}

	public Ship< Integer > getShip() {
		return ship;
	}

	public void setShip( Ship< Integer > ship ) {
		this.ship = ship;
	}
}

(the beanInit method gets called by Spring, it is defined as the default bean init method).

And this is the Ship class:

public class Ship< T > {
	private T data;

	public T getData() {
		return data;
	}

	public void setData( T data ) {
		this.data = data;
	}
}

Here is the (important part of the) Spring context configuration:

<bean class="com.test.spring.GenericInjectTest">
	<property name="ship.data" value="123" />
</bean>

Looking at the bean definition, we can see that I want to set the property "ship.data" to "123". Since ship is parameterized with "Integer" in the GenericInjectTest class, I would think that Spring is able to detect the correct runtime type, which is Integer, and would convert the "123" to a real Integer.

But this is not the case. When I run the program, the line

System.out.println( ship.getData() );

prints "123". But the next line

System.out.println( ship.getData().getClass() );

throws the exception

java.lang.ClassCastException: java.lang.String cannot be cast to java.lang.Integer

So Spring has injected a String for the property "ship.data", not an Integer.


Is it the case, that Spring can't handle those generics, or am I doing anything wrong here?


Thanks a lot for your help

[denis.zhdanov]

Minor spring bug. Feel free to register it at the spring jira.

[patb]

Hi,

thanks for your answer,

Minor spring bug. Feel free to register it at the spring jira.

well, I did not expect such an answer, I'd rather say it is my own failure

Is this a known bug? I'm just wondering, because you sounded so confident in saying that.

[denis.zhdanov]

Hi,

thanks for your answer,

well, I did not expect such an answer, I'd rather say it is my own failure

Is this a known bug? I'm just wondering, because you sounded so confident in saying that.

I'm confident about that just because I debugged the test-case you mentioned and checked the processing (spring 2.5.6). I see that generic type of the bean field is not processed during context initialization, hence, I can be absolutely sure that it's a spring bug. However, I don't have an information whether it's known.

[patb]

Hello,

ok, I will post it on Spring Jira.


Thanks a lot

[jamestastic]

What is the reference number in Jira? I would like to comment on the Jira issue.

I have traced this back to the following method in CachedIntrospectionResults.java:

private CachedIntrospectionResults(Class beanClass) throws BeansException {}

The problem is that Introspector.getBeanInfo(beanClass), which is part of Java, returns a BeanInfo with a PropertyDescriptor specifying Object as the class for the "data" property. I think Spring will have to specifically check the bean instance to find out what the property instance class really is.

[patb]

Hi,

great to see that you were able to look into it - my understanding of Spring is not yet sufficient for that

The JIRA link to this issue is: http://jira.springframework.org/browse/SPR-5561

Do you know any kind of nice workaround for this?

Thanks a lot

[denis.zhdanov]

Hi,

...

Do you know any kind of nice workaround for this?

Thanks a lot

You can create a separate bean of necessary type (Integer) and inject it to the target property using 'ref' attribute instead of 'value'.

[al0]

Or even better - use inner bean

<bean class="com.test.spring.GenericInjectTest">
	<property name="ship.data"> 
              <bean class="java.lang.Integer">
                     <constructor-arg value="123"/>
              </bean>
        </property>
</bean>

[patb]

Thanks to all who offered help, now I'm waiting for the bug fix. Meanwhile, maybe someone is able to do a quick fix for it, if it's not too complicated? I'm just too new to Spring to try it myself

You can create a separate bean of necessary type (Integer) and inject it to the target property using 'ref' attribute instead of 'value'.

Or even better - use inner bean

Thanks a lot, this works of course.

Actually, my original intention was not to inject a bean to a generic property, that has been parameterized with a simple class (like Integer), but to inject a bean to a generic property, that has been parameterized with an enum.

But since my example above reproduces the same error and is more simple, I decided to use that to describe the potential bug in Spring.

So, instead of something like

private Ship< Integer > ship = new Ship< Integer >();

I have such a declaration:

private Ship< MyEnum > ship = new Ship< MyEnum >();

As we know, Spring has the nice ability to detect the enum type and inject the correct type, for example:

<bean class="com.test.spring.GenericInjectTest">
  <property name="ship.data" value="RADAR" />
</bean>

Where "RADAR" is from the enum "MyEnum" ("MyEnum.RADAR") and Spring can detect that.

There is a workaround, using this kind of bean definition:

<bean class="com.test.spring.GenericInjectTest">
  <property name="ship.data">
    <util:constant static-field="com.test.spring.GenericInjectTest$MyEnum.RADAR" />
  </property>
</bean>

But this drives me nuts, it clutters the XML code so heavily when you have to set a lot of Enums, especially when the package names are a lot longer... the direct use of the enum name for the property would be more elegant.

But I think for that, there is no workaround, since it depends on the type-detection thing for the generic parameterization, which is apparently broken at the moment