Force Spring to use a specific class in place of another one at runtime

Question

My Spring app depends on a library that I need to override a class from, let's say com.dep.A. I want to create class B which extends A, and tell spring that every time A is trying to be loaded (by any code or dependency) I want to load B instead.

Is there a spring setting that would achieve this ? If not, would AspectJ be an option (I think Spring has an AspectJ feature) ?

EDIT: a bit more specific on the use case If I were only able to plug myself into the execution flow of a specific method of class A, I'd be pretty please. In fact, forget about B extending A: what I am really trying to achieve is to intercept the execution of method A.originalMethod(MyObject o), and use o in my custom method B.interceptOriginalMethod().

Answer 1

Since the class is from a different library, it does not sound like it is a spring bean, and then Spring can't help you. It basically comes down to how the object is constructed, if the new operator is not called by you, but code inside the library you only have two options (Unless the Jar is signed, then I think there are no options).

1. Load time weaving. Which allows you to insert a cutpoint and replace the code of the method. This requires starting the JVM with a java agent, and should be a last resort.
2. If you have the source code, you can use class shadowing. You simply copy the class source into your source tree (same package name). This works because /classes is almost always before libraries in the classpath, so when the classloader try to load the class it will find your modified version, hence shadowing the original.

I have used option 2 a couple of times to fix minor issues in different open source libraries, when I was too lazy to rebuild the entire library from source. You have to be carefully when you upgrade the library, in case they have change the underlying code.

Answer 2

Use @Autowired with @Qualifier to load a bean by name instead of type. Below is the example

@Autowired
@Qualifier("b")
private A a;

The above code will load the reference of class B, provided B is also a spring bean.

Answer 3

Though @Klaus Groenbaek answer is correct (and I'll mark it as so), I have resolved my problem using another approach.

I was lucky enough that com.dep.A is annotated with Spring's @ConditionalOnMissingBean.

Therefore, simply create bean Bextending A and in your main application class declare:

@Bean 
public A a(){ 
  return new A(); 
}

Spring will load your class instead of the original one.