Synthetic Class in Java

Question

Java has the ability to create classes at runtime. These classes are known as Synthetic Classes or Dynamic Proxies.

See http://java.sun.com/j2se/1.5.0/docs/guide/reflection/proxy.html for more information.

Other open-source libraries, such as CGLIB and ASM also allow you to generate synthetic classes, and are more powerful than the libraries provided with the JRE.

Synthetic classes are used by AOP (Aspect Oriented Programming) libraries such as Spring AOP and AspectJ, as well as ORM libraries such as Hibernate.

Well I found the answer to the first question on google:

A class may be marked as synthetic if it is generated by the compiler, that is, it does not appear in the source code.

This is just a basic definition but I found it in a forum thread and there was no explanation. Still looking for a better one...

For example, When you have a switch statement, java creates a variable that starts with a $. If you want to see an example of this, peek into the java reflection of a class that has a switch statement in it. You will see these variables when you have at least one switch statement anywhere in the class.

To answer your question, I don't believe you are able to access(other than reflection) the synthetic classes.

If you are analyzing a class that you don't know anything about (via reflection) and need to know very specific and low-level things about that class, you may end up using Java reflection methods that have to do with synthetic classes. The only "use" here is get more information about the class in order to use it appropriately in your code.

(If you're doing this, you're probably building a framework of some sorts that other developers could use. )

Otherwise, if you are not using reflection, there are no practical uses of synthetic classes that I know of.

synthetic classes / methods / fields:

These things are important for the VM. Have a look at following code snippet:

class MyOuter {

  private MyInner inner;

  void createInner() {
    // The Compiler has to create a synthetic method
    // to construct a new MyInner because the constructor
    // is private.
    // --> synthetic "constructor" method
    inner = new MyInner();

    // The Compiler has to create a synthetic method
    // to doSomething on MyInner object because this
    // method is private.
    // --> synthetic "doSomething" method
    inner.doSomething();
  }

  private class MyInner {
    // the inner class holds a syntetic ref_pointer to
    // the outer "parent" class
    // --> synthetic field
    private MyInner() {
    }
    private void doSomething() {
    }
  }
}