Dynamic Proxy

 

http://www.javaworld.com/javaworld/jw-11-2000/jw-1110-proxy.html

 

• Interestingly, you can have a proxy class that implements multiple interfaces. However, there are a few restrictions on the interfaces you implement. It is important to keep those restrictions in mind when creating your dynamic proxy:

1. The proxy interface must be an interface. In other words, it cannot be a class (or an abstract class) or a primitive.
2. The array of interfaces passed to the proxy constructor must not contain duplicates of the same interface. Sun specifies that, and it makes sense that you wouldn't be trying to implement the same interface twice at the same time. For example, an array { IPerson.class, IPerson.class } would be illegal, but the code { IPerson.class, IEmployee.class } would not. The code calling the constructor should check for that case and filter out duplicates.
3. All the interfaces must be visible to the ClassLoader specified during the construction call. Again, that makes sense. The ClassLoadermust be able to load the interfaces for the proxy.
4. All the nonpublic interfaces must be from the same package. You cannot have a private interface from package com.xyz and the proxy class in package com.abc. If you think about it, it is the same way when programming a regular Java class. You couldn't implement a nonpublic interface from another package with a regular class either.
5. The proxy interfaces cannot have a conflict of methods. You can't have two methods that take the same parameters but return different types. For example, the methods public void foo() and public String foo() cannot be defined in the same class because they have the same signature, but return different types (see The Java Language Specification). Again, that is the same for a regular class.
6. The resulting proxy class cannot exceed the limits of the VM, such as the limitation on the number of interfaces that can be implemented.