The document discusses creating proxy classes in Java at runtime. It explains that proxy classes are created using the Proxy class and implement interfaces specified at runtime. The Proxy class caches generated proxy classes and returns existing classes if they have been generated before for the same interfaces in a class loader. Properties of proxy classes are that they are public, final, extend Proxy and implement interfaces as specified when created.

1. Understanding Dynamic Proxies
How to create proxy classes in runtime
Osoco
Rafael Luque

2. Introduction
Contents
1 Introduction
2 Creating a Proxy Class
Proxy Class Properties
3 Creating a Proxy Instance
4 Examples
Rafael Luque (Osoco) Java Dynamic Proxies 04/2009 2 / 17

3. Introduction
Introduction
Dynamic Proxy Class
A class that implements a list of interfaces specified at runtime, such
that a method invocation through one of the interfaces on an instance
of the class will be encoded and dispatched to another object through
a uniform interface.
Usage
Create a proxy object for a list of interfaces without writing the proxy
class at compile-time.
Rafael Luque (Osoco) Java Dynamic Proxies 04/2009 3 / 17

4. Introduction
Introduction
Dynamic Proxy Class
A class that implements a list of interfaces specified at runtime, such
that a method invocation through one of the interfaces on an instance
of the class will be encoded and dispatched to another object through
a uniform interface.
Usage
Create a proxy object for a list of interfaces without writing the proxy
class at compile-time.
Rafael Luque (Osoco) Java Dynamic Proxies 04/2009 3 / 17

5. Creating a Proxy Class
Contents
1 Introduction
2 Creating a Proxy Class
Proxy Class Properties
3 Creating a Proxy Instance
4 Examples
Rafael Luque (Osoco) Java Dynamic Proxies 04/2009 4 / 17

6. Creating a Proxy Class
Creating a Proxy Class
• Proxy classes and instances are created using static methods of
java.lang.reflect.Proxy.
• Proxy.getProxyClass() returns the java.lang.Class
object for a proxy class given a class loader and an array of
interfaces.
• The proxy class will be defined in the specified class loader and
will implement all the interfaces.
• Dynamic Proxy Class API implementations keep a cache of
generated proxy classes:
• If a proxy class for the same permutation of interfaces has already
been defined by the class loader, then the existing proxy class will
be returned; otherwise, a proxy class for those interfaces will be
generated dynamically.
Rafael Luque (Osoco) Java Dynamic Proxies 04/2009 5 / 17

7. Creating a Proxy Class
Creating a Proxy Class
• Proxy classes and instances are created using static methods of
java.lang.reflect.Proxy.
• Proxy.getProxyClass() returns the java.lang.Class
object for a proxy class given a class loader and an array of
interfaces.
• The proxy class will be defined in the specified class loader and
will implement all the interfaces.
• Dynamic Proxy Class API implementations keep a cache of
generated proxy classes:
• If a proxy class for the same permutation of interfaces has already
been defined by the class loader, then the existing proxy class will
be returned; otherwise, a proxy class for those interfaces will be
generated dynamically.
Rafael Luque (Osoco) Java Dynamic Proxies 04/2009 5 / 17

8. Creating a Proxy Class
Creating a Proxy Class
• Proxy classes and instances are created using static methods of
java.lang.reflect.Proxy.
• Proxy.getProxyClass() returns the java.lang.Class
object for a proxy class given a class loader and an array of
interfaces.
• The proxy class will be defined in the specified class loader and
will implement all the interfaces.
• Dynamic Proxy Class API implementations keep a cache of
generated proxy classes:
• If a proxy class for the same permutation of interfaces has already
been defined by the class loader, then the existing proxy class will
be returned; otherwise, a proxy class for those interfaces will be
generated dynamically.
Rafael Luque (Osoco) Java Dynamic Proxies 04/2009 5 / 17

9. Creating a Proxy Class
Creating a Proxy Class
• Proxy classes and instances are created using static methods of
java.lang.reflect.Proxy.
• Proxy.getProxyClass() returns the java.lang.Class
object for a proxy class given a class loader and an array of
interfaces.
• The proxy class will be defined in the specified class loader and
will implement all the interfaces.
• Dynamic Proxy Class API implementations keep a cache of
generated proxy classes:
• If a proxy class for the same permutation of interfaces has already
been defined by the class loader, then the existing proxy class will
be returned; otherwise, a proxy class for those interfaces will be
generated dynamically.
Rafael Luque (Osoco) Java Dynamic Proxies 04/2009 5 / 17

10. Creating a Proxy Class
Creating a Proxy Class
• Proxy classes and instances are created using static methods of
java.lang.reflect.Proxy.
• Proxy.getProxyClass() returns the java.lang.Class
object for a proxy class given a class loader and an array of
interfaces.
• The proxy class will be defined in the specified class loader and
will implement all the interfaces.
• Dynamic Proxy Class API implementations keep a cache of
generated proxy classes:
• If a proxy class for the same permutation of interfaces has already
been defined by the class loader, then the existing proxy class will
be returned; otherwise, a proxy class for those interfaces will be
generated dynamically.
Rafael Luque (Osoco) Java Dynamic Proxies 04/2009 5 / 17

11. Creating a Proxy Class
Proxy.getProxyClass Method
Proxy.getProxyClass() method
public static Class getProxyClass(
ClassLoader loader,
Class[] interfaces)
throws IllegalArgumentException
Rafael Luque (Osoco) Java Dynamic Proxies 04/2009 6 / 17

12. Creating a Proxy Class Proxy Class Properties
Proxy Class Properties
• Proxy classes are public, final, and not abstract.
• A proxy class extends java.lang.reflect.Proxy.
• The unqualified name of a proxy class is unspecified.
• If a proxy class implements a non-public interface, then it will be
defined in the same package as that interface. Otherwise, the
package of a proxy class is also unspecified.
• A proxy class implements exactly the interfaces specified at its
creation, in the same order.
• The Proxy.isProxyClass will return true if it is passed a proxy
class.
Rafael Luque (Osoco) Java Dynamic Proxies 04/2009 7 / 17

13. Creating a Proxy Class Proxy Class Properties
Proxy Class Properties
• Proxy classes are public, final, and not abstract.
• A proxy class extends java.lang.reflect.Proxy.
• The unqualified name of a proxy class is unspecified.
• If a proxy class implements a non-public interface, then it will be
defined in the same package as that interface. Otherwise, the
package of a proxy class is also unspecified.
• A proxy class implements exactly the interfaces specified at its
creation, in the same order.
• The Proxy.isProxyClass will return true if it is passed a proxy
class.
Rafael Luque (Osoco) Java Dynamic Proxies 04/2009 7 / 17

14. Creating a Proxy Class Proxy Class Properties
Proxy Class Properties
• Proxy classes are public, final, and not abstract.
• A proxy class extends java.lang.reflect.Proxy.
• The unqualified name of a proxy class is unspecified.
• If a proxy class implements a non-public interface, then it will be
defined in the same package as that interface. Otherwise, the
package of a proxy class is also unspecified.
• A proxy class implements exactly the interfaces specified at its
creation, in the same order.
• The Proxy.isProxyClass will return true if it is passed a proxy
class.
Rafael Luque (Osoco) Java Dynamic Proxies 04/2009 7 / 17

15. Creating a Proxy Class Proxy Class Properties
Proxy Class Properties
• Proxy classes are public, final, and not abstract.
• A proxy class extends java.lang.reflect.Proxy.
• The unqualified name of a proxy class is unspecified.
• If a proxy class implements a non-public interface, then it will be
defined in the same package as that interface. Otherwise, the
package of a proxy class is also unspecified.
• A proxy class implements exactly the interfaces specified at its
creation, in the same order.
• The Proxy.isProxyClass will return true if it is passed a proxy
class.
Rafael Luque (Osoco) Java Dynamic Proxies 04/2009 7 / 17

16. Creating a Proxy Class Proxy Class Properties
Proxy Class Properties
• Proxy classes are public, final, and not abstract.
• A proxy class extends java.lang.reflect.Proxy.
• The unqualified name of a proxy class is unspecified.
• If a proxy class implements a non-public interface, then it will be
defined in the same package as that interface. Otherwise, the
package of a proxy class is also unspecified.
• A proxy class implements exactly the interfaces specified at its
creation, in the same order.
• The Proxy.isProxyClass will return true if it is passed a proxy
class.
Rafael Luque (Osoco) Java Dynamic Proxies 04/2009 7 / 17

17. Creating a Proxy Class Proxy Class Properties
Proxy Class Properties
• Proxy classes are public, final, and not abstract.
• A proxy class extends java.lang.reflect.Proxy.
• The unqualified name of a proxy class is unspecified.
• If a proxy class implements a non-public interface, then it will be
defined in the same package as that interface. Otherwise, the
package of a proxy class is also unspecified.
• A proxy class implements exactly the interfaces specified at its
creation, in the same order.
• The Proxy.isProxyClass will return true if it is passed a proxy
class.
Rafael Luque (Osoco) Java Dynamic Proxies 04/2009 7 / 17

18. Creating a Proxy Instance
Contents
1 Introduction
2 Creating a Proxy Class
Proxy Class Properties
3 Creating a Proxy Instance
4 Examples
Rafael Luque (Osoco) Java Dynamic Proxies 04/2009 8 / 17

19. Creating a Proxy Instance
Invocation Handler
• Each proxy instance has an associated invocation handler object,
which implements the interface
java.lang.reflect.InvocationHandler.
• A method invocation on a proxy instance will be dispatched to the
invoke method of the instance’s invocation handler.
• Invoke method receives the proxy instance, a
java.lang.reflect.Method object identifying the method
that was invoked and an array of type Object containing the
arguments.
Rafael Luque (Osoco) Java Dynamic Proxies 04/2009 9 / 17

20. Creating a Proxy Instance
Invocation Handler
• Each proxy instance has an associated invocation handler object,
which implements the interface
java.lang.reflect.InvocationHandler.
• A method invocation on a proxy instance will be dispatched to the
invoke method of the instance’s invocation handler.
• Invoke method receives the proxy instance, a
java.lang.reflect.Method object identifying the method
that was invoked and an array of type Object containing the
arguments.
Rafael Luque (Osoco) Java Dynamic Proxies 04/2009 9 / 17

21. Creating a Proxy Instance
Invocation Handler
• Each proxy instance has an associated invocation handler object,
which implements the interface
java.lang.reflect.InvocationHandler.
• A method invocation on a proxy instance will be dispatched to the
invoke method of the instance’s invocation handler.
• Invoke method receives the proxy instance, a
java.lang.reflect.Method object identifying the method
that was invoked and an array of type Object containing the
arguments.
Rafael Luque (Osoco) Java Dynamic Proxies 04/2009 9 / 17

22. Creating a Proxy Instance
Proxy Instance I
• Each proxy class has one public constructor that takes as
argument an implementation of the interface
InvocationHandler.
• You can instantiate the proxy class using the reflection API:
Proxy for the Foo interface
Class proxyClass = Proxy.getProxyClass(
Foo.class.getClassLoader(), new Class[] { Foo.class });
InvocationHandler handler = new MyInvocationHandler(...);
Foo f = (Foo) proxyClass.
getConstructor(new Class[] { InvocationHandler.class }).
newInstance(new Object[] { handler });
Rafael Luque (Osoco) Java Dynamic Proxies 04/2009 10 / 17

23. Creating a Proxy Instance
Proxy Instance II
• Better, you can use the Proxy.newProxyInstance method:
Proxy using Proxy.newProxyInstance
InvocationHandler handler = new MyInvocationHandler(...);
Foo f = (Foo) Proxy.newProxyInstance(
Foo.class.getClassLoader(),
new Class[] { Foo.class },
handler);
• This method combines the actions of calling
Proxy.getProxyClass with invoking the constructor with an
invocation handler.
Rafael Luque (Osoco) Java Dynamic Proxies 04/2009 11 / 17

24. Creating a Proxy Instance
Proxy Instance Properties I
• Given a proxy instance proxy and one of the interfaces
implemented by its proxy class Foo, the following expression will
return true:
proxy instanceof Foo
and the following cast operation will succeed:
(Foo) proxy
• The static Proxy.getInvocationHandler method will return
the invocation handler associated with the proxy instance passed
as its argument.
Rafael Luque (Osoco) Java Dynamic Proxies 04/2009 12 / 17

25. Creating a Proxy Instance
Proxy Instance Properties II
• An interface method invocation on a proxy instance will be
encoded and dispatched to the invocation handler’s invoke
method.
• An invocation of the hashCode, equals, or toString methods
declared in java.lang.Object on a proxy instance will be also
encoded and dispatched.
Rafael Luque (Osoco) Java Dynamic Proxies 04/2009 13 / 17

26. Examples
Contents
1 Introduction
2 Creating a Proxy Class
Proxy Class Properties
3 Creating a Proxy Instance
4 Examples
Rafael Luque (Osoco) Java Dynamic Proxies 04/2009 14 / 17

27. Examples
DebugProxy Example I
A proxy that prints out a message before and after each method
invocation on an object that implements an arbitrary list of interfaces.
1 import j a v a . l a n g . r e f l e c t . Proxy ;
2
3 public class DebugProxy
4 implements j a v a . l a n g . r e f l e c t . I n v o c a t i o n H a n d l e r {
5
6 private Object obj ;
7
8 public s t a t i c O b j e c t newInstance ( O b j e c t o b j ) {
9 r e t u r n Proxy . newProxyInstance (
10 o b j . g e t C l a s s ( ) . getClassLoader ( ) ,
11 obj . getClass ( ) . g e t I n t e r f a c e s ( ) ,
12 new DebugProxy ( o b j ) ) ;
13 }
14
15 p r i v a t e DebugProxy ( O b j e c t o b j ) {
16 this . obj = obj ;
17 }
18
Rafael Luque (Osoco) Java Dynamic Proxies 04/2009 15 / 17

28. Examples
DebugProxy Example II
19 public O b j e c t i n v o k e ( O b j e c t proxy , Method m, O b j e c t [ ] args )
20 throws Throwable {
21
22 Object r e s u l t ;
23 try {
24 System . o u t . p r i n t l n ( ‘ ‘ b e f o r e method ’ ’ ) ;
25 r e s u l t = m. i n v o k e ( obj , args ) ;
26 } catch ( I n v o c a t i o n T a r g e t E x c e p t i o n e ) {
27 throw e . g e t T a r g e t E x c e p t i o n ( ) ;
28 } catch ( E x c e p t i o n e ) {
29 throw new RuntimeException ( ) ;
30 } finally {
31 System . o u t . p r i n t l n ( ‘ ‘ a f t e r method ’ ’ ) ;
32 }
33 return r e s u l t ;
34
35 }
36
37 }
Rafael Luque (Osoco) Java Dynamic Proxies 04/2009 16 / 17

29. Understanding Dynamic Proxies
How to create proxy classes in runtime
Osoco
Rafael Luque