How to create Java proxy classes in runtime

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