Understanding Java Dynamic Proxies

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

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

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

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

Understanding Java Dynamic Proxies

Recommended

More Related Content

What's hot (20)

Similar to Understanding Java Dynamic Proxies (20)

Recently uploaded (20)

Understanding Java Dynamic Proxies