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

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