Understanding Java Dynamic Proxies

Understanding Dynamic ProxiesHow to create proxy classes in runtime

OsocoRafael Luque

Introduction

Contents

1 Introduction

2 Creating a Proxy ClassProxy Class Properties

3 Creating a Proxy Instance

4 Examples

Rafael Luque (Osoco) Java Dynamic Proxies 04/2009 2 / 17

Introduction

Dynamic Proxy ClassA class that implements a list of interfaces specified at runtime, suchthat a method invocation through one of the interfaces on an instanceof the class will be encoded and dispatched to another object througha uniform interface.

Create a proxy object for a list of interfaces without writing the proxyclass at compile-time.

Introduction

Dynamic Proxy ClassA class that implements a list of interfaces specified at runtime, suchthat a method invocation through one of the interfaces on an instanceof the class will be encoded and dispatched to another object througha uniform interface.

Create a proxy object for a list of interfaces without writing the proxyclass at compile-time.

Creating a Proxy Class

Contents

1 Introduction

4 Examples

• Proxy classes and instances are created using static methods ofjava.lang.reflect.Proxy.

• Proxy.getProxyClass() returns the java.lang.Classobject for a proxy class given a class loader and an array ofinterfaces.

• The proxy class will be defined in the specified class loader andwill implement all the interfaces.

• Dynamic Proxy Class API implementations keep a cache ofgenerated proxy classes:

• If a proxy class for the same permutation of interfaces has alreadybeen defined by the class loader, then the existing proxy class willbe returned; otherwise, a proxy class for those interfaces will begenerated dynamically.

Proxy.getProxyClass Method

Proxy.getProxyClass() methodpublic static Class getProxyClass(

ClassLoader loader,Class[] interfaces)

throws IllegalArgumentException

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, thepackage of a proxy class is also unspecified.

• A proxy class implements exactly the interfaces specified at itscreation, in the same order.

• The Proxy.isProxyClass will return true if it is passed a proxyclass.

Creating a Proxy Instance

Contents

1 Introduction

4 Examples

Invocation Handler

• Each proxy instance has an associated invocation handler object,which implements the interfacejava.lang.reflect.InvocationHandler.

• A method invocation on a proxy instance will be dispatched to theinvoke method of the instance’s invocation handler.

• Invoke method receives the proxy instance, ajava.lang.reflect.Method object identifying the methodthat was invoked and an array of type Object containing thearguments.

Invocation Handler

Proxy Instance I

• Each proxy class has one public constructor that takes asargument an implementation of the interfaceInvocationHandler.

• You can instantiate the proxy class using the reflection API:

Proxy for the Foo interfaceClass 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 });

Proxy Instance II

• Better, you can use the Proxy.newProxyInstance method:

Proxy using Proxy.newProxyInstanceInvocationHandler handler = new MyInvocationHandler(...);Foo f = (Foo) Proxy.newProxyInstance(

Foo.class.getClassLoader(),new Class[] { Foo.class },handler);

• This method combines the actions of callingProxy.getProxyClass with invoking the constructor with aninvocation handler.

Proxy Instance Properties I

• Given a proxy instance proxy and one of the interfacesimplemented by its proxy class Foo, the following expression willreturn true:

proxy instanceof Foo

and the following cast operation will succeed:

(Foo) proxy

• The static Proxy.getInvocationHandler method will returnthe invocation handler associated with the proxy instance passedas its argument.

Proxy Instance Properties II

• An interface method invocation on a proxy instance will beencoded and dispatched to the invocation handler’s invokemethod.

• An invocation of the hashCode, equals, or toString methodsdeclared in java.lang.Object on a proxy instance will be alsoencoded and dispatched.

Examples

Contents

1 Introduction

4 Examples

Examples

DebugProxy Example I

A proxy that prints out a message before and after each methodinvocation on an object that implements an arbitrary list of interfaces.

1 import java . lang . r e f l e c t . Proxy ;2

3 public class DebugProxy4 implements java . lang . r e f l e c t . Invocat ionHand ler {5

6 private Object ob j ;7

8 public s t a t i c Object newInstance ( Object ob j ) {9 return Proxy . newProxyInstance (

10 ob j . getClass ( ) . getClassLoader ( ) ,11 ob j . getClass ( ) . g e t I n t e r f a c e s ( ) ,12 new DebugProxy ( ob j ) ) ;13 }14

15 private DebugProxy ( Object ob j ) {16 th is . ob j = ob j ;17 }18

Examples

DebugProxy Example II

19 public Object invoke ( Object proxy , Method m, Object [ ] args )20 throws Throwable {21

22 Object r e s u l t ;23 t ry {24 System . out . p r i n t l n ( ‘ ‘ before method ’ ’ ) ;25 r e s u l t = m. invoke ( obj , args ) ;26 } catch ( Invoca t ionTarge tExcept ion e ) {27 throw e . getTargetExcept ion ( ) ;28 } catch ( Except ion e ) {29 throw new RuntimeException ( ) ;30 } f i n a l l y {31 System . out . 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

Understanding Dynamic ProxiesHow to create proxy classes in runtime

OsocoRafael Luque

Understanding Java Dynamic Proxies

Technology

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