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

proxy class proxy classes

existing proxy class

proxy object

proxy classcreating

class object

specied class loader

permutation of interfaces

array of interfaces