(1) Dynamic Proxies in Java And so on into winter Till even I have ceased To come as a foot printer, And only some slight beast So mousy or foxy Shall print there as my proxy. Robert Frost “Closed for Good” Joe Dane April 28, 2004

(2) What’s a Proxy? “The agency for another who acts through the agent; authority to act for another” (Webster) “Provide a surrogate or placeholder for another object to control access to it” (GoF) Types of proxies mentioned: remote “virtual” (e.g. loading on demand) protection “smart references” - e.g. ref counting, locking, etc.

(3) “static” proxies Interface “real” server Proxy Factory Client implements Forwards touses By way of Generates or obtains

(4) General Proxy comments Using static proxies means tracking changes in interfaces, writing dummy methodsm recompiling. Also implies that we can’t use a framework that doesn’t know about our interfaces. Can’t use a Constructor GoF doesn’t mention this Must use a factory or registry

(5) Dynamic What? “Proxy”, because we create an object to “stand in” for another object “Dynamic” because the object is defined at runtime. Means we must have JVM support. Bottom line: at runtime, create objects which wrap other objects (services) and add functionality.

(6) Why proxies? Enhance services w/o changing either clients or servers If you need to look at the method, you probably don’t want a D.P. Explicit control over method invocation. No need to explicitly implement all methods in an interface. You’re probably already using them. App servers, Axis, hibernate

(7) Some Examples Logging, failover “Memoization” XML data binding Enforce security restrictions Transaction management

(8) Our model Services Interfaces which define behaviors Servers Programs which provide (implement) one or more services Clients Obtain references to services (cf factory pattern) No knowledge of server details Services may be local or remote

(9) Digression: Reflection Basics java.lang.reflect package Classes which represent the basic units of Java. Class, Method, Constructor From a Class, we can get a Method. From a Method, we can ‘invoke’ on an object and arguments. - object.methodname(arg1, arg2, …) - method.invoke(object, args[]) Syntax: Foo.class

(10) Creating a proxy Things you need A ClassLoader An array of interfaces to implement An InvocationHandler What’s the InvocationHandler? Basically, a single method. Could do anything Doesn’t usually implement any essential business logic Dispatches to an underlying service, possibly after (or before) providing some additional service. Note: underlying service may itself be a proxy!

(11) Creating a proxy (short) TheInterface object = (TheInterface) Proxy.newProxyInstance( TheInterface.class.getClassLoader(), new Class[] { TheInterface.class }, new SimpleHandler() );

(12) Creating a proxy (long) try { Class proxyClass = Proxy.getProxyClass( TheInterface.class.getClassLoader(), new Class[] { TheInterface.class } ); Constructor constructor = proxyClass.getConstructor(new Class[] { InvocationHandler.class }); InvocationHandler handler = new SimpleHandler(); TheInterface object = (TheInterface) constructor.newInstance(new Object[] { handler }); } catch (Exception e) { … }

(13) Proxy Properties Can be cast to any interface used in creating the proxy class. instanceof works as expected Proxy.isProxyClass() is true for the class Class name is undetermined, but will probably be something like Proxy$1. Method invocation is encoded and dispatched to the handler’s invoke method. Is Serializable This is huge! Implications for RMI, JNDI, J2EE

(14) Simple proxy use Client Factory Get proxy reference Service Proxy handler Service

(15) Advanced proxy use Client P1P2 P3 Service

(16) Downsides Loss of static guarantees Can make code difficult to understand Can’t proxy classes, only interfaces Slow? Requires JDK 1.3 All invocations go through the handler, even toString, equals(), etc., which were not in any interface. The creation syntax is ugly.

(17) Upsides Proxy handlers don’t have to be updated when service interfaces change. SOC: We can add behavior w/o changing servers or clients. Serializable means we can store proxies anywhere we like. c.f. App Servers: WebLogic, JBOSS, etc. Slow? Not really.

(18) Proxy examples Simple example Simple (but slightly more realistic) example Logging and profiling method calls A “memoizing” handler Load balancing and fail-over Some timing examples A “real world” example, time permitting

(19) References java.lang.reflect API docs Sun’s web page describing dynamic proxies. Contains simple examples and one more complicated example. Good starting point. Post to RMI-USERS mailing list describing how to use dynamic proxies to eliminate the need for rmic. Mind bending, not for the faint hearted. Related projects: cglib, BCEL, AoP