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This article is to share with you about the use of Proxy in Java dynamic agents, the editor thinks it is very practical, so I share it with you to learn. I hope you can get something after reading this article.

Dynamic proxies can provide access to another object while hiding the specific facts of the actual object, which hides the actual object from the client. Dynamic proxies can do some other processing for requests, and consider using proxies when direct access to certain classes is not allowed, or some special handling of access is required. Support for dynamic proxies is currently provided in the Java development package, but now only the implementation of the interface is supported.

Mainly through the java.lang.reflect.Proxy class and java.lang.reflect.InvocationHandler interface. The Proxy class is mainly used to obtain dynamic proxy objects, and the InvocationHandler interface is used to constrain the caller's behavior.

"write a proxy for the ArrayList class whose internal implementation is exactly the same as in ArrayList, and can calculate the time for each method to run." This is a question on an examination question, there is no answer, let's take a look at the realization:

Package example; import java.lang.reflect.InvocationHandler; import java.lang.reflect.Method; import java.lang.reflect.Proxy; import java.util.ArrayList; import java.util.List; import java.util.concurrent.TimeUnit / * *-- * @ description TODO * @ author fancy * @ email fancydeepin@yeah.net * @ date 2012-8-27

*-- * / public class ProxyApp {public static void main (String [] args) {/ / ArrayList proxy, which calculates the time required for each method call List arrayListProxy = (List) Proxy.newProxyInstance (ArrayList.class.getClassLoader () / * define the class loader of the proxy class, which is used to create the proxy object. It does not have to be ArrayList, but it can also be another class loader * / ArrayList.class.getInterfaces (), / * list of interfaces to be implemented by the proxy class * / new InvocationHandler () {/ * assign the call handler for method calls The anonymous inner class * / private ArrayList target = new ArrayList () is used here. / / Target object (real operating object) / * * method description: * * handle method calls on the proxy instance and return the result * @ param proxy proxy object (note that it is not the target object) * @ param method the proxied method * @ param args the parameter set of the proxied method * @ return returns the result of the method call * / @ Override public Object invoke (Object proxy Method method, Object [] args) throws Throwable {long beginTime = System.currentTimeMillis () / / start time TimeUnit.MICROSECONDS.sleep (1); Object obj = method.invoke (target, args); / / the method actually called, and accept the return value of the method long endTime = System.currentTimeMillis () / / end time System.out.println ("[" + method.getName () + "] spend" + (endTime-beginTime) + "ms"); return obj; / / returns the return value of the actual called method}}) ArrayListProxy.add (2); arrayListProxy.add (4); System.out.println ("- iteration -"); for (int I: arrayListProxy) {System.out.print (I + "\ t");}

Print the output in the background:

[add] spend 2 ms

[add] spend 1 ms

-iteration-

[iterator] spend 1 ms

2 4

From a code point of view, anonymous inner classes are used so that InvocationHandler can only be used once, and if such a same InvocationHandler is needed in multiple places, it can be abstracted into a separate class:

Package test; import java.lang.reflect.InvocationHandler; import java.lang.reflect.Method; import java.util.concurrent.TimeUnit; public class MyInvocationHandler implements InvocationHandler {private Object target; / / Target object public MyInvocationHandler (Object target) {this.target = target;} @ Override public Object invoke (Object proxy, Method method, Object [] args) throws Throwable {long beginTime = System.currentTimeMillis () TimeUnit.MICROSECONDS.sleep (1); Object obj = method.invoke (target, args); long endTime = System.currentTimeMillis (); System.out.println ("[" + method.getName () + "] spend" + (endTime-beginTime) + "ms"); return obj;}}

The client call is changed to:

Package example; import java.lang.reflect.Proxy; import java.util.ArrayList; import java.util.List; / *-- * @ description TODO * @ author fancy * @ email fancydeepin@yeah.net * @ date 2012-8-27

*-- * / public class ProxyApp {public static void main (String [] args) {/ / ArrayList proxy, which calculates the time required for each method call List arrayListProxy = (List) Proxy.newProxyInstance (ArrayList.class.getClassLoader () / * A class loader that defines a proxy class, which is used to create a proxy object, not necessarily ArrayList, but also other class loaders * / ArrayList.class.getInterfaces (), / * list of interfaces to be implemented by the proxy class * / new MyInvocationHandler (new ArrayList ()) / * call handlers for assigning method calls. Anonymous inner classes are used here * /) ArrayListProxy.add (2); arrayListProxy.add (4); System.out.println ("- iteration -"); for (int I: arrayListProxy) {System.out.print (I + "\ t");}

Judging from the above code, the client knows the actual target object of the agent and how to create such a proxy object. If you want to hide all this information from the client, you can move the code to a class and encapsulate it:

Package example; import java.lang.reflect.InvocationHandler; import java.lang.reflect.Method; import java.lang.reflect.Proxy; import java.util.ArrayList; import java.util.List; import java.util.concurrent.TimeUnit / * *-- * @ description TODO * @ author fancy * @ email fancydeepin@yeah.net * @ date 2012-8-27

*-- * / public class ProxyUtil {public enum ArrayListProxy {PROXY; private Object target; ArrayListProxy () {this.target = new ArrayList () } public List getInstance () {return (List) Proxy.newProxyInstance (ArrayList.class.getClassLoader (), ArrayList.class.getInterfaces (), new InvocationHandler () {@ Override public Object invoke (Object proxy, Method method) Object [] args) throws Throwable {long beginTime = System.currentTimeMillis () TimeUnit.MICROSECONDS.sleep (1); Object obj = method.invoke (target, args); long endTime = System.currentTimeMillis (); System.out.println ("[" + method.getName () + "] spend" + (endTime-beginTime) + "ms") Return obj;}});

The client call is changed to:

Package example; import java.util.List; import example.ProxyUtil.ArrayListProxy; / *-- * @ description TODO * @ author fancy * @ email fancydeepin@yeah.net * @ date 2012-8-27

*-- * / public class ProxyApp {public static void main (String [] args) {List arrayListProxy = ArrayListProxy.PROXY.getInstance (); arrayListProxy.add (2); arrayListProxy.add (4) System.out.println ("- iteration -"); for (int I: arrayListProxy) {System.out.print (I + "\ t");}

Enumeration enum is used in the above code, so if you don't want to use enumerations, just use a normal class to implement it.

The above is the use of Proxy in the Java dynamic agent. The editor believes that there are some knowledge points that we may see or use in our daily work. I hope you can learn more from this article. For more details, please follow the industry information channel.

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