AOP为Aspect Oriented Programming的缩写，意为：面向切面编程。利用AOP可以对业务逻辑的各个部分进行隔离，从而使得业务逻辑各部分之间的耦合度降低，提高程序的可重用性，同时提高了开发的效率。这里介绍AOP的基本技术JDK动态代理和Cglib
应用场景
public interface Greeting {
void sayHello();
}
class Programmer implements Greeting {
@Override
public void sayHello() {
System.out.println("Hello,I'm programmer");
}
}
class Student implements Greeting {
@Override
public void sayHello() {
System.out.println("Hello,I'm student");
}
}
class Teacher implements Greeting {
@Override
public void sayHello() {
System.out.println("Hello,I'm teacher");
}
}
public class Client {
public static void main(String args[]) {
Greeting greeting=new Student();
greeting.sayHello();
greeting=new Teacher();
greeting.sayHello();
greeting=new Programmer();
greeting.sayHello();
}
}
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需要在sayHello方法前后，分别输出call before,call after.
方法一（写死代码）
我们可以用最简单的方式，写死代码来完成这个功能。
class Programmer implements Greeting {
@Override
public void sayHello() {
System.out.println("call before");
System.out.println("Hello,I'm programmer");
System.out.println("call after");
}
}
class Student implements Greeting {
@Override
public void sayHello() {
System.out.println("call before");
System.out.println("Hello,I'm student");
System.out.println("call after");
}
}
class Teacher implements Greeting {
@Override
public void sayHello() {
System.out.println("call before");
System.out.println("Hello,I'm teacher");
System.out.println("call after");
}
}
public class Client1 {
public static void main(String args[]) {
Greeting greeting=new Student();
greeting.sayHello();
greeting=new Teacher();
greeting.sayHello();
greeting=new Programmer();
greeting.sayHello();
}
}
ebIPpt.jpg
这种写死代码的方式，如果类文件多至几百个，那么就需要改几百处代码。同时，如果我们引用的是第三方jar，这个时候我们不太容易去改变第三方的jar。
方法二（静态代理）
我们不去写死原来文件代码，我们给greeting接口写一个静态代理
public class GreetingStaticProxy implements Greeting {
private Greeting greeting;
public GreetingStaticProxy(Greeting greeting) {
this.greeting = greeting;
}
@Override
public void sayHello() {
before();
greeting.sayHello();
after();
}
private void after() {
System.out.println("call after");
}
private void before() {
System.out.println("call before");
}
}
public class Client2 {
public static void main(String args[]) {
Greeting studentStaticProxy=new GreetingStaticProxy(new Student());
studentStaticProxy.sayHello();
Greeting teacherStaticProxy=new GreetingStaticProxy(new Teacher());
teacherStaticProxy.sayHello();
Greeting programmerStaticProxy=new GreetingStaticProxy(new Programmer());
programmerStaticProxy.sayHello();
}
}
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我们通过给greeting接口提供一个静态代理GreetingStaticProxy成功的解决了写死代码的问题，但是如果接口增多，比如增加一个Complain接口，那么就需要增加一个ComplainStaticProxy。最终XxxxStaticProxy类会越来越多。如何才能让这些代理类尽可能的减少？最好只有一个代理类，我们可以用JDK的动态代理。
方法三（JDK动态代理）
JDK动态代理关键类，InvocationHandler，Proxy这两个类。
public class DynamicProxy {
public static <T> T createProxy(final T t) {
return (T) Proxy.newProxyInstance(Thread.currentThread().getContextClassLoader(), t.getClass().getInterfaces(), new InvocationHandler() {
@Override
public Object invoke(Object proxy, Method method, Object[] args) throws Throwable {
before();
Object result = method.invoke(t, args);
after();
return result;
}
private void after() {
System.out.println("call after");
}
private void before() {
System.out.println("call before");
}
});
}
}
public class Client3 {
public static void main(String args[]) {
Greeting studentDynaminProxy = DynamicProxy.createProxy(new Student());
studentDynaminProxy.sayHello();
Greeting teacherDynicProxy = DynamicProxy.createProxy(new Teacher());
teacherDynicProxy.sayHello();
Greeting programmerDynicProxy = DynamicProxy.createProxy(new Programmer());
programmerDynicProxy.sayHello();
}
}
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及时增加了很多Greeting之外的接口，我们都可以通过DynamicProxy一个代理类来进行代理。但是这样做仍然存在一个问题，就是JDK提供的代理类只能代理接口，而不能代理没有接口的类。有什么方案可以解决这个问题么？
方法四(Cglib代理)
Cglib代理关键类，MethodInterceptor,Enhancer这两个类
public class CglibDynamicProxy {
public static <T> T createProxy(Class<T> tClass) {
return (T) Enhancer.create(tClass, new MethodInterceptor() {
public Object intercept(Object targerObject, Method method, Object[] params, MethodProxy methodProxy) throws Throwable {
before();
Object result = methodProxy.invokeSuper(targerObject, params);
after();
return result;
}
private void after() {
System.out.println("call after");
}
private void before() {
System.out.println("call before");
}
});
}
}
public class Client4 {
public static void main(String args[]) {
Student studentCglibProxy=CglibDynamicProxy.createProxy(Student.class);
studentCglibProxy.sayHello();
Teacher teacherCglibProxy=CglibDynamicProxy.createProxy(Teacher.class);
teacherCglibProxy.sayHello();
Programmer programmerCglibProxy=CglibDynamicProxy.createProxy(Programmer.class);
programmerCglibProxy.sayHello();
}
}
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Spring AOP就是基于JDK的动态代理和Cglib来实现的。
