利用JDK动态代理机制实现简单拦截器

利用JDK动态代理机制实现简单的多层拦截器

首先JDK动态代理是基于接口实现的,所以我们先定义一个接口

public interface Executer {
    public Object execute(String param);
}

然后我们写一个类来实现该接口,我们将该类成为目标类。接下来我们将要对execute方法进行拦截！

public class SampleExecuter implements Executer {

    public Object execute(String param) {
        System.out.println("SampleExecuter.execute()");
        return "SampleExecuter.execute()" + param;
    }

}

我们需要定义拦截器接口 Interceptor

public interface Interceptor {

    public Object intercept(Invocation invocation) throws Throwable;
    
}

定义类 Invocation 该类负责调用我们目标对象的方法

public class Invocation {

    private Object target;
    
    private Method method;
    
    private Object[] args;

    public Invocation(Object target, Method method, Object[] args) {
        this.target = target;
        this.method = method;
        this.args = args;
    }
    
    public Object getTarget() {
        return target;
    }

    public void setTarget(Object target) {
        this.target = target;
    }

    public Method getMethod() {
        return method;
    }

    public void setMethod(Method method) {
        this.method = method;
    }

    public Object[] getArgs() {
        return args;
    }

    public void setArgs(Object[] args) {
        this.args = args;
    }
    
    public Object proceed() throws IllegalAccessException, InvocationTargetException {
        return method.invoke(target, args);
    }
}

代理类的实现

public class PlugIn implements InvocationHandler {

    private Object target;
    private Interceptor interceptor;
    
    public PlugIn(Object target, Interceptor interceptor) {
        this.target = target;
        this.interceptor = interceptor;
    }
    
    public Object invoke(Object proxy, Method method, Object[] args)
            throws Throwable {
        return interceptor.intercept(new Invocation(target, method, args));
    }
    
    /**
     * 通过JDK动态代理Proxy.newProxyInstance返回代理对象
     * @param target 目标对象
     * @param interceptor 拦截器对象
     * @return 代理对象
     */
    public static Object warp(Object target, Interceptor interceptor) {
        Class<?> type = target.getClass();
        return Proxy.newProxyInstance(type.getClassLoader(), type.getInterfaces(),
                                        new PlugIn(target, interceptor));
    }
    
}

InterceptorChain 用来注册拦截器,并将拦截器与目标对象结合生成代理对象

public class InterceptorChain {

    private List<Interceptor> interceptors = new ArrayList<Interceptor>();
    
    public void addInterceptor(Interceptor interceptor) {
        interceptors.add(interceptor);
    }
    
    public Object pluginAll(Object target) {
        for (Interceptor interceptor : interceptors) {
            target = PlugIn.warp(target, interceptor);
        }
        return target;
    }
}

最后写俩个拦截器来测试我们的程序吧

public class IntOne implements Interceptor {

    public Object intercept(Invocation invocation) throws IllegalAccessException, InvocationTargetException {
        System.out.println("IntOne.intercept()-begin");
        Object result = invocation.proceed();
        System.out.println("IntOne.intercept()-end");
        return result;
    }

}

public class IntTwo implements Interceptor {

    public Object intercept(Invocation invocation) throws IllegalAccessException, InvocationTargetException {
        System.out.println("IntTwo.intercept()-begin");
        Object result = invocation.proceed();
        System.out.println("IntTwo.intercept()-end");
        return result;
    }

}

public class Demo {

    public static void main(String[] args) {
        SampleExecuter target = new SampleExecuter();
        InterceptorChain chain = new InterceptorChain();
        
        chain.addInterceptor(new IntOne());
        chain.addInterceptor(new IntTwo());
        
        Executer executer = (Executer) chain.pluginAll(target);
        Object result = executer.execute("params");
        System.out.println(result);
    }

}

执行结果

IntTwo.intercept()-begin
IntOne.intercept()-begin
SampleExecuter.execute()
IntOne.intercept()-end
IntTwo.intercept()-end
SampleExecuter.execute()params