责任链模式

责任链模式的定义：
Avoid coupling the sender of a request to its receiver by giving more than one object a chance to handle the request.
Chain the receiving objects and pass the request along the chaing until an object handles it;
使用多个对象都有机会处理请求，从而避免了请求的发送者和接受者之间的耦合关系。将这些对象连成一条链，并沿着这条链传递该请求，
知道有对象处理它为止。

//抽象处理者
public abstract class Handler{
    private Handler nextHandler;
    public final Response handleMessage(Request request){
        Resonpse response=null;
        if(this.getHandlerLevel().equals(requset.getRequestLevel())){
            response=this.echo(request);
        }else{
            if(this.nextHandler !=null){
                response=this.nextHandler.handleMessage(request);
            }else{
                //.....
            }
        }
        return response;
    }
    public void setNext(Handler _handler){
        this.nextHandler=_handler;
    }
    protected abstract Level getHandlerLevel();
    protected abstract Response echo(Request request);
}

//具体处理者
public class ConcreteHandler1 extends Handler{
    protected Response echo(Request request){
        return null;
    }
    protected Level getHandlerLevel(){
        return null;
    }
}

public class ConcreteHandler2 extends Handler{
    protected Response echo(Request request){
        return null;
    }
    protected Level getHandlerLevel(){
        return null;
    }
}

public class ConcreteHandler3 extends Handler{
    protected Response echo(Request request){
        return null;
    }
    protected Level getHandlerLevel(){
        return null;
    }
}

public class Level{}

public class Request{}

public class Response{}

//场景类
public class Client{
    public static void main(String[] args){
        Handler handler1=new ConcreteHanlder1();
        Handler handler2=new ConcreteHanlder2();
        Handler handler3=new ConcreteHanlder3();
        handler1.setNext(hanlder2);
        handler2.setNext(handler3);
        Response response=handler1.handlerMessage(new Request());
    }
}

责任链模式优点：将请求和处理分开。请求者可以不用知道是谁处理的，处理者可以不用知道请求的全貌
　　         缺点：一是性能问题，每个请求都是从链头遍历到链尾，特别是链比较长时，性能是一个非常大的问题
　　　　　　　　二是调试不很方便，特别是链比较长时
　　　　  注意事项：链中节点数量要控制，避免出现超长链的情况，一般的做法是在Handler中设置一个最大的节点数量，在setNext方法
中判断是否已经超过其阈值，超过则不允许该链建立，避免无意识的破坏系统性能