深入了解Java设计模式之职责链模式-Java开发

Java设计模式中有很多种类别，例如单例模式、装饰模式、观察者模式等。本文将为大家详细介绍其中的职责链模式，感兴趣的可以了解一下

定义

使多个对象都有机会处理请求，从而避免请求的发送者和接受者之间的耦合关系，将这个对象连成一条链，并沿着这条链传递该请求，直到有一个对象处理它为止。

解决的问题

请求和处理分开、实现解耦、提高系统的灵活性

可以动态的调整请求的链条，增加系统灵活性

核心要点

职责链模式每个执行者都包含了另一个执行者的引用。如果一个对象不能处理该请求，会把请求传递给下一个执行者。

客户端需要动态的调整，执行者的上下级。

Handler 里面聚合它自己，在 HandlerRequest 里判断是否合适，如果没达到条件则向下传递，向谁传递之前 set 进去。

类图

代码实现

抽象父类

/**
 * 处理请求的抽象类
 *
 * @author Promsing(张有博)
 * @version 1.0.0
 * @since 2022/9/7 - 9:29
 */
public abstract class Handler {

    //维持一个下一个执行者的引用
    protected Handler handler;
    
    protected void setHandler(Handler handler) {
        this.handler = handler;
    }

    //处理请求的抽象方法
    public abstract void  processRequest(int request);
}

执行者-三个

/**
 *  只处理数字1-10
 *
 * @author Promsing(张有博)
 * @version 1.0.0
 * @since 2022/9/7 - 9:31
 */
public class ConcreteHandlerA extends Handler{
    @Override
    public void processRequest(int request) {
        if (request>=0 && request<10 ){
            System.out.println("ConcreteHandlerA已经处理完毕了  "+request);
            return;
        }
        if (handler!=null){
            //下一位处理
            handler.processRequest(request);
        }
    }
}

/**
 * 只处理数字10-20
 *
 * @author Promsing(张有博)
 * @version 1.0.0
 * @since 2022/9/7 - 9:31
 */
public class ConcreteHandlerB extends Handler{
    @Override
    public void processRequest(int request) {
        if (request>=10 && request<20 ){
            System.out.println("ConcreteHandlerB已经处理完毕了  "+request);
            return;
        }
        if (handler!=null){
            //下一位处理
            handler.processRequest(request);
        }
    }
}

/**
 *只处理数字30+
 *
 * @author Promsing(张有博)
 * @version 1.0.0
 * @since 2022/9/7 - 9:31
 */
public class ConcreteHandlerC extends Handler{
    @Override
    public void processRequest(int request) {
        if (request>=30 ){
            System.out.println("ConcreteHandlerC已经处理完毕了  "+request);
            return;
        }
        if (handler!=null){
            //下一位处理
            handler.processRequest(request);
        }
    }
}

客户端

public class Main {

    public static void main(String[] args) {
        //创建执行者
        Handler h1=new ConcreteHandlerA();
        Handler h2=new ConcreteHandlerB();
        Handler h3=new ConcreteHandlerC();

        //设置向下级的顺序,可根据配置动态设置上下级
        h1.setHandler(h2);
        h2.setHandler(h3);
        int[] requests={8,11,23,50,7,19,28,40};

        //循环处理请求，不同的数值，交给不同的执行者
        for (int request : requests) {

            h1.processRequest(request);
        }
    }

}

拓展

SpringMVC中DispatchServlet使用职责链

 protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception {
        HttpServletRequest processedRequest = request;//请求
        HandlerExecutionChain mappedHandler = null;//执行链

        try {
            ModelAndView mv = null;
            Exception dispatchException = null;

            try {

                // Determine handler for the current request.
                mappedHandler = getHandler(processedRequest); //根据请求-获得执行链
                //判断mappedHandler是否为空，空：404
                if (mappedHandler == null) {
                    noHandlerFound(processedRequest, response);
                    return;
                }

                // 确定当前请求的处理程序适配器
                HandlerAdapter ha = getHandlerAdapter(mappedHandler.getHandler());

                //职责链的前置拦截
                if (!mappedHandler.applyPreHandle(processedRequest, response)) {
                    return;
                }

                //执行handler方法-controller
                mv = ha.handle(processedRequest, response, mappedHandler.getHandler());

                //职责链的后置拦截
                mappedHandler.applyPostHandle(processedRequest, response, mv);
            }
            catch (Exception ex) {
                dispatchException = ex;
            }
            //对结果集进行处理
            processDispatchResult(processedRequest, response, mappedHandler, mv, dispatchException);
        }
        catch (Exception ex) {
            //执行完成了拦截器
            triggerAfterCompletion(processedRequest, response, mappedHandler, ex);
        }
       
        finally {
            //资源释放 
        }
    }

拦截器的方法

	boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {
		HandlerInterceptor[] interceptors = getInterceptors(); //获得拦截器
		if (!ObjectUtils.isEmpty(interceptors)) {
			for (int i = 0; i < interceptors.length; i++) {//循环遍历执行前置拦截
				HandlerInterceptor interceptor = interceptors[i];
				if (!interceptor.preHandle(request, response, this.handler)) {
					triggerAfterCompletion(request, response, null);
					return false;
				}
				this.interceptorIndex = i;
			}
		}
		return true;
	}


	void applyPostHandle(HttpServletRequest request, HttpServletResponse response, @Nullable ModelAndView mv)
			throws Exception {

		HandlerInterceptor[] interceptors = getInterceptors(); //获得拦截器
		if (!ObjectUtils.isEmpty(interceptors)) {
			for (int i = interceptors.length - 1; i >= 0; i--) { //循环遍历执行后置拦截
				HandlerInterceptor interceptor = interceptors[i];
				interceptor.postHandle(request, response, this.handler, mv);
			}
		}
	
        
        
        	void triggerAfterCompletion(HttpServletRequest request, HttpServletResponse response, @Nullable Exception ex)
			throws Exception {

		HandlerInterceptor[] interceptors = getInterceptors(); //获得拦截器
		if (!ObjectUtils.isEmpty(interceptors)) {
			for (int i = this.interceptorIndex; i >= 0; i--) {
				HandlerInterceptor interceptor = interceptors[i];
				try {   //循环遍历执行完成拦截
					interceptor.afterCompletion(request, response, this.handler, ex);
				}
				catch (Throwable ex2) {
					logger.error("HandlerInterceptor.afterCompletion threw exception", ex2);
				}
			}
		}
	}