1. 定义
   责任链模式（Chain Of Responsibility Pattern）顾名思义，就是为请求创建一条处理链路，链路上的每个处理器都判断是否可以处理请求，如果不能处理则往后走，依次从链头走到链尾，直到有处理器可以处理请求。
2. 类型
   2.1 请求只处理一次
   每个节点都有机会处理请求，但是请求只要处理成功就结束了。
   

场景
流程审批、扑克牌
代码示例
原来

public class Apply {public boolean apply(int requireDay) {if (requireDay <= 1) {return true;} else if (requireDay <= 3) {applyByLeader(requireDay);} else {applyByManager(requireDay);}}public boolean applyByLeader(int requireDay) {//...}public boolean applyByManager(int requireDay) {//...}
}

改造后
BaseHandler.java

public abstract class BaseHandler {protected BaseHandler successor;public void setSuccessor(BaseHandler successor) {this.successor = successor;}public abstract boolean apply(int requireDay);public void print() {System.out.println(this.getClass().getSimpleName() + " process.");}
}

AutoHandler.java

public class AutoHandler extends BaseHandler {@Overridepublic boolean apply(int requireDay) {super.print();if (requireDay <= 1) {return true;}return successor.apply(requireDay);}
}

LeaderHandler.java

public class LeaderHandler extends BaseHandler {@Overridepublic boolean apply(int requireDay) {super.print();if (requireDay <= 3) {return true;}return successor.apply(requireDay);}
}

ManagerHandler.java

public class ManagerHandler extends BaseHandler {private static final int MAX_DAY = 996;@Overridepublic boolean apply(int requireDay) {super.print();return requireDay <= MAX_DAY;}
}

HandlerClient.java

public class HandlerClient {private static final AutoHandler AUTO_HANDLER;static {ManagerHandler managerHandler = new ManagerHandler();LeaderHandler leaderHandler = new LeaderHandler();leaderHandler.setSuccessor(managerHandler);AUTO_HANDLER = new AutoHandler();AUTO_HANDLER.setSuccessor(leaderHandler);}public static void main(String[] args) {AUTO_HANDLER.apply(1);System.out.println();AUTO_HANDLER.apply(3);System.out.println();AUTO_HANDLER.apply(5);}
}

输出：

AutoHandler process.
AutoHandler process.
LeaderHandler process.
AutoHandler process.
LeaderHandler process.
ManagerHandler process.

2.2 请求处理多次

每个节点都有机会处理请求，节点处理完之后继续往后走，直到链尾。
场景
• 过滤器 / 拦截器
• JavaEE 的 Servlet 规范定义的 Filter
代码示例
请求如果成功通过 process 处理，则进入下一个 process，如果不通过则被过滤掉，这里不再累述代码。
3. 项目实践
有个根据配置构造ODPS查询语句的代码，配置片段如下：

{"name": "Document no","code": "service_order_code","isBasicField": true,"fromRerating": false,"classType": "java.lang.String"
}

原来是通过 if-else 来实现的，代码如下所示：

现在要新增非空校验的字段 notNull，现在配置如下：

{"name": "Document no","code": "service_order_code","isBasicField": true,"fromRerating": false,"classType": "java.lang.String","notNull": true
}

发现又得往 if-else 里面硬塞分支，有代码洁癖的我怎么能容忍自己写这种代码？最近也从同事那里了解到责任链模式的厉害之处，索性直接给它优化掉，这里我截取下关键代码片段。
首先声明抽象处理类

/*** 责任链抽象处理器*/
public abstract class AbstractHandler<T, V> {protected AbstractHandler<T, V> successor;public void setSuccessor(AbstractHandler<T, V> handler) {this.successor = handler;}/*** 处理方法** @param context 上下文* @return R*/public abstract String process(Context<T, V> context);
}

各具体处理类安排上

/*** 责任链入口，扩展字段处理*/
public class FirstExtendFieldHandler extends AbstractHandler<AdjustmentTemplateDTO, String> {/*** 处理方法** @param context 上下文* @return R*/@Overridepublic String process(Context<AdjustmentTemplateDTO, String> context) {AdjustmentTemplateDTO request = context.getRequest();if (!request.getBasicField()) {String tmpField = request.getFromRerating() ? String.format(GET_JSON_OBJECT, NEW_PARAM, PREFIX + request.getCode()) :String.format(GET_JSON_OBJECT, OLD_PARAM, PREFIX + request.getCode());context.setResult(tmpField);}return successor.process(context);}
}
/*** 非空字段处理器*/
public class SecondNotNullFieldHandler extends AbstractHandler<AdjustmentTemplateDTO, String> {/*** 处理方法** @param context 上下文* @return R*/@Overridepublic String process(Context<AdjustmentTemplateDTO, String> context) {AdjustmentTemplateDTO request = context.getRequest();if (!request.getBasicField()) {return successor.process(context);}if (request.getNotNull() == null || !request.getNotNull()) {String tmpField = (request.getFromRerating() ? NEW : OLD) + request.getCode();context.setResult(tmpField);return successor.process(context);} else {String oldValue = OLD + request.getCode();context.setResult(oldValue);oldValue = successor.process(context);String newValue = NEW + request.getCode();context.setResult(newValue);newValue = successor.process(context);String finalField = String.format(OdpsConstants.IF, oldValue, newValue, oldValue);context.setResult(finalField);return finalField;}}
}/*** 时间字段处理*/
public class ThirdTimeFormatHandler extends AbstractHandler<AdjustmentTemplateDTO, String> {@Overridepublic String process(Context<AdjustmentTemplateDTO, String> context) {AdjustmentTemplateDTO request = context.getRequest();String finalSql = StringUtils.isNotBlank(request.getTimeFormat())? String.format(TIMESTAMP_FORMAT, context.getResult(), request.getTimeFormat()): context.getResult();context.setResult(finalSql);return finalSql;}
}

最后是负责初始化责任链的客户端

/*** 责任链客户端*/
public class HandlerChainClient {private static final FirstExtendFieldHandler FIRST_HANDLER;static {ThirdTimeFormatHandler thirdHandler = new ThirdTimeFormatHandler();SecondNotNullFieldHandler secondHandler = new SecondNotNullFieldHandler();secondHandler.setSuccessor(thirdHandler);FIRST_HANDLER = new FirstExtendFieldHandler();FIRST_HANDLER.setSuccessor(secondHandler);}/*** 调用责任链进行处理** @param request 请求参数* @return result*/public static String process(AdjustmentTemplateDTO request) {AdjustmentContext context = new AdjustmentContext();context.setRequest(request);return FIRST_HANDLER.process(context);}
}

最后，业务代码里又臭又长的 if-else 变成了一行代码。

HandlerChainClient.process(request);

4. 优点
   • 解耦。请求发送者无需知道请求在何时、何处以及如何被处理，实现了发送者与处理者的解耦。
   • 灵活、可插拔。可以看到想要添加一个处理流程，只需实现BaseHandler，然后注入到对应的位置即可；删除一个流程也是一样，只需要将本节点的位置替换成下一个节点即可，客户端无需感知处理器的变化。
   • 代码优雅，责任链相比 if-else 是更加优雅的。
5. 缺点
   • 类的数量变多了，组链时要注意避免出现环状结构，导致出现死循环。