【设计模式】责任链模式

基本概念

为请求创建一个接收者对象的链，属于行为型模式，在这种模式中，通常每个接收者都包含对另一个接收者的引用。如果一个对象不能处理该请求，那么它会把相同的请求传给下一个接收者，依此类推

• 意图: 请求的接收者形成一条链，沿着这个链传递请求
• 解决什么问题：将请求的发送者和请求的接收者进行解耦，请求的发送者只关心请求的发送，具体的处理流程不需要关心
• 何时使用：处理消息的时候需要过滤很多道
• 如何实现：拦截的类实现统一接口
• 应用场景：JavaWeb中的Filter JS中的事件冒泡

举个例子

我们创建抽象类 AbstractLogger，带有详细的日志记录级别。然后我们创建三种类型的记录器，都扩展了 AbstractLogger。每个记录器消息的级别是否属于自己的级别，如果是则相应打印出来，否则将不打印并把消息传给下一个记录器

样例类图

//创建抽象logger
public abstract class AbstractLogger {
   public static int INFO = 1;
   public static int DEBUG = 2;
   public static int ERROR = 3;

   protected int level;

   //责任链中的下一个元素
   protected AbstractLogger nextLogger;

   public void setNextLogger(AbstractLogger nextLogger){
      this.nextLogger = nextLogger;
   }

   public void logMessage(int level, String message){
      if(this.level <= level){
         write(message);
      }
      if(nextLogger !=null){
         nextLogger.logMessage(level, message);
      }
   }

   abstract protected void write(String message);
    
}


//定义三个具体的logger类 重写write方法
public class ConsoleLogger extends AbstractLogger {

   public ConsoleLogger(int level){
      this.level = level;
   }

   @Override
   protected void write(String message) {        
      System.out.println("Standard Console::Logger: " + message);
   }
}

public class ErrorLogger extends AbstractLogger {

   public ErrorLogger(int level){
      this.level = level;
   }

   @Override
   protected void write(String message) {        
      System.out.println("Error Console::Logger: " + message);
   }
}

public class FileLogger extends AbstractLogger {

   public FileLogger(int level){
      this.level = level;
   }

   @Override
   protected void write(String message) {        
      System.out.println("File::Logger: " + message);
   }
}



public class ChainPatternDemo {
    
   private static AbstractLogger getChainOfLoggers(){

      AbstractLogger errorLogger = new ErrorLogger(AbstractLogger.ERROR);
      AbstractLogger fileLogger = new FileLogger(AbstractLogger.DEBUG);
      AbstractLogger consoleLogger = new ConsoleLogger(AbstractLogger.INFO);

      errorLogger.setNextLogger(fileLogger);
      fileLogger.setNextLogger(consoleLogger);

      return errorLogger;    
   }

   public static void main(String[] args) {
      AbstractLogger loggerChain = getChainOfLoggers();

      loggerChain.logMessage(AbstractLogger.INFO, 
         "This is an information.");

      loggerChain.logMessage(AbstractLogger.DEBUG, 
         "This is an debug level information.");

      loggerChain.logMessage(AbstractLogger.ERROR, 
         "This is an error information.");
   }
}

程序输出

Standard Console::Logger: This is an information.

File::Logger: This is an debug level information.

Standard Console::Logger: This is an debug level information.

Error Console::Logger: This is an error information.

File::Logger: This is an error information.

Standard Console::Logger: This is an error information.

本文例子来自http://www.runoob.com/design-pattern/chain-of-responsibility-pattern.html