Java 设计模式-责任链模式

模式介绍

责任链模式（Chain of Responsibility Pattern）为请求创建了一个接收者对象的链，对请求的发送者和接收者进行解耦。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。

使用场景

1、有多个对象可以处理同一个请求，具体哪个对象处理该请求由运行时刻自动确定。
2、Servlet 的 FilterChain 就是使用的这种模式。

系统实现

/**
 * AbstractLogger抽象类
 */
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);   
}

/**
 * ConsoleLogger类
 */
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);
   }
}

/**
 * ErrorLogger类
 */
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);
   }
}

/**
 * FileLogger类
 */
public class FileLogger extends AbstractLogger {
   public FileLogger(int level){
      this.level = level;
   }
 
   @Override
   protected void write(String message) {    
      System.out.println("File::Logger: " + message);
   }
}

/**
 * 客户端
 */
public class Client {
   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 a debug level information.");
      loggerChain.logMessage(AbstractLogger.ERROR,"This is an error information.");
   }
}

结果：
Standard Console::Logger: This is an information.
File::Logger: This is a debug level information.
Standard Console::Logger: This is a 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.