15 Chain Of Responsibility

通过责任链(Chain Of Responsibility)模式，你可以为某个请求创建一个对象链。每个对象依序检查此请求，并对其进行处理，或者将它传给链中的下一个对象。

Logger

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，如果是则相应地打印出来，否则将不打印并把消息传给下一个记录器。

AbstractLogger

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

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.java

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.java

public class FileLogger extends AbstractLogger {

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

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

ChainPatternDemo.java

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 a debug level information.");

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

dsaf