责任链模式（Chain of Responsibility Pattern）

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2023-12-01

顾名思义，责任链模式为请求创建了一系列接收者对象。此模式根据请求类型将请求的发送方和接收方解耦。这种模式属于行为模式。

在此模式中，通常每个接收器包含对另一个接收器的引用。如果一个对象无法处理请求，则它将该对象传递给下一个接收器，依此类推。

实现 (Implementation)

我们创建了一个抽象类AbstractLogger具有一定级别的日志记录。然后我们创建了三种扩展AbstractLogger的记录器。每个记录器检查消息级别到其级别并相应地打印，否则不打印并将消息传递给下一个记录器。

Step 1

创建一个抽象记录器类。

AbstractLogger.java

public abstract class AbstractLogger {
   public static int INFO = 1;
   public static int DEBUG = 2;
   public static int ERROR = 3;
   protected int level;
   //next element in chain or responsibility
   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);
}

Step 2

创建扩展记录器的具体类。

ConsoleLogger.java

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);
   }
}

Step 3

创建不同类型的记录器。为每个记录器分配错误级别并设置下一个记录器。每个记录器中的下一个记录器代表链的一部分。

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

Step 4

验证输出。

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.