Design Pattern 简明教程

Chain of Responsibility Pattern

顾名思义，职责链模式为请求创建了一条接收者对象链。此模式根据请求类型将请求的发送方和接收方分离。此模式属于行为模式。

在此模式中，每个接收器通常包含对另一个接收器的引用。如果一个对象无法处理请求，则将其传递给下一个接收器，依此类推。

Implementation

我们创建了一个具有日志记录级别的抽象类 AbstractLogger。然后，我们创建了三种类型的日志记录器，扩展了 AbstractLogger。每个日志记录器根据其级别检查消息级别并相应打印，否则不打印并将该消息传递给其下一个日志记录器。

Step 1

创建抽象记录器类。

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。将错误级别分配给它们并在每个记录器中设置下一个记录器。每个记录器中的下一个记录器表示链的一部分。

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.