Chain of Responsibility Pattern

责任链模式

责任链模式顾名思义，对于一个请求会去创建一条接受者链。这种模式会解耦一种类型的请求中接受者和发送者，该模式模式属于行为模式的一种。
在该模式当中，一般情况下，每个接受者都会有另外一个接受者的引用，如果该接受者无法处理该请求，请求会通过引用传入下一个接受者。

实现

我们创建一个抽象类AbstaractLogger，该类中含有日志的记录等级。然后我们会创建三个logger实现AbstractLogger。每一个logger都会检查并且比较消息的等级与该logger等级，判断打印或者不打印，并且传递消息到下一个logger。

第一步

创建一个抽象的Logger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);
    
}

第二步

创建具体的类实现AtstractLogger

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);
   }
}

第三步

创建不同类型的logger。设置每个Logger的错误等级以及每个logger的Next logger，每一个logger的NextLogger都是当前链的一部分。

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.");
   }
}

第四步

校验输出

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.