责任链模式

以下的日志类(logging)例子演示了该模式。 每一个logging handler首先决定是否需要在该层做处理，然后将控制传递到下一个logging handler。程序的输出是:

  Writting to stdout: Entering function y.
  Writting to stdout: Step1 completed.
  Sending via e-mail:      Step1 completed.
  Writting to stdout: An error has occurred.
  Sending via e-mail:      An error has occurred.
  Writing to stderr:       An error has occurred.

注意：该例子不是日志类的推荐实现方式。

同时，需要注意的是，通常在责任链模式的实现中，如果在某一层已经处理了这个logger，那么这个logger就不会传递下去。在我们这个例子中，消息会一直传递到最底层不管它是否已经被处理。

import java.util.*;

abstract class Logger 
{
    public static int ERR = 3;
    public static int NOTICE = 5;
    public static int DEBUG = 7;
    protected int mask;

    // The next element in the chain of responsibility
    protected Logger next;
    public Logger setNext( Logger l)
    {
        next = l;
        return this;
    }

    public final void message( String msg, int priority )
    {
        if ( priority <= mask ) 
        {
            writeMessage( msg );
            if ( next != null )
            {
                next.message( msg, priority );
            }
        }
    }
    
    protected abstract void writeMessage( String msg );

}

class StdoutLogger extends Logger 
{

    public StdoutLogger( int mask ) { this.mask = mask; }

    protected void writeMessage( String msg )
    {
        System.out.println( "Writting to stdout: " + msg );
    }
}


class EmailLogger extends Logger 
{

    public EmailLogger( int mask ) { this.mask = mask; }

    protected void writeMessage( String msg )
    {
        System.out.println( "Sending via email: " + msg );
    }
}

class StderrLogger extends Logger 
{

    public StderrLogger( int mask ) { this.mask = mask; }

    protected void writeMessage( String msg )
    {
        System.out.println( "Sending to stderr: " + msg );
    }
}

public class ChainOfResponsibilityExample
{
    public static void main( String[] args )
    {
        // Build the chain of responsibility
        Logger l = new StdoutLogger( Logger.DEBUG).setNext(
                            new EmailLogger( Logger.NOTICE ).setNext(
                            new StderrLogger( Logger.ERR ) ) );

        // Handled by StdoutLogger
        l.message( "Entering function y.", Logger.DEBUG );

        // Handled by StdoutLogger and EmailLogger
        l.message( "Step1 completed.", Logger.NOTICE );

        // Handled by all three loggers
        l.message( "An error has occurred.", Logger.ERR );
    }
}

这是一个用mutex保护对于stream的操作的真实例子。

First the outputStream interface (a simplified version of the real one):

interface outputStream
   predicates
      write : (...).
      writef : (string FormatString, ...).
end interface outputStream

This class encapsulates each of the stream operations the mutex. The finally predicate is used to ensure that the mutex is released no matter how the operation goes (i.e. also in case of exceptions). The underlying mutex object is released by a finalizer in the mutex class, and for this example we leave it at that.

class outputStream_protected : outputStream
   constructors
      new : (string MutexName, outputStream Stream).
end class outputStream_protected

#include @"pfc\multiThread\multiThread.ph"

implement outputStream_protected
   facts
      mutex : mutex.
      stream : outputStream.
   clauses
      new(MutexName, Stream) :-
         mutex := mutex::createNamed(MutexName, false), % do not take ownership
         stream := Stream.
   clauses
      write(...) :-
         _ = mutex:wait(),  % ignore wait code in this simplified example
         finally(stream:write(...), mutex:release()).

   clauses
      writef(FormatString, ...) :-
         _ = mutex:wait(),  % ignore wait code in this simplified example
         finally(stream:writef(FormatString, ...), mutex:release()).
end implement outputStream_protected

Usage example.

The client uses an outputStream. Instead of receiving the pipeStream directly, it gets the protected version of it.

#include @"pfc\pipe\pipe.ph"

goal
   Stream = pipeStream::openClient("TestPipe"),
   Protected = outputStream_protected::new("PipeMutex", Stream),
   client(Protected),
   Stream:close().