人在江湖

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The following Java code illustrates the pattern with the example of a logging class. Each logging handler decides if any action is to be taken at this log level and then passes the message on to the next logging handler. The output is:

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

Note that this example should not be seen as a recommendation on how to write logging classes.

Also, note that in a 'pure' implementation of the chain of responsibility pattern, a logger would not pass responsibility further down the chain after handling a message. In this example, a message will be passed down the chain whether it is handled or not.

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 l;
    }
    public void message( String msg, int priority )
    {
        if ( priority <= mask )
        {
            writeMessage( msg );
        }
        if ( next != null )
        {
            next.message( msg, priority );
        }
    }
    abstract protected void writeMessage( String msg );
}
class StdoutLogger extends Logger
{
    public StdoutLogger( int mask ) { this.mask = mask; }
    protected void writeMessage( String msg )
    {
        System.out.println( "Writing 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.err.println( "Sending to stderr: " + msg );
    }
}
public class ChainOfResponsibilityExample
{
    public static void main( String[] args )
    {
        // Build the chain of responsibility
        Logger l,l1;
        l1 = l = new StdoutLogger( Logger.DEBUG );
        l1 = l1.setNext(new EmailLogger( Logger.NOTICE ));
        l1 = l1.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 );
    }
}

posted on 2011-02-12 23:17 人在江湖 阅读(1577) 评论(0)  编辑  收藏 所属分类: design pattern

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