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What this is

This file is included in the "Java Source Code Warehouse" project. The intent of this project is to help you "Learn Java by Example" TM.

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The source code

 * Copyright (C) The Apache Software Foundation. All rights reserved.
 * This software is published under the terms of the Apache Software
 * License version 1.1, a copy of which has been included with this
 * distribution in the LICENSE.txt file.  */


import java.util.Vector;

import org.apache.log4j.helpers.LogLog;
import org.apache.log4j.spi.LoggingEvent;
import org.apache.log4j.AppenderSkeleton;

  Sends {@link LoggingEvent} objects to a set of remote log servers,
  usually a {@link SocketNode SocketNodes}.

Acts just like {@link SocketAppender} except that instead of connecting to a given remote log server, SocketHubAppender accepts connections from the remote log servers as clients. It can accept more than one connection. When a log event is received, the event is sent to the set of currently connected remote log servers. Implemented this way it does not require any update to the configuration file to send data to another remote log server. The remote log server simply connects to the host and port the SocketHubAppender is running on.

The SocketHubAppender does not store events such that the remote side will events that arrived after the establishment of its connection. Once connected, events arrive in order as guaranteed by the TCP protocol.

This implementation borrows heavily from the {@link SocketAppender}.

The SocketHubAppender has the following characteristics:

  • If sent to a {@link SocketNode}, logging is non-intrusive as far as the log event is concerned. In other words, the event will be logged with the same time stamp, {@link org.apache.log4j.NDC}, location info as if it were logged locally.

  • SocketHubAppender does not use a layout. It ships a serialized {@link LoggingEvent} object to the remote side.

  • SocketHubAppender relies on the TCP protocol. Consequently, if the remote side is reachable, then log events will eventually arrive at remote client.

  • If no remote clients are attached, the logging requests are simply dropped.

  • Logging events are automatically buffered by the native TCP implementation. This means that if the link to remote client is slow but still faster than the rate of (log) event production, the application will not be affected by the slow network connection. However, if the network connection is slower then the rate of event production, then the local application can only progress at the network rate. In particular, if the network link to the the remote client is down, the application will be blocked.

    On the other hand, if the network link is up, but the remote client is down, the client will not be blocked when making log requests but the log events will be lost due to client unavailability.

    The single remote client case extends to multiple clients connections. The rate of logging will be determined by the slowest link.

  • If the JVM hosting the SocketHubAppender exits before the SocketHubAppender is closed either explicitly or subsequent to garbage collection, then there might be untransmitted data in the pipe which might be lost. This is a common problem on Windows based systems.

    To avoid lost data, it is usually sufficient to {@link #close} the SocketHubAppender either explicitly or by calling the {@link org.apache.log4j.LogManager#shutdown} method before exiting the application.

@author Mark Womack */ public class SocketHubAppender extends AppenderSkeleton { /** The default port number of the ServerSocket will be created on. */ static final int DEFAULT_PORT = 4560; private int port = DEFAULT_PORT; private Vector oosList = new Vector(); private ServerMonitor serverMonitor = null; private boolean locationInfo = false; public SocketHubAppender() { } /** Connects to remote server at address and port. */ public SocketHubAppender(int _port) { port = _port; startServer(); } /** Set up the socket server on the specified port. */ public void activateOptions() { startServer(); } /** Close this appender.

This will mark the appender as closed and call then {@link #cleanUp} method. */ synchronized public void close() { if(closed) return; LogLog.debug("closing SocketHubAppender " + getName()); this.closed = true; cleanUp(); LogLog.debug("SocketHubAppender " + getName() + " closed"); } /** Release the underlying ServerMonitor thread, and drop the connections to all connected remote servers. */ public void cleanUp() { // stop the monitor thread LogLog.debug("stopping ServerSocket"); serverMonitor.stopMonitor(); serverMonitor = null; // close all of the connections LogLog.debug("closing client connections"); while (oosList.size() != 0) { ObjectOutputStream oos = (ObjectOutputStream)oosList.elementAt(0); if(oos != null) { try { oos.close(); } catch(IOException e) { LogLog.error("could not close oos.", e); } oosList.removeElementAt(0); } } } /** Append an event to all of current connections. */ public void append(LoggingEvent event) { // if no event or no open connections, exit now if(event == null || oosList.size() == 0) return; // set up location info if requested if (locationInfo) { event.getLocationInformation(); } // loop through the current set of open connections, appending the event to each for (int streamCount = 0; streamCount < oosList.size(); streamCount++) { ObjectOutputStream oos = null; try { oos = (ObjectOutputStream)oosList.elementAt(streamCount); } catch (ArrayIndexOutOfBoundsException e) { // catch this, but just don't assign a value // this should not really occur as this method is // the only one that can remove oos's (besides cleanUp). } // list size changed unexpectedly? Just exit the append. if (oos == null) break; try { oos.writeObject(event); oos.flush(); // Failing to reset the object output stream every now and // then creates a serious memory leak. // right now we always reset. TODO - set up frequency counter per oos? oos.reset(); } catch(IOException e) { // there was an io exception so just drop the connection oosList.removeElementAt(streamCount); LogLog.debug("dropped connection"); // decrement to keep the counter in place (for loop always increments) streamCount--; } } } /** The SocketHubAppender does not use a layout. Hence, this method returns false. */ public boolean requiresLayout() { return false; } /** The Port option takes a positive integer representing the port where the server is waiting for connections. */ public void setPort(int _port) { port = _port; } /** Returns value of the Port option. */ public int getPort() { return port; } /** The LocationInfo option takes a boolean value. If true, the information sent to the remote host will include location information. By default no location information is sent to the server. */ public void setLocationInfo(boolean _locationInfo) { locationInfo = _locationInfo; } /** Returns value of the LocationInfo option. */ public boolean getLocationInfo() { return locationInfo; } /** Start the ServerMonitor thread. */ private void startServer() { serverMonitor = new ServerMonitor(port, oosList); } /** This class is used internally to monitor a ServerSocket and register new connections in a vector passed in the constructor. */ private class ServerMonitor implements Runnable { private int port; private Vector oosList; private boolean keepRunning; private Thread monitorThread; /** Create a thread and start the monitor. */ public ServerMonitor(int _port, Vector _oosList) { port = _port; oosList = _oosList; keepRunning = true; monitorThread = new Thread(this); monitorThread.setDaemon(true); monitorThread.start(); } /** Stops the monitor. This method will not return until the thread has finished executing. */ public synchronized void stopMonitor() { if (keepRunning) { LogLog.debug("server monitor thread shutting down"); keepRunning = false; try { monitorThread.join(); } catch (InterruptedException e) { // do nothing? } // release the thread monitorThread = null; LogLog.debug("server monitor thread shut down"); } } /** Method that runs, monitoring the ServerSocket and adding connections as they connect to the socket. */ public void run() { ServerSocket serverSocket = null; try { serverSocket = new ServerSocket(port); serverSocket.setSoTimeout(1000); } catch (Exception e) { LogLog.error("exception setting timeout, shutting down server socket.", e); keepRunning = false; return; } try { try { serverSocket.setSoTimeout(1000); } catch (SocketException e) { LogLog.error("exception setting timeout, shutting down server socket.", e); return; } while (keepRunning) { Socket socket = null; try { socket = serverSocket.accept(); } catch (InterruptedIOException e) { // timeout occurred, so just loop } catch (SocketException e) { LogLog.error("exception accepting socket, shutting down server socket.", e); keepRunning = false; } catch (IOException e) { LogLog.error("exception accepting socket.", e); } // if there was a socket accepted if (socket != null) { try { InetAddress remoteAddress = socket.getInetAddress(); LogLog.debug("accepting connection from " + remoteAddress.getHostName() + " (" + remoteAddress.getHostAddress() + ")"); // create an ObjectOutputStream ObjectOutputStream oos = new ObjectOutputStream(socket.getOutputStream()); // add it to the oosList. OK since Vector is synchronized. oosList.addElement(oos); } catch (IOException e) { LogLog.error("exception creating output stream on socket.", e); } } } } finally { // close the socket try { serverSocket.close(); } catch (IOException e) { // do nothing with it? } } } } }

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