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		<h2>Chapter 4: Appenders</h2>
		<div class="author">
			Authors: Ceki G&#252;lc&#252;, S&#233;bastien Pennec
		</div>


		<table>
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					<p>Copyright &#169; 2000-2006, QOS.ch</p>

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		<h2>What is an Appender</h2>

		<p>
			Logback delegates the task of writing a logging event to appenders. 
			Appenders must imple-ment the <code>ch.qos.logback.core.Appender</code> interface. 
			The salient methods of this interface are summarized below:
		</p>
		<div class="source"><pre>package ch.qos.logback.core;
  
import ch.qos.logback.core.spi.ContextAware;
import ch.qos.logback.core.spi.FilterAttachable;
import ch.qos.logback.core.spi.LifeCycle;
  

public interface Appender extends LifeCycle, ContextAware, FilterAttachable {

  public String getName();
  <b>void doAppend(Object event);</b>
  public void setLayout(Layout layout);
  public Layout getLayout();
  public void setName(String name);
  
}</pre></div>

	<p>
		Most of the methods in the Appender interface are made of setter 
		and getter methods. A notable exception is the <code>doAppend()</code> 
		method taking an Object instance as its only parameter. 
		This method is perhaps the most important in the logback framework. 
		It is responsible for outputting the logging events in a suitable format 
		to the appropriate output device. Appenders are named entities. 
		This ensures that they can be referenced by name, a quality confirmed 
		to be especially significant in configuration scripts. 
		An appender can contain multiple filters, thus the <code>Appender</code>
		interface extending the <code>FilterAttachable</code> interface.
		Filters are discussed in detail in a subsequent chapter.
	</p>
	
	<p>
		Appenders are ultimately responsible for outputting logging events. 
		However, they may delegate the actual formatting of the event to a 
		<code>Layout</code> object. 
		Each layout is associated with one and only one appender, referred to 
		as the containing appender. Some appenders have a built-in or fixed 
		event format, such that they do not require a layout. For example, the 
		<code>SocketAppender</code> simply serialize logging events before 
		transmitting them over the wire.
	</p>
	
	<h2>AppenderBase</h2>
	
	<p>
		The <code>ch.qos.logback.core.AppenderSkeleton</code> class is an abstract 
		class implementing the <code>Appender</code> interface. 
		It provides basic functionality shared by all appenders, 
		such as methods for getting or setting their name, their started status, 
		their layout and their filters. 
		It is the super-class of all appenders shipped with logback. 
		Although an abstract class, AppenderBase actually implements the 
		<code>doAppend()</code> method in the <code>Append</code> interface. 
		Perhaps the clearest way to discuss AppenderBase class is by 
		presenting a bit of its actual source code.
		<code>AppenderBase</code>'s complete source code is also 
		<a href="../xref/ch/qos/logback/core/AppenderBase.html">
		available for viewing</a>.  
	</p>
	
<div class="source"><pre>public synchronized void doAppend(Object eventObject) {

  // prevent re-entry.
  if (guard) {
    return;
  }

  try {
    guard = true;

    if (!this.started) {
      if (statusRepeatCount++ &lt; ALLOWED_REPEATS) {
        addStatus(new WarnStatus(
            "Attempted to append to non started appender [" + name + "].",this));
      }
      return;
    }

    if (getFilterChainDecision(eventObject) == FilterReply.DENY) {
      return;
    }
    
    // ok, we now invoke derived class' implementation of append
    this.append(eventObject);

  } finally {
    guard = false;
  }
}</pre></div>
	
	<p>
		This implementation of the <code>doAppend()</code> method is synchronized. 
		It follows that logging to the same appender from different 
		threads is safe. While a thread, say <em>T</em>, is executing the <code>doAppend()</code> 
		method, subsequent calls by other threads are queued until <em>T</em> 
		leaves the <code>doAppend()</code> method, ensuring 
		<em>T</em>'s exclusive access to the appender. 
	</p>
	
	<p>
		EXPLAIN GUARD
	</p>
	
	<p>
		The first statement of the <code>doAppend()</code> method, once the <code>try</code> block
		is reached is to check whether the <code>started</code> field is true. 
		If it is not, <code>doAppend()</code> will send a warning message and return. 
		In other words, once stopped, it is impossible to write to a closed appender. 
		<code>Appender</code> object implement the <code>LifeCycle</code> interface, 
		which implies that they implement <code>start()</code>, <code>stop()</code>
		and <code>isStarted()</code> methods. After setting all the options of an appender,
		Joran, logback's configuration framework, calls this method to signal the appender
		to bind or activate its options.
		Indeed, depending on the appender, certain options cannot be activated because 
		of interferences with other options, or appenders can even not start at all if
		some options are missing. 
		For example, since file creation depends on truncation mode, 
		<code>FileAppender</code> cannot act on the value of its <code>File</code> option 
		until the value of the Append option is also known for certain.
		Sub-classes of <code>AppenderBase</code> are required to set the boolean variable 
		<code>started</code> to false when their <code>stop()</code> method is invoked.
	</p>
	
	<p>
		If a warning message is sent due to incorrect calls to the <code>doAppend()</code>
		method, logback's powerful <code>Status</code> error reporting system is used. In case
		several incorrect calls on <code>doAppend()</code> are issued, <code>AppenderBase</code>
		does not send an unlimited number of warnings. Once a certain limit is reached, the
		<code>AppenderBase</code> instance stops its warnings.
	</p>
	
	<p>
		The next <code>if</code> statement checks the result
		of the attached <code>Filter</code> objects. 
		Depending on the decision made by the filters 
		in the filter chain, events can be denied or alternatively accepted. 
		In the absence of a decision by the filter chain, events are accepted by default.
	</p>
	
	<p>
		Lastly, the <code>doAppend()</code> method invoke the derived classes' implementation
		of the <code>append()</code> method, which does the actual work of appending the
		event to the appropriate device.
	</p>
	
	<p>DO WE ADD A UML DIAGRAM?? YES!!</p>
	
	<p>In appenders, the term option or property is reserved for named attributes 
	that are dynamically inferred using JavaBeans introspection. </p>
	
	<h2>Logback Core</h2>
	
	<h3>WriterAppender</h3>
	
	<p>
		<code>WriterAppender</code> appends events to a <code>java.io.Writer</code>. 
		This class provides basic services that other appenders build upon. 
		Users do not usually instantiate <code>WriterAppender</code> objects directly. 
		Since <code>java.io.Writer</code> type cannot be mapped to a string, there is no 
		way to specify the target <code>Writer</code> object in a configuration script. 
		Simply put, you cannot configure a <code>WriterAppender</code> from a script. 
		However, this does not mean that <code>WriterAppender</code> lacks configurable options. 
		These options are described next.
	</p>
	
	<table>
		<tr>
			<th>Option Name</th>
			<th>Type</th>
			<th>Description</th>
		</tr>
		<tr>
			<td><b><span class="option">Encoding</span></b></td>
			<td><code>String</code></td>
			<td>
				The encoding specifies the method of conversion between 16-bit Unicode 
				characters into raw 8-bit bytes. This appender will use the local platform's 
				default encoding unless you specify otherwise using the 
				<span class="option">Encoding</span> option. 
				According to the <code>java.lang</code> package documentation, acceptable values 
				are dependent on the VM implementation although all implementations are 
				required to support at least the following encodings: 
				<em>US-ASCII</em>, <em>ISO-8859-1</em>, <em>UTF-8</em>, <em>UTF-16BE</em>,
				<em>UTF-16LE</em> and <em>UTF-16</em>. 
				By default, the <span class="option">Encoding</span> option is null such 
				that the platform's default encoding is used.
			</td>
		</tr>
		<tr>
			<td><b><span class="option">ImmediateFlush</span></b></td>
			<td><code>boolean</code></td>
			<td>
				If set to true, each write of a logging event is followed by a flush operation 
				on the underlying <code>Writer</code> object. Conversely, if the option is set to false, 
				each write will not be followed by a flush. 
				In general, skipping the flush operation improves logging throughput by roughly 15%. 
				The downside is that if the application exits abruptly, the unwritten characters 
				buffered inside the <code>Writer</code> might be lost. 
				This can be particularly troublesome as those unwritten characters may contain 
				crucial information needed in identifying the reasons behind a crash. 
				By default, the <span class="option">ImmediateFlush</span> option is set to true.
			</td>
		</tr>
	</table>
	
	<p>
		In general, if you disable immediate flushing, then make sure to flush 
		any output streams when your application exits. Otherwise, log messages 
		will be lost as illustrated by the next example. 
	</p>
	
	<em>Example 4.1: Exiting an application without flushing (logback-examples/src/main/java/chapter4/
	<a href="../xref/chapter4/ExitWoes.html">ExitWoes.java</a>)</em>
<div class="source"><pre>package chapter4;

import java.io.FileOutputStream;
import java.io.OutputStream;
import java.io.OutputStreamWriter;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

import ch.qos.logback.classic.LoggerContext;
import ch.qos.logback.core.WriterAppender;
import ch.qos.logback.core.layout.DummyLayout;

public class ExitWoes {

  public static void main(String[] args) throws Exception {
    LoggerContext lc = new LoggerContext();
    WriterAppender writerAppender = new WriterAppender();
    writerAppender.setContext(lc);
    writerAppender.setLayout(new DummyLayout());

    OutputStream os = new FileOutputStream("exitWoes1.log");
    writerAppender.setWriter(new OutputStreamWriter(os));
    writerAppender.setImmediateFlush(false);
    writerAppender.start();

    Logger logger = LoggerFactory.getLogger(ExitWoes1.class);

    logger.debug("Hello world.");
  }
}</pre></div>

	<p>
		This example creates a <code>WriterAppender</code> that uses an 
		<code>OutputStreamWriter</code> 
		wrapping a <code>FileOutputStream</code> as its underlying <code>Writer</code> object, 
		with immediate flushing disabled. It then proceeds to log a single debug message. 
		According to <code>OutputStreamWriter</code> javadocs, each invocation of a 
		<code>write()</code>
		method causes the encoding converter to be invoked on the given character(s). 
		The resulting bytes are accumulated in a buffer before being written 
		to the underlying output stream. As astonishing as this may seem, 
		running <code>ExitWoes1</code> will not produce any output in the file 
		<em>exitWoes1.log</em>
		because the Java VM does not flush output streams when it exits. 
		Calling the <code>shutdown()</code> method of a <b>XXXXXX</b> ensures that all 
		appenders in the hierarchy are closed and their buffers are flushed. 
		For most applications this is as simple as including the following statement 
		before exiting the application.
	</p>
	<p>WHAT TO DO IN LB ???</p>

	<p>
		The <code>WriterAppender</code> is the super class of four other appenders, 
		namely <code>ConsoleAppender</code>, <code>FileAppender</code> which in turn is 
		the super class of <code>RollingFileAppender</code>. 
	</p>
	<p><b>include UML??</b>
		Figure 4 2 illustrates the class diagram for WriterAppender and its subclasses
	</p>
	
	<h3>ConsoleAppender</h3>
	
	<p>
		The <code>ConsoleAppender</code>, as the name indicates, appends on the console, 
		or more precisely on <em>System.out</em> or <em>System.err</em>, the former 
		being the default target. <code>ConsoleAppender</code> formats events with 
		a layout specified by the user. Both <em>System.out</em> and <em>System.err</em> 
		are <code>java.io.PrintStream</code> objects. 
		Consequently, they are wrapped inside an <code>OutputStreamWriter</code> 
		which buffers I/O operations but not character conversions. 
	</p>
	
	<table>
			<tr>
			<th>Option Name</th>
			<th>Type</th>
			<th>Description</th>
		</tr>
		<tr>
			<td><b><span class="option">Encoding</span></b></td>
			<td><code>String</code></td>
			<td>See <code>WriterAppender</code> options.</td>
		</tr>
		<tr>
			<td><b><span class="option">ImmediateFlush</span></b></td>
			<td><code>boolean</code></td>
			<td>See <code>WriterAppender</code> options.</td>
		</tr>
		<tr>
			<td><b><span class="option">Target</span></b></td>
			<td><code>String</code></td>
			<td>
				One of the String values <em>System.out</em> or 
				<em>System.err</em>. The default target is <em>System.out</em>.
			</td>
		</tr>
	</table>
	
	<h3>FileAppendery</h3>
	
	<p>
		The <code>FileAppender</code>, a subclass of <code>WriterAppender</code>, 
		appends log events into a file. The file to write to is specified by 
		the <span class="option">File</span> option. 
		If the file already exists, it is either appended to, or truncated 
		depending on the value of the <span class="option">Append</span> option. 
		It uses a <code>FileOutputStream</code> which is wrapped by an <code>OutputStreamWriter</code>. 
		Note that <code>OutputStreamWriter</code> buffers I/O operations 
		but not character conversions. To optimize character conversions one 
		can set the <span class="option">BufferedIO</span> option to true 
		which effectively wraps the <code>OutputStreamWriter</code> with 
		a <code>BufferedWriter</code>. Options for <code>FileAppender</code> are summarized below. 
	</p>
	
	<table>
			<tr>
			<th>Option Name</th>
			<th>Type</th>
			<th>Description</th>
		</tr>
		<tr>
			<td><b><span class="option">Append</span></b></td>
			<td><code>boolean</code></td>
			<td>If true, events are appended at the end of an existing file. 
			Otherwise, if <span class="option">Append</span> is false, any existing 
			file is truncated. The <span class="option">Append</span> option is set to true by default.</td>
		</tr>
		<tr>
			<td><b><span class="option">Encoding</span></b></td>
			<td><code>String</code></td>
			<td>See <code>WriterAppender</code> options.</td>
		</tr>
		<tr>
			<td><b><span class="option">BufferedIO</span></b></td>
			<td><code>boolean</code></td>
			<td>
				The <span class="option">BufferedIO</span> option is set to false by default. 
				If set to true, the underlying <code>OutputStreamWriter</code> is wrapped 
				by a <code>BufferedWriter</code> object. 
				Setting <span class="option">BufferedIO</span> to true automatically 
				sets the <span class="option">ImmediateFlush</span> option to false. 
				The name <span class="option">BufferedIO</span> is slightly misleading because 
				buffered IO is already supported by <code>OutputStreamWriter</code>. 
				Setting <span class="option">BufferedIO</span> to true has the effect of 
				buffering I/O as well as character to raw byte conversions, saving a few 
				CPU cycles in the process.
			</td>
		</tr>
		<tr>
			<td><b><span class="option">BufferSize</span></b></td>
			<td><code>int</code></td>
			<td>Size of <code>BufferedWriter</code> buffer. The default value is 8192.</td>
		</tr>
		<tr>
			<td><b><span class="option">File</span></b></td>
			<td><code>int</code></td>
			<td>
				The name of the file to write to. If the file does not exist, it is created. <br />
				On the MS Windows platform users frequently forget to escape back slashes. 
				For example, the value <em>c:\temp\test.log</em> is not likely to be interpreted 
				properly as <em>'\t'</em> is an escape sequence interpreted as a single 
				tab character <em>(\u0009)</em>. 
				Correct values can be specified as <em>c:/temp/test.log</em> or 
				alternatively as <em>c:\\temp\\test.log</em>. 
				The <span class="option">File</span> option has no default value.
			</td>
		</tr>
	</table>
	
	<p>
		By default, <code>FileAppender</code> performs a flush operation for 
		each event, ensuring that events are immediately written to disk. 
		Setting the <span class="option">ImmediateFlush</span> option to false can drastically reduce 
		I/O activity by letting <code>OutputStreamWriter</code> buffer bytes 
		before writing them on disk. For short messages, we have observed 2 or 3 
		fold increases in logging throughput, i.e. the number of logs output 
		per unit of time. For longer messages, the throughput gains are somewhat 
		less dramatic, and range between 1.4 and 2 fold. Enabling the 
		<span class="option">BufferedIO</span> 
		option, that is buffering character to byte conversions, increases 
		performance by an additional 10% to 40% compared to only disk 
		I/O buffering (<span class="option">ImmediateFlush</span>=false). 
		Performance varies somewhat depending on the host machine as well as JDK version. 
		Throughput measurements are based on the <code>chapter4.IO</code> application.
		Please refer to the file <em>logback-examples/src/main/java/chapter4/IO.java</em> 
		for actual source code. 
		<a href="../xref/chapter4/IO.html">Online viewing</a> 
		of this file is also available. 
	</p>
	
	<h2>Logback Classic</h2>
	
	<h2>Logback Access</h2>
	
	
	
	
	
	
	
	

	</body>
</document>
