The immediateFlush variable is set to true by default. 

The encoding variable is set to null by default which results in the utilization of the system's default encoding. 

 public WriterAppender() 
{

}

This default constructor does nothing.

 public WriterAppender(Layout layout,
    OutputStream os) 
{
    this(layout, new OutputStreamWriter(os));
}

Instantiate a WriterAppender and set the output destination to a new OutputStreamWriter initialized with os as its OutputStream .

 public WriterAppender(Layout layout,
    Writer writer) 
{
    this.layout = layout;
    this.setWriter(writer);
}

Instantiate a WriterAppender and set the output destination to writer.

The writer must have been previously opened by the user.

 public  void activateOptions() 
{

}

Does nothing.

 public  void append(LoggingEvent event) 
{
    // Reminder: the nesting of calls is:
    //
    //    doAppend()
    //      - check threshold
    //      - filter
    //      - append();
    //        - checkEntryConditions();
    //        - subAppend();
    if(!checkEntryConditions()) {
      return;
    }
    subAppend(event);
}

This method is called by the AppenderSkeleton#doAppend method.

If the output stream exists and is writable then write a log statement to the output stream. Otherwise, write a single warning message to System.err.

The format of the output will depend on this appender's layout.

 protected boolean checkEntryConditions() 
{
    if(this.closed) {
      LogLog.warn("Not allowed to write to a closed appender.");
      return false;
    }
    if(this.qw == null) {
      errorHandler.error("No output stream or file set for the appender named ["+
			name+"].");
      return false;
    }
    if(this.layout == null) {
      errorHandler.error("No layout set for the appender named ["+ name+"].");
      return false;
    }
    return true;
}

This method determines if there is a sense in attempting to append.

It checks whether there is a set output target and also if there is a set layout. If these checks fail, then the boolean value false is returned.

 public synchronized  void close() 
{
    if(this.closed)
      return;
    this.closed = true;
    writeFooter();
    reset();
}

Close this appender instance. The underlying stream or writer is also closed.

Closed appenders cannot be reused.

 protected  void closeWriter() 
{
    if(qw != null) {
      try {
	qw.close();
      } catch(IOException e) {
	// There is do need to invoke an error handler at this late
	// stage.
	LogLog.error("Could not close " + qw, e);
      }
    }
}

Close the underlying java.io.Writer .

 protected OutputStreamWriter createWriter(OutputStream os) 
{
    OutputStreamWriter retval = null;
    String enc = getEncoding();
    if(enc != null) {
      try {
	retval = new OutputStreamWriter(os, enc);
      } catch(IOException e) {
	LogLog.warn("Error initializing output writer.");
	LogLog.warn("Unsupported encoding?");
      }
    }
    if(retval == null) {
      retval = new OutputStreamWriter(os);
    }
    return retval;
}

Returns an OutputStreamWriter when passed an OutputStream. The encoding used will depend on the value of the encoding property. If the encoding value is specified incorrectly the writer will be opened using the default system encoding (an error message will be printed to the loglog.

 public String getEncoding() 
{
    return encoding;
}

 public boolean getImmediateFlush() 
{
    return immediateFlush;
}

Returns value of the ImmediateFlush option.

 public boolean requiresLayout() 
{
    return true;
}

The WriterAppender requires a layout. Hence, this method returns true.

 protected  void reset() 
{
    closeWriter();
    this.qw = null;
    //this.tp = null;
}

Clear internal references to the writer and other variables. Subclasses can override this method for an alternate closing behavior.

 public  void setEncoding(String value) 
{
    encoding = value;
}

 public synchronized  void setErrorHandler(ErrorHandler eh) 
{
    if(eh == null) {
      LogLog.warn("You have tried to set a null error-handler.");
    } else {
      this.errorHandler = eh;
      if(this.qw != null) {
	this.qw.setErrorHandler(eh);
      }
    }
}

Set the ErrorHandler for this WriterAppender and also the underlying QuietWriter if any.

 public  void setImmediateFlush(boolean value) 
{
    immediateFlush = value;
}

If the ImmediateFlush option is set to true, the appender will flush at the end of each write. This is the default behavior. If the option is set to false, then the underlying stream can defer writing to physical medium to a later time.

Avoiding the flush operation at the end of each append results in a performance gain of 10 to 20 percent. However, there is safety tradeoff involved in skipping flushing. Indeed, when flushing is skipped, then it is likely that the last few log events will not be recorded on disk when the application exits. This is a high price to pay even for a 20% performance gain.

 public synchronized  void setWriter(Writer writer) 
{
    reset();
    this.qw = new QuietWriter(writer, errorHandler);
    //this.tp = new TracerPrintWriter(qw);
    writeHeader();
}

Sets the Writer where the log output will go. The specified Writer must be opened by the user and be writable.

The java.io.Writer will be closed when the appender instance is closed.

WARNING: Logging to an unopened Writer will fail.

 protected  void subAppend(LoggingEvent event) 
{
    this.qw.write(this.layout.format(event));
    if(layout.ignoresThrowable()) {
      String[] s = event.getThrowableStrRep();
      if (s != null) {
	int len = s.length;
	for(int i = 0; i <  len; i++) {
	  this.qw.write(s[i]);
	  this.qw.write(Layout.LINE_SEP);
	}
      }
    }
    if(this.immediateFlush) {
      this.qw.flush();
    }
}

Actual writing occurs here.

Most subclasses of WriterAppender will need to override this method.

 protected  void writeFooter() 
{
    if(layout != null) {
      String f = layout.getFooter();
      if(f != null && this.qw != null) {
	this.qw.write(f);
	this.qw.flush();
      }
    }
}

Write a footer as produced by the embedded layout's Layout#getFooter method.

 protected  void writeHeader() 
{
    if(layout != null) {
      String h = layout.getHeader();
      if(h != null && this.qw != null)
	this.qw.write(h);
    }
}

Write a header as produced by the embedded layout's Layout#getHeader method.