WriterAppender.javaAPI DocApache log4j 1.2.159527Sat Aug 25 00:09:42 BST 2007org.apache.log4j


public class WriterAppender extends AppenderSkeleton
WriterAppender appends log events to a {@link} or an {@link} depending on the user's choice.
Ceki Gülcü

Fields Summary
protected boolean
Immediate flush means that the underlying writer or output stream will be flushed at the end of each append operation. Immediate flush is slower but ensures that each append request is actually written. If immediateFlush is set to false, then there is a good chance that the last few logs events are not actually written to persistent media if and when the application crashes.

The immediateFlush variable is set to true by default.

protected String
The encoding to use when writing.

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

protected QuietWriter
This is the {@link QuietWriter quietWriter} where we will write to.
Constructors Summary
public WriterAppender()
This default constructor does nothing.

public WriterAppender(Layout layout, OutputStream os)
Instantiate a WriterAppender and set the output destination to a new {@link OutputStreamWriter} initialized with os as its {@link OutputStream}.

    this(layout, new OutputStreamWriter(os));
public WriterAppender(Layout layout, Writer writer)
Instantiate a WriterAppender and set the output destination to writer.

The writer must have been previously opened by the user.

    this.layout = layout;
Methods Summary
public voidactivateOptions()
Does nothing.

public voidappend(org.apache.log4j.spi.LoggingEvent event)
This method is called by the {@link 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.

    // Reminder: the nesting of calls is:
    //    doAppend()
    //      - check threshold
    //      - filter
    //      - append();
    //        - checkEntryConditions();
    //        - subAppend();

    if(!checkEntryConditions()) {
protected booleancheckEntryConditions()
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.

    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 ["+
      return false;

    if(this.layout == null) {
      errorHandler.error("No layout set for the appender named ["+ name+"].");
      return false;
    return true;
public synchronized voidclose()
Close this appender instance. The underlying stream or writer is also closed.

Closed appenders cannot be reused.


    this.closed = true;
protected voidcloseWriter()
Close the underlying {@link}.

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

    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;
public java.lang.StringgetEncoding()

    return encoding;
public booleangetImmediateFlush()
Returns value of the ImmediateFlush option.

    return immediateFlush;
public booleanrequiresLayout()
The WriterAppender requires a layout. Hence, this method returns true.

    return true;
protected voidreset()
Clear internal references to the writer and other variables. Subclasses can override this method for an alternate closing behavior.

    this.qw = null;
    // = null;
public voidsetEncoding(java.lang.String value)

    encoding = value;
public synchronized voidsetErrorHandler(org.apache.log4j.spi.ErrorHandler eh)
Set the {@link ErrorHandler} for this WriterAppender and also the underlying {@link QuietWriter} if any.

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

    immediateFlush = value;
public synchronized voidsetWriter( writer)

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

The will be closed when the appender instance is closed.

WARNING: Logging to an unopened Writer will fail.

writer An already opened Writer.

    this.qw = new QuietWriter(writer, errorHandler);
    // = new TracerPrintWriter(qw);
protected voidsubAppend(org.apache.log4j.spi.LoggingEvent event)
Actual writing occurs here.

Most subclasses of WriterAppender will need to override this method.



    if(layout.ignoresThrowable()) {
      String[] s = event.getThrowableStrRep();
      if (s != null) {
	int len = s.length;
	for(int i = 0; i < len; i++) {

    if(this.immediateFlush) {
protected voidwriteFooter()
Write a footer as produced by the embedded layout's {@link Layout#getFooter} method.

    if(layout != null) {
      String f = layout.getFooter();
      if(f != null && this.qw != null) {
protected voidwriteHeader()
Write a header as produced by the embedded layout's {@link Layout#getHeader} method.

    if(layout != null) {
      String h = layout.getHeader();
      if(h != null && this.qw != null)