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FAQ.html
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<meta name="author" value="Ceki Gulcu">
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<title>Log4j project - Frequently Asked Questions about log4j</title>
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<center>
<h2>Frequently Asked Questions about log4j</h2>
<h3>Ceki G�lc�, Paul Smith, Chris Taylor<br />
May 2002, last updated on September 15th, 2004</h3>
</center>
<br/>
<div class="index-faqSection">
Section 1. <a href="#1"> Generalities</a>
</div>
<table class="index-question">
<td nowrap="true" valign="top">Question 1.1</td>
<td><a href="#1.1">What is log4j?</a></td>
</table>
<table class="index-question">
<td nowrap="true" valign="top">Question 1.2</td>
<td><a href="#1.2">Is log4j a reliable logging system?</a></td>
</table>
<table class="index-question">
<td nowrap="true" valign="top">Question 1.3</td>
<td><a href="#1.3">What are the prerequisites for log4j?</a></td>
</table>
<table class="index-question">
<td nowrap="true" valign="top">Question 1.4</td>
<td><a href="#1.4">What are the features of log4j?</a></td>
</table>
<table class="index-question">
<td nowrap="true" valign="top">Question 1.5</td>
<td><a href="#1.5">Is there example code for using log4j?</a></td>
</table>
<table class="index-question">
<td nowrap="true" valign="top">Question 1.6</td>
<td><a href="#1.6">What documentation should I read to learn log4j?</a></td>
</table>
<table class="index-question">
<td nowrap="true" valign="top">Question 1.7</td>
<td><a href="#1.7">Is log4j thread-safe?</a></td>
</table>
<table class="index-question">
<td nowrap="true" valign="top">Question 1.8</td>
<td><a href="#1.8">What does log output look like?</a></td>
</table>
<br/>
<div class="index-faqSection">
Section 2. <a href="#2"> Using log4j</a>
</div>
<table class="index-question">
<td nowrap="true" valign="top">Question 2.1</td>
<td><a href="#2.1">What are <em>Loggers</em>?</a></td>
</table>
<table class="index-question">
<td nowrap="true" valign="top">Question 2.2</td>
<td><a href="#2.2">How can I change log behavior at runtime?</a></td>
</table>
<table class="index-question">
<td nowrap="true" valign="top">Question 2.3</td>
<td><a href="#2.3">What is the fastest way of (not) logging?</a></td>
</table>
<table class="index-question">
<td nowrap="true" valign="top">Question 2.4</td>
<td><a href="#2.4"> Are there any suggested ways for naming loggers?</a></td>
</table>
<table class="index-question">
<td nowrap="true" valign="top">Question 2.5</td>
<td><a href="#2.5">How do I get the fully-qualified name of a class in a static block?</a></td>
</table>
<table class="index-question">
<td nowrap="true" valign="top">Question 2.6</td>
<td><a href="#2.6">Can the log output format be customized? </a></td>
</table>
<table class="index-question">
<td nowrap="true" valign="top">Question 2.7</td>
<td><a href="#2.7">What are the configurable options for <code>FooBarAppender</code>?</a></td>
</table>
<table class="index-question">
<td nowrap="true" valign="top">Question 2.8</td>
<td><a href="#2.8">What is the recommended way of migrating from
java.util.logging to log4j?</a></td>
</table>
<table class="index-question">
<td nowrap="true" valign="top">Question 2.9</td>
<td><a href="#2.9">Is it possible to direct log output to
different appenders by level? </a></td>
</table>
<table class="index-question">
<td nowrap="true" valign="top">Question 2.10</td>
<td><a href="#2.10">What does the Windows NT Event Viewer complain about
missing descriptions for my event messages when I use the
<code>NTEventLogAppender</code>?</a></td>
</table>
<table class="index-question">
<td nowrap="true" valign="top">Question 2.11</td>
<td><a href="#2.11">Why can't I map my logger names to the loggers that
appear in the NT Event Log when I use the
NTEventLogAppender?</a></td>
</table>
<table class="index-question">
<td nowrap="true" valign="top">Question 2.12</td>
<td><a href="#2.12">Are there suggested approaches for logging in JSP pages?</a></td>
</table>
<br/>
<div class="index-faqSection">
Section 3. <a href="#3"> Advanced questions</a>
</div>
<table class="index-question">
<td nowrap="true" valign="top">Question 3.1</td>
<td><a href="#3.1">Can the outputs of multiple client request go to
different log files?</a></td>
</table>
<table class="index-question">
<td nowrap="true" valign="top">Question 3.2</td>
<td><a href="#3.2">Logger instances seem to be create only. Why isn't
there a method to remove logger instances?</a></td>
</table>
<table class="index-question">
<td nowrap="true" valign="top">Question 3.3</td>
<td><a href="#3.3">How do I get multiple process to log to the same file?</a></td>
</table>
<table class="index-question">
<td nowrap="true" valign="top">Question 3.4</td>
<td><a href="#3.4">How about the timesamps of events generated by multiple
processes across multiple hosts (possibly across multiple
timezones)?
</a></td>
</table>
<table class="index-question">
<td nowrap="true" valign="top">Question 3.5</td>
<td><a href="#3.5">Why can't log4j find my properties file in a J2EE or WAR
application?</a></td>
</table>
<table class="index-question">
<td nowrap="true" valign="top">Question 3.6</td>
<td><a href="#3.6">Is there a way to get log4j to automatically reload a
configuration file if it changes?</a></td>
</table>
<br/>
<div class="index-faqSection">
Section 4. <a href="#4"> Contributing to the project</a>
</div>
<table class="index-question">
<td nowrap="true" valign="top">Question 4.1</td>
<td><a href="#4.1">Why should I donate my extensions to log4j back to the
project?
</a></td>
</table>
<table class="index-question">
<td nowrap="true" valign="top">Question 4.2</td>
<td><a href="#4.2">What should I keep in mind when contributing code?</a></td>
</table>
<table class="index-question">
<td nowrap="true" valign="top">Question 4.3</td>
<td><a href="#4.3">How can I contribute a new question/answer to this
document?</a></td>
</table>
<hr/>
<div class="faqSection">
<a name="1">Section 1.</a> Generalities
</div>
<p>This section contains general questions about log4j.</p>
<div class="question">
<a name="1.1">1.1 What is log4j?</a>
</div>
<p>log4j is a tool to help the programmer output log statements to a
variety of output targets.
</p>
<p>In case of problems with an application, it is helpful to
enable logging so that the problem can be located. With log4j
it is possible to enable logging at runtime without modifying
the application binary. The log4j package is designed so that
log statements can remain in <i>shipped</i> code without
incurring a high performance cost. It follows that the speed
of logging (or rather not logging) is capital.
</p>
<p>At the same time, log output can be so voluminous that it quickly
becomes overwhelming. One of the distinctive features of log4j is the
notion of <i>hierarchical loggers</i>. Using loggers it is
possible to selectively control which log statements are output at
arbitrary granularity.
</p>
<p>log4j is designed with two three goals in mind:
reliability, speed and flexibility. There is a tight balance
between these requirements. We believe that log4j strikes the
right balance.
</p>
<div class="question">
<a name="1.2">1.2 Is log4j a reliable logging system?</a>
</div>
<p>No. log4j is not reliable. It is a best-effort
<em>fail-stop</em> logging system.
</p>
<p>By fail-stop, we mean that log4j will not throw unexpected
exceptions at run-time potentially causing your application to
crash. <b>If for any reason, log4j throws an uncaught exception,
please send an email to the <a href="mailto:log4j-user@logging.apache.org">log4j-user@logging.apache.org</a>
mailing list</b>. Uncaught exceptions are handled as serious bugs
requiring immediate attention.
</p>
<p>Moreover, log4j will not revert to System.out or System.err
when its designated output stream is not opened, is not writable or
becomes full. This avoids corrupting an otherwise working program by
flooding the user's terminal because logging fails. However, log4j
will output a single message to System.err indicating that logging can
not be performed.
</p>
<div class="question">
<a name="1.3">1.3 What are the prerequisites for log4j?</a>
</div>
<ul>
<li><p>Log4j versions upto and including 1.2.8 are
compatible with JDK 1.1.x and later. Log4j version 1.3 will
be compatilble with JDK 1.2 and later.
</p></li>
<li><p>The DOMConfigurator is based on the DOM Level 1
API. The DOMConfigurator.configure(Element) method will work
with any XML parser that will pass it a DOM tree.
</p>
<p>The DOMConfigurator.configure(String filename) method and its
variants require a JAXP compatible XML parser, for example <a href="http://xml.apache.org/">Xerces</a> or Sun's
parser. Compiling the DOMConfigurator requires the presence of a
JAXP parser in the classpath.
</p>
</li>
<li><p>The <code>org.apache.log4j.net.SMTPAppender</code>
relies on the <a href="http://java.sun.com/products/javamail/">JavaMail
API</a>. It has been tested with JavaMail API version
1.2. The JavaMail API requires the <a href="http://java.sun.com/beans/glasgow/jaf.html">JavaBeans
Activation Framework</a> package.
</p></li>
<li><p>The <code>org.apache.log4j.net.JMSAppender</code>
requires the presence of the JMS API as well as JNDI.
</p></li>
<li><p>log4j test code relies on the <a href="http://www.junit.org">JUnit</a> testing framework.
</p></li>
</ul>
<div class="question">
<a name="1.4">1.4 What are the features of log4j?</a>
</div>
<ul>
<li><p>log4j is optimized for speed.</p></li>
<li><p>log4j is based on a named logger hierarchy.</p></li>
<li><p>log4j is fail-stop but not reliable.</p></li>
<li><p>log4j is thread-safe.</p></li>
<li><p>log4j is not restricted to a predefined set of
facilities.</p></li>
<li><p>Logging behavior can be set at runtime using a
configuration file. Configuration files can be property
files or in XML format. </p></li>
<li><p>log4j is designed to handle Java Exceptions from the
start.</p></li>
<li><p>log4j can direct its output to a file, the console,
an <code>java.io.OutputStream</code>,
<code>java.io.Writer</code>, a remote server using TCP, a
remote Unix Syslog daemon, to a remote listener using JMS,
to the NT EventLog or even send e-mail. </p></li>
<li><p>log4j uses 5 levels, namely DEBUG, INFO, WARN, ERROR and
FATAL.
</p></li>
<li><p>The format of the log output can be easily changed by
extending the Layout class.
</p></li>
<li><p>The target of the log output as well as the writing
strategy can be altered by implementations of the Appender
interface. </p></li>
<li><p>log4j supports multiple output appenders per logger.
</p></li>
<li><p>log4j supports internationalization.</p></li>
</ul>
<div class="question">
<a name="1.5">1.5 Is there example code for using log4j?</a>
</div>
<p>See the <code>examples/</code> directory.</p>
<div class="question">
<a name="1.6">1.6 What documentation should I read to learn log4j?</a>
</div>
<p>Make sure to read the <a href="manual.html">short
manual</a>. It is also recommended to you read <a href="https://www.qos.ch/shop/products/clm_t.jsp">The complete
log4j manual</a>. Both documents were written by Ceki
G�lc�.
</p>
<div class="question">
<a name="1.7">1.7 Is log4j thread-safe?</a>
</div>
<p>Yes, log4j is thread-safe. Log4j components are designed to
be used in heavily multithreaded systems.</p>
<div class="question">
<a name="1.8">1.8 What does log output look like?</a>
</div>
<p>The log output can be customized in many ways. Moreover,
one can completely override the output format by implementing
one's own Layout.
</p>
<p>Here is an example output using <em>PatternLayout</em> with
the conversion pattern <b>"%r [%t] %-5p %c{2} %x - %m%n"</b>
</p>
<pre class="screen_output">
176 [main] INFO examples.Sort - Populating an array of 2 elements in reverse order.
225 [main] INFO examples.SortAlgo - Entered the sort method.
262 [main] DEBUG SortAlgo.OUTER i=1 - Outer loop.
276 [main] DEBUG SortAlgo.SWAP i=1 j=0 - Swapping intArray[0] = 1 and intArray[1] = 0
290 [main] DEBUG SortAlgo.OUTER i=0 - Outer loop.
304 [main] INFO SortAlgo.DUMP - Dump of interger array:
317 [main] INFO SortAlgo.DUMP - Element [0] = 0
331 [main] INFO SortAlgo.DUMP - Element [1] = 1
343 [main] INFO examples.Sort - The next log statement should be an error message.
346 [main] ERROR SortAlgo.DUMP - Tried to dump an uninitialized array.
at org.log4j.examples.SortAlgo.dump(SortAlgo.java:58)
at org.log4j.examples.Sort.main(Sort.java:64)
467 [main] INFO examples.Sort - Exiting main method.
</pre>
<p>The first field is the number of milliseconds elapsed since
the start of the program. The second field is the thread
outputting the log statement. The third field is the level of
the log statement. The fourth field is the rightmost two
components of the logger making the log request. The fifth
field (just before the '-') is the <em>nested diagnostic
context</em> (NDC). Note the nested diagnostic context may be
empty as in the first two statements. The text after the '-'
is the message of the statement.
</p>
<div class="faqSection">
<a name="2">Section 2.</a> Using log4j
</div>
<p>This section contains answers to questions encountered while
using log4j.</p>
<div class="question">
<a name="2.1">2.1 What are <em>Loggers</em>?</a>
</div>
<p>Lggers lie at the heart of log4j. Loggers define a hierarchy and give
the programmer <em>run-time</em> control on which statements are
printed or not.
</p>
<p>Loggers are assigned levels. A log statement is printed
depending on its level <em>and</em> its logger.
</p>
<p>Make sure to read the <a href="manual.html">log4j manual</a>
for more information.
</p>
<div class="question">
<a name="2.2">2.2 How can I change log behavior at runtime?</a>
</div>
<p>Log behavior can be set using configuration files which are parsed
at runtime. Using configuration files the programmer can define
loggers and set their levels.
</p>
<p>The <code>PropertyConfigurator</code> defines a particular format
of a configuration file. See also the <code>examples/Sort.java</code>
example and associated configuration files.
</p>
<p>Configuration files can be specified in XML. See
<code>log4j.dtd</code> and
<code>org.log4j.xml.DOMConfigurator</code> for more details.
</p>
<p>See the various Layout and Appender components for specific
configuration options.
</p>
<p>In addition to configuration files, the user may disable all
messages belonging to a set of levels. See next item.
</p>
<div class="question">
<a name="2.3">2.3 What is the fastest way of (not) logging?</a>
</div>
<p> For some logger <code>l</code>, writing,
</p>
<pre class="source">
l.debug("Entry number: " + i + " is " + String.valueOf(entry[i]));
</pre>
<p>incurs the cost of constructing the message parameter, that is
converting both integer <code>i</code> and <code>entry[i]</code> to a
String, and concatenating intermediate strings. This, regardless of
whether the message will be logged or not.
</p>
<p>If you are worried about speed, then write</p>
<pre class="source">
if(l.isDebugEnabled()) {
l.debug("Entry number: " + i + " is " + String.valueOf(entry[i]));
}
</pre>
<p>This way you will not incur the cost of parameter
construction if debugging is disabled for logger
<code>l</code>. On the other hand, if the logger is debug
enabled, you will incur the cost of evaluating whether the
logger is enabled or not, twice: once in
<code>debugEnabled</code> and once in <code>debug</code>.
This is an insignificant overhead since evaluating a logger
takes less than 1% of the time it takes to actually log a
statement.
</p>
<div class="question">
<a name="2.4">2.4 Are there any suggested ways for naming loggers?</a>
</div>
<p>Yes, there are.</p>
<p>You can name loggers by <strong>locality</strong>. It turns out
that instantiating a logger in each class, with the logger name
equal to the fully-qualified name of the class, is a useful and
straightforward approach of defining loggers. This approach has
many benefits:
</p>
<ul>
<li>It is very simple to implement.</li>
<li>It is very simple to explain to new developers.</li>
<li>It automatically mirrors your application's own modular design.
</li>
<li>It can be further refined at will.</li>
<li>Printing the logger automatically gives information on the locality
of the log statement. </li>
</ul>
<p>However, this is not the only way for naming loggers. A
common alternative is to name loggers by <strong>functional
areas</strong>. For example, the "database" logger, "RMI"
logger, "security" logger, or the "XML" logger.
</p>
<p>You may choose to name loggers by functionality and
subcategorize by locality, as in "DATABASE.com.foo.some.package.someClass" or
"DATABASE.com.foo.some.other.package.someOtherClass".
</p>
<p><em>You are totally free in choosing the names of your
loggers.</em> The log4j package merely allows you to manage your
names in a hierarchy. However, it is your responsibility to define
this hierarchy.
</p>
<p>Note by naming loggers by locality one tends to name things by
functionality, since in most cases the locality relates closely to
functionality.
</p>
<div class="question">
<a name="2.5">2.5 How do I get the fully-qualified name of a class in a static block?</a>
</div>
<p>You can easily retrieve the fully-qualified name of a class in a
static block for class X, with the statement
<code>X.class.getName()</code>. Note that <code>X</code> is the class
name and not an instance. The <code>X.class</code> statement does
<i>not</i> create a new instance of class <code>X</code>.
</p>
<p>Here is the suggested usage template:</p>
<pre class="source">
package a.b.c;
public class Foo {
final static Logger logger = Logger.getLogger(Foo.class);
... other code
}
</pre>
<div class="question">
<a name="2.6">2.6 Can the log output format be customized? </a>
</div>
<p>Yes, you can extend the <code>Layout</code> class to create
you own customized log format. Appenders can be parameterized
to use the layout of your choice.
</p>
<div class="question">
<a name="2.7">2.7 What are the configurable options for <code>FooBarAppender</code>?</a>
</div>
<p>Log4j uses JavaBeans style configuration.</p>
<p>Thus, any setter method in <code>FooBarAppender</code>
corresponds to a configurable option. For example, in <a href="api\org\apache\log4j\RollingFileAppender.html"><code>RollingFileAppender</code></a>
the <a href="api/org/apache/log4j/RollingFileAppender.html#setMaxBackupIndex(int)"><code>setMaxBackupIndex(int
maxBackups)</code></a> method corresponds to the
<code>maxBackupIndex</code> option. The first letter of the
option can be upper case, i.e. <code>MaxBackupIndex</code>
and <code>maxBackupIndex</code> are equivalent but not
<code>MAXBACKUPIndex</code> nor <code>mAXBackupIndex</code>.
</p>
<p>Layouts options are also defined by their setter methods. The same goes
for most other log4j components.
</p>
<div class="question">
<a name="2.8">2.8 What is the recommended way of migrating from
java.util.logging to log4j?</a>
</div>
<p>We suggest to just use global file search/replace. You should be able
to replace all the "java.util.Logger" references with
"org.apache.log4j.Logger", and you should be on your way.
</p>
<p>If you're on a Win32 platform, we recommend <a href="http://www.textpad.com/">Textpad</a>. You can use the
CTRL+SHIFT+O to open all *.java files from a directory including all
its sub-directories, and then use the search/replace function to
replace in all files, and then CTRL+SHIFT+S to save all. Should take
about 60 seconds! :)
</p>
<div class="question">
<a name="2.9">2.9 Is it possible to direct log output to
different appenders by level? </a>
</div>
<p>Yes it is. Setting the <b>Threshold</b> option of any appender
extending <a href="api/org/apache/log4j/AppenderSkeleton.html">AppenderSkeleton</a>,
(most log4j appenders extend AppenderSkeleton) to filter out all log
events with <em>lower</em> level than the value of the threshold
option.
</p>
<p>For example, setting the threshold of an appender to DEBUG
also allow INFO, WARN, ERROR and FATAL messages to log along
with DEBUG messages. This is usually acceptable as there is
little use for DEBUG messages without the surrounding INFO,
WARN, ERROR and FATAL messages. Similarly, setting the
threshold of an appender to ERROR will filter out DEBUG, INFO
and WARN messages but not ERROR or FATAL messages.
</p>
<p>This policy usually best encapsulates what the user
actually wants to do, as opposed to her mind-projected
solution.
</p>
<p>See <i>examples/sort4.lcf</i> for an example threshold
configuration.</p>
<p>If you must filter events by exact level match, then you can
attach a <a href="api/org/apache/log4j/varia/LevelMatchFilter.html">LevelMatchFilter</a>
to any appender to filter out logging events by exact level match.
</p>
<div class="question">
<a name="2.10">2.10 What does the Windows NT Event Viewer complain about
missing descriptions for my event messages when I use the
<code>NTEventLogAppender</code>?</a>
</div>
<p>The NT Event Viewer relies on <i>message resource</i> DLLs
to properly view an event message. The NTEventLogAppender.dll
contains these message resources, but that DLL must be copied
to %SYSTEMROOT%\SYSTEM32 for it to work properly.
</p>
<div class="question">
<a name="2.11">2.11 Why can't I map my logger names to the loggers that
appear in the NT Event Log when I use the
NTEventLogAppender?</a>
</div>
<p>Unfotunately, the logger names are hardcoded within the
message resource DLL (see previous question about
NTEventLogAppender), so there isn't any easy way to override
those dynamically... in fact, I don't think it's possible to
do it, as you'd have to modify the DLL resources for every
application. Since most native applications don't use the
Logger column anyway...
</p>
<div class="question">
<a name="2.12">2.12 Are there suggested approaches for logging in JSP pages?</a>
</div>
<p>
The suggested approach depends on your design requirements. If you or
your organization has no constraints on the use of Java in JSP pages,
simply use log4j normally in <code><% ... %></code> statements
as indicated in the Short Manual and the rest of the documentation.
</p>
<p>
However, if your design calls for a minimum amount of Java in your JSP
pages, consider using the
<a href="http://jakarta.apache.org/taglibs/doc/log-doc/intro.html">Log Taglib</a>
from the Jakarta Taglibs project. It provides logging JSP tags that invoke
log4j.
</p>
<div class="faqSection">
<a name="3">Section 3.</a> Advanced questions
</div>
<p>This section contains answers to more advanced questions about log4j.</p>
<div class="question">
<a name="3.1">3.1 Can the outputs of multiple client request go to
different log files?</a>
</div>
<p>Many developers are confronted with the problem of
distinguishing the log output originating from the same class
but different client requests. They come up with ingenious
mechanisms to fan out the log output to different files. In
most cases, this is not the right approach.
</p>
<p>It is simpler to use a nested diagnostic context
(NDC). Typically, one would <em>NDC.push()</em> client
specific information, such as the client's hostname, ID or any
other distinguishing information when starting to handle the
client's request. Thereafter, log output will automatically
include the nested diagnostic context so that you can
distinguish logs from different client requests even if they
are output to the same file.
</p>
<p>See the <code>NDC</code> and the <code>PatternLayout</code>
classes for more information. The <code>NumberCruncher</code>
example shows how the NDC can be used to distinguish the log
output from multiple clients even if they share the same log
file.
</p>
<p>For select applications, such as virtual hosting
web-servers, the NDC solution is not sufficient. As of version
0.9.0, log4j supports multiple hierarchy trees. Thus, it is
possible to log to different targets from the same logger
depending on the current context.
</p>
<div class="question">
<a name="3.2">3.2 Logger instances seem to be create only. Why isn't
there a method to remove logger instances?</a>
</div>
<p>It is quite nontrivial to define the semantics of a
"removed" logger escecially if it is still referenced by the
user. Future releases <em>may</em> include a remove method in
the Logger class.</p>
<div class="question">
<a name="3.3">3.3 How do I get multiple process to log to the same file?</a>
</div>
<p>You may have each process log to a
<a href="api/org/apache/log4j/net/SocketAppender.html"><code>SocketAppender</code></a>.
The receiving
<a href="api/org/apache/log4j/net/SocketServer.html"><code>SocketServer</code></a>
(or
<a href="api/org/apache/log4j/net/SimpleSocketServer.html"><code>SimpleSocketServer</code></a>)
can receive all the events and send them to a single
log file.
</p>
<div class="question">
<a name="3.4">3.4 How about the timesamps of events generated by multiple
processes across multiple hosts (possibly across multiple
timezones)?
</a>
</div>
<p>The timestamp is created when the logging event is created.
That is so say, when the <code>debug</code>,
<code>info</code>, <code>warn</code>, <code>error</code> or
<code>fatal</code> method is invoked. Thus, the timestamp is
unaffected by the time at which event arrive at a remote
socket server.
</p>
<p>Timestamps are stored in UTC format inside the
event. Consequently, when displayed or written to a log file,
timestamps appear in the same timezone as the host displaying
or creating the logfile. Note that because the clocks of
various machines may not be synchronized, there may be
timestamp inconsistencies between events generated on
different hosts.
</p>
<div class="question">
<a name="3.5">3.5 Why can't log4j find my properties file in a J2EE or WAR
application?</a>
</div>
<p>The short answer: the log4j classes and the properties file
are not within the scope of the same classloader.
</p>
<p>The long answer (and what to do about it): J2EE or Servlet
containers utilize Java's class loading system. Sun changed
the way classloading works with the release of Java 2. In
Java 2, classloaders are arranged in a hierarchial
parent-child relationship. When a child classloader needs to
find a class or a resource, it first delegates the request to
the parent.
</p>
<p>Log4j only uses the default <code>Class.forName()</code>
mechanism for loading classes. Resources are handled
similarly. See the documentation for
<code>java.lang.ClassLoader</code> for more details.
</p>
<p>So, if you're having problems, try loading the class or
resource yourself. If you can't find it, neither will
log4j. ;)
</p>
<div class="question">
<a name="3.6">3.6 Is there a way to get log4j to automatically reload a
configuration file if it changes?</a>
</div>
<p>Yes. Both the DOMConfigurator and the PropertyConfigurator support
automatic reloading through the <code>configureAndWatch</code> method.
See the API documentation for more details.
</p>
<p>Because the <code>configureAndWatch</code> launches a
separate wathdog thread, and because there is no way to stop
this thread in log4j 1.2, the <code>configureAndWatch</code>
method is unsafe for use in J2EE envrironments where
applications are recycled.
</p>
<div class="faqSection">
<a name="4">Section 4.</a> Contributing to the project
</div>
<p>This section includes questions about contributing to the
project</p>
<div class="question">
<a name="4.1">4.1 Why should I donate my extensions to log4j back to the
project?
</a>
</div>
<p>Contrary to the GNU Public License (GPL) the Apache
Software License does not make any claims over your
extensions. By extensions, we mean totally new code that
invokes existing log4j classes. <em>You are free to do
whatever you wish with your proprietary log4j extensions.</em>
In particular, you may choose to never release your extensions
to the wider public.
</p>
<p>We are very careful not to change the log4j client API so
that newer log4j releases are backward compatible with
previous versions. We are a lot less scrupulous with the
internal log4j API. Thus, if your extension is designed to
work with log4j version <code>n</code>, then when log4j
release version <code>n+1</code> comes out, you will probably
need to adapt your proprietary extensions to the new release.
</p>
<p>Thus, you will be forced to spend precious resources in
order to keep up with log4j changes. This is commonly referred
to as the "stupid-tax." By donating the code and making it
part of the standard distribution, you save yourself the
unnecessary maintenance work.
</p>
<p>If your extensions are useful then someone will eventually
write an extension providing the same or very similar
functionality. Your development effort will be wasted. Unless
the proprietary log4j extension is business critical, there is
little reason for not donating your extensions back to the
project.
</p>
<div class="question">
<a name="4.2">4.2 What should I keep in mind when contributing code?</a>
</div>
<ol>
<li>
<p>Write a test case for your contribution.</p>
<p>There is nothing more irritating than finding the bugs
in debugging (i.e. logging) code. Writing a test case
takes some effort but is crucial for a widely used library
such as log4j. Writing a test case will go a long way in
earning you the respect of fellow developers. See the
tests/ directory for exiting test cases.
</p>
</li>
<li>
<p>Stick to the existing indentation style even if you hate it.</p>
<p>Alternating between indentation styles makes it hard to
understand the source code. Make it a little harder on
yourself but easier on others.
</p>
<p>Log4j has adopted a rather conservative approach by
following the <a href="http://java.sun.com/docs/codeconv/">Code Conventions
for the JavaTM Programming Language</a>. <b>We use 2 (two)
spaces for indentation and no tabs.</b>
</p>
</li>
<li>
<p>Please do not both modify the code and change the
indentation in a single commit.</p>
<p>If you change the code and reformat it at the same time
and then commit, the commit notification message will be
hard to read. It will contain many diffs associated with
the reformatting in addition to logical changes.
</p>
<p>If you must reformat and change the code, then perform
each step separately. For example, reformat the code and
commit. Following that, you can change the logic and
commit. The two steps can be performed in the reverse
order just as well. You can first change the logic and
commit and only later reformat and commit.
</p>
</li>
<li>
<p>Make every effort to stick to the JDK 1.1 API.</p>
<p>One of the important advantages of log4j is its
compatibility with JDK 1.1.x.
</p>
</li>
<li>
<p>Always keep it simple, small and fast when
possible.</p>
<p>It's all about the application not about logging.</p>
</li>
<li>
<p>Identify yourself as a contributor at the top of the
relevant file.
</p>
</li>
<li>
<p>Take responsibility for your code.</p>
<p>Authoring software is very much like running a marathon. It
takes time and endurance.
</p>
</li>
<li>
<p>Did we mention sticking with the indentation style? </p>
</li>
<li><p>Did we mention writing test cases? </p>
</li>
</ol>
<div class="question">
<a name="4.3">4.3 How can I contribute a new question/answer to this
document?</a>
</div>
<p>Log4j uses <a href="http://jakarta.apache.org/velocity/anakia.html">velocity-anakia</a>
to generate its web-site, including this FAQ. We have devised
special macros to help us automatically generate labeled
question/answer pairs.
</p>
<p>If you are not a commiter, you can simply submit your new
question/answer pair to the log4j-dev@logging.apache.org
mailing list. The committers will take it from there.
</p>
<p>If you are a committer, then you must edit the
<em>/src/xdocs/faq.xml</em> file. The format of the file
should be self-evident. After you have made your changes, run
the command
</p>
<pre class="source">ant site</pre>
<p>After the appropriate transformation, your changes should
appear in the file <em>/docs/faq.html</em>.
</p>
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