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<h1>SLF4J extensions</h1>
<p>SLF4J extensions are packaged within <em>slf4j-ext.jar</em>
which ships with SLF4J. </p>
<ul>
<li><a href="#profiler">Profiler</a></li>
<li><a href="#extended_logger">Extended logger</a></li>
<li><a href="#javaagent">Logging added with Java agent (requires Java 5)</a></li>
</ul>
<h2><a name="profiler"></a>Profilers</h2>
<h3>What is a profiler?</h3>
<p>According to wikipedia, <a
href="http://en.wikipedia.org/wiki/Profiler_%28computer_science%29">profiling</a>
is the investigation of a program's behavior using information
gathered as the program runs, i.e. it is a form of dynamic program
analysis, as opposed to static code analysis. The usual goal of
performance analysis is to determine which parts of a program to
optimize for speed or memory usage.
</p>
<p>SLF4J profilers, a.k.a. poor man's profilers, will help the
developer gather performance data. Essentially, a profiler
consists of one or more stopwatches. Stopwatches are driven
(started/stopped) by statements in the <em>source code</em>. An
example should make the point clearer.
</p>
<h3>Basic example</h3>
<em>Example: Using the profiler: <a
href="xref-test/org/slf4j/profiler/BasicProfilerDemo.html">BasicProfilerDemo</a></em>
<p class="source">[omitted]
32 public class BasicProfilerDemo {
33
34 public static void main(String[] args) {
35 // create a profiler called "BASIC"
36 <b>Profiler profiler = new Profiler("BASIC");</b>
37 <b>profiler.start("A");</b>
38 doA();
39
40 <b>profiler.start("B");</b>
41 doB();
42
43 <b>profiler.start("OTHER");</b>
44 doOther();
45 <b>profiler.stop().print();</b>
46 }
[omitted]</p>
<p>Running the above example will output the following output.</p>
<p class="source">+ Profiler [BASIC]
|-- elapsed time [A] 220.487 milliseconds.
|-- elapsed time [B] 2499.866 milliseconds.
|-- elapsed time [OTHER] 3300.745 milliseconds.
|-- Total [BASIC] 6022.568 milliseconds.</p>
<p>Instantiating a profiler starts a global stopwatch. Each call to
the start() method starts a new and named stopwatch. In addition to
starting a named stopwatch, the start() method also causes the
previous stopwatch to stop. Thus, the call to
<code>profiler.start("A")</code> starts a stopwatch named "A". The
subsequent call to <code>profiler.start("B")</code> starts
stopwatch "B" and simultaneously stops the stopwatch named
"A". Invoking the <code>stop()</code> on a profiler stops the last
stopwatch as well as the global stopwatch which was started when
the profiler was instantiated.
</p>
<h3>Profiler nesting</h3>
<p>Profilers can also be nested. By nesting profilers, it is
possible to measure a task which itself has subtasks that need to
be timed and measured.
</p>
<p>Starting a nested profiler will stop any previously started
stopwatch or nested profiler associated with the parent profiler.
</p>
<p>Often times, the subtask is implemented by a different class as
the class hosting the parent profiler. Using the
<code>ProfilerRegistry</code> is a convenient way of passing a
nested profiler to an object outside the current object. Each
thread has its own profiler registry which can be retrieved by
invoking the <code>getThreadContextInstance()</code> method.
</p>
<em>Example: <a
href="xref-test/org/slf4j/profiler/NestedProfilerDemo.html">NestedProfilerDemo</a>
</em>
<p class="source">33 public class NestedProfilerDemo {
34
35 public static void main(String[] args) {
36 // create a profiler called "DEMO"
37 Profiler profiler = new Profiler("DEMO");
38
39 // register this profiler in the thread context's profiler registry
40 <b>ProfilerRegistry profilerRegistry = ProfilerRegistry.getThreadContextInstance();</b>
41 <b>profiler.registerWith(profilerRegistry);</b>
42
43 // start a stopwatch called "RANDOM"
44 profiler.start("RANDOM");
45 RandomIntegerArrayGenerator riaGenerator = new RandomIntegerArrayGenerator();
46 int n = 1000*1000;
47 int[] randomArray = riaGenerator.generate(n);
48
49 // create and start a nested profiler called "SORT_AND_PRUNE"
50 // By virtue of its parent-child relationship with the "DEMO"
51 // profiler, and the previous registration of the parent profiler,
52 // this nested profiler will be automatically registered
53 // with the thread context's profiler registry
54 <b>profiler.startNested(SortAndPruneComposites.NESTED_PROFILER_NAME);</b>
55
56 SortAndPruneComposites pruner = new SortAndPruneComposites(randomArray);
57 pruner.sortAndPruneComposites();
58
59 // stop and print the "DEMO" printer
60 profiler.stop().print();
61 }
62 }</p>
<p>Here is the relevant excerpt from the <a
href="xref-test/org/slf4j/profiler/SortAndPruneComposites.html">SortAndPruneComposites</a>
class.
</p>
<p class="source">[omitted]
6 public class SortAndPruneComposites {
7
8 static String NESTED_PROFILER_NAME = "SORT_AND_PRUNE";
9
10 final int[] originalArray;
11 final int originalArrrayLength;
12
13 public SortAndPruneComposites(int[] randomArray) {
14 this.originalArray = randomArray;
15 this.originalArrrayLength = randomArray.length;
16
17 }
18
19 public int[] sortAndPruneComposites() {
20 // retrieve previously registered profiler named "SORT_AND_PRUNE"
21 ProfilerRegistry profilerRegistry = ProfilerRegistry.getThreadContextInstance();
22 <b>Profiler sortProfiler = profilerRegistry.get(NESTED_PROFILER_NAME);</b>
23
24 // start a new stopwatch called SORT
25 sortProfiler.start("SORT");
26 int[] sortedArray = sort();
27 // start a new stopwatch called PRUNE_COMPOSITES
28 sortProfiler.start("PRUNE_COMPOSITES");
29 int result[] = pruneComposites(sortedArray);
30
31 return result;
32 }
[omitted] </p>
<p>On a Dual-Core Intel CPU clocked at 3.2 GHz, running the
<code>ProfilerDemo</code> application yields the following output:</p>
<p class="source">+ Profiler [DEMO]
|-- elapsed time [RANDOM] 70.524 milliseconds.
|---+ Profiler [SORT_AND_PRUNE]
|-- elapsed time [SORT] 665.281 milliseconds.
|-- elapsed time [PRUNE_COMPOSITES] 5695.515 milliseconds.
|-- Subtotal [SORT_AND_PRUNE] 6360.866 milliseconds.
|-- elapsed time [SORT_AND_PRUNE] 6360.866 milliseconds.
|-- Total [DEMO] 6433.922 milliseconds.</p>
<p>From the above, we learn that generating 1'000'000 random
integers takes 70 ms, sorting them 665 ms, and pruning the composite
(non-prime) integers 5695 ms, for a grand total of 6433 ms. Given
that pruning composites takes most of the CPU effort, any future
optimizations efforts would be directed at the pruning part.
</p>
<p>With just a few well-placed profiler calls we were able to
identify hot-spots in our application. Also note that passing a
profiler to a target class could be achieved by registering it in a
profiler registry and then retrieving it in the target class.
</p>
<h3>Printing using a logger</h3>
<p> Invoking <code>profiler.print</code> will always print the
output on the console. If you wish to leave the profiler code in
production, then you probably need more control over the output
destination. This can be accomplished by associating a logger of
your choice with a profiler.
</p>
<p>After you have associated a logger with a profiler, you would
invoke the <code>log()</code> method instead of <code>print()</code>
previously, as the next example illustrates.
</p>
<em>Profiler with a logger: <a
href="xref-test/org/slf4j/profiler/NestedProfilerDemo2.html">NestedProfilerDemo2</a>
</em>
<p class="source">[omitted]
17 public class NestedProfilerDemo2 {
18
19 static Logger logger = LoggerFactory.getLogger(NestedProfilerDemo2.class);
20
21 public static void main(String[] args) {
22 Profiler profiler = new Profiler("DEMO");
23 // associate a logger with the profiler
24 <b>profiler.setLogger(logger);</b>
25
26 ProfilerRegistry profilerRegistry = ProfilerRegistry.getThreadContextInstance();
27 profiler.registerWith(profilerRegistry);
28
29 profiler.start("RANDOM");
30 RandomIntegerArrayGenerator riaGenerator = new RandomIntegerArrayGenerator();
31 int n = 10*1000;
32 int[] randomArray = riaGenerator.generate(n);
33
34 profiler.startNested(SortAndPruneComposites.NESTED_PROFILER_NAME);
35
36 SortAndPruneComposites pruner = new SortAndPruneComposites(randomArray);
37 pruner.sortAndPruneComposites();
38
39 // stop and log
40 profiler.stop().<b>log()</b>;
41 }
42 } </p>
<p>The output generated by this example will depend on the logging
environment, but should be very similar to the output generated by
the previous <code>NestedProfilerDemo</code> example.
</p>
<p>The log() method logs at level DEBUG using a marker named
"PROFILER".</p>
<p>If your logging system supports markers, e.g. logback, you could
specifically enable or disable output generated by SLF4J
profilers. Here is logback configuration file disabling output for
any logging event bearing the "PROFILER" marker, even if the logger
used by the profiler is enabled for the debug level.
</p>
<em>logback configuration disabling logging from profilers, and only
profilers</em>
<p class="source">&lt;configuration>
<b>&lt;turboFilter class="ch.qos.logback.classic.turbo.MarkerFilter">
&lt;Marker>PROFILER&lt;/Marker>
&lt;OnMatch>DENY&lt;/OnMatch>
&lt;/turboFilter></b>
&lt;appender name="STDOUT"
class="ch.qos.logback.core.ConsoleAppender">
&lt;layout class="ch.qos.logback.classic.PatternLayout">
&lt;Pattern>%-5level %logger{36} - %msg%n&lt;/Pattern>
&lt;/layout>
&lt;/appender>
&lt;root>
&lt;level value="DEBUG" />
&lt;appender-ref ref="STDOUT" />
&lt;/root>
&lt;/configuration> </p>
<!-- .............................................................. -->
<h2><a name="extended_logger"></a>Extended Logger</h2>
<p>The <a
href="apidocs/org/slf4j/ext/XLogger.html"><code>XLogger</code></a>
class provides a few extra logging methods that are quite useful
for following the execution path of applications. These methods
generate logging events that can be filtered separately from other
debug logging. Liberal use of these methods is encouraged as the
output has been found to
</p>
<ul>
<li>aid in problem diagnosis in development without requiring a
debug session</li>
<li>aid in problem diagnosis in production where no debugging is
possible</li>
<li>help educate new deveopers in learning the application.</li>
</ul>
<p>The two most used methods are the <code>entry()</code> and
<code>exit()</code> methods. <code>entry()</code> should be placed
at the beginning of methods, except perhaps for simple getters and
setters. <code>entry()</code> can be called passing from 0 to 4
parameters. Typically these will be parameters passed to the
method. The <code>entry()</code> method logs with a level of TRACE
and uses a <code>Marker</code> with a name of "ENTER" which is also
a "FLOW" Marker.
</p>
<p>The <code>exit()</code> method should be placed before any
return statement or as the last statement of methods without a
return. <code>exit()</code> can be called with or without a
parameter. Typically, methods that return void will use
<code>exit()</code> while methods that return an Object will use
exit(Object obj). The <code>entry()</code> method logs with a
level of TRACE and uses a Marker with a name of "EXIT" which is
also a "FLOW" Marker.
</p>
<p>The throwing() method can be used by an application when it is
throwing an exception that is unlikely to be handled, such as a
RuntimeExcpetion. This will insure that proper diagnostics are
available if needed. The logging event generated will have a level
of ERROR and will have an associated Marker with a name of
"THROWING" which is also an "EXCEPTION" Marker.
</p>
<p>The catching() method can be used by an application when it
catches an Exception that it is not going to rethrow, either
explicitly or attached to another Exception. The logging event
generated will have a level of ERROR and will have an associated
Marker with a name of "CATCHING" which is also an "EXCEPTION"
Marker.
</p>
<p>By using these extended methods applications that standardize on
SLF4J can be assured that they will be able to perform diagnostic
logging in a standardized manner.
</p>
<p>Note thar XLogger instances are obrained to through the
<a
href="apidocs/org/slf4j/ext/XLoggerFactory.html"><code>XLoggerFactory</code></a>
utility class.</p>
<p>The following example shows a simple application using these
methods in a fairly typcial manner. The <code>throwing()</code>
method is not present since no Exceptions are explicitly thrown and
not handled.
</p>
<p class="source">package com.test;
import org.slf4j.ext.XLogger;
import org.slf4j.ext.XLoggerFactory;
import java.util.Random;
public class TestService {
private XLogger logger = XLoggerFactory.getXLogger(TestService.class
.getName());
private String[] messages = new String[] { "Hello, World",
"Goodbye Cruel World", "You had me at hello" };
private Random rand = new Random(1);
public String retrieveMessage() {
logger.entry();
String testMsg = getMessage(getKey());
logger.exit(testMsg);
return testMsg;
}
public void exampleException() {
logger.entry();
try {
String msg = messages[messages.length];
logger.error("An exception should have been thrown");
} catch (Exception ex) {
logger.catching(ex);
}
logger.exit();
}
public String getMessage(int key) {
logger.entry(key);
String value = messages[key];
logger.exit(value);
return value;
}
private int getKey() {
logger.entry();
int key = rand.nextInt(messages.length);
logger.exit(key);
return key;
}
}</p>
<p>This test application uses the preceding service to generate
logging events.
</p>
<p class="source">package com.test;
public class App {
public static void main( String[] args ) {
TestService service = new TestService();
service.retrieveMessage();
service.retrieveMessage();
service.exampleException();
}
} </p>
<p>The configuration below will cause all output to be routed to
target/test.log. The pattern for the FileAppender includes the
class name, line number and method name. Including these in the
pattern are critical for the log to be of value.
</p>
<p class="source">&lt;?xml version='1.0' encoding='UTF-8'?>
&lt;configuration&gt;
&lt;appender name="console" class="ch.qos.logback.core.ConsoleAppender"&gt;
&lt;filter class="ch.qos.logback.classic.filter.LevelFilter"&gt;
&lt;level&gt;ERROR&lt;/level&gt;
&lt;onMatch&gt;ACCEPT&lt;/onMatch&gt;
&lt;onMismatch&gt;DENY&lt;/onMismatch&gt;
&lt;/filter&gt;
&lt;layout class="ch.qos.logback.classic.PatternLayout"&gt;
&lt;Pattern&gt;%d{HH:mm:ss.SSS} %-5level %class{36}:%L %M - %msg%n&lt;/Pattern&gt;
&lt;/layout&gt;
&lt;/appender&gt;
&lt;appender name="log" class="ch.qos.logback.core.FileAppender"&gt;
&lt;File&gt;target/test.log&lt;/File&gt;
&lt;Append&gt;false&lt;/Append&gt;
&lt;layout class="ch.qos.logback.classic.PatternLayout"&gt;
&lt;Pattern&gt;%d{HH:mm:ss.SSS} %-5level %class{36}:%L %M - %msg%n&lt;/Pattern&gt;
&lt;/layout&gt;
&lt;/appender&gt;
&lt;root&gt;
&lt;level value="trace" /&gt;
&lt;appender-ref ref="log" /&gt;
&lt;/root&gt;
&lt;/configuration&gt;
</p>
<p>
Here is the output that results from the Java classes and configuration above.
</p>
<p class="source">00:07:57.725 TRACE com.test.TestService:22 retrieveMessage - entry
00:07:57.738 TRACE com.test.TestService:57 getKey - entry
00:07:57.739 TRACE com.test.TestService:59 getKey - exit with (0)
00:07:57.741 TRACE com.test.TestService:47 getMessage - entry with (0)
00:07:57.741 TRACE com.test.TestService:51 getMessage - exit with (Hello, World)
00:07:57.742 TRACE com.test.TestService:26 retrieveMessage - exit with (Hello, World)
00:07:57.742 TRACE com.test.TestService:22 retrieveMessage - entry
00:07:57.742 TRACE com.test.TestService:57 getKey - entry
00:07:57.743 TRACE com.test.TestService:59 getKey - exit with (1)
00:07:57.745 TRACE com.test.TestService:47 getMessage - entry with (1)
00:07:57.745 TRACE com.test.TestService:51 getMessage - exit with (Goodbye Cruel World)
00:07:57.746 TRACE com.test.TestService:26 retrieveMessage - exit with (Goodbye Cruel World)
00:07:57.746 TRACE com.test.TestService:32 exampleException - entry
00:07:57.750 ERROR com.test.TestService:40 exampleException - catching
java.lang.ArrayIndexOutOfBoundsException: 3
at com.test.TestService.exampleException(TestService.java:35)
at com.test.AppTest.testApp(AppTest.java:39)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at junit.framework.TestCase.runTest(TestCase.java:154)
at junit.framework.TestCase.runBare(TestCase.java:127)
at junit.framework.TestResult$1.protect(TestResult.java:106)
at junit.framework.TestResult.runProtected(TestResult.java:124)
at junit.framework.TestResult.run(TestResult.java:109)
at junit.framework.TestCase.run(TestCase.java:118)
at junit.framework.TestSuite.runTest(TestSuite.java:208)
at junit.framework.TestSuite.run(TestSuite.java:203)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at org.apache.maven.surefire.junit.JUnitTestSet.execute(JUnitTestSet.java:213)
at org.apache.maven.surefire.suite.AbstractDirectoryTestSuite.executeTestSet(AbstractDirectoryTestSuite.java:140)
at org.apache.maven.surefire.suite.AbstractDirectoryTestSuite.execute(AbstractDirectoryTestSuite.java:127)
at org.apache.maven.surefire.Surefire.run(Surefire.java:177)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at org.apache.maven.surefire.booter.SurefireBooter.runSuitesInProcess(SurefireBooter.java:338)
at org.apache.maven.surefire.booter.SurefireBooter.main(SurefireBooter.java:997)
00:07:57.750 TRACE com.test.TestService:42 exampleException - exit</p>
<p>Simply changing the root logger level to DEBUG in the example
above will reduce the output considerably.
</p>
<p class="source">00:28:06.004 ERROR com.test.TestService:40 exampleException - catching
java.lang.ArrayIndexOutOfBoundsException: 3
at com.test.TestService.exampleException(TestService.java:35)
at com.test.AppTest.testApp(AppTest.java:39)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at junit.framework.TestCase.runTest(TestCase.java:154)
at junit.framework.TestCase.runBare(TestCase.java:127)
at junit.framework.TestResult$1.protect(TestResult.java:106)
at junit.framework.TestResult.runProtected(TestResult.java:124)
at junit.framework.TestResult.run(TestResult.java:109)
at junit.framework.TestCase.run(TestCase.java:118)
at junit.framework.TestSuite.runTest(TestSuite.java:208)
at junit.framework.TestSuite.run(TestSuite.java:203)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at org.apache.maven.surefire.junit.JUnitTestSet.execute(JUnitTestSet.java:213)
at org.apache.maven.surefire.suite.AbstractDirectoryTestSuite.executeTestSet(AbstractDirectoryTestSuite.java:140)
at org.apache.maven.surefire.suite.AbstractDirectoryTestSuite.execute(AbstractDirectoryTestSuite.java:127)
at org.apache.maven.surefire.Surefire.run(Surefire.java:177)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at org.apache.maven.surefire.booter.SurefireBooter.runSuitesInProcess(SurefireBooter.java:338)
at org.apache.maven.surefire.booter.SurefireBooter.main(SurefireBooter.java:997) </p>
<h2><a name="javaagent"></a>Adding logging with Java agent</h2>
<p><b>NOTE: BETA RELEASE, NOT PRODUCTION QUALITY</b> </p>
<p>Quickstart for the impatient:</p>
<ol>
<li>Use Java 5 or later.</li>
<li>Download slf4j-ext-${project.version}.jar and javassist.jar, and put them
both in the same directory.</li>
<li>Ensure your application is properly configured with
slf4j-api-X.Y.Z.jar and a suitable backend.</li>
<li>Instead of "java ..." use "java --javaagent:PATH/slf4j-ext-${project.version}.jar=time,verbose,level=info ..."<br/>
(replace PATH with the path to the jar)
</li>
<li>That's it!</li>
</ol>
<p>In some applications logging is used to trace the actual
execution of the application as opposed to log an occasional event.
One approach is using the <a href="#extended_logger">extended
logger</a> to add statements as appropriately, but another is to use
a tool which modifies compiled bytecode to add these statements!
Many exist, and the one included in slf4j-ext is not intended to
compete with these, but merely provide a quick way to get very basic
trace information from a given application.
</p>
<p>Java 5 added the Java Instrumentation mechanism, which allows you
to provide "Java agents" that can inspect and modify the byte code
of the classes as they are loaded. This allows the original class
files to remain unchanged, and the transformations done on the byte
codes depend on the needs at launch time.
</p>
<p>Given the well-known "Hello World" example:</p>
<p class="source">public class HelloWorld {
public static void main(String args[]) {
System.out.println("Hello World");
}
}</p>
<p>a typical transformation would be similar to: (imports omitted)</p>
<p class="source">public class LoggingHelloWorld {
final static Logger _log = LoggerFactory.getLogger(LoggingHelloWorld.class.getName());
public static void main(String args[]) {
if (_log.isInfoEnabled()) {
_log.info("> main(args=" + Arrays.asList(args) + ")");
}
System.out.println("Hello World");
if (_log.isInfoEnabled()) {
_log.info("&lt; main()");
}
}
}</p>
<p>which in turn produces the following result when run similar to
"java LoggingHelloWorld 1 2 3 4":
</p>
<p class="source">1 [main] INFO LoggingHelloWorld - > main(args=[1, 2, 3, 4])
Hello World
1 [main] INFO LoggingHelloWorld - &lt; main()</p>
<p>The same effect could have been had by using this command (with
the relative path to javassist.jar and
slf4j-ext-${project.version}.jar being ../jars):</p>
<p class="source">java -javaagent:../jars/slf4j-ext-${project.version}.jar HelloWorld 1 2 3 4</p>
<p></p>
<h3>How to use</h3>
<p>The javaagent may take one or more options separated by comma. The following options
are currently supported:</p>
<dl>
<dt><b>level</b>=X</dt>
<dd>The log level to use for the generated log statements. X is
one of "info", "debug" or "trace". Default is "info".</dd>
<dt><b>time</b></dt>
<dd>Print out the current date at program start, and again when
the program ends plus the execution time in milliseconds.</dd>
<dt><b>verbose</b></dt>
<dd>Print out when a class is processed as part of being loaded</dd>
<dt><b>ignore</b>=X:Y:...</dt>
<dd>(Advanced) Provide full list of colon separated prefixes of
class names NOT to add logging to. The default list is { "sun/",
"java/", "javax/", "org/slf4j/", "ch/qos/logback/",
"org/apache/log4j/", "apple/", "com/sun/" }.
</dd>
</dl>
<p>Note: These are not finalized yet, and may change.</p>
<h3>Locations of jar files</h3>
<p>The javassist library is used for the actual byte code
manipulation and must be available to be able to add any logging
statements. slf4j-ext-${project.version} has been configured to
look for the following:
</p>
<ul>
<li>"javassist-3.4.GA.jar" relatively to
slf4j-ext-${project.version}.jar as would be if Maven had downloaded
both from the repository and slf4j-ext-${project.version}.jar was
referenced directly in the Maven repository in the
"-javaagent"-argument.</li>
<li>"javassist-3.4.GA.jar" in the same directory as slf4j-ext</li>
<li>"javassist.jar" in the same directory as slf4j-ext</li>
</ul>
<p>A warning message is printed if the javassist library was not
found by the agent.
</p>
<h3>Misc notes</h3>
<ul>
<li>A java agent does not "see" any classes already loaded by the
class loader.</li>
<li>Any exceptions in the java agent that would normally have been
printed, are silently swallowed by the JVM.</li>
<li>The javaagent does not do any logging itself, and the slf4j
backend does not need to be available to the agent. </li>
<li>You MUST remember to have slf4j-api-${project.version}.jar
explicitly provided to the program to be instrumented. If not,
the one embedded in the slf4j-ext-${project.version}.jar library
is used, and these classes cannot see the jars loaded later.
(Empirically determined, reference would be appreciated).
</li>
</ul>
<p>(The agent is an adaption of the java.util.logging version
described in <a
href="http://today.java.net/pub/a/today/2008/04/24/add-logging-at-class-load-time-with-instrumentation.html"
>http://today.java.net/pub/a/today/2008/04/24/add-logging-at-class-load-time-with-instrumentation.html</a>)
</p>
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