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Simple wrapper API around multiple logging APIs.
Overview
This package provides an API for logging in server-based applications that
can be used around a variety of different logging implementations, including
prebuilt support for the following:
- Log4J (version 1.2 or later)
from Apache's Logging project. Each named Log
instance is connected to a corresponding Log4J Logger.
-
JDK Logging API, included in JDK 1.4 or later systems. Each named
Log instance is connected to a corresponding
java.util.logging.Logger instance.
- LogKit from Apache's
Avalon project. Each named Log instance is
connected to a corresponding LogKit
Logger.
- NoOpLog implementation that simply swallows
all log output, for all named Log instances.
- SimpleLog implementation that writes all
log output, for all named Log instances, to
System.err.
Quick Start Guide
For those impatient to just get on with it, the following example
illustrates the typical declaration and use of a logger that is named (by
convention) after the calling class:
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
public class Foo {
private Log log = LogFactory.getLog(Foo.class);
public void foo() {
...
try {
if (log.isDebugEnabled()) {
log.debug("About to do something to object " + name);
}
name.bar();
} catch (IllegalStateException e) {
log.error("Something bad happened to " + name, e);
}
...
}
Unless you configure things differently, all log output will be written
to System.err. Therefore, you really will want to review the remainder of
this page in order to understand how to configure logging for your
application.
Configuring the Commons Logging Package
Choosing a LogFactory Implementation
From an application perspective, the first requirement is to retrieve an
object reference to the LogFactory instance that will be used
to create Log instances for this
application. This is normally accomplished by calling the static
getFactory() method. This method implements the following
discovery algorithm to select the name of the LogFactory
implementation class this application wants to use:
- Check for a system property named
org.apache.commons.logging.LogFactory.
- Use the JDK 1.3 JAR Services Discovery mechanism (see
http://java.sun.com/j2se/1.3/docs/guide/jar/jar.html for
more information) to look for a resource named
META-INF/services/org.apache.commons.logging.LogFactory
whose first line is assumed to contain the desired class name.
- Look for a properties file named
commons-logging.properties
visible in the application class path, with a property named
org.apache.commons.logging.LogFactory defining the
desired implementation class name.
- Fall back to a default implementation, which is described
further below.
If a commons-logging.properties file is found, all of the
properties defined there are also used to set configuration attributes on
the instantiated LogFactory instance.
Once an implementation class name is selected, the corresponding class is
loaded from the current Thread context class loader (if there is one), or
from the class loader that loaded the LogFactory class itself
otherwise. This allows a copy of commons-logging.jar to be
shared in a multiple class loader environment (such as a servlet container),
but still allow each web application to provide its own LogFactory
implementation, if it so desires. An instance of this class will then be
created, and cached per class loader.
The Default LogFactory Implementation
The Logging Package APIs include a default LogFactory
implementation class (
org.apache.commons.logging.impl.LogFactoryImpl) that is selected if no
other implementation class name can be discovered. Its primary purpose is
to create (as necessary) and return Log instances
in response to calls to the getInstance() method. The default
implementation uses the following rules:
- At most one
Log instance of the same name will be created.
Subsequent getInstance() calls to the same
LogFactory instance, with the same name or Class
parameter, will return the same Log instance.
- When a new
Log instance must be created, the default
LogFactory implementation uses the following discovery
process:
- Look for a configuration attribute of this factory named
org.apache.commons.logging.Log (for backwards
compatibility to pre-1.0 versions of this API, an attribute
org.apache.commons.logging.log is also consulted).
- Look for a system property named
org.apache.commons.logging.Log (for backwards
compatibility to pre-1.0 versions of this API, a system property
org.apache.commons.logging.log is also consulted).
- If the Log4J logging system is available in the application
class path, use the corresponding wrapper class
(Log4JLogger).
- If the application is executing on a JDK 1.4 system, use
the corresponding wrapper class
(Jdk14Logger).
- Fall back to the default simple logging implementation
(SimpleLog).
- Load the class of the specified name from the thread context class
loader (if any), or from the class loader that loaded the
LogFactory class otherwise.
- Instantiate an instance of the selected
Log
implementation class, passing the specified name as the single
argument to its constructor.
See the SimpleLog JavaDocs for detailed
configuration information for this default implementation.
Configuring the Underlying Logging System
The basic principle is that the user is totally responsible for the
configuration of the underlying logging system.
Commons-logging should not change the existing configuration.
Each individual Log implementation may
support its own configuration properties. These will be documented in the
class descriptions for the corresponding implementation class.
Finally, some Log implementations (such as the one for Log4J)
require an external configuration file for the entire logging environment.
This file should be prepared in a manner that is specific to the actual logging
technology being used.
Using the Logging Package APIs
Use of the Logging Package APIs, from the perspective of an application
component, consists of the following steps:
- Acquire a reference to an instance of
org.apache.commons.logging.Log, by calling the
factory method
LogFactory.getInstance(String name). Your application can contain
references to multiple loggers that are used for different
purposes. A typical scenario for a server application is to have each
major component of the server use its own Log instance.
- Cause messages to be logged (if the corresponding detail level is enabled)
by calling appropriate methods (
trace(), debug(),
info(), warn(), error, and
fatal()).
For convenience, LogFactory also offers a static method
getLog() that combines the typical two-step pattern:
Log log = LogFactory.getFactory().getInstance(Foo.class);
into a single method call:
Log log = LogFactory.getLog(Foo.class);
For example, you might use the following technique to initialize and
use a Log instance in an application component:
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
public class MyComponent {
protected Log log =
LogFactory.getLog(MyComponent.class);
// Called once at startup time
public void start() {
...
log.info("MyComponent started");
...
}
// Called once at shutdown time
public void stop() {
...
log.info("MyComponent stopped");
...
}
// Called repeatedly to process a particular argument value
// which you want logged if debugging is enabled
public void process(String value) {
...
// Do the string concatenation only if logging is enabled
if (log.isDebugEnabled())
log.debug("MyComponent processing " + value);
...
}
}