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Jakarta Commons Logging implemented over SLF4J.
Overview
This package contains the same public user interface as Jakarta Commons
Logging (JCL). It is intended as a 100% compatible drop-in
replacement for the original JCL version 1.0.4.
As the original JCL version 1.0.4, the present version supports
various logging APIs. It differs from the original in implementation
but not the public API. This implementation uses SLF4J under the
covers. As as such, all the logging systems that SLF4J supports,
e.g. NOP, Simple, JDK14, nlog4j are supported by this version of JCL.
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 {
static 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);
}
...
}
Configuring the Commons Logging Package
In this version of JCL, the selection of the logging system to use
is chosen by the underlying SLF4J API. Consequently, all JCL-specific
configuration parameters are ignored.
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
always returns the same factory, i.e. a unique instance of the SLF4FLogFactory class.
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 static 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);
...
}
}