Default Initialization Procedure
The log4j library does not make any assumptions about its environment. In particular, there are no default log4j appenders. Under certain well-defined circumstances however, the static inializer of the Logger class will attempt to automatically configure log4j. The Java language guarantees that the static initializer of a class is called once and only once during the loading of a class into memory. It is important to remember that different classloaders may load distinct copies of the same class. These copies of the same class are considered as totally unrelated by the JVM.
The default initialization is very useful in environments where the exact entry point to the application depends on the runtime environment. For example, the same application can be used as a stand-alone application, as an applet, or as a servlet under the control of a web-server.
The exact default initialization algorithm is defined as follows:
- Setting the log4j.defaultInitOverride system property to any other value then "false" will cause log4j to skip the default initialization procedure (this procedure).
- Set the resource string variable to the value of the log4j.configuration system property. The preferred way to specify the default initialization file is through the log4j.configuration system property. In case the system property log4j.configuration is not defined, then set the string variable resourceto its default value "log4j.properties".
- Attempt to convert the resource variable to a URL.
- If the resource variable cannot be converted to a URL, for example due to a MalformedURLException, then search for the resource from the classpath by calling org.apache.log4j.helpers.Loader.getResource(resource, Logger.class) which returns a URL. Note that the string "log4j.properties" constitutes a malformed URL.
See Loader.getResource(java.lang.String) for the list of searched locations. - If no URL could not be found, abort default initialization. Otherwise, configure log4j from the URL.
The PropertyConfigurator will be used to parse the URL to configure log4j unless the URL ends with the ".xml" extension, in which case the DOMConfiguratorwill be used. You can optionaly specify a custom configurator. The value of the log4j.configuratorClass system property is taken as the fully qualified class name of your custom configurator. The custom configurator you specify must implement the Configurator interface.
Example Configurations：Default Initialization under Tomcat
The default log4j initialization is particularly useful in web-server environments. Under Tomcat 3.x and 4.x, you should place the log4j.properties under the WEB-INF/classes directory of your web-applications. Log4j will find the properties file and initialize itself. This is easy to do and it works.
You can also choose to set the system property log4j.configuration before starting Tomcat. For Tomcat 3.x The TOMCAT_OPTS environment variable is used to set command line options. For Tomcat 4.0, set the CATALINA_OPTS environment variable instead of TOMCAT_OPTS.
Example 1
The Unix shell command
export TOMCAT_OPTS="-Dlog4j.configuration=foobar.txt"
tells log4j to use the file foobar.txt as the default configuration file. This file should be place under the WEB-INF/classes directory of your web-application. The file will be read using the PropertyConfigurator. Each web-application will use a different default configuration file because each file is relative to a web-application.
Example 2
The Unix shell command
export TOMCAT_OPTS="-Dlog4j.debug -Dlog4j.configuration=foobar.xml"
tells log4j to output log4j-internal debugging information and to use the file foobar.xml as the default configuration file. This file should be place under the WEB-INF/classes directory of your web-application. Since the file ends with a .xml extension, it will read using the DOMConfigurator. Each web-application will use a different default configuration file because each file is relative to a web-application.
Example 3
The Windows shell command
set TOMCAT_OPTS=-Dlog4j.configuration=foobar.lcf -Dlog4j.configuratorClass=com.foo.BarConfigurator
tells log4j to use the file foobar.lcf as the default configuration file. This file should be place under the WEB-INF/classes directory of your web-application. Due to the definition of the log4j.configuratorClass system property, the file will be read using the com.foo.BarConfigurator custom configurator. Each web-application will use a different default configuration file because each file is relative to a web-application.
Example 4
The Windows shell command
set TOMCAT_OPTS=-Dlog4j.configuration=file:/c:/foobar.lcf
tells log4j to use the file c:\foobar.lcf as the default configuration file. The configuration file is fully specified by the URL file:/c:/foobar.lcf. Thus, the same configuration file will be used for all web-applications.
Different web-applications will load the log4j classes through their respective classloaderss. Thus, each image of the log4j environment will act independetly and without any mutual synchronization. For example, FileAppenders defined exactly the same way in multiple web-application configurations will all attempt to write the same file. The results are likely to be less than satisfactory. You must make sure that log4j configurations of different web-applications do not use the same underlying system resource.
Initialization servlet
It is also possible to use a special servlet for log4j initialization. Here is an example,
package com.foo;
import org.apache.log4j.PropertyConfigurator;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.PrintWriter;
import java.io.IOException;
public class Log4jInit extends HttpServlet {
public
void init() {
String prefix = getServletContext().getRealPath("/");
String file = getInitParameter("log4j-init-file");
// if the log4j-init-file is not set, then no point in trying
if(file != null) {
PropertyConfigurator.configure(prefix+file);
}
}
public
void doGet(HttpServletRequest req, HttpServletResponse res) {
}
}
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Define the following servlet in the web.xml file for your web-application.
<servlet>
<servlet-name>log4j-init</servlet-name>
<servlet-class>com.foo.Log4jInit</servlet-class>
<init-param>
<param-name>log4j-init-file</param-name>
<param-value>WEB-INF/classes/log4j.lcf</param-value>
</init-param>
<load-on-startup>1</load-on-startup>
</servlet>
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Writing an initialization servlet is the most flexible way for initializing log4j. There are no constraints on the code you can place in the init() method of the servlet.
原文地址：http://logging.apache.org/log4j/1.2/manual.html