Java日志学习二：Apache Commons Logging (JCL)源码
一.Apache Commons Logginghttp://zy19982004.iteye.com/blog/1867448里提到了commons-logging的工作方式，本文将看下代码怎么实现这一工作方式。
二.Apache Commons Logging类结构
就这么简单，一个接口包，一个实现包。
三.类说明
[*]Log：A simple logging interface abstracting logging APIs，和JDBC API是一个性质的东西
[*]Level。对六种日志级别的操作，官网建议我们这样用：
http://commons.apache.org/proper/commons-logging/guide.html 写道
It is important to ensure that log message are appropriate in content and severity. The following guidelines are suggested:
fatal - Severe errors that cause premature termination. Expect these to be immediately visible on a status console. See also Internationalization.
error - Other runtime errors or unexpected conditions. Expect these to be immediately visible on a status console. See also Internationalization.
warn - Use of deprecated APIs, poor use of API, 'almost' errors, other runtime situations that are undesirable or unexpected, but not necessarily "wrong". Expect these to be immediately visible on a status console. See also Internationalization.
info - Interesting runtime events (startup/shutdown). Expect these to be immediately visible on a console, so be conservative and keep to a minimum. See also Internationalization.
debug - detailed information on the flow through the system. Expect these to be written to logs only.
trace - more detailed information. Expect these to be written to logs only.
[*]LogFactory：抽闲类，获得Log类。 getFactory()先从缓存里取LogFactory，取不到再去找，然后放进缓存。其中找的这段代码如下
public static final String FACTORY_PROPERTY = "org.apache.commons.logging.LogFactory"
protected static final String SERVICE_ID = "META-INF/services/org.apache.commons.logging.LogFactory"
public static final String FACTORY_PROPERTIES = "commons-logging.properties"
public static final String FACTORY_DEFAULT = "org.apache.commons.logging.impl.LogFactoryImpl"
public static LogFactory getFactory() throws LogConfigurationException {
//1.系统环境变量里是否定义了LogFactory实现类，System.getProperty(FACTORY_PROPERTY)
//2.利用JDK1.3 开始提供的service 发现机制，会扫描classpah 下的SERVICE_ID文件，若找到则装载里面的配置，使用里面的配置
//3.Classpath下有FACTORY_PROPERTIES文件的话，看此properties里有无定义FACTORY_PROPERTY
//4.如果123找不到LogFactory实现类，使用FACTORY_DEFAULT
}
public abstract Log getInstance(String name)
throws LogConfigurationException;
public abstract Log getInstance(Class clazz)
throws LogConfigurationException;
public static Log getLog(String name)
throws LogConfigurationException {
return (getFactory().getInstance(name));
}
public static Log getLog(Class clazz)
throws LogConfigurationException {
return (getFactory().getInstance(clazz));
}
[*]LogFactoryImpl，实现LogFactory，真正产生Log类的地方。首先从缓存里取Log，取不到就按照http://zy19982004.iteye.com/blog/1867448 中的顺序加载Log类。(i<classesToDiscover.length) && (result == null)就是这里。
public Log getInstance(Class clazz) throws LogConfigurationException {
return (getInstance(clazz.getName()));
}
public Log getInstance(String name) throws LogConfigurationException {
Log instance = (Log) instances.get(name);
if (instance == null) {
instance = newInstance(name);
instances.put(name, instance);
}
return (instance);
}
private static final String[] classesToDiscover = {
"org.apache.commons.logging.impl.Log4JLogger",
"org.apache.commons.logging.impl.Jdk14Logger",
"org.apache.commons.logging.impl.Jdk13LumberjackLogger",
"org.apache.commons.logging.impl.SimpleLog"
};
private Log discoverLogImplementation(String logCategory)
throws LogConfigurationException
{
// See if the user specified the Log implementation to use
//1).首先在classpath下寻找自己的配置文件commons-logging.properties，如果找到，则使用其中定义的Log实现类；
//2) 如果找不到commons-logging.properties文件，则在查找是否已定义系统环境变量org.apache.commons.logging.Log，找到则使用其定义的Log实现类；
String specifiedLogClassName = findUserSpecifiedLogClassName();
if (specifiedLogClassName != null) {
result = createLogFromClass(specifiedLogClassName,
logCategory,
true);
return result;
}
//3) 否则，查看classpath中是否有Log4j的包，如果发现，则自动使用Log4j作为日志实现类；
//4) 否则，使用JDK自身的日志实现类（JDK1.4以后才有日志实现类）；
//5) 否则，使用commons-logging自己提供的一个简单的日志实现类SimpleLog；
for(int i=0; (i<classesToDiscover.length) && (result == null); ++i) {
result = createLogFromClass(classesToDiscover, logCategory, true);
}
return result;
createLogFromClass方法使用Class.forName(logAdapterClassName, true, currentCL)，看classpath下是否有此logAdapterClassName
[*]Log4JLogger：包装了org.apache.log4j.Logger。
[*]Jdk14Logger：包装了JDK1.4的java.util.logging.Logger。
[*]Jdk13LumberjackLogger：包装了JDK1.3及以前版本的java.util.logging.Logger。
[*]SimpleLog：自己实现的一个简单日志。
[*]NoOpLog：不记录日志，都是空方法。
四.Log实现类
五.commons-logging-api.jar
[*]关于commons-logging-api.jar和commong-logging.jar的差别可以参考http://commons.apache.org/proper/commons-logging/guide.html。
[*]commons-logging-api.jar it does not include the wrapper Log implementations that require additional libraries such as Log4j, Avalon and Lumberjack。必然的commons-logging.jar依赖log4j，而commons-logging-api.jar不依赖任何包。
[*]commons-logging.jar 和 log4j一起，LogFactory.getLog(X)得到的毫无疑问是org.apache.commons.logging.impl.Log4JLogger。
[*]commons-logging-api.jar 和 log4j一起，得到的是org.apache.commons.logging.impl.Jdk14Logger。为什么？commons-logging-api.jar和commons-logging.jar的LogFactoryImpl classesToDiscover都是一模一样的，但commons-logging-api.jar里根本没有Log4JLogger这个类，肯定加载不到，最终使用JDK自身的日志实现类，由Jdk14Logger包装。
[*]总的来说，你想使用commons-logging和Log4j(或者Avalon Lumberjack)，请使用commons-logging.jar 和 log4j。如果你只是想使用commons-logging做一个门面，其它日志做实现，如slf4j，请使用commons-logging-api.jar和slf4j相关jar。
六.总结
[*]LogFactory实现类的查找和Log实现类的查找是两回事。但它们均可以在commons-logging.properties里定义。
[*]合理采用commons-logging-api.jar和commons-logging.jar，首选commons-logging.jar肯定不会出问题，如果追求代码的干净请合理使用commons-logging-api.jar。
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