Java日志学习二：Apache Commons Logging (JCL)源码

一.Apache Commons Logging

http://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[i], 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

Java日志学习二：Apache Commons Logging (JCL)源码

五.commons-logging-api.jar

Java日志学习二：Apache Commons Logging (JCL)源码

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。

Java日志学习二：Apache Commons Logging (JCL)源码

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