spring日志加载代码解析
2024-08-26 06:28:44
项目用的是springmvc+spring+mybatis框架，
配置日志的时候非常简单，仅仅是把commons-logging、log4j，还有slf4j-log4j三个日志相关的jar包导入项目，然后在classpath中加个log4j.properties的配置文件即可。
但是你有没有想过Log4j是什么时候被加载进虚拟机的？为什么我们没有手动加载，spring和mybatis就能自动的用起来log4j，毫不见外？
spring使用的是commons-logging，
mybatis用的是自己写的，
看一下源码，非常简单，一个个的去试，没有这个就进入下一个，直到找到了log4j
这两个工具其实都只是日志工具的规范，可以理解成接口，而log4j才是一个实实在在的日志实现。
但是slf4j在什么地方？——虽然这两个工具没用到slf4j，我还是把它放进来，因为我引入的其他工具会用到它。。比如：
下面开始看代码代码来说明一下log4j是怎么被加载到虚拟机的：
首先我们来看spring，我们从web.xml看起，加载spring：
到这里我们看到了LogFactory，这是commons-logging的一个log工厂，这里用的是这个虚类的一个静态方法，我们继续看这个方法：
再看getFactory（代码较长，可以直接跳过先看后边的结论）：
public static LogFactory getFactory() throws LogConfigurationException {
// Identify the class loader we will be using
ClassLoader contextClassLoader = getContextClassLoaderInternal();
if (contextClassLoader == null) {
// This is an odd enough situation to report about. This
// output will be a nuisance on JDK1.1, as the system
// classloader is null in that environment.
if (isDiagnosticsEnabled()) {
logDiagnostic("Context classloader is null.");
}
}
// Return any previously registered factory for this class loader
LogFactory factory = getCachedFactory(contextClassLoader);
if (factory != null) {
return factory;
}
if (isDiagnosticsEnabled()) {
logDiagnostic(
"[LOOKUP] LogFactory implementation requested for the first time for context classloader " +
objectId(contextClassLoader));
logHierarchy("[LOOKUP] ", contextClassLoader);
}
// Load properties file.
//
// If the properties file exists, then its contents are used as
// "attributes" on the LogFactory implementation class. One particular
// property may also control which LogFactory concrete subclass is
// used, but only if other discovery mechanisms fail..
//
// As the properties file (if it exists) will be used one way or
// another in the end we may as well look for it first.
Properties props = getConfigurationFile(contextClassLoader, FACTORY_PROPERTIES);
// Determine whether we will be using the thread context class loader to
// load logging classes or not by checking the loaded properties file (if any).
ClassLoader baseClassLoader = contextClassLoader;
if (props != null) {
String useTCCLStr = props.getProperty(TCCL_KEY);
if (useTCCLStr != null) {
// The Boolean.valueOf(useTCCLStr).booleanValue() formulation
// is required for Java 1.2 compatibility.
if (Boolean.valueOf(useTCCLStr).booleanValue() == false) {
// Don't use current context classloader when locating any
// LogFactory or Log classes, just use the class that loaded
// this abstract class. When this class is deployed in a shared
// classpath of a container, it means webapps cannot deploy their
// own logging implementations. It also means that it is up to the
// implementation whether to load library-specific config files
// from the TCCL or not.
baseClassLoader = thisClassLoader;
}
}
}
// Determine which concrete LogFactory subclass to use.
// First, try a global system property
if (isDiagnosticsEnabled()) {
logDiagnostic("[LOOKUP] Looking for system property [" + FACTORY_PROPERTY +
"] to define the LogFactory subclass to use...");
}
try {
String factoryClass = getSystemProperty(FACTORY_PROPERTY, null);
if (factoryClass != null) {
if (isDiagnosticsEnabled()) {
logDiagnostic("[LOOKUP] Creating an instance of LogFactory class '" + factoryClass +
"' as specified by system property " + FACTORY_PROPERTY);
}
factory = newFactory(factoryClass, baseClassLoader, contextClassLoader);
} else {
if (isDiagnosticsEnabled()) {
logDiagnostic("[LOOKUP] No system property [" + FACTORY_PROPERTY + "] defined.");
}
}
} catch (SecurityException e) {
if (isDiagnosticsEnabled()) {
logDiagnostic("[LOOKUP] A security exception occurred while trying to create an" +
" instance of the custom factory class" + ": [" + trim(e.getMessage()) +
"]. Trying alternative implementations...");
}
// ignore
} catch (RuntimeException e) {
// This is not consistent with the behaviour when a bad LogFactory class is
// specified in a services file.
//
// One possible exception that can occur here is a ClassCastException when
// the specified class wasn't castable to this LogFactory type.
if (isDiagnosticsEnabled()) {
logDiagnostic("[LOOKUP] An exception occurred while trying to create an" +
" instance of the custom factory class" + ": [" +
trim(e.getMessage()) +
"] as specified by a system property.");
}
throw e;
}
// Second, try to find a service by using the JDK1.3 class
// discovery mechanism, which involves putting a file with the name
// of an interface class in the META-INF/services directory, where the
// contents of the file is a single line specifying a concrete class
// that implements the desired interface.
if (factory == null) {
if (isDiagnosticsEnabled()) {
logDiagnostic("[LOOKUP] Looking for a resource file of name [" + SERVICE_ID +
"] to define the LogFactory subclass to use...");
}
try {
final InputStream is = getResourceAsStream(contextClassLoader, SERVICE_ID);
if( is != null ) {
// This code is needed by EBCDIC and other strange systems.
// It's a fix for bugs reported in xerces
BufferedReader rd;
try {
rd = new BufferedReader(new InputStreamReader(is, "UTF-8"));
} catch (java.io.UnsupportedEncodingException e) {
rd = new BufferedReader(new InputStreamReader(is));
}
String factoryClassName = rd.readLine();
rd.close();
if (factoryClassName != null && ! "".equals(factoryClassName)) {
if (isDiagnosticsEnabled()) {
logDiagnostic("[LOOKUP] Creating an instance of LogFactory class " +
factoryClassName +
" as specified by file '" + SERVICE_ID +
"' which was present in the path of the context classloader.");
}
factory = newFactory(factoryClassName, baseClassLoader, contextClassLoader );
}
} else {
// is == null
if (isDiagnosticsEnabled()) {
logDiagnostic("[LOOKUP] No resource file with name '" + SERVICE_ID + "' found.");
}
}
} catch (Exception ex) {
// note: if the specified LogFactory class wasn't compatible with LogFactory
// for some reason, a ClassCastException will be caught here, and attempts will
// continue to find a compatible class.
if (isDiagnosticsEnabled()) {
logDiagnostic(
"[LOOKUP] A security exception occurred while trying to create an" +
" instance of the custom factory class" +
": [" + trim(ex.getMessage()) +
"]. Trying alternative implementations...");
}
// ignore
}
}
// Third try looking into the properties file read earlier (if found)
if (factory == null) {
if (props != null) {
if (isDiagnosticsEnabled()) {
logDiagnostic(
"[LOOKUP] Looking in properties file for entry with key '" + FACTORY_PROPERTY +
"' to define the LogFactory subclass to use...");
}
String factoryClass = props.getProperty(FACTORY_PROPERTY);
if (factoryClass != null) {
if (isDiagnosticsEnabled()) {
logDiagnostic(
"[LOOKUP] Properties file specifies LogFactory subclass '" + factoryClass + "'");
}
factory = newFactory(factoryClass, baseClassLoader, contextClassLoader);
// TODO: think about whether we need to handle exceptions from newFactory
} else {
if (isDiagnosticsEnabled()) {
logDiagnostic("[LOOKUP] Properties file has no entry specifying LogFactory subclass.");
}
}
} else {
if (isDiagnosticsEnabled()) {
logDiagnostic("[LOOKUP] No properties file available to determine" + " LogFactory subclass from..");
}
}
}
// Fourth, try the fallback implementation class
if (factory == null) {
if (isDiagnosticsEnabled()) {
logDiagnostic(
"[LOOKUP] Loading the default LogFactory implementation '" + FACTORY_DEFAULT +
"' via the same classloader that loaded this LogFactory" +
" class (ie not looking in the context classloader).");
}
// Note: unlike the above code which can try to load custom LogFactory
// implementations via the TCCL, we don't try to load the default LogFactory
// implementation via the context classloader because:
// * that can cause problems (see comments in newFactory method)
// * no-one should be customising the code of the default class
// Yes, we do give up the ability for the child to ship a newer
// version of the LogFactoryImpl class and have it used dynamically
// by an old LogFactory class in the parent, but that isn't
// necessarily a good idea anyway.
factory = newFactory(FACTORY_DEFAULT, thisClassLoader, contextClassLoader);
}
if (factory != null) {
/**
* Always cache using context class loader.
*/
cacheFactory(contextClassLoader, factory);
if (props != null) {
Enumeration names = props.propertyNames();
while (names.hasMoreElements()) {
String name = (String) names.nextElement();
String value = props.getProperty(name);
factory.setAttribute(name, value);
}
}
}
return factory;
}
以上代码会先试图从classpath中加载commons-logging.properties等几个commons-logging才有的东西，直到208行代码，
我们看到最后创建了一个LogFactoryImpl的实例，然后返回了。
回头看看我们上边的代码：
看完了getFactory我们知道最后我们得到的是LogFactoryImpl的实例，
那么接下来我们该去这个类中看看它的getInstance方法了：
非常简单，在这里创建了一个Log的实例，这也是最关键的地方，我们去看看这个方法。
经过我个人进一步的代码分析logConstructor这个东西是空的，我就不再展开了，这时我们看541行这个方法，这里才是真正的创建Log实例：
这个方法名我们能看懂，就是去发现系统中的Log实现，也就是一个找的过程，后边的782行是找找系统变量中有没有commons-logging自己的指明实现类类名的变量，结果是没有的，因为我没有配。
这个方法还没完，中间是一大段的注释，我们直接看后边的重点：
这个时候开始遍历一个字符串数组，依此去系统中找有没有这个数组中的实现类，我们看看这个数组先。
大家看到了什么？log4j，现在大家基本已经知道为什么了，后边的代码如果还有兴趣可以自己去看，其实就是一个个的试着去系统中加载这些类，加载不到处理一下ClassNotFoundException然后继续找下一个。
完毕。
最后如果大家想看看commons-logging自己的日志，可以写一个Listener放在web xml第一个位置，把下边一句代码弄上：
你们会从日志中看到多了很多内容，其中有一句是：
[LogFactoryImpl@1302539661 from org.apache.catalina.loader.WebappClassLoader@1536551745] Class 'org.apache.commons.logging.impl.Log4JLogger' was found at 'jar:file:/E:/eaipweb/wtpwebapps/EAIP/WEB-INF/lib/commons-logging-1.1.1.jar!/org/apache/commons/logging/impl/Log4JLogger.class'
这回真完毕了。
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