spring日志加载代码解析

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。