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spring日志加载代码解析
阅读量:6634 次
发布时间:2019-06-25

本文共 13335 字,大约阅读时间需要 44 分钟。

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):

1 public static LogFactory getFactory() throws LogConfigurationException {  2         // Identify the class loader we will be using  3         ClassLoader contextClassLoader = getContextClassLoaderInternal();  4   5         if (contextClassLoader == null) {  6             // This is an odd enough situation to report about. This  7             // output will be a nuisance on JDK1.1, as the system  8             // classloader is null in that environment.  9             if (isDiagnosticsEnabled()) { 10                 logDiagnostic("Context classloader is null."); 11             } 12         } 13  14         // Return any previously registered factory for this class loader 15         LogFactory factory = getCachedFactory(contextClassLoader); 16         if (factory != null) { 17             return factory; 18         } 19  20         if (isDiagnosticsEnabled()) { 21             logDiagnostic( 22                     "[LOOKUP] LogFactory implementation requested for the first time for context classloader " + 23                     objectId(contextClassLoader)); 24             logHierarchy("[LOOKUP] ", contextClassLoader); 25         } 26  27         // Load properties file. 28         // 29         // If the properties file exists, then its contents are used as 30         // "attributes" on the LogFactory implementation class. One particular 31         // property may also control which LogFactory concrete subclass is 32         // used, but only if other discovery mechanisms fail.. 33         // 34         // As the properties file (if it exists) will be used one way or 35         // another in the end we may as well look for it first. 36  37         Properties props = getConfigurationFile(contextClassLoader, FACTORY_PROPERTIES); 38  39         // Determine whether we will be using the thread context class loader to 40         // load logging classes or not by checking the loaded properties file (if any). 41         ClassLoader baseClassLoader = contextClassLoader; 42         if (props != null) { 43             String useTCCLStr = props.getProperty(TCCL_KEY); 44             if (useTCCLStr != null) { 45                 // The Boolean.valueOf(useTCCLStr).booleanValue() formulation 46                 // is required for Java 1.2 compatibility. 47                 if (Boolean.valueOf(useTCCLStr).booleanValue() == false) { 48                     // Don't use current context classloader when locating any 49                     // LogFactory or Log classes, just use the class that loaded 50                     // this abstract class. When this class is deployed in a shared 51                     // classpath of a container, it means webapps cannot deploy their 52                     // own logging implementations. It also means that it is up to the 53                     // implementation whether to load library-specific config files 54                     // from the TCCL or not. 55                     baseClassLoader = thisClassLoader; 56                 } 57             } 58         } 59  60         // Determine which concrete LogFactory subclass to use. 61         // First, try a global system property 62         if (isDiagnosticsEnabled()) { 63             logDiagnostic("[LOOKUP] Looking for system property [" + FACTORY_PROPERTY + 64                           "] to define the LogFactory subclass to use..."); 65         } 66  67         try { 68             String factoryClass = getSystemProperty(FACTORY_PROPERTY, null); 69             if (factoryClass != null) { 70                 if (isDiagnosticsEnabled()) { 71                     logDiagnostic("[LOOKUP] Creating an instance of LogFactory class '" + factoryClass + 72                                   "' as specified by system property " + FACTORY_PROPERTY); 73                 } 74                 factory = newFactory(factoryClass, baseClassLoader, contextClassLoader); 75             } else { 76                 if (isDiagnosticsEnabled()) { 77                     logDiagnostic("[LOOKUP] No system property [" + FACTORY_PROPERTY + "] defined."); 78                 } 79             } 80         } catch (SecurityException e) { 81             if (isDiagnosticsEnabled()) { 82                 logDiagnostic("[LOOKUP] A security exception occurred while trying to create an" + 83                               " instance of the custom factory class" + ": [" + trim(e.getMessage()) + 84                               "]. Trying alternative implementations..."); 85             } 86             // ignore 87         } catch (RuntimeException e) { 88             // This is not consistent with the behaviour when a bad LogFactory class is 89             // specified in a services file. 90             // 91             // One possible exception that can occur here is a ClassCastException when 92             // the specified class wasn't castable to this LogFactory type. 93             if (isDiagnosticsEnabled()) { 94                 logDiagnostic("[LOOKUP] An exception occurred while trying to create an" + 95                               " instance of the custom factory class" + ": [" + 96                               trim(e.getMessage()) + 97                               "] as specified by a system property."); 98             } 99             throw e;100         }101 102         // Second, try to find a service by using the JDK1.3 class103         // discovery mechanism, which involves putting a file with the name104         // of an interface class in the META-INF/services directory, where the105         // contents of the file is a single line specifying a concrete class106         // that implements the desired interface.107 108         if (factory == null) {109             if (isDiagnosticsEnabled()) {110                 logDiagnostic("[LOOKUP] Looking for a resource file of name [" + SERVICE_ID +111                               "] to define the LogFactory subclass to use...");112             }113             try {114                 final InputStream is = getResourceAsStream(contextClassLoader, SERVICE_ID);115 116                 if( is != null ) {117                     // This code is needed by EBCDIC and other strange systems.118                     // It's a fix for bugs reported in xerces119                     BufferedReader rd;120                     try {121                         rd = new BufferedReader(new InputStreamReader(is, "UTF-8"));122                     } catch (java.io.UnsupportedEncodingException e) {123                         rd = new BufferedReader(new InputStreamReader(is));124                     }125 126                     String factoryClassName = rd.readLine();127                     rd.close();128 129                     if (factoryClassName != null && ! "".equals(factoryClassName)) {130                         if (isDiagnosticsEnabled()) {131                             logDiagnostic("[LOOKUP]  Creating an instance of LogFactory class " +132                                           factoryClassName +133                                           " as specified by file '" + SERVICE_ID +134                                           "' which was present in the path of the context classloader.");135                         }136                         factory = newFactory(factoryClassName, baseClassLoader, contextClassLoader );137                     }138                 } else {139                     // is == null140                     if (isDiagnosticsEnabled()) {141                         logDiagnostic("[LOOKUP] No resource file with name '" + SERVICE_ID + "' found.");142                     }143                 }144             } catch (Exception ex) {145                 // note: if the specified LogFactory class wasn't compatible with LogFactory146                 // for some reason, a ClassCastException will be caught here, and attempts will147                 // continue to find a compatible class.148                 if (isDiagnosticsEnabled()) {149                     logDiagnostic(150                         "[LOOKUP] A security exception occurred while trying to create an" +151                         " instance of the custom factory class" +152                         ": [" + trim(ex.getMessage()) +153                         "]. Trying alternative implementations...");154                 }155                 // ignore156             }157         }158 159         // Third try looking into the properties file read earlier (if found)160 161         if (factory == null) {162             if (props != null) {163                 if (isDiagnosticsEnabled()) {164                     logDiagnostic(165                         "[LOOKUP] Looking in properties file for entry with key '" + FACTORY_PROPERTY +166                         "' to define the LogFactory subclass to use...");167                 }168                 String factoryClass = props.getProperty(FACTORY_PROPERTY);169                 if (factoryClass != null) {170                     if (isDiagnosticsEnabled()) {171                         logDiagnostic(172                             "[LOOKUP] Properties file specifies LogFactory subclass '" + factoryClass + "'");173                     }174                     factory = newFactory(factoryClass, baseClassLoader, contextClassLoader);175 176                     // TODO: think about whether we need to handle exceptions from newFactory177                 } else {178                     if (isDiagnosticsEnabled()) {179                         logDiagnostic("[LOOKUP] Properties file has no entry specifying LogFactory subclass.");180                     }181                 }182             } else {183                 if (isDiagnosticsEnabled()) {184                     logDiagnostic("[LOOKUP] No properties file available to determine" + " LogFactory subclass from..");185                 }186             }187         }188 189         // Fourth, try the fallback implementation class190 191         if (factory == null) {192             if (isDiagnosticsEnabled()) {193                 logDiagnostic(194                     "[LOOKUP] Loading the default LogFactory implementation '" + FACTORY_DEFAULT +195                     "' via the same classloader that loaded this LogFactory" +196                     " class (ie not looking in the context classloader).");197             }198 199             // Note: unlike the above code which can try to load custom LogFactory200             // implementations via the TCCL, we don't try to load the default LogFactory201             // implementation via the context classloader because:202             // * that can cause problems (see comments in newFactory method)203             // * no-one should be customising the code of the default class204             // Yes, we do give up the ability for the child to ship a newer205             // version of the LogFactoryImpl class and have it used dynamically206             // by an old LogFactory class in the parent, but that isn't207             // necessarily a good idea anyway.208             factory = newFactory(FACTORY_DEFAULT, thisClassLoader, contextClassLoader);209         }210 211         if (factory != null) {212             /**213              * Always cache using context class loader.214              */215             cacheFactory(contextClassLoader, factory);216 217             if (props != null) {218                 Enumeration names = props.propertyNames();219                 while (names.hasMoreElements()) {220                     String name = (String) names.nextElement();221                     String value = props.getProperty(name);222                     factory.setAttribute(name, value);223                 }224             }225         }226 227         return factory;228     }

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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