从源码来理解slf4j的绑定,以及logback对配置文件的加载
前言
项目中的日志系统使用的是slf4j + logback。slf4j作为一个简单日志门面,为各种loging APIs(像java.util.logging, logback, log4j)提供一个简单统一的接口,有利于维护和各个类的日志处理方式统一。Logback作为一个具体的日志组件,完成具体的日志操作。
本博客旨在带领大家理清楚slf4j的绑定(logback如何绑定到slf4j的),logback是何时加载配置文件的。至于具体的配置则需要大家自己去查阅资料了。
路漫漫其修远兮,吾将上下而求索!
github:https://github.com/youzhibing
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slf4j + logback的使用
使用非常简单,引入依赖的jar即可,如下图
pom.xml
<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd"> <modelVersion>4.0.0</modelVersion> <groupId>com.yzb</groupId> <artifactId>mylog</artifactId> <version>0.0.1-SNAPSHOT</version> <packaging>jar</packaging> <name>mylog</name> <url>http://maven.apache.org</url> <properties> <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding> <maven.compiler.source>1.7</maven.compiler.source> <maven.compiler.target>1.7</maven.compiler.target> </properties> <dependencyManagement> <dependencies> <dependency> <groupId>org.slf4j</groupId> <artifactId>slf4j-api</artifactId> <version>1.7.7</version> </dependency> </dependencies> </dependencyManagement> <dependencies> <dependency> <groupId>org.slf4j</groupId> <artifactId>slf4j-api</artifactId> </dependency> <dependency> <groupId>ch.qos.logback</groupId> <artifactId>logback-core</artifactId> <version>1.1.7</version> </dependency> <dependency> <groupId>ch.qos.logback</groupId> <artifactId>logback-classic</artifactId> <version>1.1.7</version> </dependency> </dependencies> </project>
测试代码
public class LogTest { private static Logger LOGGER = LoggerFactory.getLogger(LogTest.class); public static void main(String[] args) { LOGGER.info("......info"); LOGGER.debug("......debug"); LOGGER.warn("......warn"); LOGGER.error("......error"); LOGGER.trace("......trace"); } }
控制台输出结果
15:24:48.840 [main] INFO com.huawei.log.LogTest - ......info 15:24:48.842 [main] DEBUG com.huawei.log.LogTest - ......debug 15:24:48.842 [main] WARN com.huawei.log.LogTest - ......warn 15:24:48.842 [main] ERROR com.huawei.log.LogTest - ......error
使用真的简单,也正是这种简单让我产生了一些疑问。
问题1:大家对spring使用的比较多的话,就知道将某个实现类注给其接口的时候,都是需要明确指出的,无论是通过配置文件的方式还是注解的方式。如下
<?xml version="1.0" encoding="UTF-8"?> <beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:context="http://www.springframework.org/schema/context" xmlns:task="http://www.springframework.org/schema/task" xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.1.xsd http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.1.xsd http://www.springframework.org/schema/task http://www.springframework.org/schema/task/spring-task-3.1.xsd"> <!-- applicationContext.xml实际是不会存在 配置文件会报错,因为缺少spring的jar包,这里只是模拟spring的依赖注入 更详细代码请看附件 --> <bean id="daoImpl" class="com.yzb.dao.impl.DaoImpl" /> <bean id="studentService" class="com.yzb.service.StudentService"> <!-- dao对应private IDao dao; 将实现daoImpl绑定到接口dao --> <property name="dao" ref="daoImpl"/> </bean> </beans>
可slf4j + logback没有其他任何的配置,工程就能跑起来,能够打印各种类型的日志,这是怎么实现的呢?
问题2:我们加上logback的配置文件,仅仅在src/main/resources(相当于classpath)下加logback.xml,发现生成了日志文件(若没有设置日志文件路径,那么日志文件生成在当前工程下),并且控制台输出结果如下:
2017-05-13 15:57:27|INFO|......info 2017-05-13 15:57:27|WARN|......warn 2017-05-13 15:57:27|ERROR|......error
仅仅在src/main/resources下配置logback.xml,就能达到这种效果,logback.xml是什么时候加载的呢?
源码解析
从LogTest.java开始
public class LogTest { private static Logger LOGGER = LoggerFactory.getLogger(LogTest.class); public static void main(String[] args) { // 下面5个方法相当于接口调用实现 LOGGER.info("......info"); LOGGER.debug("......debug"); LOGGER.warn("......warn"); LOGGER.error("......error"); LOGGER.trace("......trace"); } }
代码非常简单,很明显我们只需要看private static Logger LOGGER = LoggerFactory.getLogger(LogTest.class)的实现。
跟进getLogger方法 2步,来到
/** * Return a logger named according to the name parameter using the statically * bound {@link ILoggerFactory} instance. * * @param name The name of the logger. * @return logger */ public static Logger getLogger(String name) { // 获取日志工厂 ILoggerFactory iLoggerFactory = getILoggerFactory(); // 返回日志实例 return iLoggerFactory.getLogger(name); }
我们跟进getILoggerFactory方法
/** * Return the {@link ILoggerFactory} instance in use. * <p/> * <p/> * ILoggerFactory instance is bound with this class at compile time. // 编译时绑定工厂实例 * * @return the ILoggerFactory instance in use */ public static ILoggerFactory getILoggerFactory() { if (INITIALIZATION_STATE == UNINITIALIZED) { INITIALIZATION_STATE = ONGOING_INITIALIZATION; // 执行初始化 performInitialization(); } switch (INITIALIZATION_STATE) { case SUCCESSFUL_INITIALIZATION: // 若初始化成功,则返回日志工厂 return StaticLoggerBinder.getSingleton().getLoggerFactory(); case NOP_FALLBACK_INITIALIZATION: return NOP_FALLBACK_FACTORY; case FAILED_INITIALIZATION: throw new IllegalStateException(UNSUCCESSFUL_INIT_MSG); case ONGOING_INITIALIZATION: // support re-entrant behavior. // See also http://bugzilla.slf4j.org/show_bug.cgi?id=106 return TEMP_FACTORY; } throw new IllegalStateException("Unreachable code"); }
很显然,接着跟进performInitialization方法
private final static void performInitialization() { bind(); if (INITIALIZATION_STATE == SUCCESSFUL_INITIALIZATION) { versionSanityCheck(); } }
跟进bind方法
private final static void bind() { try { // 从classpath获取可能的日志绑定者,就是找出所有slf4j的实现,并将它们的资源路径存放到staticLoggerBinderPathSet Set<URL> staticLoggerBinderPathSet = findPossibleStaticLoggerBinderPathSet(); // 若有多个(多余1个)绑定者,就是从classpath中找到了多个slf4j的实现,那么就打印警告。这个方法就不跟进了,感兴趣的自己跟进 reportMultipleBindingAmbiguity(staticLoggerBinderPathSet); // the next line does the binding 真正的绑定,将具体的实现绑定到slf4j StaticLoggerBinder.getSingleton(); // 修改初始化状态为初始化成功 INITIALIZATION_STATE = SUCCESSFUL_INITIALIZATION; // 报告真实的绑定信息 reportActualBinding(staticLoggerBinderPathSet); fixSubstitutedLoggers(); } catch (NoClassDefFoundError ncde) { // 若有多个绑定者,则会抛此异常,Java虚拟机在编译时能找到合适的类,而在运行时不能找到合适的类导致的错误,jvm不知道用哪个StaticLoggerBinder String msg = ncde.getMessage(); if (messageContainsOrgSlf4jImplStaticLoggerBinder(msg)) { INITIALIZATION_STATE = NOP_FALLBACK_INITIALIZATION; Util.report("Failed to load class \"org.slf4j.impl.StaticLoggerBinder\"."); Util.report("Defaulting to no-operation (NOP) logger implementation"); Util.report("See " + NO_STATICLOGGERBINDER_URL + " for further details."); } else { failedBinding(ncde); throw ncde; } } catch (java.lang.NoSuchMethodError nsme) { String msg = nsme.getMessage(); if (msg != null && msg.indexOf("org.slf4j.impl.StaticLoggerBinder.getSingleton()") != -1) { INITIALIZATION_STATE = FAILED_INITIALIZATION; Util.report("slf4j-api 1.6.x (or later) is incompatible with this binding."); Util.report("Your binding is version 1.5.5 or earlier."); Util.report("Upgrade your binding to version 1.6.x."); } throw nsme; } catch (Exception e) { failedBinding(e); throw new IllegalStateException("Unexpected initialization failure", e); } }
跟进findPossibleStaticLoggerBinderPathSet方法
// We need to use the name of the StaticLoggerBinder class, but we can't reference // the class itself. private static String STATIC_LOGGER_BINDER_PATH = "org/slf4j/impl/StaticLoggerBinder.class"; // 从classpath找出所有slf4j的实现,并记录下它们的资源路径 private static Set<URL> findPossibleStaticLoggerBinderPathSet() { // use Set instead of list in order to deal with bug #138 // LinkedHashSet appropriate here because it preserves insertion order during iteration 用LinkedHashSet能够保证插入的顺序 Set<URL> staticLoggerBinderPathSet = new LinkedHashSet<URL>(); try { ClassLoader loggerFactoryClassLoader = LoggerFactory.class .getClassLoader(); Enumeration<URL> paths; if (loggerFactoryClassLoader == null) { paths = ClassLoader.getSystemResources(STATIC_LOGGER_BINDER_PATH); } else { paths = loggerFactoryClassLoader .getResources(STATIC_LOGGER_BINDER_PATH); } while (paths.hasMoreElements()) { // path的值 jar:file:/D:/repository/ch/qos/logback/logback-classic/1.1.7/logback-classic-1.1.7.jar!/org/slf4j/impl/StaticLoggerBinder.class URL path = (URL) paths.nextElement(); staticLoggerBinderPathSet.add(path); } } catch (IOException ioe) { Util.report("Error getting resources from path", ioe); } return staticLoggerBinderPathSet; }
至此,问题1的答案就很明显了,slf4j会在classpath中找所有org/slf4j/impl/StaticLoggerBinder.class的资源路径,一般而言只有一个,在本博客中就在logback的jar中,如图
那么logback与slf4j就关联起来了,接下来看logback对配置文件的加载。我们回到bind方法,跟进StaticLoggerBinder.getSingleton(),方法很简单
public static StaticLoggerBinder getSingleton() { return SINGLETON; }
很显然,执行此方法之前,对配置文件的加载已经执行完了,也就是说在编译器已经完成对配置文件的加载了。那么我们需要换目标跟进了,StaticLoggerBinder中只有一段静态块
static { SINGLETON.init(); }
那么我们跟进init方法
/** * Package access for testing purposes. */ void init() { try { try { // 上下文初始化器 new ContextInitializer(defaultLoggerContext).autoConfig(); } catch (JoranException je) { Util.report("Failed to auto configure default logger context", je); } // logback-292 if (!StatusUtil.contextHasStatusListener(defaultLoggerContext)) { StatusPrinter.printInCaseOfErrorsOrWarnings(defaultLoggerContext); } contextSelectorBinder.init(defaultLoggerContext, KEY); initialized = true; } catch (Throwable t) { // we should never get here Util.report("Failed to instantiate [" + LoggerContext.class.getName() + "]", t); } }
接着跟进上下文初始化器的autoConfig方法
public void autoConfig() throws JoranException { StatusListenerConfigHelper.installIfAsked(loggerContext); // 寻找默认配置文件 URL url = findURLOfDefaultConfigurationFile(true); if (url != null) { configureByResource(url); } else { Configurator c = EnvUtil.loadFromServiceLoader(Configurator.class); if (c != null) { try { c.setContext(loggerContext); c.configure(loggerContext); } catch (Exception e) { throw new LogbackException(String.format("Failed to initialize Configurator: %s using ServiceLoader", c != null ? c.getClass() .getCanonicalName() : "null"), e); } } else { // 没有找到配置文件,则使用默认的配置器,那么日志只会打印在控制台 BasicConfigurator basicConfigurator = new BasicConfigurator(); basicConfigurator.setContext(loggerContext); basicConfigurator.configure(loggerContext); } } }
跟进findURLOfDefaultConfigurationFile方法
public URL findURLOfDefaultConfigurationFile(boolean updateStatus) { // 获取当前实例的类加载器,目的是在classpath下寻找配置文件 ClassLoader myClassLoader = Loader.getClassLoaderOfObject(this); // 先找logback.configurationFile文件 URL url = findConfigFileURLFromSystemProperties(myClassLoader, updateStatus); if (url != null) { return url; } // logback.configurationFile文件没找到,再找logback.groovy url = getResource(GROOVY_AUTOCONFIG_FILE, myClassLoader, updateStatus); if (url != null) { return url; } // logback.groovy没找到,再找logback-test.xml url = getResource(TEST_AUTOCONFIG_FILE, myClassLoader, updateStatus); if (url != null) { return url; } // logback-test.xml没找到,最后找logback.xml return getResource(AUTOCONFIG_FILE, myClassLoader, updateStatus); }
自此,问题2的答案也清楚了,编译期间logback就完成了对配置文件的加载。
总结
编译期间,完成slf4j的绑定以及logback配置文件的加载。slf4j会在classpath中寻找org/slf4j/impl/StaticLoggerBinder.class(会在具体的日志框架如log4j、logback等中存在),找到并完成绑定;同时,logback也会在classpath中寻找配置文件,先找logback.configurationFile、没有则找logback.groovy,若logback.groovy也没有,则找logback-test.xml,若logback-test.xml还是没有,则找logback.xml,若连logback.xml也没有,那么说明没有配置logback的配置文件,那么logback则会启用默认的配置(日志信息只会打印在控制台)。
slf4j只能绑定某一个特定的日志框架,若没有绑定,则会有如下警告,说明没有找到合适的日志框架
SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder". SLF4J: Defaulting to no-operation (NOP) logger implementation SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.
若找到多个日志框架,slf4j会发出警告,并在运行时抛出NoClassDefFoundError异常
最后来一张图