从源码来理解slf4j的绑定，以及logback对配置文件的加载

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。

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的绑定已经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