slf4j绑定log4j2日志系统的过程（源码分析）

一、环境及介绍

slf4j和log4j2的介绍在文章http://blog.csdn.net/honghailiang888/article/details/52681777进行过介绍，源码分析版本log4j-api-2.2.jar、log4j-core-2.2.jar、log4j-slf4j-impl-2.2.jar、slf4j-api-1.7.21.jar

二、从使用入手

/** * @author HHL *  * @date 2016年9月27日 *  *       日志工具类类，采用slf4j方式 */public class LogUtil {// test日志public static Logger testLog = LoggerFactory.getLogger("test_logger");public static boolean isPrintStackTrace = true;// 记录test错误日志信息public static void testLogError(String errorMessage, Exception ex) {if (testLog != null) {testLog.error(errorMessage);}if (isPrintStackTrace && ex != null && testLog != null) {testLog.error(ex.getMessage(), ex);}}public static void printInfoLog(Class<?> cla, String message) {LoggerFactory.getLogger(cla).info(message);}}

配置文件内容log4j2.xml

<?xml version="1.0" encoding="UTF-8"?><Configuration status="WARN">  <Appenders>    <Console name="Console" target="SYSTEM_OUT">      <PatternLayout pattern="%d{YYYY-MM-dd HH:mm:ss.SSS} [%t] %-5level %l %msg%n"/>    </Console>        <RollingFile name="test_logger" filename="${sys:catalina.home}/logs/test_logger.log"      filepattern="${sys:catalina.home}/logs/test_logger.%d{yyyy-MM-dd-HH}.log">  <PatternLayout pattern="%d{YYYY-MM-dd HH:mm:ss.SSS} %5p [%t] %c(%F:%L) - %msg%n" />      <Policies>      <!-- 文件大小配置 -->        <SizeBasedTriggeringPolicy size="100 KB"/>      </Policies>       <!-- 共存log文件数量 -->       <DefaultRolloverStrategy max="20"/>    </RollingFile>      </Appenders>    <Loggers>    <Root level="info">      <AppenderRef ref="Console"/>    </Root>    <!-- springMVC log配置 -->    <Logger name="org.springframework.web" level="info" additivity="false">      <AppenderRef ref="Console"/>    </Logger>    <!-- mybatis log配置 -->    <Logger name="com.mango.mapper" level="trace" additivity="false">      <AppenderRef ref="Console"/>    </Logger>    <!-- log文件配置 -->    <Logger name="test_logger" level="info" additivity="true">      <AppenderRef ref="test_logger"/>    </Logger>  </Loggers></Configuration>

三、源码分析

1.从第一句入手

public static Logger testLog = LoggerFactory.getLogger("test_logger");

 /**     * 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) {        ILoggerFactory iLoggerFactory = getILoggerFactory();        return iLoggerFactory.getLogger(name);    }

/**     * Return the {@link ILoggerFactory} instance in use.     * <p/>     * <p/>     * ILoggerFactory instance is bound with this class at compile time.     *      * @return the ILoggerFactory instance in use     */    public static ILoggerFactory getILoggerFactory() {        if (INITIALIZATION_STATE == UNINITIALIZED) {            synchronized (LoggerFactory.class) {                if (INITIALIZATION_STATE == UNINITIALIZED) {                    INITIALIZATION_STATE = ONGOING_INITIALIZATION;                    performInitialization();                }            }        }        switch (INITIALIZATION_STATE) {        case SUCCESSFUL_INITIALIZATION:            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://jira.qos.ch/browse/SLF4J-97            return SUBST_FACTORY;        }        throw new IllegalStateException("Unreachable code");    }

其中初次bind的日志系统就在上述performInitialization()中

private final static void performInitialization() {        bind();        if (INITIALIZATION_STATE == SUCCESSFUL_INITIALIZATION) {            versionSanityCheck();        }    }

直接看bind()函数：

private final static void bind() {        try {            Set<URL> staticLoggerBinderPathSet = null;            // skip check under android, see also            // http://jira.qos.ch/browse/SLF4J-328            if (!isAndroid()) {                staticLoggerBinderPathSet = findPossibleStaticLoggerBinderPathSet();                reportMultipleBindingAmbiguity(staticLoggerBinderPathSet);            }            // the next line does the binding            StaticLoggerBinder.getSingleton();            INITIALIZATION_STATE = SUCCESSFUL_INITIALIZATION;            reportActualBinding(staticLoggerBinderPathSet);            fixSubstituteLoggers();            replayEvents();            // release all resources in SUBST_FACTORY            SUBST_FACTORY.clear();        } catch (NoClassDefFoundError ncde) {            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.contains("org.slf4j.impl.StaticLoggerBinder.getSingleton()")) {                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);        }    }

findPossibleStaticLoggerBinderPathSet()发现可能的binder路径，从类路径中寻找org/slf4j/impl/StaticLoggerBinder.class类：LoggerFactory.java

// 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";    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        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()) {                URL path = paths.nextElement();                staticLoggerBinderPathSet.add(path);            }        } catch (IOException ioe) {            Util.report("Error getting resources from path", ioe);        }        return staticLoggerBinderPathSet;    }

其中获取到的path为jar:file:/D:/software/Server/Tomcat/apache-tomcat-7.0.30/webapps/ETeam/WEB-INF/lib/log4j-slf4j-impl-2.2.jar!/org/slf4j/impl/StaticLoggerBinder.class，如果为多个，则会随机选取一个bind，但这里只有一个，因此只会选择一个。

总结，其实这里获取StaticLoggerBinder类的路径只是用来记录用，使用哪一个在编译时期就已经决定了(log4j-slf4j-impl-2.2.jar包起了桥接的作用)。这个时候就和log4j2关联上了。获得的loggerFactory就是

/**     * Private constructor to prevent instantiation     */    private StaticLoggerBinder() {        loggerFactory = new Log4jLoggerFactory();    }

2、看Log4jLoggerFactory.java中的getLogger是继承的AbstractLoggerAdapter类中的

@Override    public L getLogger(final String name) {        final LoggerContext context = getContext();   //获取logger环境        final ConcurrentMap<String, L> loggers = getLoggersInContext(context);        if (loggers.containsKey(name)) {            return loggers.get(name);        }        loggers.putIfAbsent(name, newLogger(name, context));        return loggers.get(name);  根据名字获取logger    }

看getContext()干了什么

@Override    protected LoggerContext getContext() {        final Class<?> anchor = ReflectionUtil.getCallerClass(FQCN, PACKAGE); 获取调用类        return anchor == null ? LogManager.getContext() : getContext(ReflectionUtil.getCallerClass(anchor));    }

最终调用到了LogManager.java中，彻底进入log4j2

/**     * Gets the {@link LoggerContext} associated with the given caller class.     *     * @param callerClass the caller class     * @return the LoggerContext for the calling class     */    protected LoggerContext getContext(final Class<?> callerClass) {        ClassLoader cl = null;        if (callerClass != null) {            cl = callerClass.getClassLoader();        }        if (cl == null) {            cl = LoaderUtil.getThreadContextClassLoader();        }        return LogManager.getContext(cl, false);    }

public static LoggerContext getContext(final ClassLoader loader, final boolean currentContext) {        return factory.getContext(FQCN, loader, null, currentContext);    }

Log4jContextFactory.java

/**     * Loads the LoggerContext using the ContextSelector.     * @param fqcn The fully qualified class name of the caller.     * @param loader The ClassLoader to use or null.     * @param currentContext If true returns the current Context, if false returns the Context appropriate     * for the caller if a more appropriate Context can be determined.     * @param externalContext An external context (such as a ServletContext) to be associated with the LoggerContext.     * @return The LoggerContext.     */    @Override    public LoggerContext getContext(final String fqcn, final ClassLoader loader, final Object externalContext,                                    final boolean currentContext) {        final LoggerContext ctx = selector.getContext(fqcn, loader, currentContext);        if (externalContext != null && ctx.getExternalContext() == null) {            ctx.setExternalContext(externalContext);        }        if (ctx.getState() == LifeCycle.State.INITIALIZED) {            ctx.start();           //初始获取，会解析配置文件log4j2.xml        }        return ctx;    }

LoggerContext.java

@Override    public void start() {        LOGGER.debug("Starting LoggerContext[name={}, {}]...", getName(), this);        if (configLock.tryLock()) {            try {                if (this.isInitialized() || this.isStopped()) {                    this.setStarting();                    reconfigure();       //解析配置文件                    if (this.config.isShutdownHookEnabled()) {                        setUpShutdownHook();                    }                    this.setStarted();                }            } finally {                configLock.unlock();            }        }        LOGGER.debug("LoggerContext[name={}, {}] started OK.", getName(), this);    }

/**     * Reconfigure the context.     */    public synchronized void reconfigure() {        final ClassLoader cl = ClassLoader.class.isInstance(externalContext) ? (ClassLoader) externalContext : null;        LOGGER.debug("Reconfiguration started for context[name={}] at {} ({}) with optional ClassLoader: {}", name,            configLocation, this, cl);        final Configuration instance = ConfigurationFactory.getInstance().getConfiguration(name, configLocation, cl);        setConfiguration(instance);        /*         * instance.start(); Configuration old = setConfiguration(instance);         * updateLoggers(); if (old != null) { old.stop(); }         */        LOGGER.debug("Reconfiguration complete for context[name={}] at {} ({}) with optional ClassLoader: {}", name,            configLocation, this, cl);    }

直接看最终的

private Configuration getConfiguration(final boolean isTest, final String name) {            final boolean named = name != null && name.length() > 0;            final ClassLoader loader = LoaderUtil.getThreadContextClassLoader();            for (final ConfigurationFactory factory : factories) {                String configName;                final String prefix = isTest ? TEST_PREFIX : DEFAULT_PREFIX;                final String [] types = factory.getSupportedTypes();                if (types == null) {                    continue;                }                for (final String suffix : types) {                    if (suffix.equals("*")) {                        continue;                    }                    configName = named ? prefix + name + suffix : prefix + suffix;                    final ConfigurationSource source = getInputFromResource(configName, loader);                    if (source != null) {                        return factory.getConfiguration(source);                    }                }            }            return null;        }

</pre><pre class="java" name="code" code_snippet_id="1933544" snippet_file_name="blog_20161017_19_6401194">/**     * File name prefix for test configurations.     */    protected static final String TEST_PREFIX = "log4j2-test";

    /**     * File name prefix for standard configurations.     */    protected static final String DEFAULT_PREFIX = "log4j2";

slf4j和log4j2的结合就此出现，log输出也是用的log4j2的。

四、没有配置的情况下，默认的配置为

/** * The default configuration writes all output to the Console using the default logging level. You configure default * logging level by setting the system property "org.apache.logging.log4j.level" to a level name. If you do not * specify the property, Log4j uses the ERROR Level. Log Events will be printed using the basic formatting provided * by each Message. */public class DefaultConfiguration extends AbstractConfiguration {    private static final long serialVersionUID = 1L;    /**     * The name of the default configuration.     */    public static final String DEFAULT_NAME = "Default";    /**     * The System Property used to specify the logging level.     */    public static final String DEFAULT_LEVEL = "org.apache.logging.log4j.level";    /**     * The default Pattern used for the default Layout.     */    public static final String DEFAULT_PATTERN = "%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n";    /**     * Constructor to create the default configuration.     */    public DefaultConfiguration() {        super(ConfigurationSource.NULL_SOURCE);                setName(DEFAULT_NAME);        final Layout<? extends Serializable> layout = PatternLayout.newBuilder()            .withPattern(DEFAULT_PATTERN)            .withConfiguration(this)            .build();        final Appender appender = ConsoleAppender.createDefaultAppenderForLayout(layout);        appender.start();        addAppender(appender);        final LoggerConfig root = getRootLogger();        root.addAppender(appender, null, null);        final String levelName = PropertiesUtil.getProperties().getStringProperty(DEFAULT_LEVEL);        final Level level = levelName != null && Level.valueOf(levelName) != null ?            Level.valueOf(levelName) : Level.ERROR;        root.setLevel(level);    }    @Override    protected void doConfigure() {    }}

输出到console中，默认级别为error级别，也可以通过system property "org.apache.logging.log4j.level"配置级别