SLF4j 日志组件自动发现原理分析

　　slf4j通过提供一个可变的接口，从而使各日志组件可以方便接入自己的实现，从而实现统一市面上各式各样的日志组件使用。从而减轻大家的开发压力，即使是下次更换日志组件，也无需更改原有其他代码。

      另外，因为java中中间件大面积使用，也急需一个统一的日志组件格式，但是却不应该限制用户使用任何日志组件。所以 slf4j就可以大展身手了！

我们来看看，slf4j是怎样将各式各样的组件统一化的：

      其实就是通过一个LoggerFactory工厂方法，实现不同的组件接入，获取logger实例！而logger则是一些通用的接口，对业务方完全统一。

　　private Logger logger = LoggerFactory.getLogger(this.getClass());

 

// 通过以上获取日志组件实例


// org.slf4j.LoggerFactory.getLogger()
    /**
     * Return a logger named corresponding to the class passed as parameter, using
     * the statically bound {@link ILoggerFactory} instance.
     *
     * <p>In case the the <code>clazz</code> parameter differs from the name of
     * the caller as computed internally by SLF4J, a logger name mismatch warning will be 
     * printed but only if the <code>slf4j.detectLoggerNameMismatch</code> system property is 
     * set to true. By default, this property is not set and no warnings will be printed
     * even in case of a logger name mismatch.
     * 
     * @param clazz the returned logger will be named after clazz
     * @return logger
     *
     *
     * @see <a href="http://www.slf4j.org/codes.html#loggerNameMismatch">Detected logger name mismatch</a> 
     */
    public static Logger getLogger(Class<?> clazz) {
        // 获取logger, 将clazzName 传入
        Logger logger = getLogger(clazz.getName());
        if (DETECT_LOGGER_NAME_MISMATCH) {
            Class<?> autoComputedCallingClass = Util.getCallingClass();
            if (autoComputedCallingClass != null && nonMatchingClasses(clazz, autoComputedCallingClass)) {
                Util.report(String.format("Detected logger name mismatch. Given name: \"%s\"; computed name: \"%s\".", logger.getName(),
                                autoComputedCallingClass.getName()));
                Util.report("See " + LOGGER_NAME_MISMATCH_URL + " for an explanation");
            }
        }
        return 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) {
        // 获取日志工厂
        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() {
        // INITIALIZATION_STATE 状态保存
        if (INITIALIZATION_STATE == UNINITIALIZED) {
            synchronized (LoggerFactory.class) {
                if (INITIALIZATION_STATE == UNINITIALIZED) {
                    // 未初始化时进行初始化
                    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");
    }
    
    private final static void performInitialization() {
        // 绑定
        bind();
        if (INITIALIZATION_STATE == SUCCESSFUL_INITIALIZATION) {
            // 版本检测
            versionSanityCheck();
        }
    }
    
    // 绑定
    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()) {
                // 列举可能使用的logger的url, 备用
                staticLoggerBinderPathSet = findPossibleStaticLoggerBinderPathSet();
                // 存在多个实现时，报告存在的歧义
                reportMultipleBindingAmbiguity(staticLoggerBinderPathSet);
            }
            // the next line does the binding
            // 接下来，已经找到 StaticLoggerBinder 了，就使用其实现的 getSingleton() 方法，返回logger实例
            StaticLoggerBinder.getSingleton();
            INITIALIZATION_STATE = SUCCESSFUL_INITIALIZATION;
            reportActualBinding(staticLoggerBinderPathSet);
            replayEvents();
        } 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);
        }
    }
    
    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;
            // 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";
            // 通过加载 org/slf4j/impl/StaticLoggerBinder 的实现类，来返回该值，即其他日志组件，需要覆盖该类，从而实现自己的日志组件的接入，当有多个组件时，由类加载时机决定使用
            // 如果多个实现，会返回一个列表，由后续逻辑决定使用哪个实现
            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;
    }

    
    /**
     * Prints a warning message on the console if multiple bindings were found on the class path.
     * No reporting is done otherwise.
     *
     */
    private static void reportMultipleBindingAmbiguity(Set<URL> binderPathSet) {
        if (isAmbiguousStaticLoggerBinderPathSet(binderPathSet)) {
            Util.report("Class path contains multiple SLF4J bindings.");
            for (URL path : binderPathSet) {
                Util.report("Found binding in [" + path + "]");
            }
            Util.report("See " + MULTIPLE_BINDINGS_URL + " for an explanation.");
        }
    }
    
    // logback 的实现 getSingleton() 如下:
    
    /**
     * The unique instance of this class.
     */
    private static StaticLoggerBinder SINGLETON = new StaticLoggerBinder();

    private static Object KEY = new Object();

    static {
        SINGLETON.init();
    }

    private boolean initialized = false;
    private LoggerContext defaultLoggerContext = new LoggerContext();
    private final ContextSelectorStaticBinder contextSelectorBinder = ContextSelectorStaticBinder.getSingleton();

    /**
     * Package access for testing purposes.
     */
    void init() {
        try {
            try {
                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);
        }
    }
    
    private StaticLoggerBinder() {
        // DEFAULT_CONTEXT_NAME = "default";
        defaultLoggerContext.setName(CoreConstants.DEFAULT_CONTEXT_NAME);
    }

    public static StaticLoggerBinder getSingleton() {
        // 返回 static 创建的实例
        return SINGLETON;
    }
    
    // 获取实例后，报告获取到的实例
    private static void reportActualBinding(Set<URL> binderPathSet) {
        // binderPathSet can be null under Android
        if (binderPathSet != null && isAmbiguousStaticLoggerBinderPathSet(binderPathSet)) {
            Util.report("Actual binding is of type [" + StaticLoggerBinder.getSingleton().getLoggerFactoryClassStr() + "]");
        }
    }
    
    // 绑定事件通知，调用 SubstituteLoggingEvent.log(), 进行消息通知
    private static void replayEvents() {
        final LinkedBlockingQueue<SubstituteLoggingEvent> queue = SUBST_FACTORY.getEventQueue();
        final int queueSize = queue.size();
        int count = 0;
        final int maxDrain = 128;
        List<SubstituteLoggingEvent> eventList = new ArrayList<SubstituteLoggingEvent>(maxDrain);
        while (true) {
            int numDrained = queue.drainTo(eventList, maxDrain);
            if (numDrained == 0)
                break;
            for (SubstituteLoggingEvent event : eventList) {
                replaySingleEvent(event);
                if (count++ == 0)
                    // 第一个事件通知时，触发 warning
                    emitReplayOrSubstituionWarning(event, queueSize);
            }
            eventList.clear();
        }
    }
    
    // 调用 substLogger.log(event)
    private static void replaySingleEvent(SubstituteLoggingEvent event) {
        if (event == null)
            return;

        SubstituteLogger substLogger = event.getLogger();
        String loggerName = substLogger.getName();
        if (substLogger.isDelegateNull()) {
            // 代理为空时，设置 logger 代理为自身
            Logger logger = getLogger(loggerName);
            substLogger.setDelegate(logger);
        }

        if (substLogger.isDelegateNOP()) {
            // nothing to do
        } else if (substLogger.isDelegateEventAware()) {
            substLogger.log(event);
        } else {
            Util.report(loggerName);
        }
    }
    
    // warning
    private static void emitReplayOrSubstituionWarning(SubstituteLoggingEvent event, int queueSize) {
        if (event.getLogger().isDelegateEventAware()) {
            emitReplayWarning(queueSize);
        } else if (event.getLogger().isDelegateNOP()) {
            // nothing to do
        } else {
            emitSubstitutionWarning();
        }
    }
    private static void emitReplayWarning(int eventCount) {
        Util.report("A number (" + eventCount + ") of logging calls during the initialization phase have been intercepted and are");
        Util.report("now being replayed. These are subject to the filtering rules of the underlying logging system.");
        Util.report("See also " + REPLAY_URL);
    }
    
    // 版本检测，检测不通过时，仅打印异常提示，不返回错误
    private final static void versionSanityCheck() {
        try {
            String requested = StaticLoggerBinder.REQUESTED_API_VERSION;

            boolean match = false;
            for (String aAPI_COMPATIBILITY_LIST : API_COMPATIBILITY_LIST) {
                if (requested.startsWith(aAPI_COMPATIBILITY_LIST)) {
                    match = true;
                }
            }
            if (!match) {
                Util.report("The requested version " + requested + " by your slf4j binding is not compatible with "
                                + Arrays.asList(API_COMPATIBILITY_LIST).toString());
                Util.report("See " + VERSION_MISMATCH + " for further details.");
            }
        } catch (java.lang.NoSuchFieldError nsfe) {
            // given our large user base and SLF4J's commitment to backward
            // compatibility, we cannot cry here. Only for implementations
            // which willingly declare a REQUESTED_API_VERSION field do we
            // emit compatibility warnings.
        } catch (Throwable e) {
            // we should never reach here
            Util.report("Unexpected problem occured during version sanity check", e);
        }
    }
    
    // 初始化后，获取日志工厂
    public ILoggerFactory getLoggerFactory() {
        if (!initialized) {
            // 初始化未完成时，返回默认 context
            return defaultLoggerContext;
        }

        if (contextSelectorBinder.getContextSelector() == null) {
            throw new IllegalStateException("contextSelector cannot be null. See also " + NULL_CS_URL);
        }
        // 其他情况动态获取 context
        return contextSelectorBinder.getContextSelector().getLoggerContext();
    }
    
    // 最后，由工厂获取 logger
    // 如，logback 的 getLogger() , 使用 ConcurrentHashMap 进行保存各记录类的实例，每次不同的类单独一个实例
  public Logger getLogger(String name) {
    Logger slf4jLogger = loggerMap.get(name);
    if (slf4jLogger != null) {
      return slf4jLogger;
    } else {
      org.apache.log4j.Logger log4jLogger;
      if(name.equalsIgnoreCase(Logger.ROOT_LOGGER_NAME))
        log4jLogger = LogManager.getRootLogger();
      else
        log4jLogger = LogManager.getLogger(name);

      Logger newInstance = new Log4jLoggerAdapter(log4jLogger);
      Logger oldInstance = loggerMap.putIfAbsent(name, newInstance);
      return oldInstance == null ? newInstance : oldInstance;
    }
  }

 

logback 类名缩写处理：

    // 如，logback 的 getLogger()
    // ch.qos.logback.classic.LoggerContext.getLogger()    
    
    @Override
    public final Logger getLogger(final String name) {

        if (name == null) {
            throw new IllegalArgumentException("name argument cannot be null");
        }

        // if we are asking for the root logger, then let us return it without
        // wasting time
        if (Logger.ROOT_LOGGER_NAME.equalsIgnoreCase(name)) {
            return root;
        }

        int i = 0;
        Logger logger = root;

        // check if the desired logger exists, if it does, return it
        // without further ado.
        Logger childLogger = (Logger) loggerCache.get(name);
        // if we have the child, then let us return it without wasting time
        if (childLogger != null) {
            return childLogger;
        }

        // if the desired logger does not exist, them create all the loggers
        // in between as well (if they don't already exist)
        String childName;
        while (true) {
            // 进行类名写
            int h = LoggerNameUtil.getSeparatorIndexOf(name, i);
            if (h == -1) {
                childName = name;
            } else {
                childName = name.substring(0, h);
            }
            // move i left of the last point
            i = h + 1;
            synchronized (logger) {
                childLogger = logger.getChildByName(childName);
                if (childLogger == null) {
                    childLogger = logger.createChildByName(childName);
                    loggerCache.put(childName, childLogger);
                    incSize();
                }
            }
            logger = childLogger;
            if (h == -1) {
                return childLogger;
            }
        }
    }

 

这样 Logger 实例就获取到了，就可以调用任意的 log() 方法了!