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探索Java日志的奥秘:底层日志系统-log4j2

前言

log4j2是apache在log4j的基础上,参考logback架构实现的一套新的日志系统(我感觉是apache害怕logback了)。
log4j2的官方文档上写着一些它的优点:

  • 在拥有全部logback特性的情况下,还修复了一些隐藏问题
  • API 分离:现在log4j2也是门面模式使用日志,默认的日志实现是log4j2,当然你也可以用logback(应该没有人会这么做)
  • 性能提升:log4j2包含下一代基于LMAX Disruptor library的异步logger,在多线程场景下,拥有18倍于log4j和logback的性能
  • 多API支持:log4j2提供Log4j 1.2, SLF4J, Commons Logging and java.util.logging (JUL) 的API支持
  • 避免锁定:使用Log4j2 API的应用程序始终可以选择使用任何符合SLF4J的库作为log4j-to-slf4j适配器的记录器实现
  • 自动重新加载配置:与Logback一样,Log4j 2可以在修改时自动重新加载其配置。与Logback不同,它会在重新配置发生时不会丢失日志事件。
  • 高级过滤: 与Logback一样,Log4j 2支持基于Log事件中的上下文数据,标记,正则表达式和其他组件进行过滤。
  • 插件架构: Log4j使用插件模式配置组件。因此,您无需编写代码来创建和配置Appender,Layout,Pattern Converter等。Log4j自动识别插件并在配置引用它们时使用它们。
  • 属性支持:您可以在配置中引用属性,Log4j将直接替换它们,或者Log4j将它们传递给将动态解析它们的底层组件。
  • Java 8 Lambda支持
  • 自定义日志级别
  • 产生垃圾少:在稳态日志记录期间,Log4j 2 在独立应用程序中是无垃圾的,在Web应用程序中是低垃圾。这减少了垃圾收集器的压力,并且可以提供更好的响应时间性能。
  • 和应用server集成:版本2.10.0引入了一个模块log4j-appserver,以改进与Apache Tomcat和Eclipse Jetty的集成。

Log4j2类图:

这次从四个地方去探索源码:启动,配置,异步,插件化

源码探索

启动

log4j2的关键组件

  • LogManager

根据配置指定LogContexFactory,初始化对应的LoggerContext

  • LoggerContext

1、解析配置文件,解析为对应的java对象。
2、通过LoggerRegisty缓存Logger配置
3、Configuration配置信息
4、start方法解析配置文件,转化为对应的java对象
5、通过getLogger获取logger对象

  • Logger

LogManaer

该组件是Log4J启动的入口,后续的LoggerContext以及Logger都是通过调用LogManager的静态方法获得。我们可以使用下面的代码获取Logger

Logger logger = LogManager.getLogger();

可以看出LogManager是十分关键的组件,因此在这个小节中我们详细分析LogManager的启动流程。
LogManager启动的入口是下面的static代码块:

/**
     * Scans the classpath to find all logging implementation. Currently, only one will be used but this could be
     * extended to allow multiple implementations to be used.
     */
    static {
        // Shortcut binding to force a specific logging implementation.
        final PropertiesUtil managerProps = PropertiesUtil.getProperties();
        final String factoryClassName = managerProps.getStringProperty(FACTORY_PROPERTY_NAME);
        if (factoryClassName != null) {
            try {
                factory = LoaderUtil.newCheckedInstanceOf(factoryClassName, LoggerContextFactory.class);
            } catch (final ClassNotFoundException cnfe) {
                LOGGER.error("Unable to locate configured LoggerContextFactory {}", factoryClassName);
            } catch (final Exception ex) {
                LOGGER.error("Unable to create configured LoggerContextFactory {}", factoryClassName, ex);
            }
        }

        if (factory == null) {
            final SortedMap<Integer, LoggerContextFactory> factories = new TreeMap<>();
            // note that the following initial call to ProviderUtil may block until a Provider has been installed when
            // running in an OSGi environment
            if (ProviderUtil.hasProviders()) {
                for (final Provider provider : ProviderUtil.getProviders()) {
                    final Class<? extends LoggerContextFactory> factoryClass = provider.loadLoggerContextFactory();
                    if (factoryClass != null) {
                        try {
                            factories.put(provider.getPriority(), factoryClass.newInstance());
                        } catch (final Exception e) {
                            LOGGER.error("Unable to create class {} specified in provider URL {}", factoryClass.getName(), provider
                                    .getUrl(), e);
                        }
                    }
                }

                if (factories.isEmpty()) {
                    LOGGER.error("Log4j2 could not find a logging implementation. "
                            + "Please add log4j-core to the classpath. Using SimpleLogger to log to the console...");
                    factory = new SimpleLoggerContextFactory();
                } else if (factories.size() == 1) {
                    factory = factories.get(factories.lastKey());
                } else {
                    final StringBuilder sb = new StringBuilder("Multiple logging implementations found: \n");
                    for (final Map.Entry<Integer, LoggerContextFactory> entry : factories.entrySet()) {
                        sb.append("Factory: ").append(entry.getValue().getClass().getName());
                        sb.append(", Weighting: ").append(entry.getKey()).append('\n');
                    }
                    factory = factories.get(factories.lastKey());
                    sb.append("Using factory: ").append(factory.getClass().getName());
                    LOGGER.warn(sb.toString());

                }
            } else {
                LOGGER.error("Log4j2 could not find a logging implementation. "
                        + "Please add log4j-core to the classpath. Using SimpleLogger to log to the console...");
                factory = new SimpleLoggerContextFactory();
            }
        }
    }

这段静态代码段主要分为下面的几个步骤:

  1. 首先根据特定配置文件的配置信息获取loggerContextFactory
  2. 如果没有找到对应的Factory的实现类则通过ProviderUtil中的getProviders()方法载入providers,随后通过provider的loadLoggerContextFactory方法载入LoggerContextFactory的实现类
  3. 如果provider中没有获取到LoggerContextFactory的实现类或provider为空,则使用SimpleLoggerContextFactory作为LoggerContextFactory。

根据配置文件载入LoggerContextFactory

// Shortcut binding to force a specific logging implementation.
        final PropertiesUtil managerProps = PropertiesUtil.getProperties();
        final String factoryClassName = managerProps.getStringProperty(FACTORY_PROPERTY_NAME);
        if (factoryClassName != null) {
            try {
                factory = LoaderUtil.newCheckedInstanceOf(factoryClassName, LoggerContextFactory.class);
            } catch (final ClassNotFoundException cnfe) {
                LOGGER.error("Unable to locate configured LoggerContextFactory {}", factoryClassName);
            } catch (final Exception ex) {
                LOGGER.error("Unable to create configured LoggerContextFactory {}", factoryClassName, ex);
            }
        }

在这段逻辑中,LogManager优先通过配置文件”log4j2.component.properties”通过配置项”log4j2.loggerContextFactory”来获取LoggerContextFactory,如果用户做了对应的配置,通过newCheckedInstanceOf方法实例化LoggerContextFactory的对象,最终的实现方式为:

public static <T> T newInstanceOf(final Class<T> clazz)
            throws InstantiationException, IllegalAccessException, InvocationTargetException {
        try {
            return clazz.getConstructor().newInstance();
        } catch (final NoSuchMethodException ignored) {
            // FIXME: looking at the code for Class.newInstance(), this seems to do the same thing as above
            return clazz.newInstance();
        }
    }

在默认情况下,不存在初始的默认配置文件log4j2.component.properties,因此需要从其他途径获取LoggerContextFactory。

通过Provider实例化LoggerContextFactory对象

代码:

if (factory == null) {
            final SortedMap<Integer, LoggerContextFactory> factories = new TreeMap<>();
            // note that the following initial call to ProviderUtil may block until a Provider has been installed when
            // running in an OSGi environment
            if (ProviderUtil.hasProviders()) {
                for (final Provider provider : ProviderUtil.getProviders()) {
                    final Class<? extends LoggerContextFactory> factoryClass = provider.loadLoggerContextFactory();
                    if (factoryClass != null) {
                        try {
                            factories.put(provider.getPriority(), factoryClass.newInstance());
                        } catch (final Exception e) {
                            LOGGER.error("Unable to create class {} specified in provider URL {}", factoryClass.getName(), provider
                                    .getUrl(), e);
                        }
                    }
                }

                if (factories.isEmpty()) {
                    LOGGER.error("Log4j2 could not find a logging implementation. "
                            + "Please add log4j-core to the classpath. Using SimpleLogger to log to the console...");
                    factory = new SimpleLoggerContextFactory();
                } else if (factories.size() == 1) {
                    factory = factories.get(factories.lastKey());
                } else {
                    final StringBuilder sb = new StringBuilder("Multiple logging implementations found: \n");
                    for (final Map.Entry<Integer, LoggerContextFactory> entry : factories.entrySet()) {
                        sb.append("Factory: ").append(entry.getValue().getClass().getName());
                        sb.append(", Weighting: ").append(entry.getKey()).append('\n');
                    }
                    factory = factories.get(factories.lastKey());
                    sb.append("Using factory: ").append(factory.getClass().getName());
                    LOGGER.warn(sb.toString());

                }
            } else {
                LOGGER.error("Log4j2 could not find a logging implementation. "
                        + "Please add log4j-core to the classpath. Using SimpleLogger to log to the console...");
                factory = new SimpleLoggerContextFactory();
            }
        }

这里比较有意思的是hasProviders和getProviders都会通过线程安全的方式去懒加载ProviderUtil这个对象。跟进lazyInit方法:

protected static void lazyInit() {
        //noinspection DoubleCheckedLocking
        if (INSTANCE == null) {
            try {
                STARTUP_LOCK.lockInterruptibly();
                if (INSTANCE == null) {
                    INSTANCE = new ProviderUtil();
                }
            } catch (final InterruptedException e) {
                LOGGER.fatal("Interrupted before Log4j Providers could be loaded.", e);
                Thread.currentThread().interrupt();
            } finally {
                STARTUP_LOCK.unlock();
            }
        }
    }

再看构造方法:

private ProviderUtil() {
        for (final LoaderUtil.UrlResource resource : LoaderUtil.findUrlResources(PROVIDER_RESOURCE)) {
            loadProvider(resource.getUrl(), resource.getClassLoader());
        }
    }

这里的懒加载其实就是懒加载Provider对象。在创建新的providerUtil实例的过程中就会直接实例化provider对象,其过程是先通过getClassLoaders方法获取provider的类加载器,然后通过loadProviders(classLoader);加载类。在providerUtil实例化的最后,会统一查找”META-INF/log4j-provider.properties”文件中对应的provider的url,会考虑从远程加载provider。而loadProviders方法就是在ProviderUtil的PROVIDERS列表中添加对一个的provider。可以看到默认的provider是org.apache.logging.log4j.core.impl.Log4jContextFactory

LoggerContextFactory = org.apache.logging.log4j.core.impl.Log4jContextFactory
Log4jAPIVersion = 2.1.0
FactoryPriority= 10

很有意思的是这里懒加载加上了锁,而且使用的是
lockInterruptibly这个方法。lockInterruptibly和lock的区别如下:

lock 与 lockInterruptibly比较区别在于:

lock 优先考虑获取锁,待获取锁成功后,才响应中断。
lockInterruptibly 优先考虑响应中断,而不是响应锁的普通获取或重入获取。
ReentrantLock.lockInterruptibly允许在等待时由其它线程调用等待线程的
Thread.interrupt 方法来中断等待线程的等待而直接返回,这时不用获取锁,而会抛出一个InterruptedException。 ReentrantLock.lock方法不允许Thread.interrupt中断,即使检测到Thread.isInterrupted,一样会继续尝试获取锁,失败则继续休眠。只是在最后获取锁成功后再把当前线程置为interrupted状态,然后再中断线程。

上面有一句注释值得注意:

/**
     * Guards the ProviderUtil singleton instance from lazy initialization. This is primarily used for OSGi support.
     *
     * @since 2.1
     */
    protected static final Lock STARTUP_LOCK = new ReentrantLock();
    // STARTUP_LOCK guards INSTANCE for lazy initialization; this allows the OSGi Activator to pause the startup and
    // wait for a Provider to be installed. See LOG4J2-373
    private static volatile ProviderUtil INSTANCE;

原来这里是为了让osgi可以阻止启动。
再回到logManager:
可以看到在加载完Provider之后,会做factory的绑定:

if (factories.isEmpty()) {
                    LOGGER.error("Log4j2 could not find a logging implementation. "
                            + "Please add log4j-core to the classpath. Using SimpleLogger to log to the console...");
                    factory = new SimpleLoggerContextFactory();
                } else if (factories.size() == 1) {
                    factory = factories.get(factories.lastKey());
                } else {
                    final StringBuilder sb = new StringBuilder("Multiple logging implementations found: \n");
                    for (final Map.Entry<Integer, LoggerContextFactory> entry : factories.entrySet()) {
                        sb.append("Factory: ").append(entry.getValue().getClass().getName());
                        sb.append(", Weighting: ").append(entry.getKey()).append('\n');
                    }
                    factory = factories.get(factories.lastKey());
                    sb.append("Using factory: ").append(factory.getClass().getName());
                    LOGGER.warn(sb.toString());

                }

到这里,logmanager的启动流程就结束了。

配置

在不使用slf4j的情况下,我们获取logger的方式是这样的:

Logger logger = logManager.getLogger(xx.class)

跟进getLogger方法:

    public static Logger getLogger(final Class<?> clazz) {
        final Class<?> cls = callerClass(clazz);
        return getContext(cls.getClassLoader(), false).getLogger(toLoggerName(cls));
    }

这里有一个getContext方法,跟进,

public static LoggerContext getContext(final ClassLoader loader, final boolean currentContext) {
        try {
            return factory.getContext(FQCN, loader, null, currentContext);
        } catch (final IllegalStateException ex) {
            LOGGER.warn(ex.getMessage() + " Using SimpleLogger");
            return new SimpleLoggerContextFactory().getContext(FQCN, loader, null, currentContext);
        }
    }

上文提到factory的具体实现是Log4jContextFactory,跟进getContext
方法:

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();
        }
        return ctx;
    }

直接看start:

public void start() {
        LOGGER.debug("Starting LoggerContext[name={}, {}]...", getName(), this);
        if (PropertiesUtil.getProperties().getBooleanProperty("log4j.LoggerContext.stacktrace.on.start", false)) {
            LOGGER.debug("Stack trace to locate invoker",
                    new Exception("Not a real error, showing stack trace to locate invoker"));
        }
        if (configLock.tryLock()) {
            try {
                if (this.isInitialized() || this.isStopped()) {
                    this.setStarting();
                    reconfigure();
                    if (this.configuration.isShutdownHookEnabled()) {
                        setUpShutdownHook();
                    }
                    this.setStarted();
                }
            } finally {
                configLock.unlock();
            }
        }
        LOGGER.debug("LoggerContext[name={}, {}] started OK.", getName(), this);
    }

发现其中的核心方法是reconfigure方法,继续跟进:

private void reconfigure(final URI configURI) {
        final ClassLoader cl = ClassLoader.class.isInstance(externalContext) ? (ClassLoader) externalContext : null;
        LOGGER.debug("Reconfiguration started for context[name={}] at URI {} ({}) with optional ClassLoader: {}",
                contextName, configURI, this, cl);
        final Configuration instance = ConfigurationFactory.getInstance().getConfiguration(this, contextName, configURI, cl);
        if (instance == null) {
            LOGGER.error("Reconfiguration failed: No configuration found for '{}' at '{}' in '{}'", contextName, configURI, cl);
        } else {
            setConfiguration(instance);
            /*
             * instance.start(); Configuration old = setConfiguration(instance); updateLoggers(); if (old != null) {
             * old.stop(); }
             */
            final String location = configuration == null ? "?" : String.valueOf(configuration.getConfigurationSource());
            LOGGER.debug("Reconfiguration complete for context[name={}] at URI {} ({}) with optional ClassLoader: {}",
                    contextName, location, this, cl);
        }
    }

可以看到每一个configuration都是从ConfigurationFactory拿出来的,我们先看看这个类的getInstance看看:

public static ConfigurationFactory getInstance() {
        // volatile works in Java 1.6+, so double-checked locking also works properly
        //noinspection DoubleCheckedLocking
        if (factories == null) {
            LOCK.lock();
            try {
                if (factories == null) {
                    final List<ConfigurationFactory> list = new ArrayList<ConfigurationFactory>();
                    final String factoryClass = PropertiesUtil.getProperties().getStringProperty(CONFIGURATION_FACTORY_PROPERTY);
                    if (factoryClass != null) {
                        addFactory(list, factoryClass);
                    }
                    final PluginManager manager = new PluginManager(CATEGORY);
                    manager.collectPlugins();
                    final Map<String, PluginType<?>> plugins = manager.getPlugins();
                    final List<Class<? extends ConfigurationFactory>> ordered =
                        new ArrayList<Class<? extends ConfigurationFactory>>(plugins.size());
                    for (final PluginType<?> type : plugins.values()) {
                        try {
                            ordered.add(type.getPluginClass().asSubclass(ConfigurationFactory.class));
                        } catch (final Exception ex) {
                            LOGGER.warn("Unable to add class {}", type.getPluginClass(), ex);
                        }
                    }
                    Collections.sort(ordered, OrderComparator.getInstance());
                    for (final Class<? extends ConfigurationFactory> clazz : ordered) {
                        addFactory(list, clazz);
                    }
                    // see above comments about double-checked locking
                    //noinspection NonThreadSafeLazyInitialization
                    factories = Collections.unmodifiableList(list);
                }
            } finally {
                LOCK.unlock();
            }
        }

        LOGGER.debug("Using configurationFactory {}", configFactory);
        return configFactory;
    }

这里可以看到ConfigurationFactory中利用了PluginManager来进行初始化,PluginManager会将ConfigurationFactory的子类加载进来,默认使用的XmlConfigurationFactory,JsonConfigurationFactory,YamlConfigurationFactory这三个子类,这里插件化加载暂时按下不表。
回到reconfigure这个方法,我们看到获取ConfigurationFactory实例之后会去调用getConfiguration方法:

public Configuration getConfiguration(final String name, final URI configLocation, final ClassLoader loader) {
        if (!isActive()) {
            return null;
        }
        if (loader == null) {
            return getConfiguration(name, configLocation);
        }
        if (isClassLoaderUri(configLocation)) {
            final String path = extractClassLoaderUriPath(configLocation);
            final ConfigurationSource source = getInputFromResource(path, loader);
            if (source != null) {
                final Configuration configuration = getConfiguration(source);
                if (configuration != null) {
                    return configuration;
                }
            }
        }
        return getConfiguration(name, configLocation);
    }

跟进getConfiguration,这里值得注意的是有很多个getConfiguration,注意甄别,如果不确定的话可以通过debug的方式来确定。

public Configuration getConfiguration(final String name, final URI configLocation) {

            if (configLocation == null) {
                final String config = this.substitutor.replace(
                    PropertiesUtil.getProperties().getStringProperty(CONFIGURATION_FILE_PROPERTY));
                if (config != null) {
                    ConfigurationSource source = null;
                    try {
                        source = getInputFromUri(FileUtils.getCorrectedFilePathUri(config));
                    } catch (final Exception ex) {
                        // Ignore the error and try as a String.
                        LOGGER.catching(Level.DEBUG, ex);
                    }
                    if (source == null) {
                        final ClassLoader loader = LoaderUtil.getThreadContextClassLoader();
                        source = getInputFromString(config, loader);
                    }
                    if (source != null) {
                        for (final ConfigurationFactory factory : factories) {
                            final String[] types = factory.getSupportedTypes();
                            if (types != null) {
                                for (final String type : types) {
                                    if (type.equals("*") || config.endsWith(type)) {
                                        final Configuration c = factory.getConfiguration(source);
                                        if (c != null) {
                                            return c;
                                        }
                                    }
                                }
                            }
                        }
                    }
                }
            } else {
                for (final ConfigurationFactory factory : factories) {
                    final String[] types = factory.getSupportedTypes();
                    if (types != null) {
                        for (final String type : types) {
                            if (type.equals("*") || configLocation.toString().endsWith(type)) {
                                final Configuration config = factory.getConfiguration(name, configLocation);
                                if (config != null) {
                                    return config;
                                }
                            }
                        }
                    }
                }
            }

            Configuration config = getConfiguration(true, name);
            if (config == null) {
                config = getConfiguration(true, null);
                if (config == null) {
                    config = getConfiguration(false, name);
                    if (config == null) {
                        config = getConfiguration(false, null);
                    }
                }
            }
            if (config != null) {
                return config;
            }
            LOGGER.error("No log4j2 configuration file found. Using default configuration: logging only errors to the console.");
            return new DefaultConfiguration();
        }

这里就会根据之前加载进来的factory进行配置的获取,具体的不再解析。
回到reconfigure,之后的步骤就是setConfiguration,入参就是刚才获取的config

private synchronized Configuration setConfiguration(final Configuration config) {
        Assert.requireNonNull(config, "No Configuration was provided");
        final Configuration prev = this.config;
        config.addListener(this);
        final ConcurrentMap<String, String> map = config.getComponent(Configuration.CONTEXT_PROPERTIES);

        try { // LOG4J2-719 network access may throw android.os.NetworkOnMainThreadException
            map.putIfAbsent("hostName", NetUtils.getLocalHostname());
        } catch (final Exception ex) {
            LOGGER.debug("Ignoring {}, setting hostName to 'unknown'", ex.toString());
            map.putIfAbsent("hostName", "unknown");
        }
        map.putIfAbsent("contextName", name);
        config.start();
        this.config = config;
        updateLoggers();
        if (prev != null) {
            prev.removeListener(this);
            prev.stop();
        }

        firePropertyChangeEvent(new PropertyChangeEvent(this, PROPERTY_CONFIG, prev, config));

        try {
            Server.reregisterMBeansAfterReconfigure();
        } catch (final Throwable t) {
            // LOG4J2-716: Android has no java.lang.management
            LOGGER.error("Could not reconfigure JMX", t);
        }
        return prev;
    }

这个方法最重要的步骤就是config.start,这才是真正做配置解析的

public void start() {
        LOGGER.debug("Starting configuration {}", this);
        this.setStarting();
        pluginManager.collectPlugins(pluginPackages);
        final PluginManager levelPlugins = new PluginManager(Level.CATEGORY);
        levelPlugins.collectPlugins(pluginPackages);
        final Map<String, PluginType<?>> plugins = levelPlugins.getPlugins();
        if (plugins != null) {
            for (final PluginType<?> type : plugins.values()) {
                try {
                    // Cause the class to be initialized if it isn't already.
                    Loader.initializeClass(type.getPluginClass().getName(), type.getPluginClass().getClassLoader());
                } catch (final Exception e) {
                    LOGGER.error("Unable to initialize {} due to {}", type.getPluginClass().getName(), e.getClass()
                            .getSimpleName(), e);
                }
            }
        }
        setup();
        setupAdvertisement();
        doConfigure();
        final Set<LoggerConfig> alreadyStarted = new HashSet<LoggerConfig>();
        for (final LoggerConfig logger : loggers.values()) {
            logger.start();
            alreadyStarted.add(logger);
        }
        for (final Appender appender : appenders.values()) {
            appender.start();
        }
        if (!alreadyStarted.contains(root)) { // LOG4J2-392
            root.start(); // LOG4J2-336
        }
        super.start();
        LOGGER.debug("Started configuration {} OK.", this);
    }

这里面有如下步骤:

  1. 获取日志等级的插件
  2. 初始化
  3. 初始化Advertiser
  4. 配置

先看一下初始化,也就是setup这个方法,setup是一个需要被复写的方法,我们以XMLConfiguration作为例子,

@Override
    public void setup() {
        if (rootElement == null) {
            LOGGER.error("No logging configuration");
            return;
        }
        constructHierarchy(rootNode, rootElement);
        if (status.size() > 0) {
            for (final Status s : status) {
                LOGGER.error("Error processing element {}: {}", s.name, s.errorType);
            }
            return;
        }
        rootElement = null;
    }

发现这里面有一个比较重要的方法constructHierarchy,跟进:

private void constructHierarchy(final Node node, final Element element) {
        processAttributes(node, element);
        final StringBuilder buffer = new StringBuilder();
        final NodeList list = element.getChildNodes();
        final List<Node> children = node.getChildren();
        for (int i = 0; i < list.getLength(); i++) {
            final org.w3c.dom.Node w3cNode = list.item(i);
            if (w3cNode instanceof Element) {
                final Element child = (Element) w3cNode;
                final String name = getType(child);
                final PluginType<?> type = pluginManager.getPluginType(name);
                final Node childNode = new Node(node, name, type);
                constructHierarchy(childNode, child);
                if (type == null) {
                    final String value = childNode.getValue();
                    if (!childNode.hasChildren() && value != null) {
                        node.getAttributes().put(name, value);
                    } else {
                        status.add(new Status(name, element, ErrorType.CLASS_NOT_FOUND));
                    }
                } else {
                    children.add(childNode);
                }
            } else if (w3cNode instanceof Text) {
                final Text data = (Text) w3cNode;
                buffer.append(data.getData());
            }
        }

        final String text = buffer.toString().trim();
        if (text.length() > 0 || (!node.hasChildren() && !node.isRoot())) {
            node.setValue(text);
        }
    }

发现这个就是一个树遍历的过程。诚然,配置文件是以xml的形式给出的,xml的结构就是一个树形结构。回到start方法,跟进doConfiguration:

protected void doConfigure() {
        if (rootNode.hasChildren() && rootNode.getChildren().get(0).getName().equalsIgnoreCase("Properties")) {
            final Node first = rootNode.getChildren().get(0);
            createConfiguration(first, null);
            if (first.getObject() != null) {
                subst.setVariableResolver((StrLookup) first.getObject());
            }
        } else {
            final Map<String, String> map = this.getComponent(CONTEXT_PROPERTIES);
            final StrLookup lookup = map == null ? null : new MapLookup(map);
            subst.setVariableResolver(new Interpolator(lookup, pluginPackages));
        }

        boolean setLoggers = false;
        boolean setRoot = false;
        for (final Node child : rootNode.getChildren()) {
            if (child.getName().equalsIgnoreCase("Properties")) {
                if (tempLookup == subst.getVariableResolver()) {
                    LOGGER.error("Properties declaration must be the first element in the configuration");
                }
                continue;
            }
            createConfiguration(child, null);
            if (child.getObject() == null) {
                continue;
            }
            if (child.getName().equalsIgnoreCase("Appenders")) {
                appenders = child.getObject();
            } else if (child.isInstanceOf(Filter.class)) {
                addFilter(child.getObject(Filter.class));
            } else if (child.getName().equalsIgnoreCase("Loggers")) {
                final Loggers l = child.getObject();
                loggers = l.getMap();
                setLoggers = true;
                if (l.getRoot() != null) {
                    root = l.getRoot();
                    setRoot = true;
                }
            } else if (child.getName().equalsIgnoreCase("CustomLevels")) {
                customLevels = child.getObject(CustomLevels.class).getCustomLevels();
            } else if (child.isInstanceOf(CustomLevelConfig.class)) {
                final List<CustomLevelConfig> copy = new ArrayList<CustomLevelConfig>(customLevels);
                copy.add(child.getObject(CustomLevelConfig.class));
                customLevels = copy;
            } else {
                LOGGER.error("Unknown object \"{}\" of type {} is ignored.", child.getName(),
                        child.getObject().getClass().getName());
            }
        }

        if (!setLoggers) {
            LOGGER.warn("No Loggers were configured, using default. Is the Loggers element missing?");
            setToDefault();
            return;
        } else if (!setRoot) {
            LOGGER.warn("No Root logger was configured, creating default ERROR-level Root logger with Console appender");
            setToDefault();
            // return; // LOG4J2-219: creating default root=ok, but don't exclude configured Loggers
        }

        for (final Map.Entry<String, LoggerConfig> entry : loggers.entrySet()) {
            final LoggerConfig l = entry.getValue();
            for (final AppenderRef ref : l.getAppenderRefs()) {
                final Appender app = appenders.get(ref.getRef());
                if (app != null) {
                    l.addAppender(app, ref.getLevel(), ref.getFilter());
                } else {
                    LOGGER.error("Unable to locate appender {} for logger {}", ref.getRef(), l.getName());
                }
            }

        }

        setParents();
    }

发现就是对刚刚获取的configuration进行解析,然后塞进正确的地方。回到start方法,可以看到昨晚配置之后就是开启logger和appender了。

异步

AsyncAppender

log4j2突出于其他日志的优势,异步日志实现。我们先从日志打印看进去。找到Logger,随便找一个log日志的方法。

public void debug(final Marker marker, final Message msg) {
        logIfEnabled(FQCN, Level.DEBUG, marker, msg, msg != null ? msg.getThrowable() : null);
    }

一路跟进

@PerformanceSensitive
    // NOTE: This is a hot method. Current implementation compiles to 29 bytes of byte code.
    // This is within the 35 byte MaxInlineSize threshold. Modify with care!
    private void logMessageTrackRecursion(final String fqcn,
                                          final Level level,
                                          final Marker marker,
                                          final Message msg,
                                          final Throwable throwable) {
        try {
            incrementRecursionDepth(); // LOG4J2-1518, LOG4J2-2031
            tryLogMessage(fqcn, level, marker, msg, throwable);
        } finally {
            decrementRecursionDepth();
        }
    }

可以看出这个在打日志之前做了调用次数的记录。跟进tryLogMessage,

@PerformanceSensitive
    // NOTE: This is a hot method. Current implementation compiles to 26 bytes of byte code.
    // This is within the 35 byte MaxInlineSize threshold. Modify with care!
    private void tryLogMessage(final String fqcn,
                               final Level level,
                               final Marker marker,
                               final Message msg,
                               final Throwable throwable) {
        try {
            logMessage(fqcn, level, marker, msg, throwable);
        } catch (final Exception e) {
            // LOG4J2-1990 Log4j2 suppresses all exceptions that occur once application called the logger
            handleLogMessageException(e, fqcn, msg);
        }
    }

继续跟进:

@Override
    public void logMessage(final String fqcn, final Level level, final Marker marker, final Message message,
            final Throwable t) {
        final Message msg = message == null ? new SimpleMessage(Strings.EMPTY) : message;
        final ReliabilityStrategy strategy = privateConfig.loggerConfig.getReliabilityStrategy();
        strategy.log(this, getName(), fqcn, marker, level, msg, t);
    }

这里可以看到在实际打日志的时候,会从config中获取打日志的策略,跟踪ReliabilityStrategy的创建,发现默认的实现类为DefaultReliabilityStrategy,跟进看实际打日志的方法

@Override
    public void log(final Supplier<LoggerConfig> reconfigured, final String loggerName, final String fqcn, final Marker marker, final Level level,
            final Message data, final Throwable t) {
        loggerConfig.log(loggerName, fqcn, marker, level, data, t);
    }

这里实际打日志的方法居然是交给一个config去实现的。。。感觉有点奇怪。。跟进看看

@PerformanceSensitive("allocation")
    public void log(final String loggerName, final String fqcn, final Marker marker, final Level level,
            final Message data, final Throwable t) {
        List<Property> props = null;
        if (!propertiesRequireLookup) {
            props = properties;
        } else {
            if (properties != null) {
                props = new ArrayList<>(properties.size());
                final LogEvent event = Log4jLogEvent.newBuilder()
                        .setMessage(data)
                        .setMarker(marker)
                        .setLevel(level)
                        .setLoggerName(loggerName)
                        .setLoggerFqcn(fqcn)
                        .setThrown(t)
                        .build();
                for (int i = 0; i < properties.size(); i++) {
                    final Property prop = properties.get(i);
                    final String value = prop.isValueNeedsLookup() // since LOG4J2-1575
                            ? config.getStrSubstitutor().replace(event, prop.getValue()) //
                            : prop.getValue();
                    props.add(Property.createProperty(prop.getName(), value));
                }
            }
        }
        final LogEvent logEvent = logEventFactory.createEvent(loggerName, marker, fqcn, level, data, props, t);
        try {
            log(logEvent, LoggerConfigPredicate.ALL);
        } finally {
            // LOG4J2-1583 prevent scrambled logs when logging calls are nested (logging in toString())
            ReusableLogEventFactory.release(logEvent);
        }
    }

可以清楚的看到try之前是在创建LogEvent,try里面做的才是真正的log(好tm累),一路跟进。

private void processLogEvent(final LogEvent event, LoggerConfigPredicate predicate) {
        event.setIncludeLocation(isIncludeLocation());
        if (predicate.allow(this)) {
            callAppenders(event);
        }
        logParent(event, predicate);
    }

接下来就是callAppender了,我们直接开始看AsyncAppender的append方法:

/**
     * Actual writing occurs here.
     *
     * @param logEvent The LogEvent.
     */
    @Override
    public void append(final LogEvent logEvent) {
        if (!isStarted()) {
            throw new IllegalStateException("AsyncAppender " + getName() + " is not active");
        }
        final Log4jLogEvent memento = Log4jLogEvent.createMemento(logEvent, includeLocation);
        InternalAsyncUtil.makeMessageImmutable(logEvent.getMessage());
        if (!transfer(memento)) {
            if (blocking) {
                if (AbstractLogger.getRecursionDepth() > 1) { // LOG4J2-1518, LOG4J2-2031
                    // If queue is full AND we are in a recursive call, call appender directly to prevent deadlock
                    AsyncQueueFullMessageUtil.logWarningToStatusLogger();
                    logMessageInCurrentThread(logEvent);
                } else {
                    // delegate to the event router (which may discard, enqueue and block, or log in current thread)
                    final EventRoute route = asyncQueueFullPolicy.getRoute(thread.getId(), memento.getLevel());
                    route.logMessage(this, memento);
                }
            } else {
                error("Appender " + getName() + " is unable to write primary appenders. queue is full");
                logToErrorAppenderIfNecessary(false, memento);
            }
        }
    }

这里主要的步骤就是:

  1. 生成logEvent
  2. 将logEvent放入BlockingQueue,就是transfer方法
  3. 如果BlockingQueue满了则启用相应的策略

同样的,这里也有一个线程用来做异步消费的事情

private class AsyncThread extends Log4jThread {

        private volatile boolean shutdown = false;
        private final List<AppenderControl> appenders;
        private final BlockingQueue<LogEvent> queue;

        public AsyncThread(final List<AppenderControl> appenders, final BlockingQueue<LogEvent> queue) {
            super("AsyncAppender-" + THREAD_SEQUENCE.getAndIncrement());
            this.appenders = appenders;
            this.queue = queue;
            setDaemon(true);
        }

        @Override
        public void run() {
            while (!shutdown) {
                LogEvent event;
                try {
                    event = queue.take();
                    if (event == SHUTDOWN_LOG_EVENT) {
                        shutdown = true;
                        continue;
                    }
                } catch (final InterruptedException ex) {
                    break; // LOG4J2-830
                }
                event.setEndOfBatch(queue.isEmpty());
                final boolean success = callAppenders(event);
                if (!success && errorAppender != null) {
                    try {
                        errorAppender.callAppender(event);
                    } catch (final Exception ex) {
                        // Silently accept the error.
                    }
                }
            }
            // Process any remaining items in the queue.
            LOGGER.trace("AsyncAppender.AsyncThread shutting down. Processing remaining {} queue events.",
                queue.size());
            int count = 0;
            int ignored = 0;
            while (!queue.isEmpty()) {
                try {
                    final LogEvent event = queue.take();
                    if (event instanceof Log4jLogEvent) {
                        final Log4jLogEvent logEvent = (Log4jLogEvent) event;
                        logEvent.setEndOfBatch(queue.isEmpty());
                        callAppenders(logEvent);
                        count++;
                    } else {
                        ignored++;
                        LOGGER.trace("Ignoring event of class {}", event.getClass().getName());
                    }
                } catch (final InterruptedException ex) {
                    // May have been interrupted to shut down.
                    // Here we ignore interrupts and try to process all remaining events.
                }
            }
            LOGGER.trace("AsyncAppender.AsyncThread stopped. Queue has {} events remaining. "
                + "Processed {} and ignored {} events since shutdown started.", queue.size(), count, ignored);
        }

        /**
         * Calls {@link AppenderControl#callAppender(LogEvent) callAppender} on all registered {@code AppenderControl}
         * objects, and returns {@code true} if at least one appender call was successful, {@code false} otherwise. Any
         * exceptions are silently ignored.
         *
         * @param event the event to forward to the registered appenders
         * @return {@code true} if at least one appender call succeeded, {@code false} otherwise
         */
        boolean callAppenders(final LogEvent event) {
            boolean success = false;
            for (final AppenderControl control : appenders) {
                try {
                    control.callAppender(event);
                    success = true;
                } catch (final Exception ex) {
                    // If no appender is successful the error appender will get it.
                }
            }
            return success;
        }

        public void shutdown() {
            shutdown = true;
            if (queue.isEmpty()) {
                queue.offer(SHUTDOWN_LOG_EVENT);
            }
            if (getState() == State.TIMED_WAITING || getState() == State.WAITING) {
                this.interrupt(); // LOG4J2-1422: if underlying appender is stuck in wait/sleep/join/park call
            }
        }
    }

直接看run方法:

  1. 阻塞获取logEvent
  2. 将logEvent分发出去
  3. 如果线程要退出了,将blockingQueue里面的event消费完在退出。

AsyncLogger

直接从AsyncLogger的logMessage看进去:

public void logMessage(final String fqcn, final Level level, final Marker marker, final Message message,
            final Throwable thrown) {

        if (loggerDisruptor.isUseThreadLocals()) {
            logWithThreadLocalTranslator(fqcn, level, marker, message, thrown);
        } else {
            // LOG4J2-1172: avoid storing non-JDK classes in ThreadLocals to avoid memory leaks in web apps
            logWithVarargTranslator(fqcn, level, marker, message, thrown);
        }
    }

跟进logWithThreadLocalTranslator,

private void logWithThreadLocalTranslator(final String fqcn, final Level level, final Marker marker,
            final Message message, final Throwable thrown) {
        // Implementation note: this method is tuned for performance. MODIFY WITH CARE!

        final RingBufferLogEventTranslator translator = getCachedTranslator();
        initTranslator(translator, fqcn, level, marker, message, thrown);
        initTranslatorThreadValues(translator);
        publish(translator);
    }

这里的逻辑很简单,就是将日志相关的信息转换成RingBufferLogEvent(RingBuffer是Disruptor的无所队列),然后将其发布到RingBuffer中。发布到RingBuffer中,那肯定也有消费逻辑。这时候有两种方式可以找到这个消费的逻辑。

  • 找disruptor被使用的地方,然后查看,但是这样做会很容易迷惑
  • 按照Log4j2的尿性,这种Logger都有对应的start方法,我们可以从start方法入手寻找

在start方法中,我们找到了一段代码:

final RingBufferLogEventHandler[] handlers = {new RingBufferLogEventHandler()};
        disruptor.handleEventsWith(handlers);

直接看看这个RingBufferLogEventHandler的实现:

public class RingBufferLogEventHandler implements
        SequenceReportingEventHandler<RingBufferLogEvent>, LifecycleAware {

    private static final int NOTIFY_PROGRESS_THRESHOLD = 50;
    private Sequence sequenceCallback;
    private int counter;
    private long threadId = -1;

    @Override
    public void setSequenceCallback(final Sequence sequenceCallback) {
        this.sequenceCallback = sequenceCallback;
    }

    @Override
    public void onEvent(final RingBufferLogEvent event, final long sequence,
            final boolean endOfBatch) throws Exception {
        event.execute(endOfBatch);
        event.clear();

        // notify the BatchEventProcessor that the sequence has progressed.
        // Without this callback the sequence would not be progressed
        // until the batch has completely finished.
        if (++counter > NOTIFY_PROGRESS_THRESHOLD) {
            sequenceCallback.set(sequence);
            counter = 0;
        }
    }

    /**
     * Returns the thread ID of the background consumer thread, or {@code -1} if the background thread has not started
     * yet.
     * @return the thread ID of the background consumer thread, or {@code -1}
     */
    public long getThreadId() {
        return threadId;
    }

    @Override
    public void onStart() {
        threadId = Thread.currentThread().getId();
    }

    @Override
    public void onShutdown() {
    }
}

顺着接口找上去,发现一个接口:

/**
 * Callback interface to be implemented for processing events as they become available in the {@link RingBuffer}
 *
 * @param <T> event implementation storing the data for sharing during exchange or parallel coordination of an event.
 * @see BatchEventProcessor#setExceptionHandler(ExceptionHandler) if you want to handle exceptions propagated out of the handler.
 */
public interface EventHandler<T>
{
    /**
     * Called when a publisher has published an event to the {@link RingBuffer}
     *
     * @param event      published to the {@link RingBuffer}
     * @param sequence   of the event being processed
     * @param endOfBatch flag to indicate if this is the last event in a batch from the {@link RingBuffer}
     * @throws Exception if the EventHandler would like the exception handled further up the chain.
     */
    void onEvent(T event, long sequence, boolean endOfBatch) throws Exception;
}

通过注释可以发现,这个onEvent就是处理逻辑,回到RingBufferLogEventHandler的onEvent方法,发现里面有一个execute方法,跟进:

public void execute(final boolean endOfBatch) {
        this.endOfBatch = endOfBatch;
        asyncLogger.actualAsyncLog(this);
    }

这个方法就是实际打日志了,AsyncLogger看起来还是比较简单的,只是使用了一个Disruptor。

插件化

之前在很多代码里面都可以看到

final PluginManager manager = new PluginManager(CATEGORY);
manager.collectPlugins(pluginPackages);

其实整个log4j2为了获得更好的扩展性,将自己的很多组件都做成了插件,然后在配置的时候去加载plugin。
跟进collectPlugins。

 public void collectPlugins(final List<String> packages) {
        final String categoryLowerCase = category.toLowerCase();
        final Map<String, PluginType<?>> newPlugins = new LinkedHashMap<>();

        // First, iterate the Log4j2Plugin.dat files found in the main CLASSPATH
        Map<String, List<PluginType<?>>> builtInPlugins = PluginRegistry.getInstance().loadFromMainClassLoader();
        if (builtInPlugins.isEmpty()) {
            // If we didn't find any plugins above, someone must have messed with the log4j-core.jar.
            // Search the standard package in the hopes we can find our core plugins.
            builtInPlugins = PluginRegistry.getInstance().loadFromPackage(LOG4J_PACKAGES);
        }
        mergeByName(newPlugins, builtInPlugins.get(categoryLowerCase));

        // Next, iterate any Log4j2Plugin.dat files from OSGi Bundles
        for (final Map<String, List<PluginType<?>>> pluginsByCategory : PluginRegistry.getInstance().getPluginsByCategoryByBundleId().values()) {
            mergeByName(newPlugins, pluginsByCategory.get(categoryLowerCase));
        }

        // Next iterate any packages passed to the static addPackage method.
        for (final String pkg : PACKAGES) {
            mergeByName(newPlugins, PluginRegistry.getInstance().loadFromPackage(pkg).get(categoryLowerCase));
        }
        // Finally iterate any packages provided in the configuration (note these can be changed at runtime).
        if (packages != null) {
            for (final String pkg : packages) {
                mergeByName(newPlugins, PluginRegistry.getInstance().loadFromPackage(pkg).get(categoryLowerCase));
            }
        }

        LOGGER.debug("PluginManager '{}' found {} plugins", category, newPlugins.size());

        plugins = newPlugins;
    }

处理逻辑如下:

  1. 从Log4j2Plugin.dat中加载所有的内置的plugin
  2. 然后将OSGi Bundles中的Log4j2Plugin.dat中的plugin加载进来
  3. 再加载传入的package路径中的plugin
  4. 最后加载配置中的plugin

逻辑还是比较简单的,但是我在看源码的时候发现了一个很有意思的东西,就是在加载log4j2 core插件的时候,也就是

PluginRegistry.getInstance().loadFromMainClassLoader()

这个方法,跟进到decodeCacheFiles:

private Map<String, List<PluginType<?>>> decodeCacheFiles(final ClassLoader loader) {
        final long startTime = System.nanoTime();
        final PluginCache cache = new PluginCache();
        try {
            final Enumeration<URL> resources = loader.getResources(PluginProcessor.PLUGIN_CACHE_FILE);
            if (resources == null) {
                LOGGER.info("Plugin preloads not available from class loader {}", loader);
            } else {
                cache.loadCacheFiles(resources);
            }
        } catch (final IOException ioe) {
            LOGGER.warn("Unable to preload plugins", ioe);
        }
        final Map<String, List<PluginType<?>>> newPluginsByCategory = new HashMap<>();
        int pluginCount = 0;
        for (final Map.Entry<String, Map<String, PluginEntry>> outer : cache.getAllCategories().entrySet()) {
            final String categoryLowerCase = outer.getKey();
            final List<PluginType<?>> types = new ArrayList<>(outer.getValue().size());
            newPluginsByCategory.put(categoryLowerCase, types);
            for (final Map.Entry<String, PluginEntry> inner : outer.getValue().entrySet()) {
                final PluginEntry entry = inner.getValue();
                final String className = entry.getClassName();
                try {
                    final Class<?> clazz = loader.loadClass(className);
                    final PluginType<?> type = new PluginType<>(entry, clazz, entry.getName());
                    types.add(type);
                    ++pluginCount;
                } catch (final ClassNotFoundException e) {
                    LOGGER.info("Plugin [{}] could not be loaded due to missing classes.", className, e);
                } catch (final LinkageError e) {
                    LOGGER.info("Plugin [{}] could not be loaded due to linkage error.", className, e);
                }
            }
        }

        final long endTime = System.nanoTime();
        final DecimalFormat numFormat = new DecimalFormat("#0.000000");
        final double seconds = (endTime - startTime) * 1e-9;
        LOGGER.debug("Took {} seconds to load {} plugins from {}",
            numFormat.format(seconds), pluginCount, loader);
        return newPluginsByCategory;
    }

可以发现加载时候是从一个文件(PLUGIN_CACHE_FILE)获取所有要获取的plugin。看到这里的时候我有一个疑惑就是,为什么不用反射的方式直接去扫描,而是要从文件中加载进来,而且文件是写死的,很不容易扩展啊。然后我找了一下PLUGIN_CACHE_FILE这个静态变量的用处,发现了PluginProcessor这个类,这里用到了注解处理器

/**
 * Annotation processor for pre-scanning Log4j 2 plugins.
 */
@SupportedAnnotationTypes("org.apache.logging.log4j.core.config.plugins.*")
public class PluginProcessor extends AbstractProcessor {

    // TODO: this could be made more abstract to allow for compile-time and run-time plugin processing

    /**
     * The location of the plugin cache data file. This file is written to by this processor, and read from by
     * {@link org.apache.logging.log4j.core.config.plugins.util.PluginManager}.
     */
    public static final String PLUGIN_CACHE_FILE =
            "META-INF/org/apache/logging/log4j/core/config/plugins/Log4j2Plugins.dat";

    private final PluginCache pluginCache = new PluginCache();

    @Override
    public boolean process(final Set<? extends TypeElement> annotations, final RoundEnvironment roundEnv) {
        System.out.println("Processing annotations");
        try {
            final Set<? extends Element> elements = roundEnv.getElementsAnnotatedWith(Plugin.class);
            if (elements.isEmpty()) {
                System.out.println("No elements to process");
                return false;
            }
            collectPlugins(elements);
            writeCacheFile(elements.toArray(new Element[elements.size()]));
            System.out.println("Annotations processed");
            return true;
        } catch (final IOException e) {
            e.printStackTrace();
            error(e.getMessage());
            return false;
        } catch (final Exception ex) {
            ex.printStackTrace();
            error(ex.getMessage());
            return false;
        }
    }
}

(不太重要的方法省略)

我们可以看到在process方法中,PluginProcessor会先收集所有的Plugin,然后在写入文件。这样做的好处就是可以省去反射时候的开销。
然后我又看了一下Plugin这个注解,发现它的RetentionPolicy是RUNTIME,一般来说PluginProcessor是搭配RetentionPolicy.SOURCE,CLASS使用的,而且既然你把自己的Plugin扫描之后写在文件中了,RetentionPolicy就没有必要是RUNTIME了吧,这个是一个很奇怪的地方。

小结

总算是把Log4j2的代码看完了,发现它的设计理念很值得借鉴,为了灵活性,所有的东西都设计成插件式。互联网技术日益发展,各种中间件层出不穷,而作为工程师的我们更需要做的是去思考代码与代码之间的关系,毫无疑问的是,解耦是最具有美感的关系。

作者:netflix

原文链接

本文为云栖社区原创内容,未经允许不得转载。

未经允许不得转载:ITyet » 探索Java日志的奥秘:底层日志系统-log4j2

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