1、概述

目前java应用日志收集都是采用日志框架(slf4j、apache commons logging)+日志系统（log4j、log4j2、LogBack、JUL等）的方式。而针对在分布式环境需要实时分析统计的日志，一般采用apache flume、facebook scribe等分布式日志收集系统。

日志框架：提供日志调用的接口，实际的日志输出委托给日志系统实现。

• JCL(Jakarta Commons Logging)：比较流行的日志框架，很多框架都依赖JCL，例如Spring等。
• SLF4j：提供新的API，初衷是配合Logback使用，但同时兼容Log4j。

日志系统：负责输出日志

• Log4j：经典的一种日志解决方案。内部把日志系统抽象封装成Logger 、appender 、pattern 等实现。我们可以通过配置文件轻松的实现日志系统的管理和多样化配置。
• Log4j2：Log4j的2.0版本，对Log4j进行了优化，比如支持参数API、支持异步appender、插件式架构等
• Logback：Log4j的替代产品，需要配合日志框架SLF4j使用
• JUL(java.util.logging)：JDK提供的日志系统，较混乱，不常用

目前我们的应用大部分都是使用了SLF4j作为门面，然后搭配log4j或者log4j2日志系统。

下面将介绍slf4j + Log4j2 日志组件的引入、以及配置和使用

2、Maven依赖

</pre><pre name="code" class="html"><dependency>
            <groupId>org.slf4j</groupId>
            <artifactId>slf4j-api</artifactId>
            <version>1.7.13</version>
        </dependency>
        <dependency>
            <groupId>org.apache.logging.log4j</groupId>
            <artifactId>log4j-slf4j-impl</artifactId>
            <version>2.4.1</version>
        </dependency>
        <!--兼容log4j-->
        <dependency>
            <groupId>org.apache.logging.log4j</groupId>
            <artifactId>log4j-1.2-api</artifactId>
            <version>2.0</version>
        </dependency>
        <dependency>
            <groupId>org.apache.logging.log4j</groupId>
            <artifactId>log4j-api</artifactId>
            <version>2.4.1</version>
        </dependency>
        <dependency>
            <groupId>org.apache.logging.log4j</groupId>
            <artifactId>log4j-core</artifactId>
            <version>2.4.1</version>
        </dependency>
        <!--log4j2 异步appender需要-->
        <dependency>
            <groupId>com.lmax</groupId>
            <artifactId>disruptor</artifactId>
            <version>3.2.0</version>
        </dependency>

3、配置

• Appenders：也被称为Handlers，负责将日志事件记录到目标位置。在将日志事件输出之前，Appenders使用Layouts来对事件进行格式化处理。
• Layouts：也被称为Formatters，它负责对日志事件中的数据进行转换和格式化。Layouts决定了数据在一条日志记录中的最终形式。
• Loggers：Logger负责捕捉事件并将其发送给合适的Appender。

当Logger记录一个事件时，它将事件转发给适当的Appender。然后Appender使用Layout来对日志记录进行格式化，并将其发送给控制台、文件或者其它目标位置。另外，Filters可以让你进一步指定一个Appender是否可以应用在一条特定的日志记录上。在日志配置中，Filters并不是必需的，但可以让你更灵活地控制日志消息的流动。

3.1 Appender

3.1.1 ConsoleAppender

ConsoleAppender是最常用的Appenders之一，它只是将日志消息显示到控制台上。许多日志框架都将其作为默认的Appender，并且在基本的配置中进行预配置。例如，在Log4j中ConsoleAppender的配置参数如下所示。

<!--这个输出控制台的配置-->
        <Console name="Console" target="SYSTEM_OUT">
            <!--控制台只输出level及以上级别的信息（onMatch），其他的直接拒绝（onMismatch）-->
            <ThresholdFilter level="DEBUG" onMatch="ACCEPT" onMismatch="DENY"/>
            <!--这个都知道是输出日志的格式-->
            <PatternLayout pattern="%d{HH:mm:ss.SSS} %-5level %class{36} %L %M - %msg%xEx%n"/>
        </Console>

3.1.2 FileAppender

FileAppenders将日志记录写入到文件中，它负责打开、关闭文件，向文件中追加日志记录，并对文件进行加锁，以免数据被破坏或者覆盖。

在Log4j中，如果想创建一个FileAppender，需要指定目标文件的名字，写入方式是追加还是覆盖，以及是否需要在写入日志时对文件进行加锁：

 <File name="File" fileName="fileAppender.log" append="true" locking="true">
            <PatternLayout pattern="%d{HH:mm:ss.SSS} %-5level %class{36} %L %M - %msg%xEx%n"/>
        </File>

3.1.3 RollingFileAppender

RollingFileAppender跟FileAppender的基本用法一样。但RollingFileAppender可以设置log文件的size（单位：KB/MB/GB）上限、数量上限，当log文件超过设置的size上限，会自动被压缩。RollingFileAppender可以理解为滚动输出日志，如果log4j 2记录的日志达到上限，旧的日志将被删除，腾出的空间用于记录新的日志。

<!--这个会打印出所有的信息，每次大小超过size，则这size大小的日志会自动存入按年份-月份建立的文件夹下面并进行压缩，作为存档-->
        <RollingFile name="RollingFile1" fileName="logs/log1.log"
                     filePattern="logs/$${date:yyyy-MM}/log-%d{MM-dd-yyyy}-%i.log">
            <PatternLayout pattern="%d{yyyy-MM-dd 'at' HH:mm:ss z} %-5level %class{36} %L %M - %msg%xEx%n"/>
            <SizeBasedTriggeringPolicy size="100MB"/>
        </RollingFile>

3.1.5 其他appender

我们已经介绍了一些经常用到的Appenders，还有很多其它Appender。它们添加了新功能或者在其它的一些Appender基础上实现了新功能。例如，Log4j中的RollingFileAppender扩展了FileAppender，它可以在满足特定条件时自动创建新的日志文件；SMTPAppender会将日志内容以邮件的形式发送出去；FailoverAppender会在处理日志的过程中，如果一个或者多个Appender失败，自动切换到其他Appender上。

如果想了解其他可以参考：https://logging.apache.org/log4j/2.0/manual/appenders.html

3.2 Layouts

Layouts将日志记录的内容从一种数据形式转换成另外一种。日志框架为纯文本、HTML、syslog、XML、JSON、序列化以及其它日志提供了Layouts。

这里贴一篇文章简单介绍下我们常用的PatternLayout ：http://wiki.jikexueyuan.com/project/log4j/log4j-patternlayout.html

其他的layouts配置可以参考：https://logging.apache.org/log4j/2.0/manual/layouts.html

<PatternLayout pattern="%d{yyyy-MM-dd 'at' HH:mm:ss z} %-5level %class{36} %L %M - %msg%xEx%n"/>

3.3 Loggers

Logger负责捕捉事件并将其发送给合适的Appender，Logger之间是有继承关系的。总是存在一个rootLogger，即使没有显示配置也是存在的，并且默认输出级别为DEBUG，其它的logger都继承自这个rootLogger。

Log4J中的继承关系是通过名称可以看出来，如"A"、"A.B"、"A.B.C"，A.B继承A，A.B.C继承A.B，比较类似于包名。

<loggers>
        <logger name="com.sankuai" level="info" includeLocation="true" additivity="true">
            <appender-ref ref="RollingFile2"/>
            <appender-ref ref="RollingFile1"/>
        </logger>
		<logger name="com.sankuai.meituan" level="error" includeLocation="true" additivity="true">
            <appender-ref ref="RollingFile2"/>
            <appender-ref ref="RollingFile1"/>
        </logger>
        <!--建立一个默认的root的logger-->
        <root level="error">
            <appender-ref ref="Console"/>
            <appender-ref ref="RollingFile1"/>
        </root>
    </loggers>

additivity是 子Logger 是否继承 父Logger 的 输出源（appender） 的标志位。具体说，默认情况下 子Logger 会继承 父Logger 的appender，也就是说 子Logger 会在 父Logger 的appender里输出。若是additivity设为false，则 子Logger 只会在自己的appender里输出，而不会在 父Logger 的appender里输出。

3.4 日志级别

DEBUG ， INFO ，WARN ，ERROR四种，分别对应Logger类的四种方法

debug(Object message ) ;

info(Object message ) ;

warn(Object message ) ;

error(Object message ) ;

如果设置级别为INFO，则优先级大于等于INFO级别（如：INFO、WARN、ERROR）的日志信息将可以被输出,

小于该级别的如：DEBUG将不会被输出

4、Log4j2 AsyncLogger与AsyncAppender

先上图

第一张图可以看出Log4j2的asyncLogger的性能较使用asyncAppender和sync模式有非常大的提升，特别是线程越多的时候。

第二张图是将log4j2的异步日志机制和其他日志系统进行对比，log4j2的asyncLogger 性能也是很有优势。

这里主要涉及了两个概念AsyncLogger和AysncAppender，是支持异步的Logger和Appender，下面分别简要介绍下这两个概念。

4.1 AsyncAppender

AsyncAppender持有其他的配置了aysnc的appender引用列表(appender需要通过配置注册成异步的)，当其他的logger需要打日志的时候（logEvent事件），asyncAppender会接收logEvent，缓存到queue中，然后用单独的线程完成从queue中取logEvent打印到目的appender，这个逻辑比较简单，看下源码就能明白这个流程。ps. AsyncAppender是Log4j 和Log4j2 都有的，不是新东西，但从上面的性能对比上还是有一点点差异的，主要的原因是：（引用官方说法）Asynchronous
Appenders already existed in Log4j 1.x, but have been enhanced to flush to disk at the end of a batch (when the queue is empty).

关于AsyncAppender能提高性能的场景，可以看下这个篇文章。 http://littcai.iteye.com/blog/316605

如何配置一个AsyncAppender：

<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="warn" name="MyApp" packages="">
  <Appenders>
    <File name="MyFile" fileName="logs/app.log">
      <PatternLayout>
        <Pattern>%d %p %c{1.} [%t] %m%n</Pattern>
      </PatternLayout>
    </File>
    <Async name="Async">
      <AppenderRef ref="MyFile"/>
    </Async>
  </Appenders>
  <Loggers>
    <Root level="error">
      <AppenderRef ref="Async"/>
    </Root>
  </Loggers>
</Configuration>

@Plugin(name = "Async", category = "Core", elementType = "appender", printObject = true)
public final class AsyncAppender extends AbstractAppender {
    private static final long serialVersionUID = 1L;
    private static final int DEFAULT_QUEUE_SIZE = 128;
    private static final String SHUTDOWN = "Shutdown";
    private static final AtomicLong THREAD_SEQUENCE = new AtomicLong(1);
    private static ThreadLocal<Boolean> isAppenderThread = new ThreadLocal<>();
    private final BlockingQueue<Serializable> queue;
    private final int queueSize;
    private final boolean blocking;
    private final long shutdownTimeout;
    private final Configuration config;
    private final AppenderRef[] appenderRefs;
    private final String errorRef;
    private final boolean includeLocation;
    private AppenderControl errorAppender;
    private AsyncThread thread;
    private AsyncAppender(final String name, final Filter filter, final AppenderRef[] appenderRefs,
            final String errorRef, final int queueSize, final boolean blocking, final boolean ignoreExceptions,
            final long shutdownTimeout, final Configuration config, final boolean includeLocation) {
        super(name, filter, null, ignoreExceptions);
        this.queue = new ArrayBlockingQueue<>(queueSize);
        this.queueSize = queueSize;
        this.blocking = blocking;
        this.shutdownTimeout = shutdownTimeout;
        this.config = config;
        this.appenderRefs = appenderRefs;
        this.errorRef = errorRef;
        this.includeLocation = includeLocation;
    }
    @Override
    public void start() {
        final Map<String, Appender> map = config.getAppenders();
        final List<AppenderControl> appenders = new ArrayList<>();
        for (final AppenderRef appenderRef : appenderRefs) {
            final Appender appender = map.get(appenderRef.getRef());
            if (appender != null) {
                appenders.add(new AppenderControl(appender, appenderRef.getLevel(), appenderRef.getFilter()));
            } else {
                LOGGER.error("No appender named {} was configured", appenderRef);
            }
        }
        if (errorRef != null) {
            final Appender appender = map.get(errorRef);
            if (appender != null) {
                errorAppender = new AppenderControl(appender, null, null);
            } else {
                LOGGER.error("Unable to set up error Appender. No appender named {} was configured", errorRef);
            }
        }
        if (appenders.size() > 0) {
            thread = new AsyncThread(appenders, queue);
            thread.setName("AsyncAppender-" + getName());
        } else if (errorRef == null) {
            throw new ConfigurationException("No appenders are available for AsyncAppender " + getName());
        }
        thread.start();
        super.start();
    }
    @Override
    public void stop() {
        super.stop();
        LOGGER.trace("AsyncAppender stopping. Queue still has {} events.", queue.size());
        thread.shutdown();
        try {
            thread.join(shutdownTimeout);
        } catch (final InterruptedException ex) {
            LOGGER.warn("Interrupted while stopping AsyncAppender {}", getName());
        }
        LOGGER.trace("AsyncAppender stopped. Queue has {} events.", queue.size());
    }
    /**
     * Actual writing occurs here.
     *
     * @param logEvent The LogEvent.
     */
    @Override
    public void append(LogEvent logEvent) {
        if (!isStarted()) {
            throw new IllegalStateException("AsyncAppender " + getName() + " is not active");
        }
        if (!(logEvent instanceof Log4jLogEvent)) {
            if (!(logEvent instanceof RingBufferLogEvent)) {
                return; // only know how to Serialize Log4jLogEvents and RingBufferLogEvents
            }
            logEvent = ((RingBufferLogEvent) logEvent).createMemento();
        }
        logEvent.getMessage().getFormattedMessage(); // LOG4J2-763: ask message to freeze parameters
        final Log4jLogEvent coreEvent = (Log4jLogEvent) logEvent;
        boolean appendSuccessful = false;
        if (blocking) {
            if (isAppenderThread.get() == Boolean.TRUE && queue.remainingCapacity() == 0) {
                // LOG4J2-485: avoid deadlock that would result from trying
                // to add to a full queue from appender thread
                coreEvent.setEndOfBatch(false); // queue is definitely not empty!
                appendSuccessful = thread.callAppenders(coreEvent);
            } else {
                final Serializable serialized = Log4jLogEvent.serialize(coreEvent, includeLocation);
                try {
                    // wait for free slots in the queue
                    queue.put(serialized);
                    appendSuccessful = true;
                } catch (final InterruptedException e) {
                    // LOG4J2-1049: Some applications use Thread.interrupt() to send
                    // messages between application threads. This does not necessarily
                    // mean that the queue is full. To prevent dropping a log message,
                    // quickly try to offer the event to the queue again.
                    // (Yes, this means there is a possibility the same event is logged twice.)
                    //
                    // Finally, catching the InterruptedException means the
                    // interrupted flag has been cleared on the current thread.
                    // This may interfere with the application's expectation of
                    // being interrupted, so when we are done, we set the interrupted
                    // flag again.
                    appendSuccessful = queue.offer(serialized);
                    if (!appendSuccessful) {
                        LOGGER.warn("Interrupted while waiting for a free slot in the AsyncAppender LogEvent-queue {}",
                                getName());
                    }
                    // set the interrupted flag again.
                    Thread.currentThread().interrupt();
                }
            }
        } else {
            appendSuccessful = queue.offer(Log4jLogEvent.serialize(coreEvent, includeLocation));
            if (!appendSuccessful) {
                error("Appender " + getName() + " is unable to write primary appenders. queue is full");
            }
        }
        if (!appendSuccessful && errorAppender != null) {
            errorAppender.callAppender(coreEvent);
        }
    }
    /**
     * Create an AsyncAppender.
     *
     * @param appenderRefs The Appenders to reference.
     * @param errorRef An optional Appender to write to if the queue is full or other errors occur.
     * @param blocking True if the Appender should wait when the queue is full. The default is true.
     * @param shutdownTimeout How many milliseconds the Appender should wait to flush outstanding log events
     *                        in the queue on shutdown. The default is zero which means to wait forever.
     * @param size The size of the event queue. The default is 128.
     * @param name The name of the Appender.
     * @param includeLocation whether to include location information. The default is false.
     * @param filter The Filter or null.
     * @param config The Configuration.
     * @param ignoreExceptions If {@code "true"} (default) exceptions encountered when appending events are logged;
     *            otherwise they are propagated to the caller.
     * @return The AsyncAppender.
     */
    @PluginFactory
    public static AsyncAppender createAppender(@PluginElement("AppenderRef") final AppenderRef[] appenderRefs,
            @PluginAttribute("errorRef") @PluginAliases("error-ref") final String errorRef,
            @PluginAttribute(value = "blocking", defaultBoolean = true) final boolean blocking,
            @PluginAttribute(value = "shutdownTimeout", defaultLong = 0L) final long shutdownTimeout,
            @PluginAttribute(value = "bufferSize", defaultInt = DEFAULT_QUEUE_SIZE) final int size,
            @PluginAttribute("name") final String name,
            @PluginAttribute(value = "includeLocation", defaultBoolean = false) final boolean includeLocation,
            @PluginElement("Filter") final Filter filter, @PluginConfiguration final Configuration config,
            @PluginAttribute(value = "ignoreExceptions", defaultBoolean = true) final boolean ignoreExceptions) {
        if (name == null) {
            LOGGER.error("No name provided for AsyncAppender");
            return null;
        }
        if (appenderRefs == null) {
            LOGGER.error("No appender references provided to AsyncAppender {}", name);
        }
        return new AsyncAppender(name, filter, appenderRefs, errorRef, size, blocking, ignoreExceptions,
                shutdownTimeout, config, includeLocation);
    }
    /**
     * Thread that calls the Appenders.
     */
    private class AsyncThread extends Thread {
        private volatile boolean shutdown = false;
        private final List<AppenderControl> appenders;
        private final BlockingQueue<Serializable> queue;
        public AsyncThread(final List<AppenderControl> appenders, final BlockingQueue<Serializable> queue) {
            this.appenders = appenders;
            this.queue = queue;
            setDaemon(true);
            setName("AsyncAppenderThread" + THREAD_SEQUENCE.getAndIncrement());
        }
        @Override
        public void run() {
            isAppenderThread.set(Boolean.TRUE); // LOG4J2-485
            while (!shutdown) {
                Serializable s;
                try {
                    s = queue.take();
                    if (s != null && s instanceof String && SHUTDOWN.equals(s.toString())) {
                        shutdown = true;
                        continue;
                    }
                } catch (final InterruptedException ex) {
                    break; // LOG4J2-830
                }
                final Log4jLogEvent event = Log4jLogEvent.deserialize(s);
                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 Serializable s = queue.take();
                    if (Log4jLogEvent.canDeserialize(s)) {
                        final Log4jLogEvent event = Log4jLogEvent.deserialize(s);
                        event.setEndOfBatch(queue.isEmpty());
                        callAppenders(event);
                        count++;
                    } else {
                        ignored++;
                        LOGGER.trace("Ignoring event of class {}", s.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 Log4jLogEvent 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);
            }
        }
    }
    /**
     * Returns the names of the appenders that this asyncAppender delegates to as an array of Strings.
     *
     * @return the names of the sink appenders
     */
    public String[] getAppenderRefStrings() {
        final String[] result = new String[appenderRefs.length];
        for (int i = 0; i < result.length; i++) {
            result[i] = appenderRefs[i].getRef();
        }
        return result;
    }
    /**
     * Returns {@code true} if this AsyncAppender will take a snapshot of the stack with every log event to determine
     * the class and method where the logging call was made.
     *
     * @return {@code true} if location is included with every event, {@code false} otherwise
     */
    public boolean isIncludeLocation() {
        return includeLocation;
    }
    /**
     * Returns {@code true} if this AsyncAppender will block when the queue is full, or {@code false} if events are
     * dropped when the queue is full.
     *
     * @return whether this AsyncAppender will block or drop events when the queue is full.
     */
    public boolean isBlocking() {
        return blocking;
    }
    /**
     * Returns the name of the appender that any errors are logged to or {@code null}.
     *
     * @return the name of the appender that any errors are logged to or {@code null}
     */
    public String getErrorRef() {
        return errorRef;
    }
    public int getQueueCapacity() {
        return queueSize;
    }
    public int getQueueRemainingCapacity() {
        return queue.remainingCapacity();
    }
}

AsyncLogger是Log4j2引入的新特性，业务代码调用Logger.log的时候直接返回，而不需要等到appender输出到日志目的地后才返回。Log4j2的Asynclogger是通过LMAX Disruptor代替queue实现的异步（无锁的并发框架，http://ifeve.com/disruptor/，Disruptor简介），达到更高的并发和lower
latency。

4.2 AsyncLogger

尽管AsyncLogger 能够大幅度的提高性能，但是也会带来一些问题，下面是翻译官方的文档的Trade-offs：

Benefits

• Higher throughput，达到相对于sync logger的6-68倍的吞吐量
• Lower logging latency，latency是调用Logger.log直到return的时间，asyncLogger的latency比syncLogger以及基于queue的aysncAppender都要低，不仅平均latency低，而且99%、95%latency 也都低于后两者
• 降低极端大的日志量时候的延迟尖峰

Drawbacks

• Error handling, 如果在打印日志的时候出现错误，使用asyncLogger，业务是不知道异常的（可以通过配置ExceptionHandler处理异常），如果打印日志是业务逻辑的一部分，不建议使用asyncLogger
• 打印一些可变的内容的时候，使用asyncLogger 会出现问题。大部分时间，不需要担心这点，Log4j确保了类似于 logger.debug("My object is {}", myObject)，使用myObject在打印日志的时刻的版本打印（Log4j 所有打印都日志都是封装到Message的实现类里，存储在
  final String里），不管之后是否改变。但是log4j也支持一些了可变的Message，如 MapMessage and StructuredDataMessage ，这些如果在打印日志时候改变，就有问题了

全局配置异步Logger

配置所有Logger都为AsyncLogger，只需要增加disruptor包，然后配置一个system property，-DLog4jContextSelector=org.apache.logging.log4j.core.async.AsyncLoggerContextSelector，Log4j的配置文件不需要修改。

混合使用同步和异步Logger

单独配置某个logger为async的，通过<asyncRoot>或者<asyncLogger>

<Configuration status="WARN">
  <Appenders>
    <!-- Async Loggers will auto-flush in batches, so switch off immediateFlush. -->
    <RandomAccessFile name="RandomAccessFile" fileName="asyncWithLocation.log"
              immediateFlush="false" append="false">
      <PatternLayout>
        <Pattern>%d %p %class{1.} [%t] %location %m %ex%n</Pattern>
      </PatternLayout>
    </RandomAccessFile>
  </Appenders>
  <Loggers>
    <!-- pattern layout actually uses location, so we need to include it -->
    <AsyncLogger name="com.foo.Bar" level="trace" includeLocation="true">
      <AppenderRef ref="RandomAccessFile"/>
    </AsyncLogger>
    <Root level="info" includeLocation="true">
      <AppenderRef ref="RandomAccessFile"/>
    </Root>
  </Loggers>
</Configuration>

ps. location的问题

当layouts配置了输出%C or $class, %F or %file, %l or %location, %L or %line, %M or %method,或者HTML locationInfo,  log4j会获取location的一个快照，而这对于sync
和async的logger都是一个耗时的操作(官方文档上说syncLogger会慢1.3~5倍，async会慢4-20倍)，所以默认都是不会输出location信息，除非Logger配置了includeLocation="true"(官方文档这么说的，但是我测试的是默认是输出的，不管了，反正当日志出现慢的时候，可以考虑通过配置includeLocation控制是否输出location信息)。