1、概述
眼下java應用日志收集都是採用日志框架(slf4j、apache commons logging)+日志系統（log4j、log4j2、LogBack、JUL等）的方式。而針對在分布式環境須要實時分析統計的日志，一般採用apache flume、facebook scribe等分布式日志收集系統。
日志框架：提供日志調用的接口，實際的日志輸出托付給日志系統實現。
- JCL(Jakarta Commons Logging)：比較流行的日志框架，非常多框架都依賴JCL，比如Spring等。
- SLF4j：提供新的API，初衷是配合Logback使用，但同一時候兼容Log4j。
日志系統：負責輸出日志
- Log4j：經典的一種日志解決方式。內部把日志系統抽象封裝成Logger 、appender 、pattern 等實現。我們能夠通過配置文件輕松的實現日志系統的管理和多樣化配置。
- Log4j2：Log4j的2.0版本號。對Log4j進行了優化。比方支持參數API、支持異步appender、插件式架構等
- Logback：Log4j的替代產品。須要配合日志框架SLF4j使用
- JUL(java.util.logging)：JDK提供的日志系統。較混亂，不經常使用
眼下我們的應用大部分都是使用了SLF4j作為門面，然后搭配log4j或者log4j2日志系統。
以下將介紹slf4j + Log4j2 日志組件的引入、以及配置和使用
2、Maven依賴
</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 異步appender須要-->
<dependency>
<groupId>com.lmax</groupId>
<artifactId>disruptor</artifactId>
<version>3.2.0</version>
</dependency>
3、配置
- Appenders：也被稱為Handlers。負責將日志事件記錄到目標位置。在將日志事件輸出之前，Appenders使用Layouts來對事件進行格式化處理。
- Layouts：也被稱為Formatters，它負責對日志事件中的數據進行轉換和格式化。Layouts決定了數據在一條日志記錄中的終於形式。
- Loggers：Logger負責捕捉事件並將其發送給合適的Appender。
當Logger記錄一個事件時。它將事件轉發給適當的Appender。
然后Appender使用Layout來對日志記錄進行格式化，並將其發送給控制台、文件或者其他目標位置。另外。Filters能夠讓你進一步指定一個Appender能否夠應用在一條特定的日志記錄上。在日志配置中，Filters並非必需的，但能夠讓你更靈活地控制日志消息的流動。
3.1 Appender
3.1.1 ConsoleAppender
ConsoleAppender是最經常使用的Appenders之中的一個。它僅僅是將日志消息顯示到控制台上。
很多日志框架都將其作為默認的Appender。而且在主要的配置中進行預配置。
比如，在Log4j中ConsoleAppender的配置參數例如以下所看到的。
| 參數 | 描寫敘述 |
|---|---|
| filter | 用於決定是否須要使用該Appender來處理日志事件 |
| layout | 用於決定怎樣對日志記錄進行格式化，默認情況下使用“%m%n”。它會在每一行顯示一條日志記錄 |
| follow | 用於決定Appender是否須要了解輸出（system.out或者system.err）的變化，默認情況是不須要跟蹤這樣的變化 |
| name | 用於設置Appender的名字 |
| ignoreExceptions | 用於決定是否須要記錄在日志事件處理過程中出現的異常 |
| target | 用於指定輸出目標位置。默認情況下使用SYSTEM_OUT。但也能夠改動成SYSTEM_ERR |
<!--這個輸出控制台的配置-->
<Console name="Console" target="SYSTEM_OUT">
<!--控制台僅僅輸出level及以上級別的信息（onMatch）。其它的直接拒絕（onMismatch）-->
<ThresholdFilter level="DEBUG" onMatch="ACCEPT" onMismatch="DENY"/>
<!--這個都知道是輸出日志的格式-->
<PatternLayout pattern="%d{HH:mm:ss.SSS} %-5level %class{36} %L %M - %msg%xEx%n"/>
</Console>
3.1.2 FileAppender
FileAppenders將日志記錄寫入到文件里。它負責打開、關閉文件。向文件里追加日志記錄，並對文件進行加鎖。以免數據被破壞或者覆蓋。
在Log4j中，假設想創建一個FileAppender，須要指定目標文件的名字。寫入方式是追加還是覆蓋。以及是否須要在寫入日志時對文件進行加鎖：
<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的基本使用方法一樣。
但RollingFileAppender能夠設置log文件的size（單位：KB/MB/GB）上限、數量上限，當log文件超過設置的size上限，會自己主動被壓縮。
RollingFileAppender能夠理解為滾動輸出日志，假設log4j 2記錄的日志達到上限，舊的日志將被刪除，騰出的空間用於記錄新的日志。
<!--這個會打印出全部的信息。每次大小超過size，則這size大小的日志會自己主動存入按年份-月份建立的目錄以下並進行壓縮，作為存檔-->
<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 其它appender
我們已經介紹了一些經經常使用到的Appenders，還有非常多其他Appender。
它們加入了新功能或者在其他的一些Appender基礎上實現了新功能。比如，Log4j中的RollingFileAppender擴展了FileAppender。它能夠在滿足特定條件時自己主動創建新的日志文件。SMTPAppender會將日志內容以郵件的形式發送出去；FailoverAppender會在處理日志的過程中，假設一個或者多個Appender失敗，自己主動切換到其他Appender上。
假設想了解其它能夠參考：https://logging.apache.org/log4j/2.0/manual/appenders.html
3.2 Layouts
Layouts將日志記錄的內容從一種數據形式轉換成第二種。日志框架為純文本、HTML、syslog、XML、JSON、序列化以及其他日志提供了Layouts。
這里貼一篇文章簡介下我們經常使用的PatternLayout ：http://wiki.jikexueyuan.com/project/log4j/log4j-patternlayout.html
其它的layouts配置能夠參考：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負責捕捉事件並將其發送給合適的Appender。Logger之間是有繼承關系的。
總是存在一個rootLogger，即使沒有顯示配置也是存在的，而且默認輸出級別為DEBUG。其他的logger都繼承自這個rootLogger。
Log4J中的繼承關系是通過名稱能夠看出來，如"A"、"A.B"、"A.B.C"，A.B繼承A。A.B.C繼承A.B，比較類似於包名。
<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>
<!--建立一個默認的root的logger-->
<root level="error">
<appender-ref ref="Console"/>
<appender-ref ref="RollingFile1"/>
</root>
</loggers>
additivity是 子Logger 是否繼承 父Logger 的 輸出源（appender） 的標志位。詳細說，默認情況下 子Logger 會繼承 父Logger 的appender，也就是說 子Logger 會在 父Logger 的appender里輸出。若是additivity設為false，則 子Logger 僅僅會在自己的appender里輸出。而不會在 父Logger 的appender里輸出。
3.4 日志級別
DEBUG ， INFO ，WARN ，ERROR四種，分別相應Logger類的四種方法
debug(Object message ) ;
info(Object message ) ;
warn(Object message ) ;
error(Object message ) ;
假設設置級別為INFO。則優先級大於等於INFO級別（如：INFO、WARN、ERROR）的日志信息將能夠被輸出,
小於該級別的如：DEBUG將不會被輸出
4、Log4j2 AsyncLogger與AsyncAppender
先上圖
第一張圖能夠看出Log4j2的asyncLogger的性能較使用asyncAppender和sync模式有很大的提升。特別是線程越多的時候。
第二張圖是將log4j2的異步日志機制和其它日志系統進行對照，log4j2的asyncLogger 性能也是非常有優勢。
這里主要涉及了兩個概念AsyncLogger和AysncAppender。是支持異步的Logger和Appender，以下分別簡要介紹下這兩個概念。
4.1 AsyncAppender
AsyncAppender持有其它的配置了aysnc的appender引用列表(appender須要通過配置注冊成異步的)。當其它的logger須要打日志的時候（logEvent事件），asyncAppender會接收logEvent，緩存到queue中，然后用單獨的線程完畢從queue中取logEvent打印到目的appender，這個邏輯比較簡單，看下源代碼就能明確這個流程。ps. AsyncAppender是Log4j 和Log4j2 都有的，不是新東西，但從上面的性能對照上還是有一點點差異的。基本的原因是：（引用官方說法）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).
關於AsyncAppender能提高性能的場景。能夠看下這個篇文章。
http://littcai.iteye.com/blog/316605
怎樣配置一個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，業務代碼調用Logger.log的時候直接返回。而不須要等到appender輸出到日志目的地后才返回。Log4j2的Asynclogger是通過LMAX Disruptor取代queue實現的異步（無鎖的並發框架，http://ifeve.com/disruptor/。Disruptor簡單介紹），達到更高的並發和lower latency。
4.2 AsyncLogger
| 1，Disruptor使用了一個RingBuffer替代隊列，用生產者消費者指針替代鎖。 2，生產者消費者指針使用CPU支持的整數自增。無需加鎖而且速度非常快。Java的實如今Unsafe package中。 |
雖然AsyncLogger 可以大幅度的提高性能。可是也會帶來一些問題。以下是翻譯官方的文檔的Trade-offs：
Benefits
- Higher throughput，達到相對於sync logger的6-68倍的吞吐量
- Lower logging latency，latency是調用Logger.log直到return的時間。asyncLogger的latency比syncLogger以及基於queue的aysncAppender都要低，不僅平均latency低，並且99%、95%latency 也都低於后兩者
- 減少極端大的日志量時候的延遲尖峰
Drawbacks
- Error handling, 假設在打印日志的時候出現錯誤，使用asyncLogger。業務是不知道異常的（能夠通過配置ExceptionHandler處理異常）。假設打印日志是業務邏輯的一部分，不建議使用asyncLogger
- 打印一些可變的內容的時候。使用asyncLogger 會出現故障。大部分時間，不須要操心這點。Log4j確保了類似於 logger.debug("My object is {}", myObject)，使用myObject在打印日志的時刻的版本號打印（Log4j 全部打印都日志都是封裝到Message的實現類里，存儲在 final String里），無論之后是否改變。可是log4j也支持一些了可變的Message，如 MapMessage and StructuredDataMessage 。這些假設在打印日志時候改變，就有問題了
全局配置異步Logger
配置全部Logger都為AsyncLogger，僅僅須要添加disruptor包，然后配置一個system property，-DLog4jContextSelector=org.apache.logging.log4j.core.async.AsyncLoggerContextSelector。Log4j的配置文件不須要改動。
混合使用同步和異步Logger
單獨配置某個logger為async的，通過<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的問題
當layouts配置了輸出%C or $class, %F or %file, %l or %location, %L or %line, %M or %method,或者HTML locationInfo, log4j會獲取location的一個快照，而這對於sync 和async的logger都是一個耗時的操作(官方文檔上說syncLogger會慢1.3~5倍。async會慢4-20倍)。所以默認都是不會輸出location信息，除非Logger配置了includeLocation="true"(官方文檔這么說的，可是我測試的是默認是輸出的，無論了。反正當日志出現慢的時候，能夠考慮通過配置includeLocation控制是否輸出location信息)。