java学习：log4j输出xml格式的日志文件(log4j2篇)

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上一篇讲解了log4j输出xml格式的日志文件，本篇讲述log4j2.x版本。log4j2.x版本相比log4j1变化很大，使用起来也更麻烦，主要是jar包的依赖的问题。先上代码，然后再列举遇到的问题。

1、Log类代码

import org.apache.logging.log4j.LogManager;  
import org.apache.logging.log4j.Logger;
import org.apache.logging.log4j.ThreadContext;  
public class Log{
public static void main(String[] args) {
Logger logger = LogManager.getRootLogger();
ThreadContext.push("Message only");
    ThreadContext.push("int", 1);
    ThreadContext.push("int-long-string", "3");
ThreadContext.put("ipAddress", "192.168.21.9");
    ThreadContext.put("key", "value");
    
logger.debug("hello world--debug");
logger.error("hello world--error");
ThreadContext.clearMap();
ThreadContext.clearAll();
    }  
}

2、配置文件log4j2.xml

<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="error" name="XMLConfig">
    <Loggers>
        <Root level="debug">
            <AppenderRef ref="MyFile"/>
        </Root>
    </Loggers>
    <Appenders>
        <Console name="CONSOLE">
            <XMLLayout complete="true" compact="false"/>
        </Console>
        <File name="MyFile" fileName="logs/testlog.xml">
            <XMLLayout complete="true" compact="false" properties="true" locationInfo="true"/>
        </File>
    </Appenders>
</Configuration>

3、运行Log输出：

<Event xmlns="http://logging.apache.org/log4j/2.0/events" timeMillis="1468682588978" thread="main" level="DEBUG" endOfBatch="false" loggerFqcn="org.apache.logging.log4j.spi.AbstractLogger" threadId="1" threadPriority="5">
    <Message>hello world--debug</Message>
    <ContextMap>
      <item key="ipAddress" value="192.168.21.9"/>
      <item key="key" value="value"/>
    </ContextMap>
    <ContextStack>
      <ContextStackItem>Message only</ContextStackItem>
      <ContextStackItem>int</ContextStackItem>
      <ContextStackItem>int-long-string</ContextStackItem>
    </ContextStack>
    <Source class="com.flyerfish.common.Log" method="main" file="Log.java" line="18"/>
  </Event>
  <Event xmlns="http://logging.apache.org/log4j/2.0/events" timeMillis="1468682589064" thread="main" level="ERROR" endOfBatch="false" loggerFqcn="org.apache.logging.log4j.spi.AbstractLogger" threadId="1" threadPriority="5">
    <Message>hello world--error</Message>
    <ContextMap>
      <item key="ipAddress" value="192.168.21.9"/>
      <item key="key" value="value"/>
    </ContextMap>
    <ContextStack>
      <ContextStackItem>Message only</ContextStackItem>
      <ContextStackItem>int</ContextStackItem>
      <ContextStackItem>int-long-string</ContextStackItem>
    </ContextStack>
    <Source class="com.flyerfish.common.Log" method="main" file="Log.java" line="19"/>
  </Event>
</Events>

下面主要讲讲遇到的问题

问题1：log4j2的jar包很多，有几十个。如果只是在普通的java类中应用，需要使用两个：log4j-api-2.6.2.jar和log4j-core-2.6.2.jar

问题2：log4j2可以自动读取配置文件，自动查找log4j2.xml文件。Log类并没有指定配置文件。需要做的，就是把log4j2.xml文件放到src目录下。然后编译，就可以运行了。

ERROR StatusLogger No log4j2 configuration file found. Using default configuration: logging only errors to the console.
23:35:25.986 [main] ERROR  - hello world--error

问题3：“java.lang.NoClassDefFoundError: com/fasterxml/jackson/core/JsonGenerationException”异常，这个问题，定位了很久，由于不熟悉java，不太会看异常代码。现在看来，其实调用栈已经明确告诉你了，遇到JsonGenerationException。初步定位是有jar依赖，中途又去弄mvn，查找依赖，尝试了很久也没成功。采取比较笨的方法，就用google搜。各个页面仔细看，过程痛苦。

Exception in thread "main" java.lang.NoClassDefFoundError: com/fasterxml/jackson/core/JsonGenerationException
at java.lang.Class.getDeclaredMethods0(Native Method)
at java.lang.Class.privateGetDeclaredMethods(Class.java:2701)
at java.lang.Class.getDeclaredMethods(Class.java:1975)
at org.apache.logging.log4j.core.config.plugins.util.PluginBuilder.createBuilder(PluginBuilder.java:147)
at org.apache.logging.log4j.core.config.plugins.util.PluginBuilder.build(PluginBuilder.java:119)
at org.apache.logging.log4j.core.config.AbstractConfiguration.createPluginObject(AbstractConfiguration.java:918)
at org.apache.logging.log4j.core.config.AbstractConfiguration.createConfiguration(AbstractConfiguration.java:858)
at org.apache.logging.log4j.core.config.AbstractConfiguration.createConfiguration(AbstractConfiguration.java:850)
at org.apache.logging.log4j.core.config.AbstractConfiguration.createConfiguration(AbstractConfiguration.java:850)
at org.apache.logging.log4j.core.config.AbstractConfiguration.doConfigure(AbstractConfiguration.java:479)
at org.apache.logging.log4j.core.config.AbstractConfiguration.initialize(AbstractConfiguration.java:219)
at org.apache.logging.log4j.core.config.AbstractConfiguration.start(AbstractConfiguration.java:231)
at org.apache.logging.log4j.core.LoggerContext.setConfiguration(LoggerContext.java:496)
at org.apache.logging.log4j.core.LoggerContext.reconfigure(LoggerContext.java:566)
at org.apache.logging.log4j.core.LoggerContext.reconfigure(LoggerContext.java:582)
at org.apache.logging.log4j.core.LoggerContext.start(LoggerContext.java:217)
at org.apache.logging.log4j.core.impl.Log4jContextFactory.getContext(Log4jContextFactory.java:242)
at org.apache.logging.log4j.core.impl.Log4jContextFactory.getContext(Log4jContextFactory.java:45)
at org.apache.logging.log4j.LogManager.getContext(LogManager.java:174)
at org.apache.logging.log4j.LogManager.getLogger(LogManager.java:618)
at org.apache.logging.log4j.LogManager.getRootLogger(LogManager.java:652)
at testlog4j2.Log.main(Log.java:10)
Caused by: java.lang.ClassNotFoundException: com.fasterxml.jackson.core.JsonGenerationException
at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
... 22 more

最终找到的原因，log4j2有好几个库的依赖，在官网上可以找到。不同Layout，依赖也不同。XMLLayout依赖：

Jackson core, databind and dataformat XML
And com.fasterxml.woodstox:woodstox-core:5.0.2

jar包可以到http://mvnrepository.com/ 搜。下载三个jar包，jackson-annotations-2.8.0.jar,jackson-core-2.8.0.jar和jackson-dtatbind-2.8.0.jar。添加上，再运行。

问题4：添加了jackson库运行，结果出现如下错误。有颜色不一样，刚开始未注意，不断感叹多坎坷。后来发现，还少了一个库woodstox-core。加上问题依旧，想到woodstox-core还有依赖的库。于是又找到了jackson-dataformat-xml.2.8.0.jar,jackson-module-jaxb-annotations-2.8.0.jar和stax2-api-4.0.0.jar，才总算解决了运行的错误。

2016-07-16 23:46:34,996 main ERROR Unable to invoke factory method in class class org.apache.logging.log4j.core.layout.XmlLayout for element XMLLayout. java.lang.reflect.InvocationTargetException
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.apache.logging.log4j.core.config.plugins.util.PluginBuilder.build(PluginBuilder.java:132)
at org.apache.logging.log4j.core.config.AbstractConfiguration.createPluginObject(AbstractConfiguration.java:918)
at org.apache.logging.log4j.core.config.AbstractConfiguration.createConfiguration(AbstractConfiguration.java:858)
at org.apache.logging.log4j.core.config.AbstractConfiguration.createConfiguration(AbstractConfiguration.java:850)
at org.apache.logging.log4j.core.config.AbstractConfiguration.createConfiguration(AbstractConfiguration.java:850)
at org.apache.logging.log4j.core.config.AbstractConfiguration.doConfigure(AbstractConfiguration.java:479)
at org.apache.logging.log4j.core.config.AbstractConfiguration.initialize(AbstractConfiguration.java:219)
at org.apache.logging.log4j.core.config.AbstractConfiguration.start(AbstractConfiguration.java:231)
at org.apache.logging.log4j.core.LoggerContext.setConfiguration(LoggerContext.java:496)
at org.apache.logging.log4j.core.LoggerContext.reconfigure(LoggerContext.java:566)
at org.apache.logging.log4j.core.LoggerContext.reconfigure(LoggerContext.java:582)
at org.apache.logging.log4j.core.LoggerContext.start(LoggerContext.java:217)
at org.apache.logging.log4j.core.impl.Log4jContextFactory.getContext(Log4jContextFactory.java:242)
at org.apache.logging.log4j.core.impl.Log4jContextFactory.getContext(Log4jContextFactory.java:45)
at org.apache.logging.log4j.LogManager.getContext(LogManager.java:174)
at org.apache.logging.log4j.LogManager.getLogger(LogManager.java:618)
at org.apache.logging.log4j.LogManager.getRootLogger(LogManager.java:652)
at testlog4j2.Log.main(Log.java:10)
Caused by: java.lang.NoClassDefFoundError: com/fasterxml/jackson/dataformat/xml/XmlMapper
at java.lang.ClassLoader.defineClass1(Native Method)
at java.lang.ClassLoader.defineClass(ClassLoader.java:763)
at java.security.SecureClassLoader.defineClass(SecureClassLoader.java:142)
at java.net.URLClassLoader.defineClass(URLClassLoader.java:467)
at java.net.URLClassLoader.access$100(URLClassLoader.java:73)
at java.net.URLClassLoader$1.run(URLClassLoader.java:368)
at java.net.URLClassLoader$1.run(URLClassLoader.java:362)
at java.security.AccessController.doPrivileged(Native Method)
at java.net.URLClassLoader.findClass(URLClassLoader.java:361)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
at org.apache.logging.log4j.core.layout.XmlLayout.<init>(XmlLayout.java:198)
at org.apache.logging.log4j.core.layout.XmlLayout.createLayout(XmlLayout.java:287)
... 22 more
Caused by: java.lang.ClassNotFoundException: com.fasterxml.jackson.dataformat.xml.XmlMapper
at java.net.URLClassLoader.findClass(URLClassLoader.java:381)
at java.lang.ClassLoader.loadClass(ClassLoader.java:424)
at sun.misc.Launcher$AppClassLoader.loadClass(Launcher.java:331)
at java.lang.ClassLoader.loadClass(ClassLoader.java:357)
... 36 more

结论：java异常打印，基本已经告诉你了哪里错误，比如上面的红色部分，是com.fasterxml.jackson.dataformat.xml.XmlMapper类找不到定义，不要被这么长的出错信息吓到，而不仔细去看。学会看了，处理起来就容易了。

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