springboot换log4j2写日志源码分析
上一篇文章（springboot默认日志框架选择解析）从源码阶段分析了springboot的默认日志框架为logback，spring-boot包中，logging.logback下面有默认的日志配置xml；若使用logback做为日志框架，则添加相关配置即可；那么怎么样用log4j2来作为springboot的日志框架呢？为什么要选择log4j2作为日志框架，搜索一下就能知道；
还是要从LoggingSystem入手，如下代码所示，我们需要在程序启动前添加参数（第二代码块）；这样springboot启动时，就会使用配置的Log4J2LoggingSystem；当然，还需要在grdle中引入log4j2相关的包，否则会报找不到Class的异常；compile "org.apache.logging.log4j:log4j-api:2.14.0"；compile "org.apache.logging.log4j:log4j-core:2.14.0" 引用加入gradle
public static LoggingSystem get(ClassLoader classLoader) {
String loggingSystemClassName = System.getProperty(SYSTEM_PROPERTY);
if (StringUtils.hasLength(loggingSystemClassName)) {
if (NONE.equals(loggingSystemClassName)) {
return new NoOpLoggingSystem();
}
return get(classLoader, loggingSystemClassName);
}
LoggingSystem loggingSystem = SYSTEM_FACTORY.getLoggingSystem(classLoader);
Assert.state(loggingSystem != null, "No suitable logging system located");
return loggingSystem;
}
1，启动函数添加system property
public static void main(String[] args) {
System.setProperty("org.springframework.boot.logging.LoggingSystem", "org.springframework.boot.logging.log4j2.Log4J2LoggingSystem");
SpringApplication.run(Bootstrap.class, args);
}
2，添加log4j2.xml到resources目录下，log4j2的名称不能改：注意设置logPath，logFile
<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="WARN">
<Properties>
<property name="logPath">/Users/dengyouhua/logs</property>
<property name="logFile">com-fenghua-util</property>
</Properties>
<Appenders>
<Console name="console_log" target="SYSTEM_OUT">
<PatternLayout pattern="%d{HH:mm:ss.SSS} [%t] %-5level %logger{36} - %msg%n"/>
</Console>
<RollingFile name="file_log" fileName="${logPath}/${logFile}.log" append="true"
filePattern="${logPath}/${logFile}-%d{yyyy-MM-dd}-%i.log">
<PatternLayout pattern="[%d{HH:mm:ss:SSS}] [%p] - %l - %m%n"/>
<Policies>
<TimeBasedTriggeringPolicy/>
<SizeBasedTriggeringPolicy size="500MB"/>
</Policies>
</RollingFile>
</Appenders>
<Loggers>
<Root level="debug">
<AppenderRef ref="file_log"/>
<AppenderRef ref="console_log"/>
</Root>
</Loggers>
</Configuration>
3，gradle中需要排掉以下包，该方式是gradle直接全局排掉包；单独去排包，会漏排，特别是在使用spring家族的包时
configurations {
compile.exclude module: "spring-boot-starter-logging"
}
到此，你的日志就是真的在使用log4j2打印了。
接下来我们就从源码层面来解释一下，为啥需要上面那么多步； 第一步中，添加系统参数，这个在上一篇以及开头中讲到，要手动设置springboot的日志框架；这样才不会走默认的logback的选项； 第二步添加log4j2.xml的配置，这个也没什么好说的，用来配置日志打印的相关参数。
第三步为什么需要排掉包，org.apache.commons.logging.LogAdapter 这个类决定了必须把spring-boot-starter-logging包给排掉；因为该包会自动引入logback,slf4j相关的包；
重点就是LogAdapter的表态构造里面的逻辑的； 若不排掉上面包，则isPresent(LOG4J_SLF4J_PROVIDER) && isPresent(SLF4J_SPI)必为真，那么logApi，则为LogApi.SLF4J_LAL
private static final String LOG4J_SPI = "org.apache.logging.log4j.spi.ExtendedLogger";
private static final String LOG4J_SLF4J_PROVIDER = "org.apache.logging.slf4j.SLF4JProvider";
private static final String SLF4J_SPI = "org.slf4j.spi.LocationAwareLogger";
private static final String SLF4J_API = "org.slf4j.Logger";
private static final LogApi logApi;
static {
if (isPresent(LOG4J_SPI)) {
if (isPresent(LOG4J_SLF4J_PROVIDER) && isPresent(SLF4J_SPI)) {
// log4j-to-slf4j bridge -> we'll rather go with the SLF4J SPI;
// however, we still prefer Log4j over the plain SLF4J API since
// the latter does not have location awareness support.
logApi = LogApi.SLF4J_LAL;
}
else {
// Use Log4j 2.x directly, including location awareness support
logApi = LogApi.LOG4J;
}
}
else if (isPresent(SLF4J_SPI)) {
// Full SLF4J SPI including location awareness support
logApi = LogApi.SLF4J_LAL;
}
else if (isPresent(SLF4J_API)) {
// Minimal SLF4J API without location awareness support
logApi = LogApi.SLF4J;
}
else {
// java.util.logging as default
logApi = LogApi.JUL;
}
}
private LogAdapter() {
}
/**
* Create an actual {@link Log} instance for the selected API.
* @param name the logger name
*/
public static Log createLog(String name) {
switch (logApi) {
case LOG4J:
return Log4jAdapter.createLog(name);
case SLF4J_LAL:
return Slf4jAdapter.createLocationAwareLog(name);
case SLF4J:
return Slf4jAdapter.createLog(name);
default:
return JavaUtilAdapter.createLog(name);
}
}
这样在createLog时，会调用Slf4jAdapter.createLocationAwareLog，该类是LogAdapter的一个内部类，如下所示：
private static class Slf4jAdapter {
public static Log createLocationAwareLog(String name) {
Logger logger = LoggerFactory.getLogger(name);
return (logger instanceof LocationAwareLogger ?
new Slf4jLocationAwareLog((LocationAwareLogger) logger) : new Slf4jLog<>(logger));
}
public static Log createLog(String name) {
return new Slf4jLog<>(LoggerFactory.getLogger(name));
}
}
最终会调用到LoggerFactory中的findPossibleStaticLoggerBinderPathSet方法，该方法加载了一个指定类org/slf4j/impl/StaticLoggerBinder.class；如下所示：
private static String STATIC_LOGGER_BINDER_PATH = "org/slf4j/impl/StaticLoggerBinder.class";
static Set<URL> findPossibleStaticLoggerBinderPathSet() {
// use Set instead of list in order to deal with bug #138
// LinkedHashSet appropriate here because it preserves insertion order
// during iteration
Set<URL> staticLoggerBinderPathSet = new LinkedHashSet<URL>();
try {
ClassLoader loggerFactoryClassLoader = LoggerFactory.class.getClassLoader();
Enumeration<URL> paths;
if (loggerFactoryClassLoader == null) {
paths = ClassLoader.getSystemResources(STATIC_LOGGER_BINDER_PATH);
} else {
paths = loggerFactoryClassLoader.getResources(STATIC_LOGGER_BINDER_PATH);
}
while (paths.hasMoreElements()) {
URL path = paths.nextElement();
staticLoggerBinderPathSet.add(path);
}
} catch (IOException ioe) {
Util.report("Error getting resources from path", ioe);
}
return staticLoggerBinderPathSet;
}
StaticLoggerBinder类中指定了LoggerContext为ch.qos.logback.classic.LoggerContext，所以还是使用的logback为写日志的组件；该类位于org.slf4j.impl，logback-classic包中；
所以第三步排掉包为必须的步骤，否则还是走默认的logback日志框架；
总结：
springboot 设置log4j2为日志框架，需要以上三步即可完成；简单总结如下：
1，启动函数添加System.setProperty("org.springframework.boot.logging.LoggingSystem", "org.springframework.boot.logging.log4j2.Log4J2LoggingSystem");
2，添加log4j2.xml的配置
3，添加log4j2的compile "org.apache.logging.log4j:log4j-api:2.14.0"；compile "org.apache.logging.log4j:log4j-core:2.14.0"依赖包，并全局排除spring-boot-starter-logging
