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SpringBoot源码中使用的日志API为JCL(Java common log),JCL实际上只是一个日志门面,没有具体的日志功能实现。如果将SpringBoot中的spring-boot-start-logging依赖排除,可以看到启动项目时打印的是红色字体的JUL日志。
<dependency>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter</artifactId>
<exclusions>
<exclusion>
<groupId>org.springframework.boot</groupId>
<artifactId>spring-boot-starter-logging</artifactId>
</exclusion>
</exclusions>
</dependency>
如果不刻意的排除spring-boot-starter-logging,SpringBoot默认引入的logback日志,SpringBoot在一站式启动过程中,会发布各种通知事件,如项目正在启动事件,准备环境事件等,logback的加载就是通过事件监听者来响应上述事件从而触发的,关于SpringBoot的事件编程模型原理可参考文章:SpringBoot揭密:SpringBoot里的观察者设计模式,SpringBoot事件编程模型解析,这里不再赘述。
触发加载的原理与SpringBoot配置文件触发加载的过程是同理的,SpringBoot引入了一些新的监听器,从spring.factories文件中就能看到具体的监听器:
# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.ClearCachesApplicationListener,\
org.springframework.boot.builder.ParentContextCloserApplicationListener,\
org.springframework.boot.cloud.CloudFoundryVcapEnvironmentPostProcessor,\
org.springframework.boot.context.FileEncodingApplicationListener,\
org.springframework.boot.context.config.AnsiOutputApplicationListener,\
org.springframework.boot.context.config.ConfigFileApplicationListener,\
org.springframework.boot.context.config.DelegatingApplicationListener,\
org.springframework.boot.context.logging.ClasspathLoggingApplicationListener,\
org.springframework.boot.context.logging.LoggingApplicationListener,\
org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener
其中ConfigFileApplicationListener就是加载SpringBoot配置文件的监听器,LoggingApplicationListener就是加载日志的监听器。可以看到类中onApplicationEvent方法就是处理监听到的各种事件的:
@Override
public void onApplicationEvent(ApplicationEvent event) {
if (event instanceof ApplicationStartingEvent) {
onApplicationStartingEvent((ApplicationStartingEvent) event);
}
else if (event instanceof ApplicationEnvironmentPreparedEvent) {
onApplicationEnvironmentPreparedEvent((ApplicationEnvironmentPreparedEvent) event);
}
else if (event instanceof ApplicationPreparedEvent) {
onApplicationPreparedEvent((ApplicationPreparedEvent) event);
}
else if (event instanceof ContextClosedEvent
&& ((ContextClosedEvent) event).getApplicationContext().getParent() == null) {
onContextClosedEvent();
}
else if (event instanceof ApplicationFailedEvent) {
onApplicationFailedEvent();
}
}
先是第一个事件:ApplicationStartingEvent
private void onApplicationStartingEvent(ApplicationStartingEvent event) {
this.loggingSystem = LoggingSystem.get(event.getSpringApplication().getClassLoader());
this.loggingSystem.beforeInitialize();
}
LoggingSystem.get方法就是记载具体日志的过程:
public static LoggingSystem get(ClassLoader classLoader) {
String loggingSystem = System.getProperty(SYSTEM_PROPERTY);
if (StringUtils.hasLength(loggingSystem)) {
if (NONE.equals(loggingSystem)) {
return new NoOpLoggingSystem();
}
return get(classLoader, loggingSystem);
}
return SYSTEMS.entrySet().stream().filter((entry) -> ClassUtils.isPresent(entry.getKey(), classLoader))
.map((entry) -> get(classLoader, entry.getValue())).findFirst()
.orElseThrow(() -> new IllegalStateException("No suitable logging system located"));
}
这里其实是遍历SYSTEMS里面的entry,判断key代表的class是否存在,如果存在就把value代表的那个LoggingSystem给加载了,看下SYSTEMS里面都有啥:
private static final Map<String, String> SYSTEMS;
static {
Map<String, String> systems = new LinkedHashMap<>();
systems.put("ch.qos.logback.core.Appender", "org.springframework.boot.logging.logback.LogbackLoggingSystem");
systems.put("org.apache.logging.log4j.core.impl.Log4jContextFactory","org.springframework.boot.logging.log4j2.Log4J2LoggingSystem");
systems.put("java.util.logging.LogManager", "org.springframework.boot.logging.java.JavaLoggingSystem");
SYSTEMS = Collections.unmodifiableMap(systems);
}
这里默认添加了3个日志框架,依次是logback、log4j2和jdk的log,因为spring-boot-starter-logging默认依赖了logback,因此,logback会被初始化使用,再这里产生了实例LogbackLoggingSystem。
第二个事件:ApplicationEnvironmentPreparedEvent,开始通过LogBackLoggingSystem配置LogBack,如果配置了自定义的日志配置文件就加载自己的文件,否则加载默认的配置文件。
Logback整合完成后,SpringBoot中的JCL API打印的日志是如何转变成logback格式的呢,这里看下spring-boot-starter-logging pom中引入的依赖:
<dependencies>
<dependency>
<groupId>ch.qos.logback</groupId>
<artifactId>logback-classic</artifactId>
<version>1.2.3</version>
<scope>compile</scope>
</dependency>
<dependency>
<groupId>org.apache.logging.log4j</groupId>
<artifactId>log4j-to-slf4j</artifactId>
<version>2.13.3</version>
<scope>compile</scope>
</dependency>
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>jul-to-slf4j</artifactId>
<version>1.7.30</version>
<scope>compile</scope>
</dependency>
</dependencies>
这里实际上涉及到纷繁复杂的日志体系中的日志转换,大致转换链路过程就是JCL->JUL->slf4j->logback classic->logback以及log4j->slf4j->logback classic->logback