23.SpringBoot学习笔记--SpringBoot日志关系

Spring Boot 底层中关于日志框架的依赖关系如下图。

Spring Boot 使用 spring-boot-starter-logging 完成日志功能

<dependency>
  <groupId>org.springframework.bootgroupId>
  <artifactId>spring-boot-starter-loggingartifactId>
  <version>2.3.0.RELEASEversion>
  <scope>compilescope>
dependency>

由上面关于日志框架的依赖关系的图可知，Spring Boot 底层使用 SLF4J + Logback 的方式进行日志记录。Spring Boot 也把其他的日志框架都统一为 SLF4J。

下面是 spring-jcl-5.2.6.RELEASE.jar 包的部分目录结构

可以发现报名与 Apache Commons Logging 日志框架的包名一致，但是查看 LogFactory 类中的部分代码：

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;
    }
}

会发现 Apache Commons Logging 日志框架被统一为 SLF4J。

在 spring-boot-starter-2.3.0.RELEASE.pom 文件中发现

<dependency>
  <groupId>org.springframeworkgroupId>
  <artifactId>spring-coreartifactId>
  <scope>compilescope>
dependency>

并没有排除 Apache Commons Logging 日志框架，这是因为从 Spring 5 开始，底层日志系统依赖的是 spring-jcl，和传统的 JCL (common-logging.jar) 有区别，首先是尝试加载 org.apache.logging.log4j.spi.ExtendedLogger (Log4j API)，然后尝试加载 org.slf4j.spi.LocationAwareLogger (SLF4J SPI)，再次就是尝试org.slf4j.Logger (SLF4J API)，如果还是没有，就加载 JUL。