springboot默認日志框架選擇源碼解析(推薦)

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背景：

今天新生成一個springboot項目，然而啟動日志，還有mybatis的詳細日志無法打印出來，自寫程序中打印的日志可以輸出；網上找瞭很多資料，都沒法解決問題；於是決定跟一下源碼，弄清springboot日志相關的邏輯。

環境配置：macbook； intellij idea community edition 2020.03 ； gradle 6.8.3 jdk1.8 ；

gradle引用包如下：

dependencies {
  compile "com.alibaba:fastjson:1.2.75"
  compile "mysql:mysql-connector-java"
 
  //spring
  compile("org.springframework.boot:spring-boot-starter")
  compile("org.mybatis.spring.boot:mybatis-spring-boot-starter:2.1.4")
  compile("org.springframework.boot:spring-boot-starter-web")
  compile("org.springframework.boot:spring-boot-starter-actuator")
 
  //lombok
  compileOnly 'org.projectlombok:lombok'
  annotationProcessor 'org.projectlombok:lombok'
 
  //test
  testCompile('org.springframework.boot:spring-boot-starter-test')
  testImplementation 'io.projectreactor:reactor-test'
}

springboot 默認日志使用的是logback(引入spring-boot-starter包後，就自動引入瞭logback-core,logback-classic兩個包，當然還有slf4j的包)，當springboot啟動時，org.springframework.boot.context.logging.LoggingApplicationListener，該類211行註冊的監控事件會被ApplicationStartingEvent觸發；如下代碼所示，會調用onApplicationStartingEvent初始化loggingSystem，而使用哪個日志組件，就要看loggingSystem初始化的值瞭

@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();
    }
  }
 
  private void onApplicationStartingEvent(ApplicationStartingEvent event) {
    this.loggingSystem = LoggingSystem.get(event.getSpringApplication().getClassLoader());
    this.loggingSystem.beforeInitialize();
  }

如下圖是org.springframework.boot.logging.LoggingSystem類裡面get函數的內容：首先會從system.getProperty中獲取className，新生成的項目，取到的這個值都為空，SYSTEM_PROPERTY是一個固定值，就是該類的名字；那麼loggingSystem的值就是從SYSTEM_FACTORY.getLoggingSystem(classLoader);獲取到的；接下來我們得看LoggingSystemFactory.fromSpringFactories.getLoggingSystem取的值是什麼瞭；

public static final String SYSTEM_PROPERTY = LoggingSystem.class.getName();
private static final LoggingSystemFactory SYSTEM_FACTORY = LoggingSystemFactory.fromSpringFactories();
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;
  }
 
  private static LoggingSystem get(ClassLoader classLoader, String loggingSystemClassName) {
    try {
      Class<?> systemClass = ClassUtils.forName(loggingSystemClassName, classLoader);
      Constructor<?> constructor = systemClass.getDeclaredConstructor(ClassLoader.class);
      constructor.setAccessible(true);
      return (LoggingSystem) constructor.newInstance(classLoader);
    }
    catch (Exception ex) {
      throw new IllegalStateException(ex);
    }
  }

LoggingSystemFactory是一個接口，它的實現類在spring-boot-start有4個，其中3個是在內部內類實現，DelegatingLoggingSystemFactory（JavaLoggingSystem，Log4J2LoggingSystem，LogbackLoggingSystem，內部類實現）。上面SYSTEM_FACTORY的實現就是DelegatingLoggingSystemFactory這個類，如下代碼中delegates的值為JavaLoggingSystem，Log4J2LoggingSystem，LogbackLoggingSystem；三個類具體的加載邏輯在SpringFactoriesLoader.loadFactories函數中，最終返回的loggingSystem就是前面函數返回列表中的第一個；SpringFactoriesLoader.loadFactories 才是決定springboot默認會使用哪個日志組件關鍵：該類是spring的核心組件類，在spring-core包中，org.springframework.core.io.support.SpringFactoriesLoader；loggingSystem的值=LogbackLoggingSystem

public interface LoggingSystemFactory {
 
  /**
  * Return a logging system implementation or {@code null} if no logging system is
  * available.
  * @param classLoader the class loader to use
  * @return a logging system
  */
  LoggingSystem getLoggingSystem(ClassLoader classLoader);
 
  /**
  * Return a {@link LoggingSystemFactory} backed by {@code spring.factories}.
  * @return a {@link LoggingSystemFactory} instance
  */
  static LoggingSystemFactory fromSpringFactories() {
   return new DelegatingLoggingSystemFactory(
      (classLoader) -> SpringFactoriesLoader.loadFactories(LoggingSystemFactory.class, classLoader));
  }
 
}
class DelegatingLoggingSystemFactory implements LoggingSystemFactory {
 
  private final Function<ClassLoader, List<LoggingSystemFactory>> delegates;
 
  /**
  * Create a new {@link DelegatingLoggingSystemFactory} instance.
  * @param delegates a function that provides the delegates
  */
  DelegatingLoggingSystemFactory(Function<ClassLoader, List<LoggingSystemFactory>> delegates) {
   this.delegates = delegates;
  }
 
  @Override
  public LoggingSystem getLoggingSystem(ClassLoader classLoader) {
   List<LoggingSystemFactory> delegates = (this.delegates != null) ? this.delegates.apply(classLoader) : null;
   if (delegates != null) {
     for (LoggingSystemFactory delegate : delegates) {
      LoggingSystem loggingSystem = delegate.getLoggingSystem(classLoader);
      if (loggingSystem != null) {
        return loggingSystem;
      }
     }
   }
   return null;
  }
 
}

總結：雖然springboot會加載JavaLoggingSystem，Log4J2LoggingSystem，LogbackLoggingSystem三個日志實現類，但在選擇時，還是會使用LogbackLoggingSystem作為它的日志框架

到此這篇關於springboot默認日志框架選擇源碼解析的文章就介紹到這瞭,更多相關springboot日志框架內容請搜索WalkonNet以前的文章或繼續瀏覽下面的相關文章希望大傢以後多多支持WalkonNet！

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