slf4j日志的一些理解

slf4j的一些理解

1.log4j

/**
 * @description: 定制静态日志基础类
 * @author: mufeng
 * @create: 2020/4/7 10:26
 */
public class BaseLogger {
    static {
        InputStream in = BaseLogger.class.getClassLoader().getResourceAsStream("log4j.properties");
        Properties properties=new Properties();
        try {
            properties.load(in);
            PropertyConfigurator.configure(properties);
            in.close();
            logger = Logger.getLogger("Logger");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
    private static Logger logger;
    static final String FQCN = BaseLogger.class.getName();
    public static void info(String msg, Object... args){
        if(logger.isInfoEnabled()){
            FormattingTuple ft = MessageFormatter.arrayFormat(msg, args);
            logger.log(FQCN, Level.INFO,ft.getMessage(),ft.getThrowable());
        }
    }
}

Logger logger= LoggerFactory.getLogger(Test1.class);
    @Test
    public void test1(){
        BaseLogger.info("12{}","13");
    }
    @Test
    public void test2(){
        logger.info("12{}","13");
    }

日志输出：

test1：

11:05:59,145 INFO Test1:21 - 1213

test2：

11:05:59,145 INFO Test1:21 - 1213

可以看出日志输出是一样的。

实现原理：

通过Logger logger= LoggerFactory.getLogger(Test1.class)追溯源代码

LoggerFactory中 getILoggerFactory执行performInitialization方法，会调用bind方法

public static ILoggerFactory getILoggerFactory() {
        if (INITIALIZATION_STATE == 0) {
            Class var0 = LoggerFactory.class;
            synchronized(LoggerFactory.class) {
                if (INITIALIZATION_STATE == 0) {
                    INITIALIZATION_STATE = 1;
                    performInitialization();
                }
            }
        }
​
        switch(INITIALIZATION_STATE) {
        case 1:
            return SUBST_FACTORY;
        case 2:
            throw new IllegalStateException("org.slf4j.LoggerFactory in failed state. Original exception was thrown EARLIER. See also http://www.slf4j.org/codes.html#unsuccessfulInit");
        case 3:
            return StaticLoggerBinder.getSingleton().getLoggerFactory();
        case 4:
            return NOP_FALLBACK_FACTORY;
        default:
            throw new IllegalStateException("Unreachable code");
        }
    }

findPossibleStaticLoggerBinderPathSet找出类路径所有路径集合，StaticLoggerBinder.getSingleton();存在多个会选择一个。（http://www.slf4j.org/codes.html#multiple_bindings解释：by the JVM and for all practical purposes should be considered random）

日志输出可以看出：

SLF4J: Class path contains multiple SLF4J bindings.
SLF4J: Found binding in [jar:file:/D:/persist/maven_Repositories/org/slf4j/slf4j-log4j12/1.7.26/slf4j-log4j12-1.7.26.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: Found binding in [jar:file:/D:/persist/maven_Repositories/ch/qos/logback/logback-classic/1.2.3/logback-classic-1.2.3.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: See http://www.slf4j.org/codes.html#multiple_bindings for an explanation.
SLF4J: Actual binding is of type [org.slf4j.impl.Log4jLoggerFactory]

private static final void bind() {
    String msg;
    try {
        Set<URL> staticLoggerBinderPathSet = null;
        if (!isAndroid()) {
            staticLoggerBinderPathSet = findPossibleStaticLoggerBinderPathSet();
            reportMultipleBindingAmbiguity(staticLoggerBinderPathSet);
        }
​
        StaticLoggerBinder.getSingleton();
        INITIALIZATION_STATE = 3;
        reportActualBinding(staticLoggerBinderPathSet);
        fixSubstituteLoggers();
        replayEvents();
        SUBST_FACTORY.clear();
    } catch (NoClassDefFoundError var2) {
        msg = var2.getMessage();
        if (!messageContainsOrgSlf4jImplStaticLoggerBinder(msg)) {
            failedBinding(var2);
            throw var2;
        }
​
        INITIALIZATION_STATE = 4;
        Util.report("Failed to load class "org.slf4j.impl.StaticLoggerBinder".");
        Util.report("Defaulting to no-operation (NOP) logger implementation");
        Util.report("See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.");
    } catch (NoSuchMethodError var3) {
        msg = var3.getMessage();
        if (msg != null && msg.contains("org.slf4j.impl.StaticLoggerBinder.getSingleton()")) {
            INITIALIZATION_STATE = 2;
            Util.report("slf4j-api 1.6.x (or later) is incompatible with this binding.");
            Util.report("Your binding is version 1.5.5 or earlier.");
            Util.report("Upgrade your binding to version 1.6.x.");
        }
​
        throw var3;
    } catch (Exception var4) {
        failedBinding(var4);
        throw new IllegalStateException("Unexpected initialization failure", var4);
    }
​
}

得出：logger为Log4jLoggerFactory类getLogger方法得到的实例（Log4jLoggerAdapter）

public Logger getLogger(String name) {
        Logger slf4jLogger = (Logger)this.loggerMap.get(name);
        if (slf4jLogger != null) {
            return slf4jLogger;
        } else {
            org.apache.log4j.Logger log4jLogger;
            if (name.equalsIgnoreCase("ROOT")) {
                log4jLogger = LogManager.getRootLogger();
            } else {
                log4jLogger = LogManager.getLogger(name);
            }
​
            Logger newInstance = new Log4jLoggerAdapter(log4jLogger);
            Logger oldInstance = (Logger)this.loggerMap.putIfAbsent(name, newInstance);
            return (Logger)(oldInstance == null ? newInstance : oldInstance);
        }
    }

logger.info()实际调用Log4jLoggerAdapter类中的info方法

log4j怎么找到log4j.properties配置文件？

LogManager类存在static静态块，获取了配置信息，LogManager.getLogger(name)获取调起

（PropertyConfigurator.doConfigure方法解析log4j.properties）

 static {
    // By default we use a DefaultRepositorySelector which always returns 'h'.
    Hierarchy h = new Hierarchy(new RootLogger((Level) Level.DEBUG));
    repositorySelector = new DefaultRepositorySelector(h);
​
    /** Search for the properties file log4j.properties in the CLASSPATH.  */
    String override =OptionConverter.getSystemProperty(DEFAULT_INIT_OVERRIDE_KEY,
                               null);
​
    // if there is no default init override, then get the resource
    // specified by the user or the default config file.
    if(override == null || "false".equalsIgnoreCase(override)) {
​
      String configurationOptionStr = OptionConverter.getSystemProperty(
                              DEFAULT_CONFIGURATION_KEY, 
                              null);
​
      String configuratorClassName = OptionConverter.getSystemProperty(
                                                   CONFIGURATOR_CLASS_KEY, 
                           null);
​
      URL url = null;
​
      // if the user has not specified the log4j.configuration
      // property, we search first for the file "log4j.xml" and then
      // "log4j.properties"
      if(configurationOptionStr == null) {  
    url = Loader.getResource(DEFAULT_XML_CONFIGURATION_FILE);
    if(url == null) {
      url = Loader.getResource(DEFAULT_CONFIGURATION_FILE);
    }
      } else {
    try {
      url = new URL(configurationOptionStr);
    } catch (MalformedURLException ex) {
      // so, resource is not a URL:
      // attempt to get the resource from the class path
      url = Loader.getResource(configurationOptionStr); 
    }   
      }
      
      // If we have a non-null url, then delegate the rest of the
      // configuration to the OptionConverter.selectAndConfigure
      // method.
      if(url != null) {
        LogLog.debug("Using URL ["+url+"] for automatic log4j configuration.");
        try {
            OptionConverter.selectAndConfigure(url, configuratorClassName,
                       LogManager.getLoggerRepository());
        } catch (NoClassDefFoundError e) {
            LogLog.warn("Error during default initialization", e);
        }
      } else {
        LogLog.debug("Could not find resource: ["+configurationOptionStr+"].");
      }
    } else {
        LogLog.debug("Default initialization of overridden by " + 
            DEFAULT_INIT_OVERRIDE_KEY + "property."); 
    }  
  }