Java日誌框架：SLF4J詳解

SLF4J概述

The Simple Logging Facade for Java (SLF4J) serves as a simple facade or abstraction for various logging frameworks, such as java.util.logging, logback and log4j. SLF4J allows the end-user to plug in the desired logging framework at deployment time.

Simple Logging Facade for Java（SLF4J）可用做各類日誌框架的簡單外觀或抽象，例如java.util.logging，logback和log4j。SLF4J容許最終用戶在部署時插入所需的日誌記錄框架。

Binding with a logging framework at deployment time

The SLF4J distribution ships with several jar files referred to as SLF4J bindings, with each binding corresponding to a supported framework.

• slf4j-log4j12-1.7.27.jar

  Binding for log4j version 1.2, a widely used logging framework. You also need to place log4j.jar on your class path.

• slf4j-jdk14-1.7.27.jar

  Binding for java.util.logging, also referred to as JDK 1.4 logging

• slf4j-nop-1.7.27.jar

  Binding for NOP, silently discarding all logging.

• slf4j-simple-1.7.27.jar

  Binding for Simple implementation, which outputs all events to System.err. Only messages of level INFO and higher are printed. This binding may be useful in the context of small applications.

• slf4j-jcl-1.7.27.jar

  Binding for Jakarta Commons Logging. This binding will delegate all SLF4J logging to JCL.

• logback-classic-1.2.3.jar (requires logback-core-1.2.3.jar)

  There are also SLF4J bindings external to the SLF4J project, Logback's ch.qos.logback.classic.Loggerclass is a direct implementation of SLF4J's org.slf4j.Logger interface.

To switch logging frameworks, just replace slf4j bindings on your class path. For example, to switch from java.util.logging to log4j, just replace slf4j-jdk14-1.7.27.jar with slf4j-log4j12-1.7.27.jar.

要切換日誌框架，只需替換類路徑上的slf4j綁定。例如，要從java.util.logging切換到log4j，只需將slf4j-jdk14-1.7.27.jar替換爲slf4j-log4j12-1.7.27.jar便可。

SLF4J does not rely on any special class loader machinery. In fact, each SLF4J binding is hardwired at compile time to use one and only one specific logging framework. For example, the slf4j-log4j12-1.7.27.jar binding is bound at compile time to use log4j. In your code, in addition to slf4j-api-1.7.27.jar, you simply drop one and only one binding of your choice onto the appropriate class path location. Do not place more than one binding on your class path.

Consolidate logging via SLF4J（經過SLF4J整合日誌記錄）

SLF4J caters for this common use-case by providing bridging modules for JCL, java.util.logging and log4j.

SLF4J經過爲JCL，java.util.logging和log4j提供橋接模塊來知足這種常見用例。

Mapped Diagnostic Context (MDC) support

Mapped Diagnostic Context is essentially a map maintained by the logging framework where the application code provides key-value pairs which can then be inserted by the logging framework in log messages. MDC data can also be highly helpful in filtering messages or triggering certain actions.

映射診斷上下文本質上是由日誌框架維護的映射，其中應用程序代碼提供鍵值對，而後日誌消息中的日誌記錄框架能夠插入鍵值對。 MDC數據在過濾消息或觸發某些操做方面也很是有用。

SLF4J supports MDC, or mapped diagnostic context. If the underlying logging framework offers MDC functionality, then SLF4J will delegate to the underlying framework's MDC. Note that at this time, only log4j and logback offer MDC functionality. If the underlying framework does not offer MDC, for example java.util.logging, then SLF4J will still store MDC data but the information therein will need to be retrieved by custom user code.

SLF4J支持MDC或映射診斷上下文。 若是底層日誌記錄框架提供了MDC功能，那麼SLF4J將委託給底層框架的MDC。 請注意，目前只有log4j和logback提供MDC功能。 若是底層框架不提供MDC，例如java.util.logging，則SLF4J仍將存儲MDC數據，但其中的信息須要由自定義用戶代碼檢索。

原理分析

經過上圖可知SLF4J可以抽象各類具體日誌框架，是由StaticLoggerBinder類完成。接下來重點看看bind()方法是如何查找StaticLoggerBinder類，源碼以下：

private final static void bind() {
    try {
        // 查找類路徑下全部的StaticLoggerBinder類
        Set<URL> staticLoggerBinderPathSet = findPossibleStaticLoggerBinderPathSet();
        // 若是存在多個StaticLoggerBinder類，則打印日誌
        reportMultipleBindingAmbiguity(staticLoggerBinderPathSet);
        // 獲取StaticLoggerBinder實例，若是不存在，則拋出NoClassDefFoundError異常
        StaticLoggerBinder.getSingleton();
        INITIALIZATION_STATE = SUCCESSFUL_INITIALIZATION;
        // 打印實際使用StaticLoggerBinder類
        reportActualBinding(staticLoggerBinderPathSet);
        fixSubstitutedLoggers();
    } catch (NoClassDefFoundError ncde) {
        String msg = ncde.getMessage();
        if (messageContainsOrgSlf4jImplStaticLoggerBinder(msg)) {
            INITIALIZATION_STATE = NOP_FALLBACK_INITIALIZATION;
            Util.report("Failed to load class \"org.slf4j.impl.StaticLoggerBinder\".");
            Util.report("Defaulting to no-operation (NOP) logger implementation");
            Util.report("See " + NO_STATICLOGGERBINDER_URL + " for further details.");
        } else {
            failedBinding(ncde);
            throw ncde;
        }
    } catch (java.lang.NoSuchMethodError nsme) {
        String msg = nsme.getMessage();
        if (msg != null && msg.indexOf("org.slf4j.impl.StaticLoggerBinder.getSingleton()") != -1) {
            INITIALIZATION_STATE = FAILED_INITIALIZATION;
            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 nsme;
    } catch (Exception e) {
        failedBinding(e);
        throw new IllegalStateException("Unexpected initialization failure", e);
    }
}

private static String  STATIC_LOGGER_BINDER_PATH = "org/slf4j/impl/StaticLoggerBinder.class";

private static Set<URL> findPossibleStaticLoggerBinderPathSet() {
    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 = (URL) paths.nextElement();
            staticLoggerBinderPathSet.add(path);
        }
    } catch (IOException ioe) {
        Util.report("Error getting resources from path", ioe);
    }
    return staticLoggerBinderPathSet;
}
複製代碼

SLF4J與logback集成

1. maven依賴包以下：

   <!-- slf4j-api -->
   <dependency>
       <groupId>org.slf4j</groupId>
       <artifactId>slf4j-api</artifactId>
       <version>1.7.12</version>
   </dependency>
   <!-- logback -->
   <dependency> 
       <groupId>ch.qos.logback</groupId> 
       <artifactId>logback-core</artifactId> 
       <version>1.1.3</version> 
   </dependency>
   <!-- logback-classic（已含有對slf4j的集成包） --> 
   <dependency> 
       <groupId>ch.qos.logback</groupId> 
       <artifactId>logback-classic</artifactId> 
       <version>1.1.3</version> 
   </dependency>
   複製代碼

2. 編寫logback的配置文件logback.xml，內容以下:

   <?xml version="1.0" encoding="UTF-8"?>
   <configuration>
     <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
       <encoder>
         <pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n</pattern>
       </encoder>
     </appender>
     <root level="DEBUG">          
       <appender-ref ref="STDOUT" />
     </root>  
   </configuration>
   複製代碼

3. 使用方式

   private static final Logger logger=LoggerFactory.getLogger(LogbackTest.class);
   public static void main(String[] args){
       if(logger.isDebugEnabled()){
           logger.debug("slf4j-logback debug message");
       }
       if(logger.isInfoEnabled()){
           logger.info("slf4j-logback info message");
       }
       if(logger.isTraceEnabled()){
           logger.trace("slf4j-logback trace message");
       }
   }
   複製代碼

4. org.slf4j.impl.StaticLoggerBinder類實現

   

   ---

   

SLF4J與log4集成

1. maven依賴包以下：

   <!-- slf4j -->
   <dependency>
       <groupId>org.slf4j</groupId>
       <artifactId>slf4j-api</artifactId>
       <version>1.7.12</version>
   </dependency>
   
   <!-- slf4j-log4j -->
   <dependency>
       <groupId>org.slf4j</groupId>
       <artifactId>slf4j-log4j12</artifactId>
       <version>1.7.12</version>
   </dependency>
   
   <!-- log4j -->
   <dependency>
       <groupId>log4j</groupId>
       <artifactId>log4j</artifactId>
       <version>1.2.17</version>
   </dependency>
   複製代碼

2. 編寫log4j.properties配置文件,放到類路徑下

   log4j.rootLogger = debug, console
   log4j.appender.console = org.apache.log4j.ConsoleAppender
   log4j.appender.console.layout = org.apache.log4j.PatternLayout
   log4j.appender.console.layout.ConversionPattern = %-d{yyyy-MM-dd HH:mm:ss} %m%n
   複製代碼

3. 使用方式

   private static Logger logger=LoggerFactory.getLogger(Log4j2Slf4jTest.class);
   public static void main(String[] args){
       if(logger.isTraceEnabled()){
           logger.trace("slf4j-log4j2 trace message");
       }
       if(logger.isDebugEnabled()){
           logger.debug("slf4j-log4j2 debug message");
       }
       if(logger.isInfoEnabled()){
           logger.info("slf4j-log4j2 info message");
       }
   }
   複製代碼

4. org.slf4j.impl.StaticLoggerBinder類實現

   

   ---

   

參考資料

jcl與jul、log4j一、log4j二、logback的集成原理