java 日志框架 slf4j_slf4j-simple-优快云博客

本文链接：https://blog.youkuaiyun.com/kangaroo_07/article/details/115548866

SLF4J是一个日志标准，提供日志接口并允许获取具体日志对象。其不实现日志输出，但有如slf4j-simple、logback等实现。通过测试，展示了slf4j与log4j、logback的桥接及日志输出。当系统中同时引入多个SLF4J实现时，会警告并选择一个实现进行日志输出。

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SLF4J 本身不实现日志输出，其本身是一个日志标准，主要提供以下两点功能：

（1）提供日志接口

（2）提供获取具体日志对象的方法

主要实现有：

slf4j-simple、logback都是slf4j的具体实现，log4j并不直接实现slf4j，但是有专门的一层桥接slf4j-log4j12来实现slf4j。

功能测试：slf4j的引入

<dependency>
   <groupId>org.slf4j</groupId>
   <artifactId>slf4j-api</artifactId>
   <version>1.7.30</version>
</dependency>

测试代码如下：注意Logger、LoggerFactory均为slf4j包所属类。

package com.test;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;

/**
 * Hello world!
 *
 */
public class App 
{
    private static Logger logger = LoggerFactory.getLogger(App.class);
    public static void main( String[] args )
    {
        logger.info("这是一个测试日志");
    }
}

运行效果：没有日志输出，从而验证了slf4j本身不做日志输出，而需要其实现完成日志输出

SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.

Process finished with exit code 0

测试一.引入log4j依赖如下：不再需要slf4j-api依赖，slf4j-log4j12中包含了对slf4j-api的依赖

<!--
<dependency>
   <groupId>org.slf4j</groupId>
   <artifactId>slf4j-api</artifactId>
   <version>1.7.30</version>
</dependency>
-->
<dependency>
   <groupId>org.slf4j</groupId>
   <artifactId>slf4j-log4j12</artifactId>
   <version>1.7.30</version>
</dependency>

同时创建log4j.properties文件示例如下：

### 设置###
log4j.rootLogger = trace,stdout,D,E

### 输出信息到控制抬 ###
log4j.appender.stdout = org.apache.log4j.ConsoleAppender
log4j.appender.stdout.Target = System.out
log4j.appender.stdout.layout = org.apache.log4j.PatternLayout
#log4j.appender.stdout.layout.ConversionPattern = [%-5p] %d{yyyy-MM-dd HH:mm:ss,SSS} method:%l %m%n
log4j.appender.stdout.layout.ConversionPattern = [%-5p] - %d{[yyyy-MM-dd HH:mm:ss:SSS]} %M: %l %m%n
### 输出DEBUG 级别以上的日志到=E://logs/error.log ###
log4j.appender.D = org.apache.log4j.DailyRollingFileAppender
log4j.appender.D.File = E://logs/log.log
log4j.appender.D.Append = true
log4j.appender.D.Threshold = DEBUG
log4j.appender.D.layout = org.apache.log4j.PatternLayout
log4j.appender.D.layout.ConversionPattern = %-d{yyyy-MM-dd HH:mm:ss}  [ %t:%r ] - [ %p ]  %m%n

### 输出ERROR 级别以上的日志到=E://logs/error.log ###
log4j.appender.E = org.apache.log4j.DailyRollingFileAppender
log4j.appender.E.File =E://logs/error.log
log4j.appender.E.Append = true
log4j.appender.E.Threshold = ERROR
log4j.appender.E.layout = org.apache.log4j.PatternLayout
log4j.appender.E.layout.ConversionPattern = %-d{yyyy-MM-dd HH:mm:ss}  [ %t:%r ] - [ %p ]  %m%n

运行效果如下，正确打印日志：

[INFO ] - [2021-04-09 14:18:54:248] main: com.test.App.main(App.java:14) 这是一个测试日志

Process finished with exit code 0

测试二.引入logback 依赖：也再需要slf4j-api依赖，logback-classic中包含了对slf4j-api的依赖


    <!--
     <dependency>
      <groupId>org.slf4j</groupId>
      <artifactId>slf4j-api</artifactId>
      <version>1.7.30</version>
    </dependency>

    <dependency>
      <groupId>org.slf4j</groupId>
      <artifactId>slf4j-log4j12</artifactId>
      <version>1.7.30</version>
    </dependency>
    -->
    <dependency>
      <groupId>ch.qos.logback</groupId>
      <artifactId>logback-classic</artifactId>
      <version>1.2.3</version>
    </dependency>

同时新建logback.xml示例如下：


<?xml version="1.0" encoding="UTF-8"?>
<configuration>
    <!--在slf4j中，从小到大的日志级别依旧是trace、debug、info、warn、error-->
    <appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
        <!--这里定义了DEBUG，也就是控制台不会输出比DEGUG级别小的日志-->
        <filter class="ch.qos.logback.classic.filter.ThresholdFilter">
            <level>DEBUG</level>
        </filter>
        <encoder>
            <pattern>[%thread] [%cyan(%-5level)] %cyan(%logger{30}) - %msg%n</pattern>
        </encoder>
    </appender>
    <root level="debug">
        <appender-ref ref="STDOUT"/>
    </root>
</configuration>

运行效果如下，正确打印日志：

[main] [INFO ] com.test.App - 这是一个测试日志

Process finished with exit code 0

测试三.引入slf4j-simple依赖：

<!--
    <dependency>
      <groupId>org.slf4j</groupId>
      <artifactId>slf4j-api</artifactId>
      <version>1.7.30</version>
    </dependency>
    
    <dependency>
      <groupId>org.slf4j</groupId>
      <artifactId>slf4j-log4j12</artifactId>
      <version>1.7.30</version>
    </dependency>

    <dependency>
      <groupId>ch.qos.logback</groupId>
      <artifactId>logback-classic</artifactId>
      <version>1.2.3</version>
    </dependency>
    -->
   <dependency>
      <groupId>org.slf4j</groupId>
      <artifactId>slf4j-simple</artifactId>
      <version>1.7.30</version>
    </dependency>

运行效果如下，正确打印日志：

[main] INFO com.test.App - 这是一个测试日志

Process finished with exit code 0

以上实验说明，我们在做日志的时候使用slf4j接口，具体日志输出交由其实现类去实现，后期维护、更换都很方便。

此时有个问题，slf4j有多个实现类，如果同时引入了多种实现，能否正常输出日志呢？

那么我们继续做实验：

测试四 同时引入多个实现：

<!--<dependency>
      <groupId>org.slf4j</groupId>
      <artifactId>slf4j-api</artifactId>
      <version>1.7.30</version>
    </dependency>-->

    <dependency>
      <groupId>org.slf4j</groupId>
      <artifactId>slf4j-log4j12</artifactId>
      <version>1.7.30</version>
    </dependency>

    <dependency>
      <groupId>ch.qos.logback</groupId>
      <artifactId>logback-classic</artifactId>
      <version>1.2.3</version>
    </dependency>
   <dependency>
      <groupId>org.slf4j</groupId>
      <artifactId>slf4j-simple</artifactId>
      <version>1.7.30</version>
    </dependency>

运行效果如下: 报告引入了多个slf4j实现，最后选择了一个实现日志输出，正常输出了日志

SLF4J: Class path contains multiple SLF4J bindings.
SLF4J: Found binding in [jar:file:/D:/mvn_repository/org/slf4j/slf4j-log4j12/1.7.30/slf4j-log4j12-1.7.30.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: Found binding in [jar:file:/D:/mvn_repository/ch/qos/logback/logback-classic/1.2.3/logback-classic-1.2.3.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: Found binding in [jar:file:/D:/mvn_repository/org/slf4j/slf4j-simple/1.7.30/slf4j-simple-1.7.30.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]
[INFO ] - [2021-04-09 14:48:52:020] main: com.test.App.main(App.java:14) 这是一个测试日志

Process finished with exit code 0

我们就看一看这后面的实现逻辑是什么：先看LoggerFactory.getLogger(App.class)

public class App 
{
    private static Logger logger = LoggerFactory.getLogger(App.class);
    public static void main( String[] args )
    {
        logger.info("这是一个测试日志");
    }
}

继续看getLogger方法

public static Logger getLogger(Class<?> clazz) {
        Logger logger = getLogger(clazz.getName());
        if (DETECT_LOGGER_NAME_MISMATCH) {
            Class<?> autoComputedCallingClass = Util.getCallingClass();
            if (autoComputedCallingClass != null && nonMatchingClasses(clazz, autoComputedCallingClass)) {
                Util.report(String.format("Detected logger name mismatch. Given name: \"%s\"; computed name: \"%s\".", logger.getName(),
                                autoComputedCallingClass.getName()));
                Util.report("See " + LOGGER_NAME_MISMATCH_URL + " for an explanation");
            }
        }
        return logger;
    }

performInitaalization()

    public static ILoggerFactory getILoggerFactory() {
        if (INITIALIZATION_STATE == UNINITIALIZED) {
            synchronized (LoggerFactory.class) {
                if (INITIALIZATION_STATE == UNINITIALIZED) {
                    INITIALIZATION_STATE = ONGOING_INITIALIZATION;
                    performInitialization();
                }
            }
        }
        switch (INITIALIZATION_STATE) {
        case SUCCESSFUL_INITIALIZATION:
            return StaticLoggerBinder.getSingleton().getLoggerFactory();
        case NOP_FALLBACK_INITIALIZATION:
            return NOP_FALLBACK_FACTORY;
        case FAILED_INITIALIZATION:
            throw new IllegalStateException(UNSUCCESSFUL_INIT_MSG);
        case ONGOING_INITIALIZATION:
            // support re-entrant behavior.
            // See also http://jira.qos.ch/browse/SLF4J-97
            return SUBST_FACTORY;
        }
        throw new IllegalStateException("Unreachable code");
    }
}

private final static void performInitialization() {
        bind();
        if (INITIALIZATION_STATE == SUCCESSFUL_INITIALIZATION) {
            versionSanityCheck();
        }
    }

bind()

private final static void bind() {
        try {
            Set<URL> staticLoggerBinderPathSet = null;
            // skip check under android, see also
            // http://jira.qos.ch/browse/SLF4J-328
            if (!isAndroid()) {
                staticLoggerBinderPathSet = findPossibleStaticLoggerBinderPathSet();
                reportMultipleBindingAmbiguity(staticLoggerBinderPathSet);
            }
            // the next line does the binding
            StaticLoggerBinder.getSingleton();
            INITIALIZATION_STATE = SUCCESSFUL_INITIALIZATION;
            reportActualBinding(staticLoggerBinderPathSet);
        } 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.contains("org.slf4j.impl.StaticLoggerBinder.getSingleton()")) {
                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);
        } finally {
            postBindCleanUp();
        }
    }

findPossibleStaticLoggerBinderPathSet() 会去classpath下找STATIC_LOGGER_BINDER_PATH，STATIC_LOGGER_BINDER_PATH值为"org/slf4j/impl/StaticLoggerBinder.class,即所有slf4j的实现，在提供的jar包路径下，一定是有"org/slf4j/impl/StaticLoggerBinder.class"存在的，我们可以看一下：

static Set<URL> findPossibleStaticLoggerBinderPathSet() {
        // use Set instead of list in order to deal with bug #138
        // LinkedHashSet appropriate here because it preserves insertion order
        // during iteration
        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 = paths.nextElement();
                staticLoggerBinderPathSet.add(path);
            }
        } catch (IOException ioe) {
            Util.report("Error getting resources from path", ioe);
        }
        return staticLoggerBinderPathSet;
    }

不能避免在系统中同时引入多个slf4j的实现，所以接收的地方是一个Set。大家应该注意到，上部分在演示同时引入logback、slf4j-simple、log4j的时候会有警告。

private static void reportMultipleBindingAmbiguity(Set<URL> binderPathSet) {
        if (isAmbiguousStaticLoggerBinderPathSet(binderPathSet)) {
            Util.report("Class path contains multiple SLF4J bindings.");
            for (URL path : binderPathSet) {
                Util.report("Found binding in [" + path + "]");
            }
            Util.report("See " + MULTIPLE_BINDINGS_URL + " for an explanation.");
        }
    }

当集合size大于1时，会报告有多个实现：

private static boolean isAmbiguousStaticLoggerBinderPathSet(Set<URL> binderPathSet) {
        return binderPathSet.size() > 1;
    }

报告选择的实现类：

private static void reportActualBinding(Set<URL> binderPathSet) {
        // binderPathSet can be null under Android
        if (binderPathSet != null && isAmbiguousStaticLoggerBinderPathSet(binderPathSet)) {
            Util.report("Actual binding is of type [" + StaticLoggerBinder.getSingleton().getLoggerFactoryClassStr() + "]");
        }
    }