源码解读SLF4J绑定日志实现的原理

一、导读

我们使用log4j框架时，经常会用slf4j-api。在运行时，经常会遇到如下的错误提示：

SLF4J: Class path contains multiple SLF4J bindings.
SLF4J: Found binding in [jar:file:/Users/abc/maven-repository/org/slf4j/slf4j-simple/1.7.26/slf4j-simple-1.7.26.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: Found binding in [jar:file:/Users/abc/maven-repository/org/apache/logging/log4j/log4j-slf4j-impl/2.7/log4j-slf4j-impl-2.7.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.SimpleLoggerFactory]

也即classpath下有多个日志实现，slf4j-api在绑定时出现了冲突。那么这时slf4j-api到底会绑定哪一个具体的实现呢？

 

官方文档有如下说明：

 

也就是说，当有多个日志实现时，SLF4J会在编译期随机选择其中的一个实现。那么，它到底是如何随机选择的呢？下面我们通过源码来分析。

请尊重作者劳动成果，转载请标明原文链接：https://www.cnblogs.com/waterystone/p/11329645.html

 

二、源码分析

2.1 LoggerFactory.getLogger(this.getClass())

我们通过LoggerFactory.getLogger(this.getClass())拿到Logger对象，其代码如下：

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;
}

　　

2.2 LoggerFactory.bind()

getLogger() -> getILoggerFactory()-> performInitialization() -> bind() ，bind()即绑定日志实现的函数，其源码如下：

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(); //在classpath下查找有多少个日志实现
            reportMultipleBindingAmbiguity(staticLoggerBinderPathSet); //如果有多个日志实现，打印出来
        }
        // the next line does the binding。classpath下每个日志实现jar都会有org.slf4j.impl.StaticLoggerBinder类，这里会随机加载其中的一个。
        StaticLoggerBinder.getSingleton();
        INITIALIZATION_STATE = SUCCESSFUL_INITIALIZATION;
        reportActualBinding(staticLoggerBinderPathSet);
        fixSubstituteLoggers();
        replayEvents();
        // release all resources in SUBST_FACTORY
        SUBST_FACTORY.clear();
    } catch (NoClassDefFoundError ncde) { //如果在classpath下没有找到一个org.slf4j.impl.StaticLoggerBinder
        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);
    }
}

　　

2.3 LoggerFactory.findPossibleStaticLoggerBinderPathSet()

接下来，我们再看看findPossibleStaticLoggerBinderPathSet()如何在classpath下查找有多少个日志实现的源码：

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) { //用ClassLoader去查找classpath下有多少个org.slf4j.impl.StaticLoggerBinder类
            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;
}

　　

通过源码我们可以看到，findPossibleStaticLoggerBinderPathSet()会用ClassLoader查找当前classpath下有多少个org.slf4j.impl.StaticLoggerBinder类，每个path的URL能定位到其具体jar包位置。每个日志实现jar包都会有个org.slf4j.impl.StaticLoggerBinder类，反过来，那么classpath下有多少个StaticLoggerBinder类，就会有多少个相应的jar包，也即有多少个日志实现。所以findPossibleStaticLoggerBinderPathSet()通过扫描classpath下的org.slf4j.impl.StaticLoggerBinder类就能找到有多少个日志实现。

 

接下来，我们再看看是不是每个日志实现jar包都会有个org.slf4j.impl.StaticLoggerBinder类呢？答案是YES。

 

三、具体日志实现

3.1 slf4j-simple

 

3.2 slf4j-nop

 

3.3 logback-classic

 

3.4 log4j-slf4j-impl

 

四、总结

• 每个日志实现jar都会有org.slf4j.impl.StaticLoggerBinder类的实现；
• SLF4J会通过ClassLoad扫描当前classpath下有多少个org.slf4j.impl.StaticLoggerBinder类，也就找到了有多少个日志实现（通过这样，我们只需在项目中加入日志实现的jar包，编译时即可自动加载，业务代码无须显式依赖，实现解耦）；
• 如果有多个org.slf4j.impl.StaticLoggerBinder类，SLF4J会在LoggerFactory.bind()里调用StaticLoggerBinder.getSingleton()随机加载一个日志实现jar的StaticLoggerBinder。
• 发现一个有趣的现象，在本地idea上运行时，是加载pom.xml里声明的第一个日志实现；但是如果打包好后通过java -jar启动时，其加载的日志实现确实是随机的（是在编译打包时随机加载一个日志实现，所以一旦编译打包好后其加载的那个日志实现就会固定不变）。所以，我们在具体使用时一定要通过排除依赖的方式来确定日志实现，不要由于日志实现的不确定性引入难以排查、不必要的坑。