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SLF4J源码解析（一）

阅读源码之前，首先提几个问题

SLF4J是如何整合不同的日志框架的
Class Path中为什么只能有且仅有一种日志框架的binding

这段文字摘录自官网：In your code, in addition to slf4j-api-1.8.0-beta2.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.

源码版本

org.slf4j:slf4j-api:1.7.25
org.apache.logging.log4j:log4j-slf4j-impl:2.11.1
ch.qos.logback:logback-classic:1.2.3
org.slf4j:slf4j-jcl:1.7.25

源码解析

带着上面的两个问题看下源码

bind()方法中通过调用findPossibleStaticLoggerBinderPathSet()方法来查找日志框架的绑定

// org/slf4j/slf4j-api/1.7.25/slf4j-api-1.7.25.jar!/org/slf4j/LoggerFactory.class
private static final void bind() {
    // ...
    if (!isAndroid()) {
        // 查找日志框架的绑定
        staticLoggerBinderPathSet = findPossibleStaticLoggerBinderPathSet();
        reportMultipleBindingAmbiguity(staticLoggerBinderPathSet);
    }
    // ...
}
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findPossibleStaticLoggerBinderPathSet()方法中通过ClassLoader或者loggerFactoryClassLoader来获取名为"STATIC_LOGGER_BINDER_PATH"的Resources "STATIC_LOGGER_BINDER_PATH"的值在文件一开始已经定义过了

private static String STATIC_LOGGER_BINDER_PATH = "org/slf4j/impl/StaticLoggerBinder.class";
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// org/slf4j/slf4j-api/1.7.25/slf4j-api-1.7.25.jar!/org/slf4j/LoggerFactory.class
static Set<URL> findPossibleStaticLoggerBinderPathSet() {
    LinkedHashSet staticLoggerBinderPathSet = new LinkedHashSet();

    try {
        ClassLoader loggerFactoryClassLoader = LoggerFactory.class.getClassLoader();
        Enumeration paths;
        // 通过ClassLoader或者loggerFactoryClassLoader来获取Resources
        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 var4) {
        Util.report("Error getting resources from path", var4);
    }

    return staticLoggerBinderPathSet;
}
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这里拿整合Log4j举例，"org/slf4j/impl/StaticLoggerBinder.class"就在log4j的log4j-slf4j-impl库中，这个类负责初始化Log4j相关的类。StaticLoggerBinder实现了LoggerFactoryBinder接口。

// org/apache/logging/log4j/log4j-slf4j-impl/2.11.1/log4j-slf4j-impl-2.11.1.jar!/org/slf4j/impl/StaticLoggerBinder.class

// 这里的包名定义为org.slf4j.impl，使得上述第2步的Class Loader可以加载到这个类
package org.slf4j.impl;

public final class StaticLoggerBinder implements LoggerFactoryBinder {
// log4j init...
}
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其他日志框架也定义了StaticLoggerBinder类，且实现了LoggerFactoryBinder接口。例如： logback，jcl

// ch/qos/logback/logback-classic/1.2.3/logback-classic-1.2.3.jar!/org/slf4j/impl/StaticLoggerBinder.class

// 这里的包名定义为org.slf4j.impl，使得上述第2步的Class Loader可以加载到这个类
package org.slf4j.impl;

public class StaticLoggerBinder implements LoggerFactoryBinder {
// logback init...
}
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// org/slf4j/slf4j-jcl/1.7.25/slf4j-jcl-1.7.25.jar!/org/slf4j/impl/StaticLoggerBinder.class

// 这里的包名定义为org.slf4j.impl，使得上述第2步的Class Loader可以加载到这个类
package org.slf4j.impl;

public class StaticLoggerBinder implements LoggerFactoryBinder {
// jcl init...
}
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问题答案

SLF4J是如何整合不同的日志框架的 SLF4J通过加载各个底层日志框架桥接库的org/slf4j/impl/StaticLoggerBinder.class来加载初始化对应的日志框架。
Class Path中为什么只能有且仅有一种日志框架的binding slf4j-api中的LoggerFactory中，在上述第2步返回staticLoggerBinderPathSet后，会立马调用 reportMultipleBindingAmbiguity(staticLoggerBinderPathSet)方法来检测是否有多个binding。如果有多个binding，就输出错误信息"Class path contains multiple SLF4J bindings."

private static void reportMultipleBindingAmbiguity(Set<URL> binderPathSet) {
    if (isAmbiguousStaticLoggerBinderPathSet(binderPathSet)) {
        Util.report("Class path contains multiple SLF4J bindings.");
        Iterator i$ = binderPathSet.iterator();

        while(i$.hasNext()) {
            URL path = (URL)i$.next();
            Util.report("Found binding in [" + path + "]");
        }

        Util.report("See http://www.slf4j.org/codes.html#multiple_bindings for an explanation.");
    }

}
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