提出问题
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);
}
// ...
}
- 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";
// 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;
}
- 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...
}
- 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...
}
// 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...
}
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.");
}
}
最后
代码中使用SLF4J来记录日志,可以任意切换底层日志框架而不需要修改代码,只需要更新依赖以及日志配置文件即可。
来源:oschina
链接:https://my.oschina.net/xiaominmin/blog/4274980