按照绑定实现类的方式是基于约定原则：判断分以下几个步骤
1.LoggerFactory扫描实现类路径有几个实现类，即在org/slf4j/impl/下有几个StaticLoggerBinder.class
2.如果有多个实现类，向开发人员报告多个实现类的路径
3.如果有多个实现类，向开发人员报告真正绑定的是哪一个实现类
4.如果没有实现类，怎么办？
具体代码实现
//要扫描的文件路径
private static String STATIC_LOGGER_BINDER_PATH = "org/slf4j/impl/StaticLoggerBinder.class";
//扫描所有的StaticLoggerBinder.class路径放入到set集合
private static Set findPossibleStaticLoggerBinderPathSet() {
// use Set instead of list in order to deal with bug #138
// LinkedHashSet appropriate here because it preserves insertion order during iteration
Set staticLoggerBinderPathSet = new LinkedHashSet();
try {
ClassLoader loggerFactoryClassLoader = LoggerFactory.class
.getClassLoader();
Enumeration 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;
}
//报告所有的实现类路径
private static void reportMultipleBindingAmbiguity(Set staticLoggerBinderPathSet) {
if (isAmbiguousStaticLoggerBinderPathSet(staticLoggerBinderPathSet)) {
Util.report("Class path contains multiple SLF4J bindings.");
Iterator iterator = staticLoggerBinderPathSet.iterator();
while (iterator.hasNext()) {
URL path = (URL) iterator.next();
Util.report("Found binding in [" + path + "]");
}
Util.report("See " + MULTIPLE_BINDINGS_URL + " for an explanation.");
}
}
//报告真正用到的实现类名称
private static void reportActualBinding(Set staticLoggerBinderPathSet) {
if (isAmbiguousStaticLoggerBinderPathSet(staticLoggerBinderPathSet)) {
Util.report("Actual binding is of type ["+StaticLoggerBinder.getSingleton().getLoggerFactoryClassStr()+"]");
}
}
//去拿到实现类的实例，因为StaticLoggerBinder类不一定存在，所以要捕获NoClassDefFoundError异常
private final static void bind() {
try {
Set staticLoggerBinderPathSet = findPossibleStaticLoggerBinderPathSet();
reportMultipleBindingAmbiguity(staticLoggerBinderPathSet);
// the next line does the binding
StaticLoggerBinder.getSingleton();
INITIALIZATION_STATE = SUCCESSFUL_INITIALIZATION;
reportActualBinding(staticLoggerBinderPathSet);
emitSubstituteLoggerWarning();
} 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);
}
}
到此，就真正拿到实现类的实例了
slf自动绑定实现类过程判断