1. 整合需要的Jar包
1.1 代码使用SLF4J-API时
1.2 代码使用其他API时
1.3 一个样板例子
Spring全家桶使用得commons-logging API，当我们自己代码想要使用或者切换到SLF4J时；可以使用下面得插件
<!--目前存在commons-logging 及 slf4j两套log api-->
<dependency>
<groupId>org.slf4j</groupId><!--jcl这个插件也是利用了commons-logging载入具体实现时利用得SPI机制来搞得-->
<artifactId>jcl-over-slf4j</artifactId>
</dependency>
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-api</artifactId>
</dependency>
<dependency><!--logback 实现了SLF4J-API接口-->
<groupId>ch.qos.logback</groupId>
<artifactId>logback-classic</artifactId>
<version>1.2.3</version>
</dependency>
2. SLF4J源代码分析(bind的过程及涉及到的JVM机制）
2.1 ver-1.7.25---ClassLoader检测
起始：
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;
}
//从类命获取
public static Logger getLogger(String name) {
ILoggerFactory iLoggerFactory = getILoggerFactory();
return iLoggerFactory.getLogger(name);
}
//获取日志工厂
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");
}
//日志工厂初始化
private final static void performInitialization() {
bind();
if (INITIALIZATION_STATE == SUCCESSFUL_INITIALIZATION) {
versionSanityCheck();
}
}
//具体实现绑定
private final static void bind() {
try {
Set<URL> staticLoggerBinderPathSet = null;
if (!isAndroid()) {//不是安卓系统得时候,需要检查是否有多个org/slf4j/impl/StaticLoggerBinder.class文件；保证SLF4J-API下没有多个兼容插件(1.8后以SPI机制替换)
staticLoggerBinderPathSet = findPossibleStaticLoggerBinderPathSet();
reportMultipleBindingAmbiguity(staticLoggerBinderPathSet);
}
// 真正执行绑定,StaticLoggerBinder不同兼容包实现不一样，但目的一样这个类是单例实例持有一个LoggerFactory得引用；而这个loggerfatory里面都是具体框架logger得适配器类实例
StaticLoggerBinder.getSingleton();
INITIALIZATION_STATE = SUCCESSFUL_INITIALIZATION;
reportActualBinding(staticLoggerBinderPathSet);
fixSubstituteLoggers();
replayEvents();
// release all resources in SUBST_FACTORY
SUBST_FACTORY.clear();
} 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);
}
}
//获取
2.2 ver-1.8 SPI机制
起始:
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;
}
//转从name获取
public static Logger getLogger(String name) {
ILoggerFactory iLoggerFactory = getILoggerFactory();
return iLoggerFactory.getLogger(name);
}
//先获取具体得实现
public static ILoggerFactory getILoggerFactory() {
return getProvider().getLoggerFactory();
}
//先check当前LoggerFactory得状态；如果是未初始化则进行初始化
static SLF4JServiceProvider getProvider() {
if (INITIALIZATION_STATE == UNINITIALIZED) {
synchronized (LoggerFactory.class) {
if (INITIALIZATION_STATE == UNINITIALIZED) {
INITIALIZATION_STATE = ONGOING_INITIALIZATION;
performInitialization();
}
}
}
switch (INITIALIZATION_STATE) {
case SUCCESSFUL_INITIALIZATION:
return PROVIDER;
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_PROVIDER;
}
throw new IllegalStateException("Unreachable code");
}
//封装一下免得难看
private final static void performInitialization() {
bind();
if (INITIALIZATION_STATE == SUCCESSFUL_INITIALIZATION) {
versionSanityCheck();
}
}
//接口实现者的绑定
private final static void bind() {
try {
List<SLF4JServiceProvider> providersList = findServiceProviders();//SPI机制
reportMultipleBindingAmbiguity(providersList);//如果有多个实现类，则BugReport
if (providersList != null && !providersList.isEmpty()) {
PROVIDER = providersList.get(0);//获取第一个实现类，注意Linux和windows都是获取磁盘node值最小的Jar里面的实现类
PROVIDER.initialize();//接口初始化,不同日志框架的兼容包实现不一样Log4J或者logback或者jul等等，
INITIALIZATION_STATE = SUCCESSFUL_INITIALIZATION;
reportActualBinding(providersList);
fixSubstituteLoggers();
replayEvents();
// release all resources in SUBST_FACTORY
SUBST_PROVIDER.getSubstituteLoggerFactory().clear();
} else {
INITIALIZATION_STATE = NOP_FALLBACK_INITIALIZATION;
Util.report("No SLF4J providers were found.");
Util.report("Defaulting to no-operation (NOP) logger implementation");
Util.report("See " + NO_PROVIDERS_URL + " for further details.");
Set<URL> staticLoggerBinderPathSet = findPossibleStaticLoggerBinderPathSet();
reportIgnoredStaticLoggerBinders(staticLoggerBinderPathSet);
}
} catch (Exception e) {
failedBinding(e);
throw new IllegalStateException("Unexpected initialization failure", e);
}
}
//获取实现类List,SPI机制
private static List<SLF4JServiceProvider> findServiceProviders() {
//SPI机制,主要由ServiceLoader这个工具类负责实施
//主要工作流程：从每个Jar包的META-INFO目录下的services文件夹下面获取指定接口的实现类
//要去文件名和接口的全类名一致；
//返回值是所有的Jar包内的所有的实现类Class实例；
ServiceLoader<SLF4JServiceProvider> serviceLoader=ServiceLoader.load(SLF4JServiceProvider.class);
List<SLF4JServiceProvider> providerList = new ArrayList<SLF4JServiceProvider>();
for (SLF4JServiceProvider provider : serviceLoader) {
providerList.add(provider);
}
return providerList;
}
3 吐槽
- Java技术生态里最让人蛋疼得一点是，JDK制定得行业标准时不但慢而且还不一定好使；
- 比如日志标准(先有了Log4J等等JDK才出日志得API还不好使),比如JavaEE标准（被Spring怼成了傻子）等等；
- 等JDK标准出来了，别人已经写了一些框架占了相当大部分市场；
- 而且同一个标准或者工具有很多种实现，不同实现还要加上自己得特定API。。。
- 还有JDK也缺少包管理机制,只有靠maven和gradle是JDK体系外实现解决这个问题。。。