slf4j的含义为
Simple logging facade for Java，其为简单的为java实现的日志打印工具，本文则对其源码进行简单的分析
JAVA调用SLF4J
public class Test{
private static fianl Logger log = LoggerFactory.getLogger(Test.class) ;
public static void main(String[] args){
log.debug();
log.info();
log.error();
log.fatal();
}
}
注意调用slf4j接口用的是
LoggerFactory.getLogger()方法，与log4j调用的LogManager.getLogger()有点区别
Logger接口
内部属性概览
final public String ROOR_LOGGER_NAME = "ROOT" ;
public boolean isTraceEnabled();
public void trace(String msg);
....
....
简单的看也就是定义了相应的级别输出方法，比如trace()/info()/error()/debug()等
LoggerFactory内部属性
其为final类型的class。罗列部分属性，具体如下
//代表日志工具的初始化状态
static final int UNINITIALIZED = 0;
static final int ONGOING_INITILIZATION = 1;
static final int FAILED_INITILIZATION = 2;
static final int SUCCESSFUL_INITILIZATION = 3;
static final int NOP_FALLBACK_INITILIZATION = 4;
//返回的均为NOPLogger类，即不打印任何日志信息
static SubstituteLoggerFactory TEMP_FACTORY = new SubstituteLoggerFactory();
static NOPLoggerFactory NOP_FALLBACK_FACTORY = new NOPLoggerFactory();
LoggerFactory#getLogger()-获取日志对象
其为静态方法，源码如下
public static Logger getLogger(String name) {
ILoggerFactory iLoggerFactory = getILoggerFactory();
return iLoggerFactory.getLogger(name);
}
继续查看下LoggerFactory#getILoggerFactory()方法
LoggerFactory#getILoggerFactory()-获取真实的日志工厂
源码如下
public static ILoggerFactory getILoggerFactory() {
//初次获取，初始化状态为0
if (INITIALIZATION_STATE == UNINITIALIZED) {
//状态置为1
INITIALIZATION_STATE = ONGOING_INITILIZATION;
//进行初始化
performInitialization();
}
switch (INITIALIZATION_STATE) {
case SUCCESSFUL_INITILIZATION:
//返回初始化成功的日志对象
return StaticLoggerBinder.getSingleton().getLoggerFactory();
case NOP_FALLBACK_INITILIZATION:
return NOP_FALLBACK_FACTORY;
case FAILED_INITILIZATION:
throw new IllegalStateException(UNSUCCESSFUL_INIT_MSG);
case ONGOING_INITILIZATION:
// support re-entrant behavior.
// See also http://bugzilla.slf4j.org/show_bug.cgi?id=106
return TEMP_FACTORY;
}
throw new IllegalStateException("Unreachable code");
}
继续观察LoggerFactory#performInitialization()方法
LoggerFactory#performInitialization()-初始化日志操作
private final static void performInitialization() {
//检查项目部署的环境即classpath下有无StaticLoggerBinder.class
singleImplementationSanityCheck();
//有则开始绑定
bind();
if (INITIALIZATION_STATE == SUCCESSFUL_INITILIZATION) {
//对slf4j支持的版本进行确认
versionSanityCheck();
}
}
分别看下LoggerFactory#singleImplementationSanityCheck()方法和LoggerFactory#bind()方法
LoggerFactory#singleImplementationSanityCheck()-特定类存在判断
特定类指的是org/slf4j/impl/StaticLoggerBinder.class。具体源码如下
private static void singleImplementationSanityCheck() {
try {
//获取类加载器
ClassLoader loggerFactoryClassLoader = LoggerFactory.class
.getClassLoader();
//存放特定类的个数，当无相应的资源返回为空集合，但不为null
Enumeration paths;
if (loggerFactoryClassLoader == null) {
paths = ClassLoader.getSystemResources(STATIC_LOGGER_BINDER_PATH);
} else {
paths = loggerFactoryClassLoader
.getResources(STATIC_LOGGER_BINDER_PATH);
}
List implementationList = new ArrayList();
//对获取org/slf4j/impl/StaticLoggerBinder.class的资源进行遍历
while (paths.hasMoreElements()) {
URL path = (URL) paths.nextElement();
implementationList.add(path);
}
//打印成功日志
if (implementationList.size() > 1) {
Util.report("Class path contains multiple SLF4J bindings.");
for (int i = 0; i < implementationList.size(); i++) {
Util.report("Found binding in [" + implementationList.get(i) + "]");
}
Util.report("See " + MULTIPLE_BINDINGS_URL + " for an explanation.");
}
} catch (IOException ioe) {
Util.report("Error getting resources from path", ioe);
}
}
主要任务是判断是否存在
org/slf4j/impl/StaticLoggerBinder.class，其在为bind()方法作预备调查，此方法目的是打印一些帮助信息。注意此处不建议拥有多个StaticLoggerBinder类，一般要求只有单个StaticLoggerBinder类存在，不然则会导致日志输出不了
LoggerFactory#bind()-绑定获取真实的日志处理类
bind()方法的处理逻辑显得就很有意思了
private final static void bind() {
try {
//获取StaticLoggerBinder单例
// the next line does the binding
StaticLoggerBinder.getSingleton();
INITIALIZATION_STATE = SUCCESSFUL_INITILIZATION;
emitSubstituteLoggerWarning();
} catch (NoClassDefFoundError ncde) {
//对无此类的异常处理
String msg = ncde.getMessage();
//如果错误信息含有org/slf4j/impl/StaticLoggerBinder信息则设置初始化状态为4
if (msg != null && msg.indexOf("org/slf4j/impl/StaticLoggerBinder") != -1) {
INITIALIZATION_STATE = NOP_FALLBACK_INITILIZATION;
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 {
//状态为2，并抛出异常
failedBinding(ncde);
throw ncde;
}
} catch(java.lang.NoSuchMethodError nsme) {
//对StaticLoggerBinder类无getSingleton()方法做处理
String msg = nsme.getMessage();
if (msg != null && msg.indexOf("org.slf4j.impl.StaticLoggerBinder.getSingleton()") != -1) {
INITIALIZATION_STATE = FAILED_INITILIZATION;
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. or 2.0.x");
}
throw nsme;
} catch (Exception e) {
failedBinding(e);
throw new IllegalStateException("Unexpected initialization failure", e);
}
}
bind()方法对
StaticLoggerBinder#getSingleton()方法做了异常捕获处理，处理逻辑如下：
对不存在StaticLoggerBinder类的
NoClassDefFoundError异常，如果错误信息含有org/slf4j/impl/StaticLoggerBinder信息则不抛出异常，但设置状态为NOP_FALLBACK_INITILIZATION(4);反之则直接抛出异常，并设置状态为FAILED_INITILIZATION(2)对存在StaticLoggerBinder类但不存在getSingleton()方法的
NoSuchMethodError异常，均抛出异常，并设置状态为FAILED_INITILIZATION(2)
LoggerFactory#versionSanityCheck()-日志版本要求验证
源码如下
private final static void versionSanityCheck() {
try {
//此处为1.6
String requested = StaticLoggerBinder.REQUESTED_API_VERSION;
//判断是否与API的版本一致，此处为true
boolean match = false;
for (int i = 0; i < API_COMPATIBILITY_LIST.length; i++) {
if (requested.startsWith(API_COMPATIBILITY_LIST[i])) {
match = true;
}
}
if (!match) {
Util.report("The requested version " + requested
+ " by your slf4j binding is not compatible with "
+ Arrays.asList(API_COMPATIBILITY_LIST).toString());
Util.report("See " + VERSION_MISMATCH + " for further details.");
}
} catch (java.lang.NoSuchFieldError nsfe) {
// given our large user base and SLF4J's commitment to backward
// compatibility, we cannot cry here. Only for implementations
// which willingly declare a REQUESTED_API_VERSION field do we
// emit compatibility warnings.
} catch (Throwable e) {
// we should never reach here
Util.report("Unexpected problem occured during version sanity check", e);
}
}
目前的slf4j版本为1.6
小结
slf4j相当于是一个抽象接口，其会判断classpath下是否存在
StaticLoggerBinder类，并针对此类进行相应的逻辑处理，于此我们可以判断出，其可以很好的被其他日志API接入，比如logback等
下节内容预告
针对返回状态为
SUCCESSFUL_INITILIZATION、NOP_FALLBACK_INITILIZATION、FAILED_INITILIZATION、ONGOING_INITILIZATION时，创建的为何种ILoggerFactory，详情见SLF4J源码解析-LoggerFactory(二)