使用方式：
在系统开发中，统一按照slf4j的API进行开发，在部署时，选择不同的日志系统包，即可自动转换到不同的日志系统上。比如：选择JDK自带的日志系统，则只需要将slf4j-api-1.5.10.jar和slf4j-jdk14-1.5.10.jar放置到classpath中即可，如果中途无法忍受JDK自带的日志系统了，想换成log4j的日志系统，仅需要用slf4j-log4j12-1.5.10.jar替换slf4j-jdk14-1.5.10.jar即可（当然也需要log4j的jar及配置文件）
使用场景：
我们开发的是类库或者嵌入式组件，可以考虑使用slf4j，因为我们并不能决定用户选择哪种日志系统（不同软件开发公司会钟情于不同的日志系统）；但是如果我们开发独立应用，面向的是最终客户，则无需考虑slf4j，因为最终客户只关心功能实现，不会在意开发公司具体使用什么日志系统的。
应用举例
我们先举个实际例子，让大家有个直观认识，建立一个简单测试类，如下：
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package chb.test.slf4j;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
/**
* @author chb
*
*/
public class TestSlf4j {
Logger log = LoggerFactory.getLogger(TestSlf4j.class);
public void testLog(){
log.info("this is a test log");
}
/**
* @param args
*/
public static void main(String[] args) {
TestSlf4j slf = new TestSlf4j();
slf.testLog();
}
}
package chb.test.slf4j;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
/**
* @author chb
*
*/
public class TestSlf4j {
Logger log = LoggerFactory.getLogger(TestSlf4j.class);
public void testLog(){
log.info("this is a test log");
}
/**
* @param args
*/
public static void main(String[] args) {
TestSlf4j slf = new TestSlf4j();
slf.testLog();
}
}
1>JDK自带的log输出
首先，我们在classpath中加入slf4j-api-1.5.10.jar和slf4j-jdk14-1.5.10.jar两个包，然后运行main函数，输出信息如下：
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2010-1-5 21:44:47 chb.test.slf4j.TestSlf4j testLog
信息: this is a test log
2010-1-5 21:44:47 chb.test.slf4j.TestSlf4j testLog
信息: this is a test log
2>slg4j提供的simple log
然后，我们用slf4j-simple-1.5.10.jar替换slf4j-jdk14-1.5.10.jar，选择使用slf4j提供的simple log，输出信息如下：
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0 [main] INFO chb.test.slf4j.TestSlf4j - this is a test log
0 [main] INFO chb.test.slf4j.TestSlf4j - this is a test log
3>log4j日志输出
再然后，我们再用slf4j-log4j12-1.5.10.jar替换slf4j-simple-1.5.10.jar（记得classpath也需要增加log4j依赖jar包），同时增加一个log4j.properties文件，内容如下：
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log4j.debug=true
log4j.rootLogger=DEBUG,stdout
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.Target=System.out
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%d{ABSOLUTE} %5p - %m%n
log4j.debug=true
log4j.rootLogger=DEBUG,stdout
log4j.appender.stdout=org.apache.log4j.ConsoleAppender
log4j.appender.stdout.Target=System.out
log4j.appender.stdout.layout=org.apache.log4j.PatternLayout
log4j.appender.stdout.layout.ConversionPattern=%d{ABSOLUTE} %5p - %m%n
我们再稍微修改一下main函数，加载一下log4j.properties，如；
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public static void main(String[] args) {
System.setProperty("log4j.configuration", "log4j.properties");
TestSlf4j slf = new TestSlf4j();
slf.testLog();
}
public static void main(String[] args) {
System.setProperty("log4j.configuration", "log4j.properties");
TestSlf4j slf = new TestSlf4j();
slf.testLog();
}
运行main函数，输出结果如下：
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log4j: Parsing for [root] with value=[DEBUG,stdout].
log4j: Level token is [DEBUG].
log4j: Category root set to DEBUG
log4j: Parsing appender named "stdout".
log4j: Parsing layout options for "stdout".
log4j: Setting property [conversionPattern] to [%d{ABSOLUTE} %5p - %m%n ].
log4j: End of parsing for "stdout".
log4j: Setting property [target] to [System.out].
log4j: Parsed "stdout" options.
log4j: Finished configuring.
22:01:40,831 INFO - this is a test log
log4j: Parsing for [root] with value=[DEBUG,stdout].
log4j: Level token is [DEBUG].
log4j: Category root set to DEBUG
log4j: Parsing appender named "stdout".
log4j: Parsing layout options for "stdout".
log4j: Setting property [conversionPattern] to [%d{ABSOLUTE} %5p - %m%n ].
log4j: End of parsing for "stdout".
log4j: Setting property [target] to [System.out].
log4j: Parsed "stdout" options.
log4j: Finished configuring.
22:01:40,831 INFO - this is a test log
原理介绍--静态绑定
大家看到要使用哪种日志系统，只需要将对应的日志系统所需要的jar包文件（包括slf4j提供的jar包和日志系统自身依赖的jar包，例如：slf4j-log4j12-1.5.10.jar和log4j.1.2.jar）放入classpath即可，slf4j可以自动探测具体使用哪种日志系统，这种技术被称为静态绑定。
在实际使用中，我们通过LoggerFactory.getLogger()获得logger，查看LoggerFactory的源代码会发现如下两点，
LoggerFactory通过StaticLoggerBinder.getSingleton().getLoggerFactory()获得LogFactory，然后再通过该LogFactory来获取logger的
但是StaticLoggerBinder类并不在slf4j-api-1.5.10.jar中，分析与具体日志系统相关的jar包，会发现每个jar包都有一个StaticLoggerBinder类的实现(如slf4j-log4j12-1.5.10.jar、slf4j-simple-1.5.10.jar、slf4j-jdk14-1.5.10.jar均有StaticLoggerBinder类实现)，这就很明白了，slf4j在启动时会动态到classpath中查找StaticLoggerBinder类，找到之后就可以生成对应日志系统的日志文件了。
这里就有一个问题了，slf4j是如何将自己的通用日志格式转成不同的日志系统的格式的呢？
我们再分析每个日志系统相关的源代码，会发现不同日志系统包都会有一个Adapter，用来在slf4j和不同日志系统之间做转换。
下面是LoggerFactory.java的源代码，大家可以参考一下，为了理解方便，我已经将代码顺序做了调整：
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/**
* Return a logger named corresponding to the class passed as parameter, using
* the statically bound {@link ILoggerFactory} instance.
*
* @param clazz
* the returned logger will be named after clazz
* @return logger
*/
public static Logger getLogger(Class clazz) {
return getLogger(clazz.getName());
}
public static Logger getLogger(String name) {
ILoggerFactory iLoggerFactory = getILoggerFactory();
return iLoggerFactory.getLogger(name);
}
/**
* Return the {@link ILoggerFactory} instance in use.
*
* <p>
* ILoggerFactory instance is bound with this class at compile time.
*
* @return the ILoggerFactory instance in use
*/
public static ILoggerFactory getILoggerFactory() {
if (INITIALIZATION_STATE == UNINITIALIZED) {
INITIALIZATION_STATE = ONGOING_INITILIZATION;
performInitialization();
}
switch (INITIALIZATION_STATE) {
case SUCCESSFUL_INITILIZATION:
return getSingleton().getLoggerFactory();
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");
}
private final static void performInitialization() {
bind();
versionSanityCheck();
singleImplementationSanityCheck();
}
private final static void bind() {
try {
// the next line does the binding
getSingleton();
INITIALIZATION_STATE = SUCCESSFUL_INITILIZATION;
emitSubstituteLoggerWarning();
} catch (NoClassDefFoundError ncde) {
INITIALIZATION_STATE = FAILED_INITILIZATION;
String msg = ncde.getMessage();
if (msg != null && msg.indexOf("org/slf4j/impl/StaticLoggerBinder") != -1) {
Util
.reportFailure("Failed to load class \"org.slf4j.impl.StaticLoggerBinder\".");
Util.reportFailure("See " + NO_STATICLOGGERBINDER_URL
+ " for further details.");
}
throw ncde;
} catch (Exception e) {
INITIALIZATION_STATE = FAILED_INITILIZATION;
// we should never get here
Util.reportFailure("Failed to instantiate logger ["
+ getSingleton().getLoggerFactoryClassStr() + "]", e);
}
}
private final static StaticLoggerBinder getSingleton() {
if (GET_SINGLETON_METHOD == GET_SINGLETON_INEXISTENT) {
return StaticLoggerBinder.getSingleton();
}
if (GET_SINGLETON_METHOD == GET_SINGLETON_EXISTS) {
return StaticLoggerBinder.getSingleton();
}
try {
StaticLoggerBinder singleton = StaticLoggerBinder.getSingleton();
GET_SINGLETON_METHOD = GET_SINGLETON_EXISTS;
return singleton;
} catch (NoSuchMethodError nsme) {
GET_SINGLETON_METHOD = GET_SINGLETON_INEXISTENT;
return StaticLoggerBinder.getSingleton();
}
}
/**
* Return a logger named corresponding to the class passed as parameter, using
* the statically bound {@link ILoggerFactory} instance.
*
* @param clazz
* the returned logger will be named after clazz
* @return logger
*/
public static Logger getLogger(Class clazz) {
return getLogger(clazz.getName());
}
public static Logger getLogger(String name) {
ILoggerFactory iLoggerFactory = getILoggerFactory();
return iLoggerFactory.getLogger(name);
}
/**
* Return the {@link ILoggerFactory} instance in use.
*
* <p>
* ILoggerFactory instance is bound with this class at compile time.
*
* @return the ILoggerFactory instance in use
*/
public static ILoggerFactory getILoggerFactory() {
if (INITIALIZATION_STATE == UNINITIALIZED) {
INITIALIZATION_STATE = ONGOING_INITILIZATION;
performInitialization();
}
switch (INITIALIZATION_STATE) {
case SUCCESSFUL_INITILIZATION:
return getSingleton().getLoggerFactory();
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");
}
private final static void performInitialization() {
bind();
versionSanityCheck();
singleImplementationSanityCheck();
}
private final static void bind() {
try {
// the next line does the binding
getSingleton();
INITIALIZATION_STATE = SUCCESSFUL_INITILIZATION;
emitSubstituteLoggerWarning();
} catch (NoClassDefFoundError ncde) {
INITIALIZATION_STATE = FAILED_INITILIZATION;
String msg = ncde.getMessage();
if (msg != null && msg.indexOf("org/slf4j/impl/StaticLoggerBinder") != -1) {
Util
.reportFailure("Failed to load class \"org.slf4j.impl.StaticLoggerBinder\".");
Util.reportFailure("See " + NO_STATICLOGGERBINDER_URL
+ " for further details.");
}
throw ncde;
} catch (Exception e) {
INITIALIZATION_STATE = FAILED_INITILIZATION;
// we should never get here
Util.reportFailure("Failed to instantiate logger ["
+ getSingleton().getLoggerFactoryClassStr() + "]", e);
}
}
private final static StaticLoggerBinder getSingleton() {
if (GET_SINGLETON_METHOD == GET_SINGLETON_INEXISTENT) {
return StaticLoggerBinder.getSingleton();
}
if (GET_SINGLETON_METHOD == GET_SINGLETON_EXISTS) {
return StaticLoggerBinder.getSingleton();
}
try {
StaticLoggerBinder singleton = StaticLoggerBinder.getSingleton();
GET_SINGLETON_METHOD = GET_SINGLETON_EXISTS;
return singleton;
} catch (NoSuchMethodError nsme) {
GET_SINGLETON_METHOD = GET_SINGLETON_INEXISTENT;
return StaticLoggerBinder.getSingleton();
}
}
作者在 2011-07-06 09:33:49 发布以下内容
准确的说，slf4j并不是一种具体的日志系统，而是一个用户日志系统的facade，允许用户在部署最终应用时方便的变更其日志系统。
