Java Logging: slf4j + logback
Java日志方案有很多，包括：java.util.logging、Apache的commons-logging和log4j、slf4j以及logback. 一个大型项目会用到众多第三方jar包，这些jar包可能会用到上述各种日志方案，如何在新的项目中使用slf4j+logback的组合，让所有其他jar包的日志也输出到logback，并避免冲突和异常？
SLF4J is a simple facade for logging systems allowing the end-user to plug-in the desired logging system at deployment time.
Logback is intended as a successor to the popular log4j project. It was designed by Ceki Gülcü, log4j’s founder. It builds upon a decade of experience gained in designing industrial-strength logging systems. The resulting product, i.e. logback, is faster and has a smaller footprint than all existing logging systems, sometimes by a wide margin. Just as importantly, logback offers unique and rather useful features missing in other logging systems.
下面这张图能够很好地说明slf4j和各个日志框架集成的原理：
slf4j-api.jar里的org.slf4j.LoggerFactory会调org.slf4j.impl.StaticLoggerBinder，这个StaticLoggerBinder在logback-classic.jar里实现，这样就把slf4j的日志绑定到logback了。这时，如果classpath里同时还有slf4j-log4j12.jar那么会报multiple SLF4j bindings错误，因为slf4j-log4j12.jar里也有StaticLoggerBinder实现。
SLF4J: Class path contains multiple SLF4J bindings.
SLF4J: Found binding in [jar:file:/D:/mvn_repository/ch/qos/logback/logback-classic/1.1.2/logback-classic-1.1.2.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: Found binding in [jar:file:/D:/mvn_repository/org/slf4j/slf4j-log4j12/1.7.6/slf4j-log4j12-1.7.6.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: See http://www.slf4j.org/codes.html#multiple_bindings for an explanation.
SLF4J: Actual binding is of type [ch.qos.logback.classic.util.ContextSelectorStaticBinder]
应用里只能把slf4j绑定到一个最终日志实现方案，可以是logback,log4j,comming-logging或java.util.logging等，本篇主要介绍slf4j绑定logback, 上面这个图就是绑定到logback.
如果应用引用其他的jar是使用log4j,comming-logging或java.util.logging记日志的，要把这些日志也路由到logback上来，首先把这些日志调用路由到slf4j，slf4j再到logback.具体实现是用jcl-over-slf4j.jar替换commons-logging,用log4j-over-slf4j.jar替换log4j.jar, 用jul-to-slf4j.jar替换java.util.logging. 原理也很简单，log4j-over-slf4j.jar里有log4j.jar同名的类，所以有log4j-over-slf4j.jar了就要把log4j.jar删了。但jul-to-slf4j.jar的原理比较特殊一些，因为java.util.logging是jdk自带的，没有办法替换，想了解具体怎么做的可以看下实现代码代码。
maven pom.xml配置:
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>slf4j-api</artifactId>
<version>1.7.6</version>
</dependency>
<dependency>
<groupId>ch.qos.logback</groupId>
<artifactId>logback-classic</artifactId>
<version>1.1.2</version>
</dependency>
<dependency>
<groupId>ch.qos.logback</groupId>
<artifactId>logback-core</artifactId>
<version>1.1.2</version>
</dependency>
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>log4j-over-slf4j</artifactId>
<version>1.7.6</version>
</dependency>
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>jcl-over-slf4j</artifactId>
<version>1.7.6</version>
</dependency>
<dependency>
<groupId>org.slf4j</groupId>
<artifactId>jul-to-slf4j</artifactId>
<version>1.7.6</version>
</dependency>
logback的配置：
<?xml version="1.0" encoding="UTF-8"?>
<configuration>
<!-- Send debug messages to System.out -->
<appender name="STDOUT" class="ch.qos.logback.core.ConsoleAppender">
<!-- By default, encoders are assigned the type ch.qos.logback.classic.encoder.PatternLayoutEncoder -->
<encoder>
<pattern>%d{HH:mm:ss.SSS} [%thread] %-5level %logger{5} - %msg%n</pattern>
</encoder>
</appender>
<!-- Send debug messages to a file -->
<appender name="FILE"
class="ch.qos.logback.core.rolling.RollingFileAppender">
<file>D:/logs/java_toolbox_lbk.log</file>
<encoder class="ch.qos.logback.classic.encoder.PatternLayoutEncoder">
<Pattern>%d{yyyy-MM-dd_HH:mm:ss.SSS} [%thread] %-5level %logger{36} - %msg%n</Pattern>
</encoder>
<rollingPolicy class="ch.qos.logback.core.rolling.FixedWindowRollingPolicy">
<FileNamePattern>D:/logs/java_toolbox_lbk.%i.log.zip</FileNamePattern>
<MinIndex>1</MinIndex>
<MaxIndex>10</MaxIndex>
</rollingPolicy>
<triggeringPolicy class="ch.qos.logback.core.rolling.SizeBasedTriggeringPolicy">
<MaxFileSize>2MB</MaxFileSize>
</triggeringPolicy>
</appender>
<logger name="com.weibo.keeplooking.logging" level="INFO" additivity="false">
<appender-ref ref="STDOUT" />
<appender-ref ref="FILE" />
</logger>
<!-- By default, the level of the root level is set to DEBUG -->
<root level="DEBUG">
<appender-ref ref="STDOUT" />
</root>
</configuration>
测试代码：
import org.junit.Test;
import org.slf4j.bridge.SLF4JBridgeHandler;
public class VariousLogging {
private static final java.util.logging.Logger jucLogger = java.util.logging.Logger.getLogger(VariousLogging.class
.getName());
private static final org.apache.log4j.Logger log4jLogger = org.apache.log4j.Logger.getLogger(VariousLogging.class);
private static final org.apache.commons.logging.Log commonLogger = org.apache.commons.logging.LogFactory
.getLog(VariousLogging.class);
private static final org.slf4j.Logger slf4jLogger = org.slf4j.LoggerFactory.getLogger(VariousLogging.class);
static {
// optionally remove existing handlers attached to j.u.l root logger
SLF4JBridgeHandler.removeHandlersForRootLogger(); // (since SLF4J 1.6.5)
// add SLF4JBridgeHandler to j.u.l's root logger, should be done once during
// the initialization phase of your application
SLF4JBridgeHandler.install();
}
private void logWithJuc() {
jucLogger.info("java util logging...");
}
private void logWithLog4j() {
log4jLogger.info("log4j...");
}
private void logWithCommonLogging() {
commonLogger.info("common logging...");
}
private void logWithSlf4j() {
slf4jLogger.info("slf4j...");
}
@Test
public void testLogging() {
logWithJuc();
logWithLog4j();
logWithCommonLogging();
logWithSlf4j();
}
}
输出结果：
23:16:49.378 [main] INFO c.w.k.l.VariousLogging - java util logging…
23:16:49.382 [main] INFO c.w.k.l.VariousLogging - log4j…
23:16:49.382 [main] INFO c.w.k.l.VariousLogging - common logging…
23:16:49.382 [main] INFO c.w.k.l.VariousLogging - slf4j…
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