Java 日志系列（三）：日志使用示例及常见报错

承接前面两篇文章（《Java 日志系列一》和《Java 日志系列二》），本文将介绍几种主流日志框架的使用示例和常见的报错。为了便于读者理解，文中的报错案例力求信息完整，并给出了测试代码，感兴趣的读者，可以通过示例快速实践。

《Java 日志系列一：详解主流日志框架Log4j、Log4j 2、JUL、Commons Logging和Slf4j&Logback》
《Java 日志系列二：Java 日志使用中需要遵循的规范及注意事项》

1.日志框架使用示例

1.1 Log4j 使用示例

maven 依赖：

<dependency><groupId>log4j</groupId><artifactId>log4j</artifactId><version>1.2.17</version>
</dependency>

配置文件：
log4j.properties

### 设置
log4j.rootLogger = debug,stdout,INFO
### 输出信息到控制台，测试的时候便于观察，实际应用中不配置 ###
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 = [%-5p] %d{yyyy-MM-dd HH:mm:ss,SSS} method:%l%n%m%n
### 输出 INFO 级别以上的日志到指定文件
log4j.appender.INFO = org.apache.log4j.DailyRollingFileAppender
log4j.appender.INFO.File = ~/Code/logs/log4j.log
log4j.appender.INFO.Append = true
log4j.appender.INFO.Threshold = info 
log4j.appender.INFO.layout = org.apache.log4j.PatternLayout
log4j.appender.INFO.layout.ConversionPattern = %-d{yyyy-MM-dd HH:mm:ss}  [ %t:%r ] - [ %p ]  %m%n

测试代码：

public class Log4jTest {private static final Logger LOGGER = Logger.getLogger(Log4jTest.class);public static void main(String[] args) {LOGGER.info("log test");try {((Object) null).toString();} catch (Exception e) {LOGGER.info("exception info", e);}LOGGER.info("This is log4j. Thread=" + Thread.currentThread().getName());}
}

日志输出：

2019-10-31 21:05:36  [ main:0 ] - [ INFO ]  log test
2019-10-31 21:05:36  [ main:5 ] - [ INFO ]  exception info
java.lang.NullPointerExceptionat Log4jTest.main(Log4jTest.java:18)
2019-10-31 21:05:36  [ main:9 ] - [ INFO ]  This is log4j. Thread=main

1.2 commons-logging 使用示例

本例中，使用 Commons-Logging (也称为 JCL) 作为日志门面提供统一的日志接口，Log4j 作为具体的日志实现。

maven 依赖：

<dependency><groupId>commons-logging</groupId><artifactId>commons-logging</artifactId><version>1.2</version>
</dependency>
<dependency><groupId>log4j</groupId><artifactId>log4j</artifactId><version>1.2.17</version>
</dependency>

配置文件

1-commons-logging.properties

org.apache.commons.logging.Log=org.apache.commons.logging.impl.Log4JLogger

2-log4j.properties

### 设置
log4j.rootLogger = debug,stdout,INFO
### 输出信息到控制台 ###
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 = [%-5p] %d{yyyy-MM-dd HH:mm:ss,SSS} method:%l%n%m%n
### 输出 INFO 级别以上的日志到指定文件
log4j.appender.INFO = org.apache.log4j.DailyRollingFileAppender
log4j.appender.INFO.File = ～/Code/logs/jcllog4j.log
log4j.appender.INFO.Append = true
log4j.appender.INFO.Threshold = info 
log4j.appender.INFO.layout = org.apache.log4j.PatternLayout
log4j.appender.INFO.layout.ConversionPattern = %-d{yyyy-MM-dd HH:mm:ss}  [ %t:%r ] - [ %p ]  %m%n

测试代码：

public class JclTest {private static final Log LOGGER = LogFactory.getLog(JclTest.class);public static void main(String[] args) {LOGGER.info("log test");try {((Object) null).toString();} catch (Exception e) {LOGGER.info("exception info", e);}LOGGER.info("This is jcl log. Thread=" + Thread.currentThread().getName());}
}

日志输出：

2019-10-31 21:30:42  [ main:0 ] - [ INFO ]  log test
2019-10-31 21:30:42  [ main:4 ] - [ INFO ]  exception info
java.lang.NullPointerExceptionat JclTest.main(JclTest.java:19)
2019-10-31 21:30:42  [ main:6 ] - [ INFO ]  This is jcl log. Thread=main

1.3 slf4j 使用示例

本例中，使用 Slf4j 作为日志门面提供统一的日志接口，Logback 作为具体的日志实现。

maven 依赖：

<dependency><groupId>org.slf4j</groupId><artifactId>slf4j-api</artifactId><version>1.7.28</version>
</dependency>
<dependency><groupId>ch.qos.logback</groupId><artifactId>logback-classic</artifactId><version>1.2.3</version>
</dependency>
<dependency><groupId>ch.qos.logback</groupId><artifactId>logback-core</artifactId><version>1.2.3</version>
</dependency>

配置文件：
logback.xml

<configuration debug="true" scan="true" scanPeriod="1 seconds"><contextName>logback</contextName><property name="app.name" value="~/Code"/><appender name="stdout" class="ch.qos.logback.core.ConsoleAppender"><filter class="ch.qos.logback.classic.filter.ThresholdFilter"><level>DEBUG</level></filter><encoder><pattern>%d [%thread] %-5level %logger{36} [%file : %line] - %msg%n</pattern></encoder></appender><appender name="file" class="ch.qos.logback.core.rolling.RollingFileAppender"><file>${app.name}/logs/slf4jlogback.log</file><rollingPolicy class="ch.qos.logback.core.rolling.TimeBasedRollingPolicy"><fileNamePattern>${app.name}/logs/slf4jlogback.%d{yyyy-MM-dd.HH}.log.gz</fileNamePattern><maxHistory>60</maxHistory><totalSizeCap>20GB</totalSizeCap></rollingPolicy><triggeringPolicy class="ch.qos.logback.core.rolling.SizeBasedTriggeringPolicy"><maxFileSize>100MB</maxFileSize></triggeringPolicy><encoder><pattern>%d [%thread] %-5level %logger{36} [%file : %line] - %msg%n</pattern></encoder></appender><root level="info"><appender-ref ref="stdout"/><appender-ref ref="file"/></root>
</configuration>

测试代码：

public class Slf4jTest {private static final Logger LOGGER = LoggerFactory.getLogger(Slf4jTest.class);public static void main(String[] args) {LOGGER.info("log test");try {((Object) null).toString();} catch (Exception e) {LOGGER.info("exception info", e);}LOGGER.info("This is slf4jLogback log. Thread=" + Thread.currentThread().getName());}
}

日志输出：

019-10-31 21:26:17,806 [main] INFO  Slf4jTest [Slf4jTest.java : 17] - log test
2019-10-31 21:26:17,811 [main] INFO  Slf4jTest [Slf4jTest.java : 22] - exception info
java.lang.NullPointerException: nullat Slf4jTest.main(Slf4jTest.java:19)
2019-10-31 21:26:17,811 [main] INFO  Slf4jTest [Slf4jTest.java : 24] - This is slf4jLogback log. Thread=main

2.常见报错案例

2.1 找不到绑定器

如下图所示，应用使用的是 Slf4j 标准接口，目标日志框架为 Log4j，但是没有引入绑定器(slf4j-log4j12)。
在这里插入图片描述

maven 依赖：

<dependencies><!-- slf4j --><dependency><groupId>org.slf4j</groupId><artifactId>slf4j-api</artifactId><version>1.7.28</version></dependency><!-- Log4j2 --><dependency><groupId>log4j</groupId><artifactId>log4j</artifactId><version>1.2.17</version></dependency><!-- binder --><!--<dependency>--><!--<groupId>org.slf4j</groupId>--><!--<artifactId>slf4j-log4j12</artifactId>--><!--<version>1.7.28</version>--><!--</dependency>-->
</dependencies>

运行测试代码将会报错，提示加载绑定器失败，具体内容如下：

SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder".
SLF4J: Defaulting to no-operation (NOP) logger implementation
SLF4J: See http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.

2.2 配置了多个绑定器

如下图所示，应用使用的是 Slf4j 标准接口，目标日志框架为 Log4j，但是同时引入了两个日志绑定器。

在这里插入图片描述

存在问题的 maven 依赖如下所示：


<!-- slf4j -->
<dependency><groupId>org.slf4j</groupId><artifactId>slf4j-api</artifactId><version>1.7.28</version>
</dependency>
<!-- Log4j2 -->
<dependency><groupId>log4j</groupId><artifactId>log4j</artifactId><version>1.2.17</version>
</dependency>
<!-- binder -->
<dependency><groupId>org.slf4j</groupId><artifactId>slf4j-log4j12</artifactId><version>1.7.28</version>
</dependency>
<dependency><groupId>org.apache.logging.log4j</groupId><artifactId>log4j-slf4j-impl</artifactId><version>2.12.1</version>
</dependency>

这种情况下，将无法按照预期打印日志，应用会加载默认的日志配置，将日志打印到控制台，详细的错误信息如下：

SLF4J: Class path contains multiple SLF4J bindings.
SLF4J: Found binding in [jar:file:/Users/test/.m2/repository/org/slf4j/slf4j-log4j12/1.7.28/slf4j-log4j12-1.7.28.jar!/org/slf4j/impl/StaticLoggerBinder.class]
SLF4J: Found binding in [jar:file:/Users/test/.m2/repository/org/apache/logging/log4j/log4j-slf4j-impl/2.12.1/log4j-slf4j-impl-2.12.1.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 [org.slf4j.impl.Log4jLoggerFactory]

2.3 循环依赖

如下图，应用使用的是 Slf4j 标准接口，目标日志框架为 Log4j，但是同时引入了两个日志绑定器: log4j-over-slf4j 和 slf4j-log4j12，如此将会出现循环依赖。

在这里插入图片描述

存在循环依赖问题的 maven 配置如下：

<!-- slf4j -->
<dependency><groupId>org.slf4j</groupId><artifactId>slf4j-api</artifactId><version>1.7.28</version>
</dependency>
<!-- Log4j2 -->
<dependency><groupId>log4j</groupId><artifactId>log4j</artifactId><version>1.2.17</version>
</dependency>
<!-- binder -->
<dependency><groupId>org.slf4j</groupId><artifactId>slf4j-log4j12</artifactId><version>1.7.28</version>
</dependency>
<dependency><groupId>org.slf4j</groupId><artifactId>log4j-over-slf4j</artifactId><version>1.7.28</version>
</dependency>

出现循环依赖问题时，日志系统将无法正常启动，进而导致应用无法启动，详细的错误信息如下：

SLF4J: Detected both log4j-over-slf4j.jar AND bound slf4j-log4j12.jar on the class path, preempting StackOverflowError. 
SLF4J: See also http://www.slf4j.org/codes.html#log4jDelegationLoop for more details.
Exception in thread "main" java.lang.ExceptionInInitializerErrorat org.slf4j.impl.StaticLoggerBinder.<init>(StaticLoggerBinder.java:72)at org.slf4j.impl.StaticLoggerBinder.<clinit>(StaticLoggerBinder.java:45)at org.slf4j.LoggerFactory.bind(LoggerFactory.java:150)at org.slf4j.LoggerFactory.performInitialization(LoggerFactory.java:124)at org.slf4j.LoggerFactory.getILoggerFactory(LoggerFactory.java:412)at org.slf4j.LoggerFactory.getLogger(LoggerFactory.java:357)at org.slf4j.LoggerFactory.getLogger(LoggerFactory.java:383)at Log4jTest.<clinit>(Log4jTest.java:13)
Caused by: java.lang.IllegalStateException: Detected both log4j-over-slf4j.jar AND bound slf4j-log4j12.jar on the class path, preempting StackOverflowError. See also http://www.slf4j.org/codes.html#log4jDelegationLoop for more details.at org.slf4j.impl.Log4jLoggerFactory.<clinit>(Log4jLoggerFactory.java:54)... 8 more