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>log4jgroupId>
  <artifactId>log4jartifactId>
  <version>1.2.17version>
dependency>
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• 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
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• 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());
    }
}
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• 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.NullPointerException
        at Log4jTest.main(Log4jTest.java:18)
2019-10-31 21:05:36  [ main:9 ] - [ INFO ]  This is log4j. Thread=main
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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-logginggroupId>
  <artifactId>commons-loggingartifactId>
  <version>1.2version>
dependency>
<dependency>
  <groupId>log4jgroupId>
  <artifactId>log4jartifactId>
  <version>1.2.17version>
dependency>
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• 配置文件

1-commons-logging.properties

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

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• 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());
    }
}
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• 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.NullPointerException
        at JclTest.main(JclTest.java:19)
2019-10-31 21:30:42  [ main:6 ] - [ INFO ]  This is jcl log. Thread=main     
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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.slf4jgroupId>
  <artifactId>slf4j-apiartifactId>
  <version>1.7.28version>
dependency>
<dependency>
  <groupId>ch.qos.logbackgroupId>
  <artifactId>logback-classicartifactId>
  <version>1.2.3version>
dependency>
<dependency>
  <groupId>ch.qos.logbackgroupId>
  <artifactId>logback-coreartifactId>
  <version>1.2.3version>
dependency>
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• 配置文件：
  logback.xml

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

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• 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: null
        at Slf4jTest.main(Slf4jTest.java:19)
2019-10-31 21:26:17,811 [main] INFO  Slf4jTest [Slf4jTest.java : 24] - This is slf4jLogback log. Thread=main
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---

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)。

• maven 依赖：

<dependencies>
  
  <dependency>
    <groupId>org.slf4jgroupId>
    <artifactId>slf4j-apiartifactId>
    <version>1.7.28version>
  dependency>
  
  <dependency>
    <groupId>log4jgroupId>
    <artifactId>log4jartifactId>
    <version>1.2.17version>
  dependency>
  
  
  
  
  
  
dependencies>
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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.
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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。

存在问题的 maven 依赖如下所示：

<dependency>
    <groupId>org.slf4jgroupId>
    <artifactId>slf4j-apiartifactId>
    <version>1.7.28version>
dependency>

<dependency>
    <groupId>log4jgroupId>
    <artifactId>log4jartifactId>
    <version>1.2.17version>
dependency>

<dependency>
    <groupId>org.slf4jgroupId>
    <artifactId>slf4j-log4j12artifactId>
    <version>1.7.28version>
dependency>
<dependency>
    <groupId>org.apache.logging.log4jgroupId>
    <artifactId>log4j-slf4j-implartifactId>
    <version>2.12.1version>
dependency>
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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]
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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。

存在循环依赖问题的 maven 配置如下：

<dependency>
    <groupId>org.slf4jgroupId>
    <artifactId>slf4j-apiartifactId>
    <version>1.7.28version>
dependency>

<dependency>
    <groupId>log4jgroupId>
    <artifactId>log4jartifactId>
    <version>1.2.17version>
dependency>

<dependency>
    <groupId>org.slf4jgroupId>
    <artifactId>slf4j-log4j12artifactId>
    <version>1.7.28version>
dependency>
<dependency>
    <groupId>org.slf4jgroupId>
    <artifactId>log4j-over-slf4jartifactId>
    <version>1.7.28version>
dependency>
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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.ExceptionInInitializerError
    at 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
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