logger === [`logger` online tutorial](https://arthas.aliyun.com/doc/arthas-tutorials?language=en&id=command-logger) > Print the logger information, update the logger level ### Usage #### Print the logger information Take the following `logback.xml` as an example: ```xml app.log mylog-%d{yyyy-MM-dd}.%i.txt 100MB 60 2GB %logger{35} - %msg%n %-4relative [%thread] %-5level %logger{35} - %msg %n utf8 ``` The result of the `logger` command: ```bash [arthas@2062]$ logger name ROOT class ch.qos.logback.classic.Logger classLoader sun.misc.Launcher$AppClassLoader@2a139a55 classLoaderHash 2a139a55 level INFO effectiveLevel INFO additivity true codeSource file:/Users/hengyunabc/.m2/repository/ch/qos/logback/logback-classic/1.2.3/logback-classic-1.2.3.jar appenders name CONSOLE class ch.qos.logback.core.ConsoleAppender classLoader sun.misc.Launcher$AppClassLoader@2a139a55 classLoaderHash 2a139a55 target System.out name APPLICATION class ch.qos.logback.core.rolling.RollingFileAppender classLoader sun.misc.Launcher$AppClassLoader@2a139a55 classLoaderHash 2a139a55 file app.log name ASYNC class ch.qos.logback.classic.AsyncAppender classLoader sun.misc.Launcher$AppClassLoader@2a139a55 classLoaderHash 2a139a55 appenderRef [APPLICATION] ``` In the `appenders` section: * The target of `CONSOLE` logger is `System.out` * `APPLICATION` logger is `RollingFileAppender`, the file is `app.log` * `ASYNC` its `appenderRef` is `APPLICATION`, which means asynchronous output to the file #### View logger information for the special name ```bash [arthas@2062]$ logger -n org.springframework.web name org.springframework.web class ch.qos.logback.classic.Logger classLoader sun.misc.Launcher$AppClassLoader@2a139a55 classLoaderHash 2a139a55 level null effectiveLevel INFO additivity true codeSource file:/Users/hengyunabc/.m2/repository/ch/qos/logback/logback-classic/1.2.3/logback-classic-1.2.3.jar ``` #### View logger information for the special classloader Note that the hashcode changes, you need to check the current ClassLoader information first, and extract the hashcode corresponding to the ClassLoader. if you use`-c`, you have to manually type hashcode by `-c `. ```bash [arthas@2062]$ logger -c 2a139a55 name ROOT class ch.qos.logback.classic.Logger classLoader sun.misc.Launcher$AppClassLoader@2a139a55 classLoaderHash 2a139a55 level DEBUG effectiveLevel DEBUG additivity true codeSource file:/Users/hengyunabc/.m2/repository/ch/qos/logback/logback-classic/1.2.3/logback-classic-1.2.3.jar appenders name CONSOLE class ch.qos.logback.core.ConsoleAppender classLoader sun.misc.Launcher$AppClassLoader@2a139a55 classLoaderHash 2a139a55 target System.out name APPLICATION class ch.qos.logback.core.rolling.RollingFileAppender classLoader sun.misc.Launcher$AppClassLoader@2a139a55 classLoaderHash 2a139a55 file app.log name ASYNC class ch.qos.logback.classic.AsyncAppender classLoader sun.misc.Launcher$AppClassLoader@2a139a55 classLoaderHash 2a139a55 appenderRef [APPLICATION] ``` For classloader with only one instance, it can be specified by `--classLoaderClass` using class name, which is more convenient to use. `logger --classLoaderClass sun.misc.Launcher$AppClassLoader` * PS: Here the classLoaderClass in java 8 is sun.misc.Launcher$AppClassLoader, while in java 11 it's jdk.internal.loader.ClassLoaders$AppClassLoader. The value of `--classloaderclass` is the class name of classloader. It can only work when it matches a unique classloader instance. The purpose is to facilitate the input of general commands. However, `-c ` is dynamic. #### Update logger level ```bash [arthas@2062]$ logger --name ROOT --level debug update logger level success. ``` #### Speecify classloader to update logger level By default，logger command will be executed under SystemClassloader, if the application is a traditional `war`, or using spring boot fat jar, then it needs to specify classloader。 You can first use `sc -d yourClassName` to check specified classloader hashcode，then specify classloader when updating logger level: ```bash [arthas@2062]$ logger -c 2a139a55 --name ROOT --level debug ``` #### View the logger information without appenders By default, the `logger` command only prints information about the logger with appenders. If you want to see information about loggers without `appender`, you can use the parameter `--include-no-appender`. Note that the output will usually be very long. ```bash [arthas@2062]$ logger --include-no-appender name ROOT class ch.qos.logback.classic.Logger classLoader sun.misc.Launcher$AppClassLoader@2a139a55 classLoaderHash 2a139a55 level DEBUG effectiveLevel DEBUG additivity true codeSource file:/Users/hengyunabc/.m2/repository/ch/qos/logback/logback-classic/1.2.3/logback-classic-1.2.3.jar appenders name CONSOLE class ch.qos.logback.core.ConsoleAppender classLoader sun.misc.Launcher$AppClassLoader@2a139a55 classLoaderHash 2a139a55 target System.out name APPLICATION class ch.qos.logback.core.rolling.RollingFileAppender classLoader sun.misc.Launcher$AppClassLoader@2a139a55 classLoaderHash 2a139a55 file app.log name ASYNC class ch.qos.logback.classic.AsyncAppender classLoader sun.misc.Launcher$AppClassLoader@2a139a55 classLoaderHash 2a139a55 appenderRef [APPLICATION] name com class ch.qos.logback.classic.Logger classLoader sun.misc.Launcher$AppClassLoader@2a139a55 classLoaderHash 2a139a55 level null effectiveLevel DEBUG additivity true codeSource file:/Users/hengyunabc/.m2/repository/ch/qos/logback/logback-classic/1.2.3/logback-classic-1.2.3.jar name com.alibaba class ch.qos.logback.classic.Logger classLoader sun.misc.Launcher$AppClassLoader@2a139a55 classLoaderHash 2a139a55 level null effectiveLevel DEBUG additivity true codeSource file:/Users/hengyunabc/.m2/repository/ch/qos/logback/logback-classic/1.2.3/logback-classic-1.2.3.jar ... ```