arthas/tutorials/katacoda/arthas-advanced-en/ognl.md

The ognl command can execute code dynamically.

Invoke the static method

ognl '@java.lang.System@out.println("hello ognl")'{{execute T2}}

The Terminal 1 will print hello ognl.

Find the ClassLoader of the UserController

sc -d com.example.demo.arthas.user.UserController | grep classLoaderHash{{execute T2}}

$ sc -d com.example.demo.arthas.user.UserController | grep classLoaderHash
 classLoaderHash   1be6f5c3

Please write down your classLoaderHash here since it's dynamic. In the case here, it's 1be6f5c3.

if you use-c, you have to manually type hashcode by -c <hashcode>.

$ ognl -c 1be6f5c3 @com.example.demo.arthas.user.UserController@logger

For classloader with only one instance, it can be specified by --classLoaderClass using class name, which is more convenient to use.

$ ognl --classLoaderClass org.springframework.boot.loader.LaunchedURLClassLoader  @org.springframework.boot.SpringApplication@logger
@Slf4jLocationAwareLog[
    FQCN=@String[org.apache.commons.logging.LogAdapter$Slf4jLocationAwareLog],
    name=@String[org.springframework.boot.SpringApplication],
    logger=@Logger[Logger[org.springframework.boot.SpringApplication]],
]

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 <hashcode> is dynamic.

Get static fields of static classes

Get the logger field of the UserController class:

ognl --classLoaderClass org.springframework.boot.loader.LaunchedURLClassLoader @com.example.demo.arthas.user.UserController@logger{{execute T2}}

Control the number of expansion layers of the return value with the -x parameter. such as:

ognl --classLoaderClass org.springframework.boot.loader.LaunchedURLClassLoader -x 2 @com.example.demo.arthas.user.UserController@logger{{execute T2}}

Execute multi-line expressions

Return a list:

ognl '#value1=@System@getProperty("java.home"), #value2=@System@getProperty("java.runtime.name"), {#value1, #value2}'{{execute T2}}

$ ognl '#value1=@System@getProperty("java.home"), #value2=@System@getProperty("java.runtime.name"), {#value1, #value2}'
@ArrayList[
    @String[/Library/Java/JavaVirtualMachines/jdk1.8.0_162.jdk/Contents/Home/jre],
    @String[Java(TM) SE Runtime Environment],
]

More

The ognl expression in Arthas is an important feature, and the ognl expression can be used in many commands.

For some more complicated usages, refer to:

• For special usage of OGNL, please refer to: https://github.com/alibaba/arthas/issues/71
• Official Guide to OGNL Expressions: https://commons.apache.org/proper/commons-ognl/language-guide.html