arthas/tutorials/katacoda/command-dump-en/dump.md

Dump the bytecode for the particular classes to the specified directory.

Options

Name	Specification
class-pattern	class name pattern
[c:]	hashcode of the class loader that loaded the target class
[classLoaderClass:]	The class name of the ClassLoader that executes the expression.
[d:]	set the destination directory for class files
[E]	turn on regex match, the default behavior is wild card match

Usage

dump java.lang.String{{execute T2}}

$ dump java.lang.String
 HASHCODE  CLASSLOADER  LOCATION
 null                   /Users/admin/logs/arthas/classdump/java/lang/String.class
Affect(row-cnt:1) cost in 119 ms.

dump demo.*{{execute T2}}

$ dump demo.*
 HASHCODE  CLASSLOADER                                    LOCATION
 3d4eac69  +-sun.misc.Launcher$AppClassLoader@3d4eac69    /Users/admin/logs/arthas/classdump/sun.misc.Launcher$AppClassLoader-3d4eac69/demo/MathGame.class
             +-sun.misc.Launcher$ExtClassLoader@66350f69
Affect(row-cnt:1) cost in 39 ms.

dump -d /tmp/output java.lang.String{{execute T2}}

$ dump 
 HASHCODE  CLASSLOADER  LOCATION
 null                   /tmp/output/java/lang/String.class
Affect(row-cnt:1) cost in 138 ms.

• Specify 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 <hashcode>.

$ dump -c 3d4eac69 demo.*

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

dump --classLoaderClass sun.misc.Launcher$AppClassLoader demo.*{{execute T2}}

$ dump --classLoaderClass sun.misc.Launcher$AppClassLoader demo.*
 HASHCODE  CLASSLOADER                                    LOCATION
 3d4eac69  +-sun.misc.Launcher$AppClassLoader@3d4eac69    /Users/admin/logs/arthas/classdump/sun.misc.Launcher$AppClassLoader-3d4eac69/demo/MathGame.class
             +-sun.misc.Launcher$ExtClassLoader@66350f69
Affect(row-cnt:1) cost in 39 ms.

• PS: Here the classLoaderClass in java 8 is sun.misc.Launcher$AppClassLoader, while in java 11 it's jdk.internal.loader.ClassLoaders$AppClassLoader. Currently katacoda using java 8.

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.