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

Search classes loaded by JVM.

sc stands for search class. This command can search all possible classes loaded by JVM and show their information. The supported options are: [d]、[E]、[f] and [x:].

Supported Options

Name	Specification
class-pattern	pattern for the class name
method-pattern	pattern for the method name
[d]	print the details of the current class, including its code source, class specification, its class loader and so on. If a class is loaded by more than one class loader, then the class details will be printed several times
[E]	turn on regex match, the default behavior is wildcards match
[f]	print the fields info of the current class, MUST be used with -d together
[x:]	specify the depth of recursive traverse the static fields, the default value is '0' - equivalent to use toString to output
[c:]	The hash code of the special class's classLoader
[classLoaderClass:]	The class name of the ClassLoader that executes the expression.
[n:]	Maximum number of matching classes with details (100 by default)

class-patten supports full qualified class name, e.g. com.taobao.test.AAA and com/taobao/test/AAA. It also supports the format of 'com/taobao/test/AAA', so that it is convenient to directly copy class name from the exception stack trace without replacing '/' to '.'.

sc turns on matching sub-class match by default, that is, sc will also search the sub classes of the target class too. If exact-match is desired, pls. use options disable-sub-class true.

Usage

• Wildcards match search

  sc demo.*{{execute T2}}

  $ sc demo.*
  demo.MathGame
  Affect(row-cnt:1) cost in 55 ms.
  

• View class details

  sc -d demo.MathGame{{execute T2}}

  $ sc -d demo.MathGame
  class-info        demo.MathGame
  code-source       /private/tmp/math-game.jar
  name              demo.MathGame
  isInterface       false
  isAnnotation      false
  isEnum            false
  isAnonymousClass  false
  isArray           false
  isLocalClass      false
  isMemberClass     false
  isPrimitive       false
  isSynthetic       false
  simple-name       MathGame
  modifier          public
  annotation
  interfaces
  super-class       +-java.lang.Object
  class-loader      +-sun.misc.Launcher$AppClassLoader@3d4eac69
                      +-sun.misc.Launcher$ExtClassLoader@66350f69
  classLoaderHash   3d4eac69
  
  Affect(row-cnt:1) cost in 875 ms.
  

Take a note of the classLoaderHash here:3d4eac69, and use it to replace <classLoaderHash> and execute the following command.

• 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>.

$ sc -c 3d4eac69 -d demo*

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

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

• 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.

• View class fields

  sc -d -f demo.MathGame{{execute T2}}

  $ sc -d -f demo.MathGame
  class-info        demo.MathGame
  code-source       /private/tmp/math-game.jar
  name              demo.MathGame
  isInterface       false
  isAnnotation      false
  isEnum            false
  isAnonymousClass  false
  isArray           false
  isLocalClass      false
  isMemberClass     false
  isPrimitive       false
  isSynthetic       false
  simple-name       MathGame
  modifier          public
  annotation
  interfaces
  super-class       +-java.lang.Object
  class-loader      +-sun.misc.Launcher$AppClassLoader@3d4eac69
                      +-sun.misc.Launcher$ExtClassLoader@66350f69
  classLoaderHash   3d4eac69
  fields            modifierprivate,static
                    type    java.util.Random
                    name    random
                    value   java.util.Random@522b4
                            08a
  
                    modifierprivate
                    type    int
                    name    illegalArgumentCount
  
  
  Affect(row-cnt:1) cost in 19 ms.