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

View hierarchy, urls and classes-loading info for the class-loaders.

classloader can search and print out the URLs for a specified resource from one particular classloader. It is quite handy when analyzing ResourceNotFoundException.

Options

Name	Specification
[l]	list all classloader instances
[t]	print classloader's hierarchy
[a]	list all the classes loaded by all the classloaders (use it with great caution since the output can be huge)
[c:]	print classloader's hashcode
[classLoaderClass:]	The class name of the ClassLoader that executes the expression.
[c: r:]	using ClassLoader to search resource
[c: load:]	using ClassLoader to load class

Usage

First visit the jsp page: https://HOST_SUBDOMAIN-80-KATACODA_HOST.environments.katacoda.com/hello

List all ClassLoaders

classloader -l{{execute T2}}

$ classloader -l
 name                                                             loadedCount  hash      parent
 BootstrapClassLoader                                             2724         null      null
 com.taobao.arthas.agent.ArthasClassloader@411ce1ab               2009         411ce1ab  sun.misc.Launcher$ExtClassLoader@7494e528
 com.taobao.arthas.agent.ArthasClassloader@22ae1234               1253         22ae1234  sun.misc.Launcher$ExtClassLoader@7494e528
 org.apache.jasper.servlet.JasperLoader@65361d9a                  1            65361d9a  TomcatEmbeddedWebappClassLoader
                                                                                           context: ROOT
                                                                                           delegate: true
                                                                                         ----------> Parent Classloader:
                                                                                         org.springframework.boot.loader.LaunchedURLClassLoader@1be6f5c3

 TomcatEmbeddedWebappClassLoader                                  0            8546cd5   org.springframework.boot.loader.LaunchedURLClassLoader@1be6f5c3
   context: ROOT
   delegate: true
 ----------> Parent Classloader:
 org.springframework.boot.loader.LaunchedURLClassLoader@1be6f5c3

 org.springframework.boot.loader.LaunchedURLClassLoader@1be6f5c3  5416         1be6f5c3  sun.misc.Launcher$AppClassLoader@3d4eac69
 sun.misc.Launcher$AppClassLoader@3d4eac69                        45           3d4eac69  sun.misc.Launcher$ExtClassLoader@7494e528
 sun.misc.Launcher$ExtClassLoader@7494e528                        4            7494e528  null

• The number of classes loaded by TomcatEmbeddedWebappClassLoader is 0, so in spring boot embedded tomcat, it is just an empty ClassLoader, all the classes are loaded by LaunchedURLClassLoader

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>.

$ classloader -c 65361d9a

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

$ classloader --classLoaderClass org.apache.jasper.servlet.JasperLoader

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.

List all classes loaded in ClassLoader

List all classes loaded by org.apache.jasper.servlet.JasperLoader:

classloader -a --classLoaderClass org.apache.jasper.servlet.JasperLoader{{execute T2}}

$ classloader -a --classLoaderClass org.apache.jasper.servlet.JasperLoader
 hash:1698045338, org.apache.jasper.servlet.JasperLoader@65361d9a
 org.apache.jsp.jsp.hello_jsp

Check the structure of classloader

sc -d org.apache.jsp.jsp.hello_jsp{{execute T2}}

View the ClassLoader tree

classloader -t{{execute T2}}

$ classloader -t
+-BootstrapClassLoader
+-sun.misc.Launcher$ExtClassLoader@28cbbddd
  +-com.taobao.arthas.agent.ArthasClassloader@8c25e55
  +-sun.misc.Launcher$AppClassLoader@55f96302
    +-org.springframework.boot.loader.LaunchedURLClassLoader@1be6f5c3
      +-TomcatEmbeddedWebappClassLoader
          context: ROOT
          delegate: true
        ----------> Parent Classloader:
        org.springframework.boot.loader.LaunchedURLClassLoader@1be6f5c3

        +-org.apache.jasper.servlet.JasperLoader@21ae0fe2

Show the URLs of the URLClassLoader

For example, the hashcode of spring LaunchedURLClassLoader viewed above is 1be6f5c3, and all its urls can be listed by specifying classloader using the -c or--classLoaderClass parameter and then executing the following command:

classloader --classLoaderClass org.springframework.boot.loader.LaunchedURLClassLoader{{execute T2}}

$ classloader --classLoaderClass org.springframework.boot.loader.LaunchedURLClassLoader
jar:file:/home/scrapbook/tutorial/demo-arthas-spring-boot.jar!/BOOT-INF/classes!/
jar:file:/home/scrapbook/tutorial/demo-arthas-spring-boot.jar!/BOOT-INF/lib/spring-boot-starter-aop-1.5
.13.RELEASE.jar!/
...

Load the resource file in the specified ClassLoader

Load the specified resource file: classloader --classLoaderClass org.springframework.boot.loader.LaunchedURLClassLoader -r logback-spring.xml{{execute T2}}

$ classloader --classLoaderClass org.springframework.boot.loader.LaunchedURLClassLoader -r logback-spring.xml
 jar:file:/home/scrapbook/tutorial/demo-arthas-spring-boot.jar!/BOOT-INF/classes!/logback-spring.xml

Use the classloader to load .class resource

classloader --classLoaderClass org.springframework.boot.loader.LaunchedURLClassLoader -r java/lang/String.class{{execute T2}}

$ classloader --classLoaderClass org.springframework.boot.loader.LaunchedURLClassLoader -r java/lang/String.class
 jar:file:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/rt.jar!/java/lang/String.class

Try to load the specified class

For example, try loading ch.qos.logback.classic.spi.StackTraceElementProxy with spring LaunchedURLClassLoader :

First check with sc ch.qos.logback.classic.spi.StackTraceElementProxy{{execute T2}}, you can see that it's unloaded:

classloader -c <classLoaderHash> --load java.lang.String

Affect(row-cnt:0) cost in 18 ms.

So use spring LaunchedURLClassLoader to try to load:

classloader --classLoaderClass org.springframework.boot.loader.LaunchedURLClassLoader --load ch.qos.logback.classic.spi.StackTraceElementProxy{{execute T2}}

$ classloader --classLoaderClass org.springframework.boot.loader.LaunchedURLClassLoader --load ch.qos.logback.classic.spi.StackTraceElementProxy
load class success.
 class-info        ch.qos.logback.classic.spi.StackTraceElementProxy
 code-source       file:/home/scrapbook/tutorial/demo-arthas-spring-boot.jar!/BOOT-INF/lib/logback-classic-1.
                   1.11.jar!/
 name              ch.qos.logback.classic.spi.StackTraceElementProxy
 isInterface       false
 isAnnotation      false
 isEnum            false
 isAnonymousClass  false
 isArray           false
 isLocalClass      false
 isMemberClass     false
 isPrimitive       false
 isSynthetic       false
 simple-name       StackTraceElementProxy
 modifier          public
 annotation
 interfaces        java.io.Serializable
 super-class       +-java.lang.Object
 class-loader      +-org.springframework.boot.loader.LaunchedURLClassLoader@5674cd4d
                     +-sun.misc.Launcher$AppClassLoader@70dea4e
                       +-sun.misc.Launcher$ExtClassLoader@56a96482
 classLoaderHash   5674cd4d

Finally check with sc ch.qos.logback.classic.spi.StackTraceElementProxy{{execute T2}}, and you can see that it has been loaded:

ch.qos.logback.classic.spi.StackTraceElementProxy
Affect(row-cnt:1) cost in 19 ms.