arthas/tutorials/katacoda/case-classloader-cn/case-classloader.md

下面介绍classloader命令的功能。

先访问一个jsp网页，触发jsp的加载： https://HOST_SUBDOMAIN-80-KATACODA_HOST.environments.katacoda.com/hello

列出所有ClassLoader

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

• TomcatEmbeddedWebappClassLoader 加载的class数量是0，所以在spring boot embedded tomcat里，它只是一个空壳，所有的类加载都是LaunchedURLClassLoader完成的

请记下你的classLoaderHash，后面需要使用它。在这里，它是 65361d9a。

注意：请使用你的classLoaderHash值覆盖 <classLoaderHash> ，然后手动执行下面所有所述命令：

列出ClassLoader里加载的所有类

列出上面的org.apache.jasper.servlet.JasperLoader加载的类：

classloader -a -c <classLoaderHash>

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

查看类的classloader层次

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

查看ClassLoader树

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

查看URLClassLoader实际的urls

比如上面查看到的spring LaunchedURLClassLoader的 hashcode是1be6f5c3，可以通过-c参数来指定classloader，从而查看URLClassLoader实际的urls：

classloader -c <classLoaderHash>

$ classloader -c 1be6f5c3
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!/
...

加载指定ClassLoader里的资源文件

查找指定的资源文件： classloader -c <classLoaderHash> -r logback-spring.xml

$ classloader -c 1be6f5c3 -r logback-spring.xml
 jar:file:/home/scrapbook/tutorial/demo-arthas-spring-boot.jar!/BOOT-INF/classes!/logback-spring.xml

也可以尝试查找类的class文件：

classloader -c <classLoaderHash> -r java/lang/String.class

$ classloader -c 1b6d3586 -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

尝试加载指定的类

比如用上面的spring LaunchedURLClassLoader 尝试加载 ch.qos.logback.classic.spi.StackTraceElementProxy ：

首先使用sc ch.qos.logback.classic.spi.StackTraceElementProxy{{execute T2}}查看，可发现未加载：

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

因而使用spring LaunchedURLClassLoader 尝试加载：

classloader -c <classLoaderHash> --load ch.qos.logback.classic.spi.StackTraceElementProxy

$ classloader -c 1be6f5c3 --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

再次使用sc ch.qos.logback.classic.spi.StackTraceElementProxy{{execute T2}}查看，发现已经加载：

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