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

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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}}
```bash
$ 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>`
```bash
$ 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`
```bash
$ 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:
```bash
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`
```bash
$ 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:
```bash
ch.qos.logback.classic.spi.StackTraceElementProxy
Affect(row-cnt:1) cost in 19 ms.
```