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下面介绍`classloader`命令的功能。
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先访问一个jsp网页，触发jsp的加载： https://[[HOST_SUBDOMAIN]]-80-[[KATACODA_HOST]].environments.katacoda.com/hello
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### 列出所有ClassLoader
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`classloader -l`{{execute T2}}
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```bash
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$ classloader -l
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name loadedCount hash parent
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BootstrapClassLoader 2724 null null
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com.taobao.arthas.agent.ArthasClassloader@411ce1ab 2009 411ce1ab sun.misc.Launcher$ExtClassLoader@7494e528
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com.taobao.arthas.agent.ArthasClassloader@22ae1234 1253 22ae1234 sun.misc.Launcher$ExtClassLoader@7494e528
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org.apache.jasper.servlet.JasperLoader@65361d9a 1 65361d9a TomcatEmbeddedWebappClassLoader
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context: ROOT
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delegate: true
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----------> Parent Classloader:
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org.springframework.boot.loader.LaunchedURLClassLoader@1be6f5c3
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TomcatEmbeddedWebappClassLoader 0 8546cd5 org.springframework.boot.loader.LaunchedURLClassLoader@1be6f5c3
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context: ROOT
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delegate: true
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----------> Parent Classloader:
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org.springframework.boot.loader.LaunchedURLClassLoader@1be6f5c3
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org.springframework.boot.loader.LaunchedURLClassLoader@1be6f5c3 5416 1be6f5c3 sun.misc.Launcher$AppClassLoader@3d4eac69
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sun.misc.Launcher$AppClassLoader@3d4eac69 45 3d4eac69 sun.misc.Launcher$ExtClassLoader@7494e528
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sun.misc.Launcher$ExtClassLoader@7494e528 4 7494e528 null
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```
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* 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
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### 列出ClassLoader里加载的所有类
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列出上面的`org.apache.jasper.servlet.JasperLoader`加载的类：
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`classloader -a --classLoaderClass org.apache.jasper.servlet.JasperLoader`{{execute T2}}
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```bash
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$ classloader -a --classLoaderClass apache.jasper.servlet.JasperLoader
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hash:1698045338, org.apache.jasper.servlet.JasperLoader@65361d9a
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org.apache.jsp.jsp.hello_jsp
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```
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* 注：同ognl, 也可用`-c <hashcode>`而不用`--classLoaderClass`指定
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### 反编译jsp的代码
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`jad org.apache.jsp.jsp.hello_jsp`{{execute T2}}
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```bash
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$ jad org.apache.jsp.jsp.hello_jsp
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ClassLoader:
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+-org.apache.jasper.servlet.JasperLoader@65361d9a
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+-TomcatEmbeddedWebappClassLoader
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context: ROOT
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...
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```
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### 查看ClassLoader树
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`classloader -t`{{execute T2}}
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```
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$ classloader -t
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+-BootstrapClassLoader
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+-sun.misc.Launcher$ExtClassLoader@28cbbddd
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+-com.taobao.arthas.agent.ArthasClassloader@8c25e55
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+-sun.misc.Launcher$AppClassLoader@55f96302
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+-org.springframework.boot.loader.LaunchedURLClassLoader@1be6f5c3
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+-TomcatEmbeddedWebappClassLoader
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context: ROOT
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delegate: true
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----------> Parent Classloader:
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org.springframework.boot.loader.LaunchedURLClassLoader@1be6f5c3
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+-org.apache.jasper.servlet.JasperLoader@21ae0fe2
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```
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注意：请使用你的classLoaderHash值覆盖 `<classLoaderHash>` ，然后手动执行下面相关命令：
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### 列出ClassLoader的urls
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比如上面查看到的spring LaunchedURLClassLoader的 hashcode是`1be6f5c3`，可以通过`-c`或者`--classLoaderClass`参数来列出它的所有urls：
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`classloader --classLoaderClass org.springframework.boot.loader.LaunchedURLClassLoader`{{execute T2}}
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```
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$ classloader --classLoaderClass org.springframework.boot.loader.LaunchedURLClassLoader
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jar:file:/home/scrapbook/tutorial/demo-arthas-spring-boot.jar!/BOOT-INF/classes!/
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jar:file:/home/scrapbook/tutorial/demo-arthas-spring-boot.jar!/BOOT-INF/lib/spring-boot-starter-aop-1.5
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.13.RELEASE.jar!/
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...
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```
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### 加载指定ClassLoader里的资源文件
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查找指定的资源文件： `classloader --classLoaderClass org.springframework.boot.loader.LaunchedURLClassLoader -r logback-spring.xml`{{execute T2}}
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```
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$ classloader --classLoaderClass org.springframework.boot.loader.LaunchedURLClassLoader -r logback-spring.xml
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jar:file:/home/scrapbook/tutorial/demo-arthas-spring-boot.jar!/BOOT-INF/classes!/logback-spring.xml
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```
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### 尝试加载指定的类
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比如用上面的spring LaunchedURLClassLoader 尝试加载 `java.lang.String` ：
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`classloader --classLoaderClass org.springframework.boot.loader.LaunchedURLClassLoader --load java.lang.String`{{execute T2}}
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```
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$ classloader --classLoaderClass org.springframework.boot.loader.LaunchedURLClassLoader --load java.lang.String
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load class success.
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class-info java.lang.String
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code-source
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name java.lang.String
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isInterface false
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isAnnotation false
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isEnum false
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isAnonymousClass false
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isArray false
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isLocalClass false
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isMemberClass false
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isPrimitive false
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isSynthetic false
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simple-name String
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modifier final,public
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annotation
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interfaces java.io.Serializable,java.lang.Comparable,java.lang.CharSequence
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super-class +-java.lang.Object
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class-loader
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classLoaderHash null
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```
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