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> 查看classloader的继承树，urls，类加载信息
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`classloader` 命令将 JVM 中所有的classloader的信息统计出来，并可以展示继承树，urls等。
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可以让指定的classloader去getResources，打印出所有查找到的resources的url。对于`ResourceNotFoundException`比较有用。
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### 参数说明
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|参数名称|参数说明|
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|---:|:---|
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|[l]|按类加载实例进行统计|
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|[t]|打印所有ClassLoader的继承树|
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|[a]|列出所有ClassLoader加载的类，请谨慎使用|
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|`[c:]`|ClassLoader的hashcode|
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|`[classLoaderClass:]`|指定执行表达式的 ClassLoader 的 class name|
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|`[c: r:]`|用ClassLoader去查找resource|
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|`[c: load:]`|用ClassLoader去加载指定的类|
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### 使用参考
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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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注意hashcode是变化的，需要先查看当前的ClassLoader信息，提取对应ClassLoader的hashcode。
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如果你使用`-c`，你需要手动输入hashcode：`-c <hashcode>`
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```bash
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$ classloader -c 65361d9a
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```
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对于只有唯一实例的ClassLoader可以通过`--classLoaderClass`指定class name，使用起来更加方便：
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```bash
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$ classloader --classLoaderClass org.apache.jasper.servlet.JasperLoader
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```
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`--classLoaderClass` 的值是ClassLoader的类名，只有匹配到唯一的ClassLoader实例时才能工作，目的是方便输入通用命令，而`-c <hashcode>`是动态变化的。
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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 org.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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### 查看类的classloader层次
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`sc -d org.apache.jsp.jsp.hello_jsp`{{execute T2}}
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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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### 查看URLClassLoader实际的urls
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比如上面查看到的spring LaunchedURLClassLoader的 hashcode是`1be6f5c3`，可以通过`-c`参数来指定classloader，或者直接使用`--classLoaderClass`，从而查看URLClassLoader实际的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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也可以尝试查找类的class文件：
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`classloader --classLoaderClass org.springframework.boot.loader.LaunchedURLClassLoader -r java/lang/String.class`{{execute T2}}
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```bash
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$ classloader --classLoaderClass org.springframework.boot.loader.LaunchedURLClassLoader -r java/lang/String.class
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jar:file:/Library/Java/JavaVirtualMachines/jdk1.8.0_60.jdk/Contents/Home/jre/lib/rt.jar!/java/lang/String.class
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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 `ch.qos.logback.classic.spi.StackTraceElementProxy` ：
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首先使用`sc ch.qos.logback.classic.spi.StackTraceElementProxy`{{execute T2}}查看，可发现未加载：
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|
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```bash
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|
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Affect(row-cnt:0) cost in 18 ms.
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```
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因而使用spring LaunchedURLClassLoader 尝试加载：
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`classloader --classLoaderClass org.springframework.boot.loader.LaunchedURLClassLoader --load ch.qos.logback.classic.spi.StackTraceElementProxy`{{execute T2}}
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```bash
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$ classloader --classLoaderClass org.springframework.boot.loader.LaunchedURLClassLoader --load ch.qos.logback.classic.spi.StackTraceElementProxy
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|
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load class success.
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class-info ch.qos.logback.classic.spi.StackTraceElementProxy
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|
code-source file:/home/scrapbook/tutorial/demo-arthas-spring-boot.jar!/BOOT-INF/lib/logback-classic-1.
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1.11.jar!/
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name ch.qos.logback.classic.spi.StackTraceElementProxy
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|
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isInterface false
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isAnnotation false
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isEnum false
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|
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isAnonymousClass false
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isArray false
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|
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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 StackTraceElementProxy
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modifier public
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annotation
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interfaces java.io.Serializable
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super-class +-java.lang.Object
|
|
|
class-loader +-org.springframework.boot.loader.LaunchedURLClassLoader@5674cd4d
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|
|
+-sun.misc.Launcher$AppClassLoader@70dea4e
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|
|
+-sun.misc.Launcher$ExtClassLoader@56a96482
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|
|
classLoaderHash 5674cd4d
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```
|
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|
|
再次使用`sc ch.qos.logback.classic.spi.StackTraceElementProxy`{{execute T2}}查看，发现已经加载：
|
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|
|
```bash
|
|
|
ch.qos.logback.classic.spi.StackTraceElementProxy
|
|
|
Affect(row-cnt:1) cost in 19 ms.
|
|
|
```
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