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classloader
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===
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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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|`[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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#### 按类加载类型查看统计信息
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```bash
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$ classloader
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name numberOfInstances loadedCountTotal
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com.taobao.arthas.agent.ArthasClassloader 1 2115
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BootstrapClassLoader 1 1861
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sun.reflect.DelegatingClassLoader 5 5
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sun.misc.Launcher$AppClassLoader 1 4
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sun.misc.Launcher$ExtClassLoader 1 1
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Affect(row-cnt:5) cost in 3 ms.
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```
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#### 按类加载实例查看统计信息
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```bash
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$ classloader -l
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name loadedCount hash parent
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BootstrapClassLoader 1861 null null
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com.taobao.arthas.agent.ArthasClassloader@68b31f0a 2115 68b31f0a sun.misc.Launcher$ExtClassLoader@66350f69
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sun.misc.Launcher$AppClassLoader@3d4eac69 4 3d4eac69 sun.misc.Launcher$ExtClassLoader@66350f69
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sun.misc.Launcher$ExtClassLoader@66350f69 1 66350f69 null
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Affect(row-cnt:4) cost in 2 ms.
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```
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#### 查看ClassLoader的继承树
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```bash
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$ classloader -t
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+-BootstrapClassLoader
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+-sun.misc.Launcher$ExtClassLoader@66350f69
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+-com.taobao.arthas.agent.ArthasClassloader@68b31f0a
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+-sun.misc.Launcher$AppClassLoader@3d4eac69
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Affect(row-cnt:4) cost in 3 ms.
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```
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#### 查看URLClassLoader实际的urls
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```bash
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$ classloader -c 3d4eac69
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file:/private/tmp/arthas-demo.jar
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file:/Users/hengyunabc/.arthas/lib/3.0.5/arthas/arthas-agent.jar
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Affect(row-cnt:9) cost in 3 ms.
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```
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#### 使用ClassLoader去查找resource
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```bash
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$ classloader -c 3d4eac69 -r META-INF/MANIFEST.MF
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jar:file:/System/Library/Java/Extensions/MRJToolkit.jar!/META-INF/MANIFEST.MF
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jar:file:/private/tmp/arthas-demo.jar!/META-INF/MANIFEST.MF
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jar:file:/Users/hengyunabc/.arthas/lib/3.0.5/arthas/arthas-agent.jar!/META-INF/MANIFEST.MF
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```
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也可以尝试查找类的class文件：
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```bash
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$ classloader -c 1b6d3586 -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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#### 使用ClassLoader去加载类
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```bash
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$ classloader -c 3d4eac69 --load demo.MathGame
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load class success.
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class-info demo.MathGame
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code-source /private/tmp/arthas-demo.jar
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name demo.MathGame
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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 MathGame
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modifier public
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annotation
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interfaces
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super-class +-java.lang.Object
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class-loader +-sun.misc.Launcher$AppClassLoader@3d4eac69
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+-sun.misc.Launcher$ExtClassLoader@66350f69
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classLoaderHash 3d4eac69
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```
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