classloader
===========
[`classloader` online tutorial](https://arthas.aliyun.com/doc/arthas-tutorials?language=en&id=command-classloader)
View hierarchy, urls and classes-loading info for the class-loaders.
`classloader` can search and print out the URLs for a specified resource from one particular classloader. It is quite handy when analyzing `ResourceNotFoundException`.
### Options
|Name|Specification|
|---:|:---|
|[l]|list all classloader instances|
|[t]|print classloader's hierarchy|
|[a]|list all the classes loaded by all the classloaders (use it with great caution since the output can be huge)|
|[c:]|print classloader's hashcode|
|`[classLoaderClass:]`| The class name of the ClassLoader that executes the expression. |
|`[c: r:]`|using ClassLoader to search resource|
|`[c: load:]`|using ClassLoader to load class|
### Usage
#### View statistics categorized by class type
```bash
$ classloader
name numberOfInstances loadedCountTotal
com.taobao.arthas.agent.ArthasClassloader 1 2115
BootstrapClassLoader 1 1861
sun.reflect.DelegatingClassLoader 5 5
sun.misc.Launcher$AppClassLoader 1 4
sun.misc.Launcher$ExtClassLoader 1 1
Affect(row-cnt:5) cost in 3 ms.
```
#### View statistics categorized by loaded classes number
```bash
$ classloader -l
name loadedCount hash parent
BootstrapClassLoader 1861 null null
com.taobao.arthas.agent.ArthasClassloader@68b31f0a 2115 68b31f0a sun.misc.Launcher$ExtClassLoader@66350f69
sun.misc.Launcher$AppClassLoader@3d4eac69 4 3d4eac69 sun.misc.Launcher$ExtClassLoader@66350f69
sun.misc.Launcher$ExtClassLoader@66350f69 1 66350f69 null
Affect(row-cnt:4) cost in 2 ms.
```
#### View class-loaders hierarchy
```bash
$ classloader -t
+-BootstrapClassLoader
+-sun.misc.Launcher$ExtClassLoader@66350f69
+-com.taobao.arthas.agent.ArthasClassloader@68b31f0a
+-sun.misc.Launcher$AppClassLoader@3d4eac69
Affect(row-cnt:4) cost in 3 ms.
```
#### Show the URLs of the URLClassLoader
```bash
$ classloader -c 3d4eac69
file:/private/tmp/arthas-demo.jar
file:/Users/hengyunabc/.arthas/lib/3.0.5/arthas/arthas-agent.jar
Affect(row-cnt:9) cost in 3 ms.
```
Note that the hashcode changes, you need to check the current ClassLoader information first, and extract the hashcode corresponding to the ClassLoader.
For ClassLoader with only unique instance, it can be specified by class name, which is more convenient to use:
```bash
$ classloader --classLoaderClass sun.misc.Launcher$AppClassLoader
file:/private/tmp/arthas-demo.jar
file:/Users/hengyunabc/.arthas/lib/3.0.5/arthas/arthas-agent.jar
Affect(row-cnt:9) cost in 3 ms.
```
#### Use the classloader to load resource
```bash
$ classloader -c 3d4eac69 -r META-INF/MANIFEST.MF
jar:file:/System/Library/Java/Extensions/MRJToolkit.jar!/META-INF/MANIFEST.MF
jar:file:/private/tmp/arthas-demo.jar!/META-INF/MANIFEST.MF
jar:file:/Users/hengyunabc/.arthas/lib/3.0.5/arthas/arthas-agent.jar!/META-INF/MANIFEST.MF
```
Use the classloader to load `.class` resource
```shell
$ 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
```
#### Use the classloader to load class
```bash
$ classloader -c 3d4eac69 --load demo.MathGame
load class success.
class-info demo.MathGame
code-source /private/tmp/arthas-demo.jar
name demo.MathGame
isInterface false
isAnnotation false
isEnum false
isAnonymousClass false
isArray false
isLocalClass false
isMemberClass false
isPrimitive false
isSynthetic false
simple-name MathGame
modifier public
annotation
interfaces
super-class +-java.lang.Object
class-loader +-sun.misc.Launcher$AppClassLoader@3d4eac69
+-sun.misc.Launcher$ExtClassLoader@66350f69
classLoaderHash 3d4eac69
```
