记录一个 ClassLoaderHandler 类加载问题排查

在目前代码里，有一个 dubbo的示例，里面有一个 com.test.dubbo.DubboPluginClassLoaderHandler

```java
public class DubboPluginClassLoaderHandler implements ClassLoaderHandler {

@Override
public Class loadClass(String name) {
if (name.startsWith("com.test.dubbo")) {
try {
Class clazz = this.getClass().getClassLoader().loadClass(name);
return clazz;
} catch (Throwable e) {
e.printStackTrace();
}
}
return null;
}

}

```

显然它是为了保证 dubbo-test-plugin-common 下的测试类 `com.test.dubbo.RpcUtils` 能被其它ClassLoader正常加载。

---

当用户自己写了一个 dubbo 的测试，里面的代码是 `com.test.dubbo.EchoService` ，这里就出现问题了。

* package 和 DubboPluginClassLoaderHandler 里的重名了，导致应用的 `com.test.dubbo.EchoService` 会被 dubbo-plugin PluginClassLoader 加载。

* PluginClassLoader 目前的加载顺序，最终会向上调用 super 去查找，PluginClassLoader 的 parent 是启动 one-java-agent 的 ClassLoader 。
* 所以最终可能 `com.test.dubbo.EchoService` 是被 one-java-agent 的 ClassLoader 加载到。而并不是应用的 ClassLoader。

---

一个异常栈：

```
Thread [main] (Suspended (breakpoint at line 26 in AgentSpyImpl))
owns: Object (id=11559)
owns: OneAgentClassLoader (id=11560)
owns: PluginClassLoader (id=11561)
owns: Object (id=11562)
AgentSpyImpl.loadClass(String) line: 26
Launcher$AppClassLoader(ClassLoader).loadClass(String) line: 49
ClassLoader.defineClass1(String, byte[], int, int, ProtectionDomain, String) line: not available [native method]
Launcher$AppClassLoader(ClassLoader).defineClass(String, byte[], int, int, ProtectionDomain) line: 763
Launcher$AppClassLoader(SecureClassLoader).defineClass(String, byte[], int, int, CodeSource) line: 142
Launcher$AppClassLoader(URLClassLoader).defineClass(String, Resource) line: 468
URLClassLoader.access$100(URLClassLoader, String, Resource) line: 74
URLClassLoader$1.run() line: 369
URLClassLoader$1.run() line: 363
AccessController.doPrivileged(PrivilegedExceptionAction, AccessControlContext) line: not available [native method]
Launcher$AppClassLoader(URLClassLoader).findClass(String) line: 362
Launcher$AppClassLoader(ClassLoader).loadClass(String, boolean) line: 424
Launcher$AppClassLoader.loadClass(String, boolean) line: 349
OneAgentClassLoader(ClassLoader).loadClass(String, boolean) line: 411
PluginClassLoader(ClassLoader).loadClass(String, boolean) line: 411
PluginClassLoader.loadClass(String, boolean) line: 144
PluginClassLoader(ClassLoader).loadClass(String) line: 357
DubboPluginClassLoaderHandler.loadClass(String) line: 16
ClassLoaderHandlerManagerImpl.loadClass(String) line: 80
AgentSpyImpl.loadClass(String) line: 26
ReLaunchURLClassLoader(ClassLoader).loadClass(String) line: 49
Class.forName0(String, boolean, ClassLoader, Class) line: not available [native method]
Class.forName(String, boolean, ClassLoader) line: 348
ClassUtils.forName(String, ClassLoader) line: 280
ClassUtils.resolveClassName(String, ClassLoader) line: 320
ServiceClassPostProcessor.resolveClass(BeanDefinition) line: 361
ServiceClassPostProcessor.resolveClass(BeanDefinitionHolder) line: 353
ServiceClassPostProcessor.registerServiceBean(BeanDefinitionHolder, BeanDefinitionRegistry, DubboClassPathBeanDefinitionScanner) line: 279
ServiceClassPostProcessor.registerServiceBeans(Set, BeanDefinitionRegistry) line: 175
ServiceClassPostProcessor.postProcessBeanDefinitionRegistry(BeanDefinitionRegistry) line: 134
PostProcessorRegistrationDelegate.invokeBeanDefinitionRegistryPostProcessors(Collection, BeanDefinitionRegistry) line: 275
PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory, List) line: 125
AnnotationConfigApplicationContext(AbstractApplicationContext).invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory) line: 706
AnnotationConfigApplicationContext(AbstractApplicationContext).refresh() line: 532
SpringApplication.refresh(ApplicationContext) line: 744
SpringApplication.refreshContext(ConfigurableApplicationContext) line: 391
SpringApplication.run(String...) line: 312
SpringApplication.run(Class[], String[]) line: 1215
SpringApplication.run(Class, String...) line: 1204
Application.main(String[]) line: 33
NativeMethodAccessorImpl.invoke0(Method, Object, Object[]) line: not available [native method]
NativeMethodAccessorImpl.invoke(Object, Object[]) line: 62
DelegatingMethodAccessorImpl.invoke(Object, Object[]) line: 43
Method.invoke(Object, Object...) line: 498
LaunchRunner.run() line: 38
Thread.run() line: 748
```

---

TODO

* 仔细定义启动 one-java-agent的 ClassLoader 和 PluginClassLoader 的行为
* 使用 ClassLoaderHandler 时要非常小心

Trace the loading path through DubboPluginClassLoaderHandler.loadClass, ClassLoaderHandlerManagerImpl.loadClass, AgentSpyImpl.loadClass, PluginClassLoader, and OneAgentClassLoader using the provided stack trace. Reproduce the com.test.dubbo.EchoService collision and define the intended class-loader behavior; done means the behavior and safe use of ClassLoaderHandler are resolved and documented.

Written by the indexing model from the issue text.