Skywalking流程分析_9(JDK类库中增强流程)
前言
之前的文章详细介绍了关于非JDK类库的静态方法、构造方法、实例方法的增强拦截流程，本文会详细分析JDK类库中的类是如何被增强拦截的
回到最开始的SkyWalkingAgent#premain
try {
/*
* 里面有个重点逻辑 把一些类注入到Boostrap类加载器中 为了解决Bootstrap类加载器不能访问App类加载器中的内容的问题
* */
agentBuilder = BootstrapInstrumentBoost.inject(pluginFinder, instrumentation, agentBuilder, edgeClasses);
} catch (Exception e) {
LOGGER.error(e, "SkyWalking agent inject bootstrap instrumentation failure. Shutting down.");
return;
}
BootstrapInstrumentBoost.inject
这里是将必要的类注入到Bootstrap ClassLoader
public static AgentBuilder inject(PluginFinder pluginFinder, Instrumentation instrumentation,
AgentBuilder agentBuilder, JDK9ModuleExporter.EdgeClasses edgeClasses) throws PluginException {
//所有要注入到 Bootstrap ClassLoader 里的类
Map<String, byte[]> classesTypeMap = new LinkedHashMap<>();
//针对于目标类是JDK核心类库的插件，这里根据插件的拦截点的不同(实例方法、静态方法、构造方法)
//使用不同的模板(XxxTemplate)来定义新的拦截器的核心处理逻辑，并且将插件本身定义的拦截器的全类名赋值给模板的
//TARGET_INTERCEPTOR字段。 最后，这些新的拦截器的核心处理逻辑都会被放入 Bootstrap ClassLoader 中
if (!prepareJREInstrumentation(pluginFinder, classesTypeMap)) {
return agentBuilder;
}
if (!prepareJREInstrumentationV2(pluginFinder, classesTypeMap)) {
return agentBuilder;
}
for (String highPriorityClass : HIGH_PRIORITY_CLASSES) {
loadHighPriorityClass(classesTypeMap, highPriorityClass);
}
for (String highPriorityClass : ByteBuddyCoreClasses.CLASSES) {
loadHighPriorityClass(classesTypeMap, highPriorityClass);
}
/**
* Prepare to open edge of necessary classes.
*/
for (String generatedClass : classesTypeMap.keySet()) {
edgeClasses.add(generatedClass);
}
/**
* Inject the classes into bootstrap class loader by using Unsafe Strategy.
* ByteBuddy adapts the sun.misc.Unsafe and jdk.internal.misc.Unsafe automatically.
*/
/**
* 把一些类注入到Boostrap类加载器中 为了解决Bootstrap类加载器不能访问App类加载器中的内容的问题
* */
ClassInjector.UsingUnsafe.Factory factory = ClassInjector.UsingUnsafe.Factory.resolve(instrumentation);
factory.make(null, null).injectRaw(classesTypeMap);
agentBuilder = agentBuilder.with(new AgentBuilder.InjectionStrategy.UsingUnsafe.OfFactory(factory));
return agentBuilder;
}
BootstrapInstrumentBoost#prepareJREInstrumentation
private static boolean prepareJREInstrumentation(PluginFinder pluginFinder,
Map<String, byte[]> classesTypeMap) throws PluginException {
TypePool typePool = TypePool.Default.of(BootstrapInstrumentBoost.class.getClassLoader());
// 所有要对JDK核心类库生效的插件
List<AbstractClassEnhancePluginDefine> bootstrapClassMatchDefines = pluginFinder.getBootstrapClassMatchDefine();
for (AbstractClassEnhancePluginDefine define : bootstrapClassMatchDefines) {
// 是否定义实例方法拦截点
if (Objects.nonNull(define.getInstanceMethodsInterceptPoints())) {
for (InstanceMethodsInterceptPoint point : define.getInstanceMethodsInterceptPoints()) {
if (point.isOverrideArgs()) {
generateDelegator(
classesTypeMap, typePool, INSTANCE_METHOD_WITH_OVERRIDE_ARGS_DELEGATE_TEMPLATE, point
.getMethodsInterceptor());
} else {
generateDelegator(
classesTypeMap, typePool, INSTANCE_METHOD_DELEGATE_TEMPLATE, point.getMethodsInterceptor());
}
}
}
// 是否定义构造器拦截点
if (Objects.nonNull(define.getConstructorsInterceptPoints())) {
for (ConstructorInterceptPoint point : define.getConstructorsInterceptPoints()) {
generateDelegator(
classesTypeMap, typePool, CONSTRUCTOR_DELEGATE_TEMPLATE, point.getConstructorInterceptor());
}
}
// 是否定义静态方法拦截点
if (Objects.nonNull(define.getStaticMethodsInterceptPoints())) {
for (StaticMethodsInterceptPoint point : define.getStaticMethodsInterceptPoints()) {
if (point.isOverrideArgs()) {
generateDelegator(
classesTypeMap, typePool, STATIC_METHOD_WITH_OVERRIDE_ARGS_DELEGATE_TEMPLATE, point
.getMethodsInterceptor());
} else {
generateDelegator(
classesTypeMap, typePool, STATIC_METHOD_DELEGATE_TEMPLATE, point.getMethodsInterceptor());
}
}
}
}
return bootstrapClassMatchDefines.size() > 0;
}
总结
- 在之前
PluginFinder对象初始化时就会将加载的所有插件做分类，要对JDK核心类库生效的插件都放入到bootstrapClassMatchDefine集合中，然后调用getBootstrapClassMatchDefine()就是拿到所有要对JDK核心类库生效的插件 - 循环所有要对JDK核心类库生效的插件，分别判断是否定义实例方法拦截点、是否定义构造器拦截点、是否定义静态方法拦截点
下面我们来进入实例方法的增强，并不重写原方法为例来分析详细的流程
private static String INSTANCE_METHOD_DELEGATE_TEMPLATE = "org.apache.skywalking.apm.agent.core.plugin.bootstrap.template.InstanceMethodInterTemplate";
generateDelegator(
classesTypeMap, typePool, INSTANCE_METHOD_DELEGATE_TEMPLATE, point.getMethodsInterceptor());
- 这里会调用
generateDelegator方法生成一个代理器 - 其中的参数
INSTANCE_METHOD_DELEGATE_TEMPLATE是一个模版类名来传入 - 模版名为
org.apache.skywalking.apm.agent.core.plugin.bootstrap.template.InstanceMethodInterTemplate,这个是模板不是作为实际的类和对象来调用的
InstanceMethodInterTemplate模板
/**
* --------CLASS TEMPLATE---------
* <p>Author, Wu Sheng </p>
* <p>Comment, don't change this unless you are 100% sure the agent core mechanism for bootstrap class
* instrumentation.</p>
* <p>Date, 24th July 2019</p>
* -------------------------------
* <p>
* This class wouldn't be loaded in real env. This is a class template for dynamic class generation.
*
* 这个类是不会被加载的,这是一个类模板用于动态类生成
*/
public class InstanceMethodInterTemplate {
/**
* This field is never set in the template, but has value in the runtime.
*/
private static String TARGET_INTERCEPTOR;
private static InstanceMethodsAroundInterceptor INTERCEPTOR;
private static IBootstrapLog LOGGER;
/**
* Intercept the target instance method.
*
* @param obj target class instance.
* @param allArguments all method arguments
* @param method method description.
* @param zuper the origin call ref.
* @return the return value of target instance method.
* @throws Exception only throw exception because of zuper.call() or unexpected exception in sky-walking ( This is a
* bug, if anything triggers this condition ).
*/
@RuntimeType
public static Object intercept(@This Object obj, @AllArguments Object[] allArguments, @SuperCall Callable<?> zuper,
@Origin Method method) throws Throwable {
EnhancedInstance targetObject = (EnhancedInstance) obj;
prepare();
MethodInterceptResult result = new MethodInterceptResult();
try {
if (INTERCEPTOR != null) {
INTERCEPTOR.beforeMethod(targetObject, method, allArguments, method.getParameterTypes(), result);
}
} catch (Throwable t) {
if (LOGGER != null) {
LOGGER.error(t, "class[{}] before method[{}] intercept failure", obj.getClass(), method.getName());
}
}
Object ret = null;
try {
if (!result.isContinue()) {
ret = result._ret();
} else {
ret = zuper.call();
}
} catch (Throwable t) {
try {
if (INTERCEPTOR != null) {
INTERCEPTOR.handleMethodException(targetObject, method, allArguments, method.getParameterTypes(), t);
}
} catch (Throwable t2) {
if (LOGGER != null) {
LOGGER.error(t2, "class[{}] handle method[{}] exception failure", obj.getClass(), method.getName());
}
}
throw t;
} finally {
try {
if (INTERCEPTOR != null) {
ret = INTERCEPTOR.afterMethod(targetObject, method, allArguments, method.getParameterTypes(), ret);
}
} catch (Throwable t) {
if (LOGGER != null) {
LOGGER.error(t, "class[{}] after method[{}] intercept failure", obj.getClass(), method.getName());
}
}
}
return ret;
}
/**
* Prepare the context. Link to the agent core in AppClassLoader.
* - 让BootstrapClassLoader 和 AgentClassloader能够相通
* - 获取到ILog生成日志对象
* - 获取到插件自定义的拦截器增强类实例
* - 替代非JDK核心类库插件运行逻辑里的interceptor = InterceptorInstanceLoader.load(instanceMethodsAroundInterceptorClassName, classLoader);
*/
private static void prepare() {
if (INTERCEPTOR == null) {
ClassLoader loader = BootstrapInterRuntimeAssist.getAgentClassLoader();
if (loader != null) {
IBootstrapLog logger = BootstrapInterRuntimeAssist.getLogger(loader, TARGET_INTERCEPTOR);
if (logger != null) {
LOGGER = logger;
INTERCEPTOR = BootstrapInterRuntimeAssist.createInterceptor(loader, TARGET_INTERCEPTOR, LOGGER);
}
} else {
LOGGER.error("Runtime ClassLoader not found when create {}." + TARGET_INTERCEPTOR);
}
}
}
}
能看出这个模板类的intercept()方法和实例方法中非jdk类库增强的intercept()方法里面的逻辑都大致差不多
下面来详细分析generateDelegator方法
generateDelegator
/**
* Generate the delegator class based on given template class. This is preparation stage level code generation.
* 根据给定的模板类生成代理器类,这是准备阶段级别的代码生成
* <p>
* One key step to avoid class confliction between AppClassLoader and BootstrapClassLoader
* 避免AppClassLoader和BootstrapClassLoader之间的类冲突的一个关键步骤
*
* @param classesTypeMap hosts injected binary of generated class 。要注入到Bootstrap类加载器中的类和字节码的映射
* key:全类名 value:字节码
* @param typePool to generate new class 。加载BootstrapInstrumentBoost的ClassLoader的类型池
* @param templateClassName represents the class as template in this generation process. The templates are
* pre-defined in SkyWalking agent core。 要进行增强的模板，里面就是环绕增强逻辑的模板
*
* @param methodsInterceptor 要进行增强逻辑的拦截增强类
*/
private static void generateDelegator(Map<String, byte[]> classesTypeMap, TypePool typePool,
String templateClassName, String methodsInterceptor) {
//要进行具体增强逻辑的拦截增强类名 + _internal
String internalInterceptorName = internalDelegate(methodsInterceptor);
try {
//某个类加载器 已经加载了10个类，但这个类加载器的classpath下有400个类，
//typePool.describe可以获取到这个类加载器的classpath下还没有加载类的描述信息
TypeDescription templateTypeDescription = typePool.describe(templateClassName).resolve();
//把模板通过ByteBuddy编译成字节码
DynamicType.Unloaded interceptorType = new ByteBuddy().redefine(templateTypeDescription, ClassFileLocator.ForClassLoader
.of(BootstrapInstrumentBoost.class.getClassLoader()))
.name(internalInterceptorName)
.field(named("TARGET_INTERCEPTOR"))
.value(methodsInterceptor)
.make();
classesTypeMap.put(internalInterceptorName, interceptorType.getBytes());
InstrumentDebuggingClass.INSTANCE.log(interceptorType);
} catch (Exception e) {
throw new PluginException("Generate Dynamic plugin failure", e);
}
}
- 此方法是将模板类交给ByteBuddy去编译成字节码，改了新的类名，并将TARGET_INTERCEPTOR属性赋值为插件拦截器全类名，然后就放入到classesTypeMap中
- 此Map是要注入到Bootstrap类加载器中的类和字节码的映射，key:全类名 value:字节码
接下来回到BootstrapInstrumentBoost.inject(pluginFinder, instrumentation, agentBuilder, edgeClasses)方法，继续分析剩下的部分
ClassInjector.UsingUnsafe.Factory.resolve(instrumentation)
/**
* 把一些必要的类注入到Boostrap类加载器中 为了解决Bootstrap类加载器不能访问App类加载器中的内容的问题
* */
ClassInjector.UsingUnsafe.Factory factory = ClassInjector.UsingUnsafe.Factory.resolve(instrumentation);
factory.make(null, null).injectRaw(classesTypeMap);
agentBuilder = agentBuilder.with(new AgentBuilder.InjectionStrategy.UsingUnsafe.OfFactory(factory));
将之前生成模版的方法中，类和字节码的映射的Mapinstrumentation,注入到Bootstrap ClassLoader,此步骤是使用模版进行字节码增强的前提
JDK类库方法的增强过程
想让我们回到ClassEnhancePluginDefine#enhanceInstance，再看一下实例方法的在进行增强是的判断逻辑
if (existedConstructorInterceptPoint) {
//获取所有构造方法的切点
for (ConstructorInterceptPoint constructorInterceptPoint : constructorInterceptPoints) {
//要增加的类是Bootstrap类加载器加载的
if (isBootstrapInstrumentation()) {
newClassBuilder = newClassBuilder.constructor(constructorInterceptPoint.getConstructorMatcher())
.intercept(SuperMethodCall.INSTANCE.andThen(MethodDelegation.withDefaultConfiguration()
.to(BootstrapInstrumentBoost
.forInternalDelegateClass(constructorInterceptPoint
.getConstructorInterceptor()))));
} else {
newClassBuilder = newClassBuilder.constructor(constructorInterceptPoint.getConstructorMatcher())
.intercept(SuperMethodCall.INSTANCE.andThen(MethodDelegation.withDefaultConfiguration()
.to(new ConstructorInter(constructorInterceptPoint
.getConstructorInterceptor(), classLoader))));
}
}
}
/**
* 3. enhance instance methods
*/
if (existedMethodsInterceptPoints) {
//获取所有实例方法的切点
for (InstanceMethodsInterceptPoint instanceMethodsInterceptPoint : instanceMethodsInterceptPoints) {
String interceptor = instanceMethodsInterceptPoint.getMethodsInterceptor();
if (StringUtil.isEmpty(interceptor)) {
throw new EnhanceException("no InstanceMethodsAroundInterceptor define to enhance class " + enhanceOriginClassName);
}
ElementMatcher.Junction<MethodDescription> junction = not(isStatic()).and(instanceMethodsInterceptPoint.getMethodsMatcher());
if (instanceMethodsInterceptPoint instanceof DeclaredInstanceMethodsInterceptPoint) {
//拦截到的方法必须是当前类上的 注解匹配到的方法有可能不是当前类上的
junction = junction.and(ElementMatchers.<MethodDescription>isDeclaredBy(typeDescription));
}
if (instanceMethodsInterceptPoint.isOverrideArgs()) {
//要增加的类是Bootstrap类加载器加载的
if (isBootstrapInstrumentation()) {
newClassBuilder = newClassBuilder.method(junction)
.intercept(MethodDelegation.withDefaultConfiguration()
.withBinders(Morph.Binder.install(OverrideCallable.class))
.to(BootstrapInstrumentBoost.forInternalDelegateClass(interceptor)));
} else {
newClassBuilder = newClassBuilder.method(junction)
.intercept(MethodDelegation.withDefaultConfiguration()
.withBinders(Morph.Binder.install(OverrideCallable.class))
.to(new InstMethodsInterWithOverrideArgs(interceptor, classLoader)));
}
} else {
//要增加的类是Bootstrap类加载器加载的
if (isBootstrapInstrumentation()) {
newClassBuilder = newClassBuilder.method(junction)
.intercept(MethodDelegation.withDefaultConfiguration()
.to(BootstrapInstrumentBoost.forInternalDelegateClass(interceptor)));
} else {
newClassBuilder = newClassBuilder.method(junction)
.intercept(MethodDelegation.withDefaultConfiguration()
.to(new InstMethodsInter(interceptor, classLoader)));
}
}
}
}
能看到，如果是要增强的类是JDK类库中的，都是调用到BootstrapInstrumentBoost.forInternalDelegateClass的方法
BootstrapInstrumentBoost.forInternalDelegateClass
public static Class forInternalDelegateClass(String methodsInterceptor) {
try {
return Class.forName(internalDelegate(methodsInterceptor));
} catch (ClassNotFoundException e) {
throw new PluginException(e.getMessage(), e);
}
}
public static String internalDelegate(String methodsInterceptor) {
return methodsInterceptor + "_internal";
}
通过Class.forName()加载插件拦截器全类名+_internal的类，这个类在Agent启动流程根据模板类生成并注入到Bootstrap ClassLoader中，所以这里是能加载到，再看一下模版InstanceMethodInterTemplate
/**
* --------CLASS TEMPLATE---------
* <p>Author, Wu Sheng </p>
* <p>Comment, don't change this unless you are 100% sure the agent core mechanism for bootstrap class
* instrumentation.</p>
* <p>Date, 24th July 2019</p>
* -------------------------------
* <p>
* This class wouldn't be loaded in real env. This is a class template for dynamic class generation.
*
* 这个类是不会被加载的,这是一个类模板用于动态类生成
*/
public class InstanceMethodInterTemplate {
/**
* This field is never set in the template, but has value in the runtime.
*/
private static String TARGET_INTERCEPTOR;
private static InstanceMethodsAroundInterceptor INTERCEPTOR;
private static IBootstrapLog LOGGER;
/**
* Intercept the target instance method.
*
* @param obj target class instance.
* @param allArguments all method arguments
* @param method method description.
* @param zuper the origin call ref.
* @return the return value of target instance method.
* @throws Exception only throw exception because of zuper.call() or unexpected exception in sky-walking ( This is a
* bug, if anything triggers this condition ).
*/
@RuntimeType
public static Object intercept(@This Object obj, @AllArguments Object[] allArguments, @SuperCall Callable<?> zuper,
@Origin Method method) throws Throwable {
EnhancedInstance targetObject = (EnhancedInstance) obj;
prepare();
MethodInterceptResult result = new MethodInterceptResult();
try {
if (INTERCEPTOR != null) {
INTERCEPTOR.beforeMethod(targetObject, method, allArguments, method.getParameterTypes(), result);
}
} catch (Throwable t) {
if (LOGGER != null) {
LOGGER.error(t, "class[{}] before method[{}] intercept failure", obj.getClass(), method.getName());
}
}
Object ret = null;
try {
if (!result.isContinue()) {
ret = result._ret();
} else {
ret = zuper.call();
}
} catch (Throwable t) {
try {
if (INTERCEPTOR != null) {
INTERCEPTOR.handleMethodException(targetObject, method, allArguments, method.getParameterTypes(), t);
}
} catch (Throwable t2) {
if (LOGGER != null) {
LOGGER.error(t2, "class[{}] handle method[{}] exception failure", obj.getClass(), method.getName());
}
}
throw t;
} finally {
try {
if (INTERCEPTOR != null) {
ret = INTERCEPTOR.afterMethod(targetObject, method, allArguments, method.getParameterTypes(), ret);
}
} catch (Throwable t) {
if (LOGGER != null) {
LOGGER.error(t, "class[{}] after method[{}] intercept failure", obj.getClass(), method.getName());
}
}
}
return ret;
}
/**
* Prepare the context. Link to the agent core in AppClassLoader.
* - 让BootstrapClassLoader 和 AgentClassloader能够相通
* - 获取到ILog生成日志对象
* - 获取到插件自定义的拦截器增强类实例
* - 替代非JDK核心类库插件运行逻辑里的interceptor = InterceptorInstanceLoader.load(instanceMethodsAroundInterceptorClassName, classLoader);
*/
private static void prepare() {
if (INTERCEPTOR == null) {
ClassLoader loader = BootstrapInterRuntimeAssist.getAgentClassLoader();
if (loader != null) {
IBootstrapLog logger = BootstrapInterRuntimeAssist.getLogger(loader, TARGET_INTERCEPTOR);
if (logger != null) {
LOGGER = logger;
INTERCEPTOR = BootstrapInterRuntimeAssist.createInterceptor(loader, TARGET_INTERCEPTOR, LOGGER);
}
} else {
LOGGER.error("Runtime ClassLoader not found when create {}." + TARGET_INTERCEPTOR);
}
}
}
}
- JDK类库的类做增强会交给模板
InstanceMethodInterTemplate生成的类来处理，实际是模板中的TARGET_INTERCEPTOR赋值为插件拦截器全类名 - 和实例方法插桩的
InstMethodsInter的intercept()方法相比这里多调用了一个prepare()方法 -
prepare()方法是让BootstrapClassLoader能够和AgentClassLoader相同的关键
prepare()
/**
* Prepare the context. Link to the agent core in AppClassLoader.
* - 让BootstrapClassLoader 和 AgentClassloader能够相通
* - 获取到ILog生成日志对象
* - 获取到插件自定义的拦截器增强类实例
* - 替代非JDK核心类库插件运行逻辑里的interceptor = InterceptorInstanceLoader.load(instanceMethodsAroundInterceptorClassName, classLoader);
*/
private static void prepare() {
if (INTERCEPTOR == null) {
ClassLoader loader = BootstrapInterRuntimeAssist.getAgentClassLoader();
if (loader != null) {
IBootstrapLog logger = BootstrapInterRuntimeAssist.getLogger(loader, TARGET_INTERCEPTOR);
if (logger != null) {
LOGGER = logger;
INTERCEPTOR = BootstrapInterRuntimeAssist.createInterceptor(loader, TARGET_INTERCEPTOR, LOGGER);
}
} else {
LOGGER.error("Runtime ClassLoader not found when create {}." + TARGET_INTERCEPTOR);
}
}
}
BootstrapInterRuntimeAssist
public class BootstrapInterRuntimeAssist {
private static final String AGENT_CLASSLOADER_DEFAULT = "org.apache.skywalking.apm.agent.core.plugin.loader.AgentClassLoader";
private static final String DEFAULT_AGENT_CLASSLOADER_INSTANCE = "DEFAULT_LOADER";
private static final String LOG_MANAGER_CLASS = "org.apache.skywalking.apm.agent.core.plugin.bootstrap.BootstrapPluginLogBridge";
private static final String LOG_MANAGER_GET_LOGGER_METHOD = "getLogger";
private static final PrintStream OUT = System.out;
public static ClassLoader getAgentClassLoader() {
try {
ClassLoader loader = Thread.currentThread().getContextClassLoader();
if (loader == null) {
return null;
}
Class<?> agentClassLoaderClass = Class.forName(AGENT_CLASSLOADER_DEFAULT, true, loader);
Field defaultLoaderField = agentClassLoaderClass.getDeclaredField(DEFAULT_AGENT_CLASSLOADER_INSTANCE);
defaultLoaderField.setAccessible(true);
ClassLoader defaultAgentClassLoader = (ClassLoader) defaultLoaderField.get(null);
return defaultAgentClassLoader;
} catch (Exception e) {
e.printStackTrace(OUT);
return null;
}
}
public static IBootstrapLog getLogger(ClassLoader defaultAgentClassLoader, String interceptor) {
try {
Class<?> logManagerClass = Class.forName(LOG_MANAGER_CLASS, true, defaultAgentClassLoader);
Method getLogger = logManagerClass.getMethod(LOG_MANAGER_GET_LOGGER_METHOD, String.class);
return (IBootstrapLog) getLogger.invoke(null, interceptor + "_internal");
} catch (Exception e) {
e.printStackTrace(OUT);
return null;
}
}
public static <T> T createInterceptor(ClassLoader defaultAgentClassLoader, String className, IBootstrapLog log) {
try {
Class<?> interceptor = Class.forName(className, true, defaultAgentClassLoader);
return (T) interceptor.newInstance();
} catch (Exception e) {
log.error(e, "Interceptor[{}] not found", className);
}
return null;
}
}
prepare()总结
- 获取到
AgentClassLoader - 用
AgentClassLoader加载桥接器BootstrapPluginLogBridge类，然后调用getLogger方法传入TARGET_INTERCEPTOR+_internal得到这个传入的实际类名的BootstrapPluginLogBridge的对象 - 将得到的
BootstrapPluginLogBridge赋给模板类的成员属性LOGGER - 用
AgentClassLoader加载插件自定义的拦截器实例
为什么要用prepare()来特意的利用桥接来绕一下呢
因为InstanceMethodInterTemplate生成的类是由BootstrapClassLoader去加载的，而日志对象和插件自定义的拦截器都是通过AgentClassLoader去加载的，prepare()方法的作用就是将BootstrapClassLoader和AgentClassLoader能够相同
举例说明:
- 如果
BootstrapClassLoader加载的由InstanceMethodInterTemplate生成的类是org.apache.skywalking.xxx.HttpClientInterceptor_internal，AgentClassLoader加载了日志用到的ILog和插件拦截器HttpClientIntercepotr -
AgentClassLoader的顶层父类加载器为BootstrapClassLoader，根据双亲委派模型，从下往上加载是可以拿到的，但是从上往下加载是拿不到的,BootstrapClassLoader中不能拿到ILog和HttpClientIntercepotr，所以需要通过prepare()方法打通BootstrapClassLoader和AgentClassLoader
总结
- 准备工作，使用对应的Template模板来生成实际的拦截增强，实际类名为xxx_internal
- prepare()方法
- 将
BootstrapClassLoader和AgentClassLoader能够相通 - 获取到日志对象ILog
- 实例化插件定义的拦截器，用来代替非JDK类库增强中的
InterceptorInstanceLoader.load(instanceMethodsAroundInterceptorClassName, classLoader)
- 将
- 后续的增强逻辑和非JDK类库的流程相同
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