JVM源码分析之javaagent原理完全解读--转

发布时间：2020-12-14 06:22:54 所属栏目：Java 来源：网络整理

导读：原文地址：http://www.infoq.com/cn/articles/javaagent-illustrated java -javaagent:myagent.jar=mode=test Test jvmtiEnv span class="token operator"gt;* jvmtienv span class="token operator"gt;= span class="token function"gt;jvmtispan class="to

原文地址：http://www.infoq.com/cn/articles/javaagent-illustrated

java -javaagent:myagent.jar=mode=test Test

jvmtiEnv <span class="token operator"&gt;*          jvmtienv <span class="token operator"&gt;= <span class="token function"&gt;jvmti<span class="token punctuation"&gt;(agent<span class="token punctuation"&gt;)<span class="token punctuation"&gt;;

jvmtiError          jvmtierror<span class="token punctuation"&gt;;

<span class="token function"&gt;memset<span class="token punctuation"&gt;(<span class="token operator"&gt;&amp;callbacks<span class="token punctuation"&gt;,<span class="token number"&gt;0<span class="token punctuation"&gt;,<span class="token keyword"&gt;sizeof<span class="token punctuation"&gt;(callbacks<span class="token punctuation"&gt;)<span class="token punctuation"&gt;)<span class="token punctuation"&gt;;

callbacks<span class="token punctuation"&gt;.ClassFileLoadHook <span class="token operator"&gt;= <span class="token operator"&gt;&amp;eventHandlerClassFileLoadHook<span class="token punctuation"&gt;;

jvmtierror <span class="token operator"&gt;= <span class="token punctuation"&gt;(<span class="token operator"&gt;*jvmtienv<span class="token punctuation"&gt;)<span class="token operator"&gt;-<span class="token operator"&gt;><span class="token function"&gt;SetEventCallbacks<span class="token punctuation"&gt;( jvmtienv<span class="token punctuation"&gt;,<span class="token operator"&gt;&amp;callbacks<span class="token punctuation"&gt;,<span class="token keyword"&gt;sizeof<span class="token punctuation"&gt;(callbacks<span class="token punctuation"&gt;)<span class="token punctuation"&gt;)<span class="token punctuation"&gt;;</span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></code></pre>

<h1 class="p1">JVMTIAgent
JVMTIAgent其实就是一个动态库，利用JVMTI暴露出来的一些接口来干一些我们想做、但是正常情况下又做不到的事情，不过为了和普通的动态库进行区分，它一般会实现如下的一个或者多个函数：

<pre class=" language-cpp"><code class=" language-cpp">JNIEXPORT jint JNICALL
Agent_OnLoad(JavaVM vm,char options,void *reserved);

JNIEXPORT jint JNICALL
Agent_OnAttach(JavaVM vm,char options,void* reserved);

JNIEXPORT void JNICALL
Agent_OnUnload(JavaVM *vm);
<ul class="ul1">
<li class="li2">Agent_OnLoad函数，如果agent是在启动时加载的，也就是在vm参数里通过-agentlib来指定的，那在启动过程中就会去执行这个agent里的Agent_OnLoad函数。
<li class="li2">Agent_OnAttach函数，如果agent不是在启动时加载的，而是我们先attach到目标进程上，然后给对应的目标进程发送load命令来加载，则在加载过程中会调用Agent_OnAttach函数。
<li class="li2">Agent_OnUnload函数，在agent卸载时调用，不过貌似基本上很少实现它。

struct _JPLISEnvironment <span class="token punctuation">{

jvmtiEnv <span class="token operator">              mJVMTIEnv<span class="token punctuation">;              <span class="token comment">/ the JVM TI environment /

JPLISAgent <span class="token operator">            mAgent<span class="token punctuation">;                 <span class="token comment">/ corresponding agent /

jboolean                mIsRetransformer<span class="token punctuation">;       <span class="token comment">/ indicates if special environment /

<span class="token punctuation">}<span class="token punctuation">;

这里解释一下几个重要项：

<span class="token function">eventHandlerClassFileLoadHook<span class="token punctuation">(  jvmtiEnv <span class="token operator">              jvmtienv<span class="token punctuation">,JNIEnv <span class="token operator">                jnienv<span class="token punctuation">,jclass                  <span class="token class-name">class_being_redefined<span class="token punctuation">,jobject                 loader<span class="token punctuation">,<span class="token keyword">const <span class="token keyword">char<span class="token operator">             name<span class="token punctuation">,jobject                 protectionDomain<span class="token punctuation">,jint                    class_data_len<span class="token punctuation">,<span class="token keyword">const <span class="token keyword">unsigned <span class="token keyword">char<span class="token operator">    class_data<span class="token punctuation">,jint<span class="token operator">                   new_class_data_len<span class="token punctuation">,<span class="token keyword">unsigned <span class="token keyword">char<span class="token operator"><span class="token operator">*         new_class_data<span class="token punctuation">) <span class="token punctuation">{
JPLISEnvironment <span class="token operator"&gt;* environment  <span class="token operator"&gt;= <span class="token constant"&gt;NULL<span class="token punctuation"&gt;;

environment <span class="token operator"&gt;= <span class="token function"&gt;getJPLISEnvironment<span class="token punctuation"&gt;(jvmtienv<span class="token punctuation"&gt;)<span class="token punctuation"&gt;;

<span class="token comment"&gt;/* if something is internally inconsistent (no agent),just silently return without touching the buffer */

<span class="token keyword"&gt;if <span class="token punctuation"&gt;( environment <span class="token operator"&gt;!= <span class="token constant"&gt;NULL <span class="token punctuation"&gt;) <span class="token punctuation"&gt;{

    jthrowable outstandingException <span class="token operator"&gt;= <span class="token function"&gt;preserveThrowable<span class="token punctuation"&gt;(jnienv<span class="token punctuation"&gt;)<span class="token punctuation"&gt;;
    <span class="token function"&gt;transformClassFile<span class="token punctuation"&gt;( environment<span class="token operator"&gt;-<span class="token operator"&gt;>mAgent<span class="token punctuation"&gt;,jnienv<span class="token punctuation"&gt;,loader<span class="token punctuation"&gt;,name<span class="token punctuation"&gt;,class_being_redefined<span class="token punctuation"&gt;,protectionDomain<span class="token punctuation"&gt;,class_data_len<span class="token punctuation"&gt;,class_data<span class="token punctuation"&gt;,new_class_data_len<span class="token punctuation"&gt;,new_class_data<span class="token punctuation"&gt;,environment<span class="token operator"&gt;-<span class="token operator"&gt;>mIsRetransformer<span class="token punctuation"&gt;)<span class="token punctuation"&gt;;

    <span class="token function"&gt;restoreThrowable<span class="token punctuation"&gt;(jnienv<span class="token punctuation"&gt;,outstandingException<span class="token punctuation"&gt;)<span class="token punctuation"&gt;;
<span class="token punctuation"&gt;}
<span class="token punctuation">}

先根据jvmtiEnv取得对应的JPLISEnvironment，因为上面我已经说到其实有两个JPLISEnvironment（并且有两个jvmtiEnv），其中一个是专门做retransform的，而另外一个用来做其他事情，根据不同的用途，在注册具体的ClassFileTransformer时也是分开的，对于作为retransform用的ClassFileTransformer，我们会注册到一个单独的TransformerManager里。

接着调用transformClassFile方法，由于函数实现比较长，这里就不贴代码了，大致意思就是调用InstrumentationImpl对象的transform方法，根据最后那个参数来决定选哪个TransformerManager里的ClassFileTransformer对象们做transform操作。

<pre class=" language-java"><code class=" language-java">private byte[]
transform( ClassLoader loader,String classname,Class classBeingRedefined,ProtectionDomain protectionDomain,byte[] classfileBuffer,boolean isRetransformer) {

    TransformerManager mgr <span class="token operator"&gt;= isRetransformer<span class="token operator"&gt;?

                                    mRetransfomableTransformerManager <span class="token operator"&gt;:
                                    mTransformerManager<span class="token punctuation"&gt;;

    <span class="token keyword"&gt;if <span class="token punctuation"&gt;(mgr <span class="token operator"&gt;== null<span class="token punctuation"&gt;) <span class="token punctuation"&gt;{

        <span class="token keyword"&gt;return null<span class="token punctuation"&gt;; <span class="token comment"&gt;// no manager,no transform

    <span class="token punctuation"&gt;} <span class="token keyword"&gt;else <span class="token punctuation"&gt;{

        <span class="token keyword"&gt;return mgr<span class="token punctuation"&gt;.<span class="token function"&gt;transform<span class="token punctuation"&gt;(   loader<span class="token punctuation"&gt;,classname<span class="token punctuation"&gt;,classBeingRedefined<span class="token punctuation"&gt;,classfileBuffer<span class="token punctuation"&gt;)<span class="token punctuation"&gt;;

    <span class="token punctuation"&gt;}

<span class="token punctuation"&gt;}

public byte[]

<span class="token function"&gt;transform<span class="token punctuation"&gt;(  ClassLoader         loader<span class="token punctuation"&gt;,<span class="token keyword"&gt;byte<span class="token punctuation"&gt;[<span class="token punctuation"&gt;]              classfileBuffer<span class="token punctuation"&gt;) <span class="token punctuation"&gt;{

    <span class="token keyword"&gt;boolean someoneTouchedTheBytecode <span class="token operator"&gt;= <span class="token boolean"&gt;false<span class="token punctuation"&gt;;
    TransformerInfo<span class="token punctuation"&gt;[<span class="token punctuation"&gt;]  transformerList <span class="token operator"&gt;= <span class="token function"&gt;getSnapshotTransformerList<span class="token punctuation"&gt;(<span class="token punctuation"&gt;)<span class="token punctuation"&gt;;
    <span class="token keyword"&gt;byte<span class="token punctuation"&gt;[<span class="token punctuation"&gt;]  bufferToUse <span class="token operator"&gt;= classfileBuffer<span class="token punctuation"&gt;;

    <span class="token comment"&gt;// order matters,gotta run 'em in the order they were added

    <span class="token keyword"&gt;for <span class="token punctuation"&gt;( <span class="token keyword"&gt;int x <span class="token operator"&gt;= <span class="token number"&gt;0<span class="token punctuation"&gt;; x <span class="token operator"&gt;< transformerList<span class="token punctuation"&gt;.length<span class="token punctuation"&gt;; x<span class="token operator"&gt;++ <span class="token punctuation"&gt;) <span class="token punctuation"&gt;{

        TransformerInfo         transformerInfo <span class="token operator"&gt;= transformerList<span class="token punctuation"&gt;[x<span class="token punctuation"&gt;]<span class="token punctuation"&gt;;
        ClassFileTransformer    transformer <span class="token operator"&gt;= transformerInfo<span class="token punctuation"&gt;.<span class="token function"&gt;transformer<span class="token punctuation"&gt;(<span class="token punctuation"&gt;)<span class="token punctuation"&gt;;
        <span class="token keyword"&gt;byte<span class="token punctuation"&gt;[<span class="token punctuation"&gt;]                  transformedBytes <span class="token operator"&gt;= null<span class="token punctuation"&gt;;

        <span class="token keyword"&gt;try <span class="token punctuation"&gt;{

            transformedBytes <span class="token operator"&gt;= transformer<span class="token punctuation"&gt;.<span class="token function"&gt;transform<span class="token punctuation"&gt;(   loader<span class="token punctuation"&gt;,bufferToUse<span class="token punctuation"&gt;)<span class="token punctuation"&gt;;

        <span class="token punctuation"&gt;}

        <span class="token keyword"&gt;catch <span class="token punctuation"&gt;(<span class="token class-name"&gt;Throwable t<span class="token punctuation"&gt;) <span class="token punctuation"&gt;{

            <span class="token comment"&gt;// don't let any one transformer mess it up for the others.
            <span class="token comment"&gt;// This is where we need to put some logging. What should go here? FIXME

        <span class="token punctuation"&gt;}


        <span class="token keyword"&gt;if <span class="token punctuation"&gt;( transformedBytes <span class="token operator"&gt;!= null <span class="token punctuation"&gt;) <span class="token punctuation"&gt;{
            someoneTouchedTheBytecode <span class="token operator"&gt;= <span class="token boolean"&gt;true<span class="token punctuation"&gt;;
            bufferToUse <span class="token operator"&gt;= transformedBytes<span class="token punctuation"&gt;;
        <span class="token punctuation"&gt;}

    <span class="token punctuation"&gt;}


    <span class="token comment"&gt;// if someone modified it,return the modified buffer.
    <span class="token comment"&gt;// otherwise return null to mean "no transforms occurred"

    <span class="token keyword"&gt;byte <span class="token punctuation"&gt;[<span class="token punctuation"&gt;] result<span class="token punctuation"&gt;;

    <span class="token keyword"&gt;if <span class="token punctuation"&gt;( someoneTouchedTheBytecode <span class="token punctuation"&gt;) <span class="token punctuation"&gt;{
        result <span class="token operator"&gt;= bufferToUse<span class="token punctuation"&gt;;
    <span class="token punctuation"&gt;}
    <span class="token keyword"&gt;else <span class="token punctuation"&gt;{
        result <span class="token operator"&gt;= null<span class="token punctuation"&gt;;
    <span class="token punctuation"&gt;}

    <span class="token keyword"&gt;return result<span class="token punctuation"&gt;;

<span class="token punctuation"&gt;}

以上是最终调到的java代码，可以看到已经调用到我们自己编写的javaagent代码里了，我们一般是实现一个ClassFileTransformer类，然后创建一个对象注册到对应的TransformerManager里。

<h1 class="p1">Class Transform的实现
这里说的class transform其实是狭义的，主要是针对第一次类文件加载时就要求被transform的场景，在加载类文件的时候发出ClassFileLoad事件，然后交给instrumenat agent来调用javaagent里注册的ClassFileTransformer实现字节码的修改。

<h1 class="p1">Class Redefine的实现
类重新定义，这是Instrumentation提供的基础功能之一，主要用在已经被加载过的类上，想对其进行修改，要做这件事，我们必须要知道两个东西，一个是要修改哪个类，另外一个是想将那个类修改成怎样的结构，有了这两个信息之后就可以通过InstrumentationImpl下面的redefineClasses方法操作了：

<pre class=" language-java"><code class=" language-java">public void redefineClasses(ClassDefinition[] definitions) throws ClassNotFoundException {

    <span class="token keyword"&gt;if <span class="token punctuation"&gt;(<span class="token operator"&gt;!<span class="token function"&gt;isRedefineClassesSupported<span class="token punctuation"&gt;(<span class="token punctuation"&gt;)<span class="token punctuation"&gt;) <span class="token punctuation"&gt;{

        <span class="token keyword"&gt;throw <span class="token keyword"&gt;new <span class="token class-name"&gt;UnsupportedOperationException<span class="token punctuation"&gt;(<span class="token string"&gt;"redefineClasses is not supported in this environment"<span class="token punctuation"&gt;)<span class="token punctuation"&gt;;

    <span class="token punctuation"&gt;}

    <span class="token keyword"&gt;if <span class="token punctuation"&gt;(definitions <span class="token operator"&gt;== null<span class="token punctuation"&gt;) <span class="token punctuation"&gt;{

        <span class="token keyword"&gt;throw <span class="token keyword"&gt;new <span class="token class-name"&gt;NullPointerException<span class="token punctuation"&gt;(<span class="token string"&gt;"null passed as 'definitions' in redefineClasses"<span class="token punctuation"&gt;)<span class="token punctuation"&gt;;

    <span class="token punctuation"&gt;}

    <span class="token keyword"&gt;for <span class="token punctuation"&gt;(<span class="token keyword"&gt;int i <span class="token operator"&gt;= <span class="token number"&gt;0<span class="token punctuation"&gt;; i <span class="token operator"&gt;< definitions<span class="token punctuation"&gt;.length<span class="token punctuation"&gt;; <span class="token operator"&gt;++i<span class="token punctuation"&gt;) <span class="token punctuation"&gt;{

        <span class="token keyword"&gt;if <span class="token punctuation"&gt;(definitions<span class="token punctuation"&gt;[i<span class="token punctuation"&gt;] <span class="token operator"&gt;== null<span class="token punctuation"&gt;) <span class="token punctuation"&gt;{

            <span class="token keyword"&gt;throw <span class="token keyword"&gt;new <span class="token class-name"&gt;NullPointerException<span class="token punctuation"&gt;(<span class="token string"&gt;"element of 'definitions' is null in redefineClasses"<span class="token punctuation"&gt;)<span class="token punctuation"&gt;;

        <span class="token punctuation"&gt;}

    <span class="token punctuation"&gt;}

    <span class="token keyword"&gt;if <span class="token punctuation"&gt;(definitions<span class="token punctuation"&gt;.length <span class="token operator"&gt;== <span class="token number"&gt;0<span class="token punctuation"&gt;) <span class="token punctuation"&gt;{

        <span class="token keyword"&gt;return<span class="token punctuation"&gt;; <span class="token comment"&gt;// short-circuit if there are no changes requested

    <span class="token punctuation"&gt;}


    <span class="token function"&gt;redefineClasses0<span class="token punctuation"&gt;(mNativeAgent<span class="token punctuation"&gt;,definitions<span class="token punctuation"&gt;)<span class="token punctuation"&gt;;

<span class="token punctuation"&gt;}

在JVM里对应的实现是创建一个VM_RedefineClasses的VM_Operation，注意执行它的时候会stop-the-world：

<pre class=" language-cpp"><code class=" language-cpp">jvmtiError

JvmtiEnv::RedefineClasses(jint class_count,const jvmtiClassDefinition* class_definitions) {

//TODO: add locking

VM_RedefineClasses op(class_count,class_definitions,jvmti_class_load_kind_redefine);

VMThread::execute(&op);

return (op.check_error());

} / end RedefineClasses /
这个过程我尽量用语言来描述清楚，不详细贴代码了，因为代码量实在有点大：

<ul class="ul1">
<li class="li2">挨个遍历要批量重定义的jvmtiClassDefinition
<li class="li2">然后读取新的字节码，如果有关注ClassFileLoadHook事件的，还会走对应的transform来对新的字节码再做修改
<li class="li2">字节码解析好，创建一个klassOop对象
<li class="li2">
对比新老类，并要求如下：
<ul class="ul2">
<li class="li2">父类是同一个
<li class="li2">实现的接口数也要相同，并且是相同的接口
<li class="li2">类访问符必须一致
<li class="li2">字段数和字段名要一致
<li class="li2">新增的方法必须是private static/final的
<li class="li2">可以删除修改方法

    <span class="token keyword"&gt;if <span class="token punctuation"&gt;(<span class="token operator"&gt;!<span class="token function"&gt;isRetransformClassesSupported<span class="token punctuation"&gt;(<span class="token punctuation"&gt;)<span class="token punctuation"&gt;) <span class="token punctuation"&gt;{

        <span class="token keyword"&gt;throw <span class="token keyword"&gt;new <span class="token class-name"&gt;UnsupportedOperationException<span class="token punctuation"&gt;( <span class="token string"&gt;"retransformClasses is not supported in this environment"<span class="token punctuation"&gt;)<span class="token punctuation"&gt;;

    <span class="token punctuation"&gt;}

    <span class="token function"&gt;retransformClasses0<span class="token punctuation"&gt;(mNativeAgent<span class="token punctuation"&gt;,classes<span class="token punctuation"&gt;)<span class="token punctuation"&gt;;

<span class="token punctuation"&gt;}</span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></span></code></pre>

不过retransform的实现其实也是通过redefine的功能来实现，在类加载的时候有比较小的差别，主要体现在究竟会走哪些transform上，如果当前是做retransform的话，那将忽略那些注册到正常的TransformerManager里的ClassFileTransformer，而只会走专门为retransform而准备的TransformerManager的ClassFileTransformer，不然想象一下字节码又被无声无息改成某个中间态了。

<pre class=" language-java"><code class=" language-java">private:

void post_all_envs() {

<span class="token keyword"&gt;if <span class="token punctuation"&gt;(_load_kind <span class="token operator"&gt;!= jvmti_class_load_kind_retransform<span class="token punctuation"&gt;) <span class="token punctuation"&gt;{

  <span class="token comment"&gt;// for class load and redefine,<span class="token comment"&gt;// call the non-retransformable agents

  JvmtiEnvIterator it<span class="token punctuation"&gt;;

  <span class="token keyword"&gt;for <span class="token punctuation"&gt;(JvmtiEnv<span class="token operator"&gt;* env <span class="token operator"&gt;= it<span class="token punctuation"&gt;.<span class="token function"&gt;first<span class="token punctuation"&gt;(<span class="token punctuation"&gt;)<span class="token punctuation"&gt;; env <span class="token operator"&gt;!= NULL<span class="token punctuation"&gt;; env <span class="token operator"&gt;= it<span class="token punctuation"&gt;.<span class="token function"&gt;next<span class="token punctuation"&gt;(env<span class="token punctuation"&gt;)<span class="token punctuation"&gt;) <span class="token punctuation"&gt;{

    <span class="token keyword"&gt;if <span class="token punctuation"&gt;(<span class="token operator"&gt;!env<span class="token operator"&gt;-<span class="token operator"&gt;><span class="token function"&gt;is_retransformable<span class="token punctuation"&gt;(<span class="token punctuation"&gt;) <span class="token operator"&gt;&amp;&amp; env<span class="token operator"&gt;-<span class="token operator"&gt;><span class="token function"&gt;is_enabled<span class="token punctuation"&gt;(JVMTI_EVENT_CLASS_FILE_LOAD_HOOK<span class="token punctuation"&gt;)<span class="token punctuation"&gt;) <span class="token punctuation"&gt;{

      <span class="token comment"&gt;// non-retransformable agents cannot retransform back,<span class="token comment"&gt;// so no need to cache the original class file bytes

      <span class="token function"&gt;post_to_env<span class="token punctuation"&gt;(env<span class="token punctuation"&gt;,<span class="token boolean"&gt;false<span class="token punctuation"&gt;)<span class="token punctuation"&gt;;

    <span class="token punctuation"&gt;}

  <span class="token punctuation"&gt;}

<span class="token punctuation"&gt;}

JvmtiEnvIterator it<span class="token punctuation"&gt;;

<span class="token keyword"&gt;for <span class="token punctuation"&gt;(JvmtiEnv<span class="token operator"&gt;* env <span class="token operator"&gt;= it<span class="token punctuation"&gt;.<span class="token function"&gt;first<span class="token punctuation"&gt;(<span class="token punctuation"&gt;)<span class="token punctuation"&gt;; env <span class="token operator"&gt;!= NULL<span class="token punctuation"&gt;; env <span class="token operator"&gt;= it<span class="token punctuation"&gt;.<span class="token function"&gt;next<span class="token punctuation"&gt;(env<span class="token punctuation"&gt;)<span class="token punctuation"&gt;) <span class="token punctuation"&gt;{

  <span class="token comment"&gt;// retransformable agents get all events

  <span class="token keyword"&gt;if <span class="token punctuation"&gt;(env<span class="token operator"&gt;-<span class="token operator"&gt;><span class="token function"&gt;is_retransformable<span class="token punctuation"&gt;(<span class="token punctuation"&gt;) <span class="token operator"&gt;&amp;&amp; env<span class="token operator"&gt;-<span class="token operator"&gt;><span class="token function"&gt;is_enabled<span class="token punctuation"&gt;(JVMTI_EVENT_CLASS_FILE_LOAD_HOOK<span class="token punctuation"&gt;)<span class="token punctuation"&gt;) <span class="token punctuation"&gt;{

    <span class="token comment"&gt;// retransformable agents need to cache the original class file

    <span class="token comment"&gt;// bytes if changes are made via the ClassFileLoadHook

    <span class="token function"&gt;post_to_env<span class="token punctuation"&gt;(env<span class="token punctuation"&gt;,<span class="token boolean"&gt;true<span class="token punctuation"&gt;)<span class="token punctuation"&gt;;

  <span class="token punctuation"&gt;}

<span class="token punctuation"&gt;}

}

<h1 class="p3">javaagent的其他小众功能
javaagent除了做字节码上面的修改之外，其实还有一些小功能，有时候还是挺有用的

<ul class="ul1">
<li class="li6">获取所有已经被加载的类：Class[] getAllLoadedClasses();?
<li class="li6">获取所有已经初始化了的类：?Class[] getInitiatedClasses(ClassLoader loader);?
<li class="li6">获取某个对象的大小：?long getObjectSize(Object objectToSize);?
<li class="li6">将某个jar加入到bootstrap classpath里优先其他jar被加载：?void appendToBootstrapClassLoaderSearch(JarFile jarfile);?
<li class="li6">将某个jar加入到classpath里供appclassloard去加载：void appendToSystemClassLoaderSearch(JarFile jarfile);?
<li class="li6">设置某些native方法的前缀，主要在找native方法的时候做规则匹配：?void setNativeMethodPrefix(ClassFileTransformer transformer,String prefix)。

（编辑：李大同）

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