weixin_40725706

这个屌丝很懒，什么也没留下！

热门标签

JVM之类的热替换原理解读，中软国际java面试

作者：weixin_40725706 | 2024-04-19 08:24:12

踩

}

class_definitions[index].klass = jcls;

}

VM_RedefineClasses op(class_count, class_definitions, jvmti_class_load_kind_retransform);

VMThread::execute(&op);

return (op.check_error());

}

上面这段主要干了两件事：

(1) 根据java层的Class对象，找到JVM层的类实例InstanceKlass，并获取类的字节码，存放在class_definitions数组中。因为可以一次替换多个类，所以这里加了一个循环体，遍历每个要修改的类。

(2) 调用VMThread::execute(&op)，进入下一步。

VMThread::execute(&op) 中会调用到 VM_RedefineClasses::doit_prologue()，最终调用到VM_RedefineClasses::load_new_class_versions（）：

jvmtiError VM_RedefineClasses::load_new_class_versions(TRAPS) {

…

InstanceKlass* the_class = get_ik(_class_defs[i].klass);

Symbol* the_class_sym = the_class->name();

ClassFileStream st((u1*)_class_defs[i].class_bytes,

_class_defs[i].class_byte_count,

“VM_RedefineClasses”,

ClassFileStream::verify);

Handle the_class_loader(THREAD, the_class->class_loader());

Handle protection_domain(THREAD, the_class->protection_domain());

state->set_class_being_redefined(the_class, _class_load_kind);

InstanceKlass* scratch_class = SystemDictionary::parse_stream(

the_class_sym,

the_class_loader,

protection_domain,

&st,

…

}

上面这个方法调用了parse_stream（），从文件流中解析类，最终触发类的重新加载：

InstanceKlass* SystemDictionary::load_shared_class(InstanceKlass* ik,

Handle class_loader,

Handle protection_domain, TRAPS) {

…

InstanceKlass* new_ik = KlassFactory::check_shared_class_file_load_hook(

ik, class_name, class_loader, protection_domain, CHECK_NULL);

if (new_ik != NULL) {

return new_ik;

}

…

return ik;

}

这里又调用了KlassFactory::check_shared_class_file_load_hook（），看名字就知道是个hook方法，它会调用post_class_file_load_hook（），利用JvmtiClassFileLoadHookPoster来通知类修改器进行类的修改。

消息的处理者为：eventHandlerClassFileLoadHook（）：

void JNICALL

eventHandlerClassFileLoadHook( jvmtiEnv * jvmtienv,

JNIEnv * jnienv,

jclass class_being_redefined,

jobject loader,

const char* name,

jobject protectionDomain,

jint class_data_len,

const unsigned char* class_data,

jint* new_class_data_len,

unsigned char** new_class_data) {

JPLISEnvironment * environment = NULL;

environment = getJPLISEnvironment(jvmtienv);

/* if something is internally inconsistent (no agent), just silently return without touching the buffer */

if ( environment != NULL ) {

jthrowable outstandingException = preserveThrowable(jnienv);

transformClassFile( environment->mAgent,

jnienv,

loader,

name,

class_being_redefined,

protectionDomain,

class_data_len,

class_data,

new_class_data_len,

new_class_data,

environment->mIsRetransformer);

restoreThrowable(jnienv, outstandingException);

}

eventHandlerClassFileLoadHook（）在收到消息后，会调用transformClassFile（）：

void

transformClassFile( JPLISAgent * agent,

JNIEnv * jnienv,

jobject loaderObject,

const char* name,

jclass classBeingRedefined,

jobject protectionDomain,

jint class_data_len,

const unsigned char* class_data,

jint* new_class_data_len,

unsigned char** new_class_data,

jboolean is_retransformer) {

…

transformedBufferObject = (*jnienv)->CallObjectMethod(

jnienv,

agent->mInstrumentationImpl,

agent->mTransform,

moduleObject,

loaderObject,

classNameStringObject,

classBeingRedefined,

protectionDomain,

classFileBufferObject,

is_retransformer);

…

}

这里会利用JNI调用 java 层InstrumentationImpl的transform（），你看，我们又绕到Java层了：

private byte[] transform( Module module,

ClassLoader loader,

String classname,

Class<?> classBeingRedefined,

ProtectionDomain protectionDomain,

byte[] classfileBuffer,

boolean isRetransformer) {

TransformerManager mgr = isRetransformer?

mRetransfomableTransformerManager :

mTransformerManager;

// module is null when not a class load or when loading a class in an

// unnamed module and this is the first type to be loaded in the package.

if (module == null) {

if (classBeingRedefined != null) {

module = classBeingRedefined.getModule();

} else {

module = (loader == null) ? jdk.internal.loader.BootLoader.getUnnamedModule()

loader.getUnnamedModule();

}

if (mgr == null) {

return null; // no manager, no transform

} else {

return mgr.transform( module,

loader,

classname,

classBeingRedefined,

protectionDomain,

classfileBuffer);

}

上面主要就是调用TransformerManager的transform（）：

public byte[] transform( Module module,

ClassLoader loader,

String classname,

Class<?> classBeingRedefined,

ProtectionDomain protectionDomain,

byte[] classfileBuffer) {

boolean someoneTouchedTheBytecode = false;

TransformerInfo[] transformerList = getSnapshotTransformerList();

byte[] bufferToUse = classfileBuffer;

// order matters, gotta run 'em in the order they were added

for ( int x = 0; x < transformerList.length; x++ ) {

TransformerInfo transformerInfo = transformerList[x];

ClassFileTransformer transformer = transformerInfo.transformer();

byte[] transformedBytes = null;

try {

transformedBytes = transformer.transform( module,

loader,

classname,

classBeingRedefined,

protectionDomain,

bufferToUse);

}

catch (Throwable t) {

// don’t let any one transformer mess it up for the others.

// This is where we need to put some logging. What should go here? FIXME

}

if ( transformedBytes != null ) {

someoneTouchedTheBytecode = true;

bufferToUse = transformedBytes;

}

// if someone modified it, return the modified buffer.

// otherwise return null to mean “no transforms occurred”

byte [] result;

if ( someoneTouchedTheBytecode ) {

result = bufferToUse;

}

else {

result = null;

}

return result;

}

看到这儿，大家还记得我们开始的时候，会将我们自定义的ClassFileTransformer对象注册到TransformerManager中吗？这里终于派上用场了，TransformerManager的transform（）方法会遍历它的注册数组，调用每个ClassFileTransformer对象的transform（）方法，并将我们修改后的类字节码返回，返回后的字节码最终又回到了上面JVM层的transformClassFile（）中，并最终交还给给class_file_load_hook 消息的发送方。

让我们回到消息的发送方：check_shared_class_file_load_hook（）中去看看：

自我介绍一下，小编13年上海交大毕业，曾经在小公司待过，也去过华为、OPPO等大厂，18年进入阿里一直到现在。

深知大多数Java工程师，想要提升技能，往往是自己摸索成长或者是报班学习，但对于培训机构动则几千的学费，着实压力不小。自己不成体系的自学效果低效又漫长，而且极易碰到天花板技术停滞不前！

因此收集整理了一份《2024年Java开发全套学习资料》，初衷也很简单，就是希望能够帮助到想自学提升又不知道该从何学起的朋友，同时减轻大家的负担。