JVM之类的热替换原理解读

àì夳堔傛蜴生んèń 2022-04-06 15:40 165阅读 0赞

## **先讲讲怎么用吧** ##

一上来就说原理还是不怎么合适的，先给大家讲下这个技术怎么用吧。但是这篇文章重点不是讲怎么用，所以我只讲个大概流程。

第一步：写个Agent类，获取Instrumentation对象

public class MyAgent {
      private static Instrumentation mInstrumentation;
    
      public static void agentmain(String agentArgs, Instrumentation inst) {
        mInstrumentation = inst;
      }
    
      // 拿到Instrumentation对象后就可以利用ClassModifierTransformer来进行类的热替换了
      public static void modifyClass(Class clazz){
        ClassFileTransformer transformer = new ClassModifierTransformer();
        mInstrumentation.addTransformer(transformer, true);
        mInstrumentation.retransformClasses(new Class[]{clazz});
        mInstrumentation.removeTransformer(transformer);
      }
    }

第二步：写个ClassFileTransformer，利用Javassist等工具进行字节码修改

public class ClassModifierTransformer implements ClassFileTransformer {
    
      @Override
      public byte[] transform(ClassLoader loader, String className, Class<?> classBeingRedefined, ProtectionDomain protectionDomain, byte[] classfileBuffer) throws IllegalClassFormatException {
        // 在这里利用Javassist等工具修改类的字节码，返回修改后类的字节数组
        return null;
      }
    }

目前已经有很多文章讲具体使用方法了，大家可以Google下，我这里先介绍两篇：

*  [基于Java Instrument的Agent实现][Java Instrument_Agent]
 *  [谈谈Java Intrumentation和相关应用][Java Intrumentation]

# **原理探究** #

热替换的核心就在于Instrumentation的两个方法：

void addTransformer(ClassFileTransformer transformer, boolean canRetransform);
    
     void retransformClasses(Class<?>... classes) throws UnmodifiableClassException;

addTransformer（）用来注册类的修改器；

retransformClasses（）会让类重新加载，从而使得注册的类修改器能够重新修改类的字节码。

下面让我们细细讲讲这两个函数：

**3.1: addTransformer（）**

addTransformer的实现在InstrumentationImpl中：

//sun.instrument.InstrumentationImpl
    
        public synchronized void addTransformer(ClassFileTransformer transformer, boolean canRetransform) {
            ......
            mRetransfomableTransformerManager.addTransformer(transformer);
            ......
        }

上面代码省略了一些，可见我们的ClassFileTransformer又被添加到了TransformerManager中，让我们跟进去看看:

//sun.instrument.TransformerManager
    
        public synchronized void addTransformer( ClassFileTransformer    transformer) {
            TransformerInfo[] oldList = mTransformerList;
            TransformerInfo[] newList = new TransformerInfo[oldList.length + 1];
            System.arraycopy(   oldList,
                                0,
                                newList,
                                0,
                                oldList.length);
            newList[oldList.length] = new TransformerInfo(transformer);
            mTransformerList = newList;
        }

ClassFileTransformer对象这次被放入了TransformerManager的一个数组中。

OK，注册完毕，很简单对不对？下面我们再来看下稍微复杂点的retransformClasses（）吧。

**3.2: retransformClasses（）**

这个方法的实现是个Native方法。

private native void retransformClasses0(long var1, Class<?>[] var3);

很多同学看到Native方法就头疼，不要急，Native方法也是人写的，不过是一段文本而已。我们来看下他的具体实现吧：

// src/java.instrument/share/native/libinstrument/InstrumentationImplNativeMethods.c
    
    JNIEXPORT void JNICALL
    Java_sun_instrument_InstrumentationImpl_retransformClasses0
      (JNIEnv * jnienv, jobject implThis, jlong agent, jobjectArray classes) {
        retransformClasses(jnienv, (JPLISAgent*)(intptr_t)agent, classes);
    }

retransformClasses（）最后会调用到 jvmtiEnv.cpp中的RetransformClasses

// src/hotspot/share/prims/jvmtiEnv.cpp
    
    jvmtiError
    JvmtiEnv::RetransformClasses(jint class_count, const jclass* classes) {
    
      int index;
      JavaThread* current_thread = JavaThread::current();
      ResourceMark rm(current_thread);
    
      jvmtiClassDefinition* class_definitions =
                                NEW_RESOURCE_ARRAY(jvmtiClassDefinition, class_count);
    
      for (index = 0; index < class_count; index++) {
        HandleMark hm(current_thread);
    
        jclass jcls = classes[index];
        oop k_mirror = JNIHandles::resolve_external_guard(jcls);
        ......
        Klass* klass = java_lang_Class::as_Klass(k_mirror);
    
        jint status = klass->jvmti_class_status();
        if (status & (JVMTI_CLASS_STATUS_ERROR)) {
          return JVMTI_ERROR_INVALID_CLASS;
        }
    
        InstanceKlass* ik = InstanceKlass::cast(klass);
        if (ik->get_cached_class_file_bytes() == NULL) {
          JvmtiClassFileReconstituter reconstituter(ik);
          if (reconstituter.get_error() != JVMTI_ERROR_NONE) {
            return reconstituter.get_error();
          }
    
          class_definitions[index].class_byte_count = (jint)reconstituter.class_file_size();
          class_definitions[index].class_bytes      = (unsigned char*)
                                                           reconstituter.class_file_bytes();
        } else {
          // it is cached, get it from the cache
          class_definitions[index].class_byte_count = ik->get_cached_class_file_len();
          class_definitions[index].class_bytes      = ik->get_cached_class_file_bytes();
        }
        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：

InstanceKlass* KlassFactory::check_shared_class_file_load_hook(
                                              InstanceKlass* ik,
                                              Symbol* class_name,
                                              Handle class_loader,
                                              Handle protection_domain, TRAPS) {
    #if INCLUDE_CDS && INCLUDE_JVMTI
      assert(ik != NULL, "sanity");
      assert(ik->is_shared(), "expecting a shared class");
    
      if (JvmtiExport::should_post_class_file_load_hook()) {
        assert(THREAD->is_Java_thread(), "must be JavaThread");
    
        // Post the CFLH
        JvmtiCachedClassFileData* cached_class_file = NULL;
        JvmtiCachedClassFileData* archived_class_data = ik->get_archived_class_data();
        assert(archived_class_data != NULL, "shared class has no archived class data");
        unsigned char* ptr =
            VM_RedefineClasses::get_cached_class_file_bytes(archived_class_data);
        unsigned char* end_ptr =
            ptr + VM_RedefineClasses::get_cached_class_file_len(archived_class_data);
        unsigned char* old_ptr = ptr;
        JvmtiExport::post_class_file_load_hook(class_name,
                                               class_loader,
                                               protection_domain,
                                               &ptr,
                                               &end_ptr,
                                               &cached_class_file);
        // 这里判断类是否被修改了
        if (old_ptr != ptr) {
          ......
          // 根据返回的类字节码指针及指针范围，构造ClassFileStream
          ClassFileStream* stream = new ClassFileStream(ptr,
                                                        end_ptr - ptr,
                                                        pathname,
                                                        ClassFileStream::verify);
          // 构建新返回的类字节码解析器
          ClassFileParser parser(stream,
                                 class_name,
                                 loader_data,
                                 protection_domain,
                                 NULL,
                                 NULL,
                                 ClassFileParser::BROADCAST, // publicity level
                                 CHECK_NULL);
          // 解析新返回的类字节码，构造出新的类实例InstanceKlass
          InstanceKlass* new_ik = parser.create_instance_klass(true /* changed_by_loadhook */,
                                                               CHECK_NULL);
          if (cached_class_file != NULL) {
            new_ik->set_cached_class_file(cached_class_file);
          }
    
          if (class_loader.is_null()) {
            ResourceMark rm;
            ClassLoader::add_package(class_name->as_C_string(), path_index, THREAD);
          }
    
          return new_ik;
        }
      }
    #endif
    
      return NULL;
    }

check\_shared\_class\_file\_load\_hook（）中会解析新返回的类字节码，构造出新的类new\_ik，最后，基于修改后的字节码构造出来的新类new\_ik 会被返回给上一层，完成类的链接等任务，最终实现了类的热替换。

## **总结** ##

所谓类的热替换，本质上，就是触发了类的重新加载，并在类加载过程中，通过Hook方法，修改类的字节码，并基于修改后的字节码，重新初始化类而已。

[Java Instrument_Agent]: https://www.jianshu.com/p/b72f66da679f
[Java Intrumentation]: http://www.fanyilun.me/2017/07/18/%E8%B0%88%E8%B0%88Java%20Intrumentation%E5%92%8C%E7%9B%B8%E5%85%B3%E5%BA%94%E7%94%A8/