用javaagent来对java字节码进行变换

来源：互联网发布：elasticsearch python 编辑：程序博客网时间：2024/05/16 08:10

1、实现变换的函数

typedef void (JNICALL *jvmtiEventClassFileLoadHook) //类字节码变换的回调接口    (jvmtiEnv *jvmti_env, //注册的jvmti     JNIEnv* jni_env, //JNI环境     jclass class_being_redefined, //     jobject loader,     const char* name,     jobject protection_domain,     jint class_data_len,     const unsigned char* class_data,     jint* new_class_data_len,     unsigned char** new_class_data);

2、注册回调事件

jint result = jvm->GetEnv((void**) &jvmti, JVMTI_VERSION_1_1);jvmtiEventCallbacks callbacks;callbacks.ClassFileLoadHook = &eventHandlerClassFileLoadHook;(*jvmtienv)->SetEventCallbacks( jvmtienv,                                                 &callbacks,                                                 sizeof(callbacks));

3、启用类字节码加载的hook事件，如果不启用，注册的回调函数将不会被调用的

jvmti->SetEventNotificationMode(JVMTI_ENABLE, JVMTI_EVENT_CLASS_FILE_LOAD_HOOK, NULL);

4、启用字节码变换的标记，如果不启用，可以调用回调函数，但是里面进行的字节码变换将不起效

jvmtiCapabilities capabilities;memset(&capabilities, 0, sizeof(capabilities));capabilities.can_retransform_classes = 1;

5、实际的实现，打印加载的类

#include <jvmti.h>#include <jni.h>#include <stdio.h>#include <string.h>extern "C" JNICALL void myClassTransform//类字节码变换的回调接口    (jvmtiEnv *jvmti_env, //注册的jvmti     JNIEnv* jni_env, //JNI环境     jclass class_being_redefined, //     jobject loader,     const char* name,     jobject protection_domain,     jint class_data_len,     const unsigned char* class_data,     jint* new_class_data_len,     unsigned char** new_class_data) {    printf("Loading class %s\n", name);    }JNIEXPORT jint JNICALL Agent_OnLoad(JavaVM *jvm,  char *options, void* reserved) {jvmtiEnv *jvmti = NULL;jint error = jvm->GetEnv((void**)&jvmti, JVMTI_VERSION_1_1);//加载jvmti环境if(error != JNI_OK) {printf("Get env fail, errorCode=%d\n", error);return JNI_ERR;}//使用字节码变换能力jvmtiCapabilities capabilities;memset(&capabilities, 0, sizeof(capabilities));capabilities.can_retransform_classes = 1;capabilities.can_retransform_any_class = 1;jvmtiError e2 = jvmti->AddCapabilities(&capabilities);if(e2 != JVMTI_ERROR_NONE) {printf("AddCapabilities error\n");return JNI_ERR;}//设置回调jvmtiEventCallbacks callbacks;memset(&callbacks, 0, sizeof(callbacks));callbacks.ClassFileLoadHook = &myClassTransform; //设置类文件变换的文件句柄e2 = jvmti->SetEventCallbacks(&callbacks, sizeof(callbacks));if(e2 != JVMTI_ERROR_NONE) {printf("setcallback error\n");return JNI_ERR;}e2 = jvmti->SetEventNotificationMode(JVMTI_ENABLE, JVMTI_EVENT_CLASS_FILE_LOAD_HOOK, NULL);printf("Register ok\n");return JNI_OK;}

6、运行，注意要用g++进行编译，如果用gcc进行编译的，jvmti的方法调用，应该形如(*jvmti)->xxx(jvmti,...)

g++ -fPIC -shared -g -o test.dylib -I$JAVA_HOME/include  -I$JAVA_HOME/include/darwin/ test.cppjavac Test.javajava -agentpath:test.dylib Test

7、关于jvm的这些hook调用的实现原理，请看下一篇博客

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