用javaagent来对java字节码进行变换

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 
#include 
#include 
#include 

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.cpp
javac Test.java
java -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