Using JVMTI to obtain bytecodes of a Java method

bytecodes.c

#include <jvmti.h>

static jvmtiEnv* jvmti;

JNIEXPORT jint JNICALL
JNI_OnLoad(JavaVM* vm, void* reserved) {
    (*vm)->GetEnv(vm, (void**)&jvmti, JVMTI_VERSION_1_0);

    jvmtiCapabilities capabilities = {0};
    capabilities.can_get_bytecodes = 1;
    capabilities.can_get_constant_pool = 1;
    jvmtiError err = (*jvmti)->AddCapabilities(jvmti, &capabilities);

    return JNI_VERSION_1_4;
}

JNIEXPORT jbyteArray JNICALL
Java_Bytecodes_getBytecodes(JNIEnv* env, jclass cls, jobject method) {
    jmethodID methodID = (*env)->FromReflectedMethod(env, method);

    jint bytecode_size;
    unsigned char* bytecodes;
    if ((*jvmti)->GetBytecodes(jvmti, methodID, &bytecode_size, &bytecodes) != 0) {
        return NULL;
    }

    jbyteArray result = (*env)->NewByteArray(env, bytecode_size);
    if (result != NULL) {
        (*env)->SetByteArrayRegion(env, result, 0, bytecode_size, bytecodes);
    }

    (*jvmti)->Deallocate(jvmti, bytecodes);
    return result;
}

Bytecodes.java

import java.lang.reflect.Method;
import java.util.Arrays;

public class Bytecodes {

    static {
        System.loadLibrary("bytecodes");
    }

    private static native byte[] getBytecodes(Method method);

    public static void main(String[] args) throws Exception {
        Method m = Object.class.getDeclaredMethod("toString");
        byte[] bytecode = getBytecodes(m);
        System.out.println(Arrays.toString(bytecode));
    }
}

Is it possible to get the bytecode of an entire Java class using JVMTI?

Yes, using RetransformClasses API.
See https://github.com/apangin/class-file-extractor (not exactly JVM TI, but idea is the same).

Shouldn't it be also possible with the JvmtiClassFileReconstituter? As I understand, instrumentation's retransform method collects class' bytecode via this reconstituter (if it's not cached). Not sure if it's portable though :D
But since we are already doing something dirty, why not skip the middle man (agent)