客观的
我正在使用 JVMTI 代理进行动态字节码检测。我必须检测那些“热”的方法，即调用 JIT 编译器的方法。为此，我听了一个CompiledLoadEvent并在其回调函数内调用RetransformClasses。这反过来又调用ClassFileLoadHook包含“热”函数和实际检测的类开始。
问题前提
目前我正在对我的类进行检测以生成一些线程。我还监听线程启动并在我的代理中打印它们。用简单的ClassFileLoadHook在类加载时（没有RetransformClasses），我的仪器工作完美并产生新线程。我得到以下输出时ClassFileLoadHook类加载时的乐器：
Running Thread: Signal Dispatcher, Priority: 9, context class loader:Not Null
Running Thread: main, Priority: 5, context class loader:Not Null
Running Thread: Thread-0, Priority: 5, context class loader:Not Null
Running Thread: Thread-1, Priority: 5, context class loader:Not Null
Running Thread: Thread-2, Priority: 5, context class loader:Not Null
Running Thread: Thread-3, Priority: 5, context class loader:Not Null
Running Thread: Thread-4, Priority: 5, context class loader:Not Null
Running Thread: Thread-6, Priority: 5, context class loader:Not Null
Running Thread: Thread-5, Priority: 5, context class loader:Not Null
Running Thread: Thread-7, Priority: 5, context class loader:Not Null
Running Thread: DestroyJavaVM, Priority: 5, context class loader:: NULL
当我通过调用来检测类文件时RetransformClasses进而ClassFileLoadHook，一切工作正常，但没有生成任何线程，因此没有发生有效的检测。 VM甚至需要很长时间才能执行原始代码。
我使用 -XX:+TraceClassLoading 仔细检查了两个仪器。在这两种情况下都会加载所有重新转换的类。即使我在运行时生成的类也会被加载，但不会发生检测。以下是类加载跟踪的输出：
[Loaded Test from __VM_RedefineClasses__]
[Loaded Test_Worker_main_0 from file:/home/saqib/workspace/test/bin]
我在运行时生成第二个类并将其加载到虚拟机中，但我没有生成任何线程。
问题
- 鉴于我对问题的理解（很有可能
我错了），为什么
ClassFileLoadHook重新改造班级
在加载时成功，但不知何故行为不正确
什么时候调用 JIT？
- 只是写下
RetransformClasses函数，带空ClassFileLoadHook回电，也需要很多时间而不
发生任何类型的错误。什么可能需要时间？
代理代码
编译加载事件回调
static int x = 1;
void JNICALL
compiled_method_load(jvmtiEnv *jvmti, jmethodID method, jint code_size,
const void* code_addr, jint map_length, const jvmtiAddrLocationMap* map,
const void* compile_info) {
jvmtiError err;
jclass klass;
char* name = NULL;
char* signature = NULL;
char* generic_ptr = NULL;
err = (*jvmti)->RawMonitorEnter(jvmti, lock);
check_jvmti_error(jvmti, err, "raw monitor enter");
err = (*jvmti)->GetMethodName(jvmti, method, &name, &signature,
&generic_ptr);
check_jvmti_error(jvmti, err, "Get Method Name");
printf("\nCompiled method load event\n");
printf("Method name %s %s %s\n\n", name, signature,
generic_ptr == NULL ? "" : generic_ptr);
if (strstr(name, "main") != NULL && x == 1) {
x++;
err = (*jvmti)->GetMethodDeclaringClass(jvmti, method, &klass);
check_jvmti_error(jvmti, err, "Get Declaring Class");
err = (*jvmti)->RetransformClasses(jvmti, 1, &klass);
check_jvmti_error(jvmti, err, "Retransform class");
}
if (name != NULL) {
err = (*jvmti)->Deallocate(jvmti, (unsigned char*) name);
check_jvmti_error(jvmti, err, "deallocate name");
}
if (signature != NULL) {
err = (*jvmti)->Deallocate(jvmti, (unsigned char*) signature);
check_jvmti_error(jvmti, err, "deallocate signature");
}
if (generic_ptr != NULL) {
err = (*jvmti)->Deallocate(jvmti, (unsigned char*) generic_ptr);
check_jvmti_error(jvmti, err, "deallocate generic_ptr");
}
err = (*jvmti)->RawMonitorExit(jvmti, lock);
check_jvmti_error(jvmti, err, "raw monitor exit");
}
类文件加载挂钩
void JNICALL
Class_File_Load_Hook(jvmtiEnv *jvmti_env, JNIEnv* jni_env,
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) {
jvmtiError err;
unsigned char* jvmti_space = NULL;
if (strstr(name, "Test") != NULL && x == 2) {
char* args = "op";
javab_main(2, args, class_data, class_data_len);
err = (*jvmti_env)->Allocate(jvmti_env, (jlong)global_pos, &jvmti_space);
check_jvmti_error(jvmti_env, err, "Allocate new class Buffer.");
(void)memcpy((void*)jvmti_space, (void*)new_class_ptr, (int)global_pos);
*new_class_data_len = (jint)global_pos;
*new_class_data = jvmti_space;
if ( new_class_ptr != NULL ) {
(void)free((void*)new_class_ptr);
}
#if DEBUG
printf("Size of the class is: %d\n", class_data_len);
for (int i = 0; i < class_data_len; i += 4) {
if (i % 16 == 0)
printf("\n");
printf("%02x%02x %02x%02x ", new_class_data[i],
new_class_data[i + 1], new_class_data[i + 2],
new_class_data[i + 3]);
}
printf("\n");
system("javap -c -v Test_debug");
#endif
x++;
}
}
Here javab_main返回已检测的char *数组是正确的。检测后的数组存储在全局变量中new_class_ptr被复制到new_class_data。为了调试仪器的输出，我还在名为 Test_debug 的文件中打印了仪器类并调用javap就可以产生想要的结果。
完整的代理文件如下：Agent.c https://gist.github.com/saqibahmed515/9864ec018edeca14e14ce518131c753c
原始代码：
for (int i = 0; i < s; i++)
for (int j = 0; j < s; j++) {
c2[i][j] = 0;
for (int k = 0; k < s; k++)
c2[i][j] += a[i][k] * b[k][j];
}
仪表代码：（等效）
Thread[] threads = new Thread[NTHREADS];
for (int i = 0; i < NTHREADS ; i++) {
final int lb = i * SIZE/NTHREADS;
final int ub = (i+1) * SIZE/NTHREADS;
threads[i] = new Thread(new Runnable() {
public void run() {
for (int i = lb; i < ub; i++)
for (int j = 0; j < SIZE; j++) {
c2[i][j] = 0;
for (int k = 0; k < SIZE; k++)
c2[i][j] += a[i][k] * b[k][j];
}
}
});
threads[i].start();
}
// wait for completion
for (int i = 0; i < NTHREADS; i++) {
try {
threads[i].join();
} catch (InterruptedException ignore) {
}
}
Java版本
openjdk version "1.8.0-internal-debug"
OpenJDK Runtime Environment (build 1.8.0-internal-debug-saqib_2016_12_26_10_52-b00)
OpenJDK 64-Bit Server VM (build 25.71-b00-debug, mixed mode)
