make_local之(thread-＞active_handles()-＞allocate_handle)

make_local

openjdk8\hotspot\src\share\vm\runtime\jniHandles.cpp

jobject JNIHandles::make_local(JNIEnv* env, oop obj) {
  if (obj == NULL) {
    return NULL;                // ignore null handles
  } else {
    JavaThread* thread = JavaThread::thread_from_jni_environment(env);
    assert(Universe::heap()->is_in_reserved(obj), "sanity check");
    // thread->active_handles() = JNIHandleBlock*
    return thread->active_handles()->allocate_handle(obj); // this file : 436
  }
}

allocate_handle

// Handle理解成访问对象的一个“句柄”  
// 每个Oop都有一个对应的Handle，这样通过对应的Handle可直接获取对应的Oop
jobject JNIHandleBlock::allocate_handle(oop obj) {
  assert(Universe::heap()->is_in_reserved(obj), "sanity check");
  if (_top == 0) { //这是进入本机函数时的第一个分配或初始化的初始块。 // 如果我们有以下任何块，则它们不再有效。
    // This is the first allocation or the initial block got zapped when
    // entering a native function. If we have any following blocks they are
    // not valid anymore.
    for (JNIHandleBlock* current = _next; current != NULL;
         current = current->_next) {
      assert(current->_last == NULL, "only first block should have _last set");
      assert(current->_free_list == NULL,
             "only first block should have _free_list set");
      current->_top = 0;
      if (ZapJNIHandleArea) current->zap();
    }
    // Clear initial block
    _free_list = NULL; // 清除初始块
    _allocate_before_rebuild = 0;
    _last = this;
    if (ZapJNIHandleArea) zap();
  }

  // Try last block
  if (_last->_top < block_size_in_oops) { // 尝试最后一个块
    oop* handle = &(_last->_handles)[_last->_top++];
    *handle = obj;
    return (jobject) handle;
  }

  // Try free list
  if (_free_list != NULL) { // 尝试空闲列表
    oop* handle = _free_list;
    _free_list = (oop*) *_free_list;
    *handle = obj;
    return (jobject) handle;
  }
  // Check if unused block follow last
  if (_last->_next != NULL) {  // 检查未使用的块是否紧随其后
    // update last and retry
    _last = _last->_next; // 最后更新并重试
    return allocate_handle(obj);
  }

  // No space available, we have to rebuild free list or expand
  if (_allocate_before_rebuild == 0) { // 没有可用空间，我们必须重建空闲列表或扩展
      rebuild_free_list();        // updates _allocate_before_rebuild counter  // 更新_allocate_before_rebuild计数器
  } else {
    // Append new block  追加新块
    Thread* thread = Thread::current();
    Handle obj_handle(thread, obj);
    // This can block, so we need to preserve obj accross call.
    _last->_next = JNIHandleBlock::allocate_block(thread);  // 这里可能会阻塞，因此我们需要在整个调用期间保留obj。
    _last = _last->_next;
    _allocate_before_rebuild--;
    obj = obj_handle();
  }
  return allocate_handle(obj);  // retry
}

zap()

void JNIHandleBlock::zap() {
  // Zap block values
  _top  = 0;
  for (int index = 0; index < block_size_in_oops; index++) {
    _handles[index] = badJNIHandle;
  }
}