Hotspot GC Root 对应调用链

GC Root

GC Root全称是garbage collection root, 即垃圾回收的根. 回到我们的葡萄比喻上来, 也就是一串葡萄的柄. 实际上JVM中的GC Root不只一个, 也就是多个这样的 “柄”.

来看看hotspot网站的解释:

garbage collection root

A pointer into the Java object heap from outside the heap. These come up, e.g., from static fields of classes, local references in activation frames, etc.

官网的解释是由heap外部指向heap内的对象的指针, 比如class的静态字段, 活跃栈帧里的本地引用等. 这里有点小疑问, 从代码上来看, 静态字段也是存在于heap对象(Class对象)中的, 这样来看, 按照heap外指针的定义来解释好像不太妥当. 可以先不关注这个问题, 等会我们从代码上来直观感受一下.

这个说法可能听起来有点奇怪, 因为GC Root的存在是客观的, 并不是因为需要, 这里的意思更多的是为什么要单独提出这么一个概念. 前面介绍过, 对象之间的相互引用会形成一个图(graph), 而要遍历存活的对象, 必须从一些点出发, 这些点, 就是GC Root了.

这样做的目的避免有闭环的出现，闭环的出现讲师灾难性，大大影响性能。

最为GC Root四种对象：

虚拟机栈中的引用对象
方法区中类静态属性引用的对象
方法区中常量引用对象
本地方法栈中JNI引用对象

GC Root 遍历的源码：

void GenCollectedHeap::process_roots(StrongRootsScope* scope,
                                     ScanningOption so,
                                     OopClosure* strong_roots,
                                     OopClosure* weak_roots,
                                     CLDClosure* strong_cld_closure,
                                     CLDClosure* weak_cld_closure,
                                     CodeBlobToOopClosure* code_roots) {
  // General roots.
  assert(Threads::thread_claim_parity() != 0, "must have called prologue code");
  assert(code_roots != NULL, "code root closure should always be set");
  // _n_termination for _process_strong_tasks should be set up stream
  // in a method not running in a GC worker.  Otherwise the GC worker
  // could be trying to change the termination condition while the task
  // is executing in another GC worker.

  if (!_process_strong_tasks->is_task_claimed(GCH_PS_ClassLoaderDataGraph_oops_do)) {
    ClassLoaderDataGraph::roots_cld_do(strong_cld_closure, weak_cld_closure);
  }

  // Only process code roots from thread stacks if we aren't visiting the entire CodeCache anyway
  CodeBlobToOopClosure* roots_from_code_p = (so & SO_AllCodeCache) ? NULL : code_roots;

  bool is_par = scope->n_threads() > 1;
  Threads::possibly_parallel_oops_do(is_par, strong_roots, roots_from_code_p);

  if (!_process_strong_tasks->is_task_claimed(GCH_PS_Universe_oops_do)) {
    Universe::oops_do(strong_roots);
  }
  // Global (strong) JNI handles
  if (!_process_strong_tasks->is_task_claimed(GCH_PS_JNIHandles_oops_do)) {
    JNIHandles::oops_do(strong_roots);
  }

  if (!_process_strong_tasks->is_task_claimed(GCH_PS_ObjectSynchronizer_oops_do)) {
    ObjectSynchronizer::oops_do(strong_roots);
  }
  if (!_process_strong_tasks->is_task_claimed(GCH_PS_FlatProfiler_oops_do)) {
    FlatProfiler::oops_do(strong_roots);
  }
  if (!_process_strong_tasks->is_task_claimed(GCH_PS_Management_oops_do)) {
    Management::oops_do(strong_roots);
  }
  if (!_process_strong_tasks->is_task_claimed(GCH_PS_jvmti_oops_do)) {
    JvmtiExport::oops_do(strong_roots);
  }

  if (!_process_strong_tasks->is_task_claimed(GCH_PS_SystemDictionary_oops_do)) {
    SystemDictionary::roots_oops_do(strong_roots, weak_roots);
  }

  // All threads execute the following. A specific chunk of buckets
  // from the StringTable are the individual tasks.
  if (weak_roots != NULL) {
    if (is_par) {
      StringTable::possibly_parallel_oops_do(weak_roots);
    } else {
      StringTable::oops_do(weak_roots);
    }
  }

  if (!_process_strong_tasks->is_task_claimed(GCH_PS_CodeCache_oops_do)) {
    if (so & SO_ScavengeCodeCache) {
      assert(code_roots != NULL, "must supply closure for code cache");

      // We only visit parts of the CodeCache when scavenging.
      CodeCache::scavenge_root_nmethods_do(code_roots);
    }
    if (so & SO_AllCodeCache) {
      assert(code_roots != NULL, "must supply closure for code cache");

      // CMSCollector uses this to do intermediate-strength collections.
      // We scan the entire code cache, since CodeCache::do_unloading is not called.
      CodeCache::blobs_do(code_roots);
    }
    // Verify that the code cache contents are not subject to
    // movement by a scavenging collection.
    DEBUG_ONLY(CodeBlobToOopClosure assert_code_is_non_scavengable(&assert_is_non_scavengable_closure, !CodeBlobToOopClosure::FixRelocations));
    DEBUG_ONLY(CodeCache::asserted_non_scavengable_nmethods_do(&assert_code_is_non_scavengable));
  }
}

Hotspot GC Root 对应调用链

GC Root

猜你喜欢