OpenJDK9 Hotspot : Object wait/notify 方法浅析

摘要：前言方法是早期提供的一种基于的线程同步方法，本文先介绍相关的数据结构类，然后从方法的内部实现入手，简单分析相关的原理和实现类用于实现的定待和唤醒，不同平台操作系统平台对应的定义在文件类的分配和释放使用了对象缓存，静态字段用于缓存当前

Object wait/notify 方法是早期 JVM 提供的一种基于 Object Monitor 的线程同步方法，本文先介绍相关的数据结构（类），然后从 wait/notify 方法的内部实现入手，简单分析 Object Monitor 相关的原理和实现

Per-thread blocking support for JSR166. Basically, park acts like wait, uppark like notify

Park 类用于实现 JavaThread 的定待（park/wait）和唤醒（unpark/notify），不同平台（操作系统）

linux 平台对应的 PlatformParker 定义在 hotspot/src/os/linux/vm/os_linux.hpp 文件

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class PlatformParker : public CHeapObj {
 protected:
  enum {
    REL_INDEX = 0,
    ABS_INDEX = 1
  };
  int _cur_index;  // which cond is in use: -1, 0, 1
  pthread_mutex_t _mutex[1];
  pthread_cond_t  _cond[2]; // one for relative times and one for abs.
 public:       // TODO-FIXME: make dtor private
  ~PlatformParker() { guarantee(0, "invariant"); }
 public:
  PlatformParker() {
    int status;
    status = pthread_cond_init(&_cond[REL_INDEX], os::Linux::condAttr());
    assert_status(status == 0, status, "cond_init rel");
    status = pthread_cond_init(&_cond[ABS_INDEX], NULL);
    assert_status(status == 0, status, "cond_init abs");
    status = pthread_mutex_init(_mutex, NULL);
    assert_status(status == 0, status, "mutex_init");
    _cur_index = -1; // mark as unused
  }
};

Parker 类的分配和释放使用了对象缓存，静态字段 FreeList 用于缓存当前所有可用的 Parker，ListLock 用于实现对 FreeList 锁，静态方法 Allocate 和 Release 用于分配和释放 Parker

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class Parker {
    ...
public:
    ...
    static Parker *Allocate(JavaThread *t);
    static void Release(Parker *e);
private:
    static Parker *volatile FreeList;
    static volatile int ListLock;
}

Allocate 方法首先尝试从 FreeList 中获取 Parker，在此之前要先获取 ListLock;如果分配失败则尝试 new 一个新的 parker，最后设置 Parker 的 AssociatedWith 字段将 Parker 和 JavaThread 关联

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Parker * Parker::Allocate (JavaThread * t) {
  guarantee (t != NULL, "invariant") ;
  Parker * p ;
  // Start by trying to recycle an existing but unassociated
  // Parker from the global free list.
  // 8028280: using concurrent free list without memory management can leak
  // pretty badly it turns out.
  Thread::SpinAcquire(&ListLock, "ParkerFreeListAllocate");
  {
    p = FreeList;
    if (p != NULL) {
      FreeList = p->FreeNext;
    }
  }
  Thread::SpinRelease(&ListLock);
  if (p != NULL) {
    guarantee (p->AssociatedWith == NULL, "invariant") ;
  } else {
    // Do this the hard way -- materialize a new Parker..
    p = new Parker() ;
  }
  p->AssociatedWith = t ;          // Associate p with t
  p->FreeNext       = NULL ;
  return p ;
}

Object 类中的很多方法都是 native 方法，wait 也不例外

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public final void wait() throws InterruptedException {
    wait(0);
}
public final native void wait(long timeout) throws InterruptedException;

wait 方法对应的 native 函数为 JVM_MonitorWait

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JVM_ENTRY(void, JVM_MonitorWait(JNIEnv* env, jobject handle, jlong ms))
  JVMWrapper("JVM_MonitorWait");
  Handle obj(THREAD, JNIHandles::resolve_non_null(handle));
  JavaThreadInObjectWaitState jtiows(thread, ms != 0);
  if (JvmtiExport::should_post_monitor_wait()) {
    JvmtiExport::post_monitor_wait((JavaThread *)THREAD, (oop)obj(), ms);
    // The current thread already owns the monitor and it has not yet
    // been added to the wait queue so the current thread cannot be
    // made the successor. This means that the JVMTI_EVENT_MONITOR_WAIT
    // event handler cannot accidentally consume an unpark() meant for
    // the ParkEvent associated with this ObjectMonitor.
  }
  ObjectSynchronizer::wait(obj, ms, CHECK);
JVM_END

它首先调用 JNIHandles::resolve_non_null 函数将 jobject 类型的 handle 转化为 oop（关于 oop 和 Handle 概念可以参考之前的系列文章），然后调用 ObjectSynchronizer 类的静态方法 wait，这里出现了第一个和 Object Monitor 相关的类 ObjectSynchronizer，先做个标记，接着往下看

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int ObjectSynchronizer::wait(Handle obj, jlong millis, TRAPS) {
  if (UseBiasedLocking) {
    BiasedLocking::revoke_and_rebias(obj, false, THREAD);
    assert(!obj->mark()->has_bias_pattern(), "biases should be revoked by now");
  }
  if (millis < 0) {
    TEVENT(wait - throw IAX);
    THROW_MSG_0(vmSymbols::java_lang_IllegalArgumentException(),
        "timeout value is negative");
  }
  ObjectMonitor* monitor = ObjectSynchronizer::inflate(THREAD,
                                                       obj(),
                                                       inflate_cause_wait);
  DTRACE_MONITOR_WAIT_PROBE(monitor, obj(), THREAD, millis);
  monitor->wait(millis, true, THREAD);
  // This dummy call is in place to get around dtrace bug 6254741.  Once
  // that"s fixed we can uncomment the following line, remove the call
  // and change this function back into a "void" func.
  // DTRACE_MONITOR_PROBE(waited, monitor, obj(), THREAD);
  return dtrace_waited_probe(monitor, obj, THREAD);
}

这里出现了第二个类 ObjectMonitor（主角），通过 ObjectSynchronizer::inflate 方法获取对象的 ObjectMonitor 后调用 monitor 的 wait 方法.