OpenJDK9 Hotspot : Object wait/notify 方法浅析

前言

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

Park

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）

PlatformParker

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

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
  }
};

allocate & release

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

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

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 ;
}

park

unpark

ObjectMonitor

ObjectSynchronizer

wait

Object 类中的很多方法都是 native 方法，wait 也不例外

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

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

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.

总结