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基础篇：详解锁原理，synchronized、volatile+

2020-07-21  本文已影响0人  潜行前行

1 锁的分类

2 synchronized底层原理

public class Test {
    public static void main(String[] args){
        synchronized(Test.class){
            System.out.println("hello");
        }
    }
}

截取部分字节码，如下

    4: monitorenter
    5: getstatic    #9    // Field java/lang/System.out:Ljava/io/PrintStream; 
    8: ldc           #15   // String hello
    10: invokevirtual #17  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
    13: aload_1
    14: monitorexit

字节码出现了4: monitorenter和14: monitorexit两个指令；字面理解就是监视进入，监视退出。可以理解为代码块执行前的加锁，和退出同步时的解锁

objectMonitor.cpp
  ObjectMonitor() {
    _header       = NULL;
    _count        = 0;   \\用来记录获取锁的线程数
    _waiters      = 0,
    _recursions   = 0;    \\锁的重入次数
    _object       = NULL;
    _owner        = NULL;  \\当前持有ObjectMonitor的线程
    _WaitSet      = NULL;  \\wait()方法调用后的线程等待队列
    _WaitSetLock  = 0 ;
    _Responsible  = NULL ;
    _succ         = NULL ;
    _cxq          = NULL ; \\阻塞等待队列
    FreeNext      = NULL ;
    _EntryList    = NULL ; \\synchronized 进来线程的排队队列
    _SpinFreq     = 0 ;
    _SpinClock    = 0 ;  \\自旋计算
    OwnerIsThread = 0 ;
  }

void ATTR ObjectMonitor::enter(TRAPS)  {
    ...
    //获取锁：cmpxchg_ptr原子操作，尝试将_owner替换为自己，并返回旧值
    cur = Atomic::cmpxchg_ptr (Self, &_owner, NULL) ;
    ...
    // 重复获取锁，次数加1，返回
    if (cur == Self) {
        _recursions ++ ;
        return ;
    }
    //首次获取锁情况处理
    if (Self->is_lock_owned ((address)cur)) {
        assert (_recursions == 0, "internal state error");
        _recursions = 1 ;
        _owner = Self ;
        OwnerIsThread = 1 ;
        return ;
    }
    ...
    //尝试自旋获取锁
    if (Knob_SpinEarly && TrySpin (Self) > 0) {
    ...

总结

    public synchronized void lock(){
        System.out.println("world");
    }
....
  public synchronized void lock();
    descriptor: ()V
    flags: (0x0029) ACC_PUBLIC, ACC_SYNCHRONIZED
    Code:
      stack=2, locals=0, args_size=0
         0: getstatic     #20                 // Field java/lang/System.out:Ljava/io/PrintStream;
         3: ldc           #26                 // String world
         5: invokevirtual #28                 // Method java/io/PrintStream.println:(Ljava/lang/String;)V

3 Object的wait和notify方法原理

class ObjectWaiter : public StackObj {
 public:
  enum TStates { TS_UNDEF, TS_READY, TS_RUN, TS_WAIT, TS_ENTER, TS_CXQ } ;
  enum Sorted  { PREPEND, APPEND, SORTED } ;
  ObjectWaiter * volatile _next;
  ObjectWaiter * volatile _prev;
  Thread*       _thread;
  ParkEvent *   _event;
  volatile int  _notified ;
  volatile TStates TState ;
  Sorted        _Sorted ;           // List placement disposition
  bool          _active ;           // Contention monitoring is enabled
 public:
  ObjectWaiter(Thread* thread);
  void wait_reenter_begin(ObjectMonitor *mon);
  void wait_reenter_end(ObjectMonitor *mon);
};

调用对象锁的wait()方法时，线程会被封装成ObjectWaiter，最后使用park方法挂起

//objectMonitor.cpp
void ObjectMonitor::wait(jlong millis, bool interruptible, TRAPS){
    ...
    //线程封装成 ObjectWaiter对象
    ObjectWaiter node(Self);
    node.TState = ObjectWaiter::TS_WAIT ;
    ...
    //一系列判断操作，当线程确实加入WaitSet时，则使用park方法挂起
    if (node._notified == 0) {
        if (millis <= 0) {
            Self->_ParkEvent->park () ;
        } else {
            ret = Self->_ParkEvent->park (millis) ;
        }
    }

而当对象锁使用notify()时

void ObjectMonitor::notify(TRAPS){
    CHECK_OWNER();
    //waitSet为空，则直接返回
    if (_WaitSet == NULL) {
        TEVENT (Empty-Notify) ;
        return ;
    }
    ...
    //通过DequeueWaiter获取_WaitSet列表中的第一个ObjectWaiter
    Thread::SpinAcquire (&_WaitSetLock, "WaitSet - notify") ;
    ObjectWaiter * iterator = DequeueWaiter() ;
    if (iterator != NULL) {
    ....
    if (Policy == 2) {      // prepend to cxq
         // prepend to cxq
         if (List == NULL) {
             iterator->_next = iterator->_prev = NULL ;
             _EntryList = iterator ;
         } else {
            iterator->TState = ObjectWaiter::TS_CXQ ;
            for (;;) {
                ObjectWaiter * Front = _cxq ;
                iterator->_next = Front ;
                if (Atomic::cmpxchg_ptr (iterator, &_cxq, Front) == Front) {
                    break ;
                }
            }
         }
     }

4 jvm对synchronized的优化

Test.class
//编译器会考虑将两次加锁合并
public void test(){
    synchronized(this){
        System.out.println("hello");   
    }
    synchronized(this){
        System.out.println("world");   
    }
}

//StringBuffer的append操作会加上synchronized，
//但是变量buf不加锁也安全的，编译器会把锁消除
public void test() {
    StringBuffer buf = new StringBuffer();
    buf.append("hello").append("world");
}

5 CAS的底层原理

int expectedValue = 1;
public boolean compareAndSet(int newValue) {
    if(expectedValue == 1){
        expectedValue = newValue;
        return ture;
    }
    return false;
}

在jdk是有提供同步版的CAS解决方案，其中使用了UnSafe.java的底层方法

//UnSafe.java
    @HotSpotIntrinsicCandidate
    public final native boolean compareAndSetInt(Object o, long offset, int expected, int x) ..
    @HotSpotIntrinsicCandidate
    public final native int compareAndExchangeInt(Object o, long offset, int expected, int x)...

我们再来看看本地方法，Unsafe.cpp中的compareAndSwapInt

//unsafe.cpp
UNSAFE_ENTRY(jboolean, Unsafe_CompareAndSwapInt(JNIEnv *env, jobject unsafe, jobject obj, jlong offset, jint e, jint x))
  UnsafeWrapper("Unsafe_CompareAndSwapInt");
  oop p = JNIHandles::resolve(obj);
  jint* addr = (jint *) index_oop_from_field_offset_long(p, offset);
  return (jint)(Atomic::cmpxchg(x, addr, e)) == e;
UNSAFE_END

在Linux的x86，Atomic::cmpxchg方法的实现如下

/**
    1 __asm__表示汇编的开始；
    2 volatile表示禁止编译器优化；//禁止指令重排
    3 LOCK_IF_MP是个内联函数，
      根据当前系统是否为多核处理器，
      决定是否为cmpxchg指令添加lock前缀 //内存屏障
*/
inline jint Atomic::cmpxchg (jint exchange_value, volatile jint* dest, jint compare_value) {
  int mp = os::is_MP();
  __asm__ volatile (LOCK_IF_MP(%4) "cmpxchgl %1,(%3)"
                    : "=a" (exchange_value)
                    : "r" (exchange_value), "a" (compare_value), "r" (dest), "r" (mp)
                    : "cc", "memory");
  return exchange_value;
}

到这一步，可以总结到：jdk提供的CAS机制，在汇编层级，会禁止变量两侧的指令优化，然后使用cmpxchg指令比较并更新变量值(原子性)，如果是多核则使用lock锁定(缓存锁、MESI)

6 CAS同步操作的问题

7 基于volatile + CAS 实现同步锁的原理

//伪代码
volatile state = 0 ;   // 0-无锁 1-加锁；volatile禁止指令重排，加入内存屏障
...
if(cas(state, 0 , 1)){ // 1 加锁成功，只有一个线程能成功加锁
    ...                // 2 同步代码块
    cas(state, 1, 0);  // 3 解锁时2的操作具有可见性
}

8 LockSupport了解一下

JavaThread* thread=JavaThread::thread_from_jni_environment(env);
...
thread->parker()->park(isAbsolute != 0, time);

class PlatformParker : public CHeapObj {
  protected:
    //互斥变量类型
    pthread_mutex_t _mutex [1] ; 
   //条件变量类型
    pthread_cond_t  _cond  [1] ;
    ...
}

class Parker : public os::PlatformParker {  
private:  
  volatile int _counter ;  
  ...  
public:  
  void park(bool isAbsolute, jlong time);  
  void unpark();  
  ...  
}

9 LockSupport.park和Object.wait区别

10 AbstractQueuedSynchronizer(AQS)

//AbstractQueuedSynchronizer.java
public class AbstractQueuedSynchronizer{
    //线程节点
    static final class Node {
        ...
        volatile Node prev;
        volatile Node next;
        volatile Thread thread;
        ...
    }    
    ....
    //head 等待队列头尾节点
    private transient volatile Node head;
    private transient volatile Node tail;
    // The synchronization state. 同步状态
    private volatile int state;  
    ...
    //提供CAS操作，状态具体的修改由子类实现
    protected final boolean compareAndSetState(int expect, int update) {
        return STATE.compareAndSet(this, expect, update);
    }
}

public final void acquire(int arg) {
        if (!tryAcquire(arg) &&
            acquireQueued(addWaiter(Node.EXCLUSIVE), arg))
            selfInterrupt();
}

在AQS还存一个ConditionObject的内部类，它的使用机制和Object.wait、notify类似

//AbstractQueuedSynchronizer.java
public class ConditionObject implements Condition, java.io.Serializable {
    //条件队列;Node 复用了AQS中定义的Node
    private transient Node firstWaiter;
    private transient Node lastWaiter;
    ...

//类似Object.wait
public final void await() throws InterruptedException{
    ...
    Node node = addConditionWaiter(); //构造Node,加入条件队列
    int savedState = fullyRelease(node);
    int interruptMode = 0;
    while (!isOnSyncQueue(node)) {
        //挂起线程
        LockSupport.park(this);
        if ((interruptMode = checkInterruptWhileWaiting(node)) != 0)
            break;
    }
    //notify唤醒线程后，加入同步队列继续竞争锁
    if (acquireQueued(node, savedState) && interruptMode != THROW_IE)
        interruptMode = REINTERRUPT;
image 
    //类似Object.notify
    private void doSignal(Node first) {
        do {
            if ( (firstWaiter = first.nextWaiter) == null)
                lastWaiter = null;
            first.nextWaiter = null;
        } while (!transferForSignal(first) &&
                 (first = firstWaiter) != null);
    }
image 
protected boolean tryAcquire(int arg);//尝试独占性加锁
protected boolean tryRelease(int arg);//对应tryAcquire释放锁
protected int tryAcquireShared(int arg);//尝试共享性加锁
protected boolean tryReleaseShared(int arg);//对应tryAcquireShared释放锁
protected boolean isHeldExclusively();//该线程是否正在独占资源，只有用到condition才需要取实现它

11 ReentrantLock的原理

image 
abstract static class Sync extends AbstractQueuedSynchronizer{
    .... 
    final boolean nonfairTryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                //直接CAS状态加锁，非公平操作
                if (compareAndSetState(0, acquires)) { 
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
    ...
    //重写了tryRelease
    protected final boolean tryRelease(int releases) {
        c = state - releases; //改变同步状态
        ...
        //修改volatile 修饰的状态变量
        setState(c); 
        return free;
    }
}

    static final class NonfairSync extends Sync {
        protected final boolean tryAcquire(int acquires) {
            return nonfairTryAcquire(acquires);
        }
    }
    ....
    static final class FairSync extends Sync {
        protected final boolean tryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                if (!hasQueuedPredecessors() &&   
                    compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            }
    ....

12 AQS排他锁的实例demo

public class TwinsLock implements Lock {

    private final Sync sync = new Sync(2);
    @Override
    public void lockInterruptibly() throws InterruptedException {  throw new RuntimeException(""); }
    @Override
    public boolean tryLock(long time, TimeUnit unit) throws InterruptedException {throw new RuntimeException("");}
    @Override
    public Condition newCondition() {  return sync.newCondition(); }
    @Override
    public void lock() {  sync.acquireShared(1); }
    @Override
    public void unlock() {  sync.releaseShared(1); } }
    @Override
    public boolean tryLock() { return sync.tryAcquireShared(1) > -1;  }
}

再来看看Sync的代码

class Sync extends AbstractQueuedSynchronizer {
        Sync(int count) {
            if (count <= 0) {
                throw new IllegalArgumentException("count must large than zero");
            }
            setState(count);
        }
        @Override
        public int tryAcquireShared(int reduceCount) {
            for (; ; ) {
                int current = getState(); 
                int newCount = current - reduceCount;
                if (newCount < 0 || compareAndSetState(current, newCount)) {
                    return newCount;
                }
            }
        }
        @Override
        public boolean tryReleaseShared(int returnCount) {
            for (; ; ) {
                int current = getState();
                int newCount = current + returnCount;
                if (compareAndSetState(current, newCount)) {
                    return true;
                }
            }
        }
        public Condition newCondition() {
            return new AbstractQueuedSynchronizer.ConditionObject();
        }
    }

13 使用锁，能防止线程死循环吗

14 ThreadLocal是否可保证资源的同步

参考文章

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