Synchronized && Lock

Posted 2020-11-19 hangtutu

想要详细学习synchronized的同学们可以看 http://www.cnblogs.com/noKing/p/9190673.html

本文参考:  http://www.cnblogs.com/dolphin0520/p/3923167.html

https://blog.csdn.net/u012403290/article/details/64910926?locationNum=11&fps=1

介绍一下Lock接口

在java.util.concurrent.locks包中,有三个接口Lock,ReadWriteLock,Condition,常用到的类有ReentrantLock(实现了Lock接口),ReentrantReadWriteLock(实现了ReadWriteLock接口)

Lock.java

public interface Lock {

    /**
     * 获取锁,如果锁被暂用,则一直等待
     */
    void lock();

    /**
     * 获取锁,如果在获取锁阶段进入了等待,可以中断此线程
     *
     * @throws InterruptedException
     */
    void lockInterruptibly() throws InterruptedException;

    /**
     * 获取锁,如果获取成功返回true,如果获取失败返回false
     *
     * @return
     */
    boolean tryLock();

    /**
     * 获取锁,如果在time时间内获得锁,就返回true;否则返回false
     *
     * @param time
     * @param unit
     * @return
     * @throws InterruptedException
     */
    boolean tryLock(long time, TimeUnit unit) throws InterruptedException;

    /**
     * 释放锁
     */
    void unlock();

    /**
     * Returns a new {@link Condition} instance that is bound to this
     * {@code Lock} instance.
     * <p>
     * <p>Before waiting on the condition the lock must be held by the
     * current thread.
     * A call to {@link Condition#await()} will atomically release the lock
     * before waiting and re-acquire the lock before the wait returns.
     * <p>
     * <p><b>Implementation Considerations</b>
     * <p>
     * <p>The exact operation of the {@link Condition} instance depends on
     * the {@code Lock} implementation and must be documented by that
     * implementation.
     *
     * @return A new {@link Condition} instance for this {@code Lock} instance
     * @throws UnsupportedOperationException if this {@code Lock}
     *                                       implementation does not support conditions
     */
    Condition newCondition();
}　　

ReentrantLock.java

public class ReentrantLock implements Lock, java.io.Serializable {
    private static final long serialVersionUID = 7373984872572414699L;

    private final Sync sync;


    abstract static class Sync extends AbstractQueuedSynchronizer {
        private static final long serialVersionUID = -5179523762034025860L;

        /**
         * Performs {@link Lock#lock}. The main reason for subclassing
         * is to allow fast path for nonfair version.
         */
        abstract void lock();

        /**
         * Performs non-fair tryLock.  tryAcquire is implemented in
         * subclasses, but both need nonfair try for trylock method.
         */
        final boolean nonfairTryAcquire(int acquires) {
            final Thread current = Thread.currentThread();
            int c = getState();
            if (c == 0) {
                if (compareAndSetState(0, acquires)) {
                    setExclusiveOwnerThread(current);
                    return true;
                }
            } else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0) // overflow
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }

        protected final boolean tryRelease(int releases) {
            int c = getState() - releases;
            if (Thread.currentThread() != getExclusiveOwnerThread())
                throw new IllegalMonitorStateException();
            boolean free = false;
            if (c == 0) {
                free = true;
                setExclusiveOwnerThread(null);
            }
            setState(c);
            return free;
        }

        protected final boolean isHeldExclusively() {
            // While we must in general read state before owner,
            // we don‘t need to do so to check if current thread is owner
            return getExclusiveOwnerThread() == Thread.currentThread();
        }

        final ConditionObject newCondition() {
            return new ConditionObject();
        }

        // Methods relayed from outer class

        final Thread getOwner() {
            return getState() == 0 ? null : getExclusiveOwnerThread();
        }

        final int getHoldCount() {
            return isHeldExclusively() ? getState() : 0;
        }

        final boolean isLocked() {
            return getState() != 0;
        }

        /**
         * Reconstitutes the instance from a stream (that is, deserializes it).
         */
        private void readObject(java.io.ObjectInputStream s)
                throws java.io.IOException, ClassNotFoundException {
            s.defaultReadObject();
            setState(0); // reset to unlocked state
        }
    }

    /**
     * 非公平锁
     */
    static final class NonfairSync extends Sync {
        private static final long serialVersionUID = 7316153563782823691L;

        /**
         * Performs lock.  Try immediate barge, backing up to normal
         * acquire on failure.
         */
        final void lock() {
            if (compareAndSetState(0, 1))
                setExclusiveOwnerThread(Thread.currentThread());
            else
                acquire(1);
        }

        protected final boolean tryAcquire(int acquires) {
            return nonfairTryAcquire(acquires);
        }
    }

    /**
     * 公平锁
     */
    static final class FairSync extends Sync {
        private static final long serialVersionUID = -3000897897090466540L;

        final void lock() {
            acquire(1);
        }

        /**
         * Fair version of tryAcquire.  Don‘t grant access unless
         * recursive call or no waiters or is first.
         */
        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;
                }
            } else if (current == getExclusiveOwnerThread()) {
                int nextc = c + acquires;
                if (nextc < 0)
                    throw new Error("Maximum lock count exceeded");
                setState(nextc);
                return true;
            }
            return false;
        }
    }

    /**
     * ReentrantLock默认是非公平锁
     */
    public ReentrantLock() {
        sync = new NonfairSync();
    }

    /**
     * ReentrantLock可以通过构造器设定公平锁或者非公平锁
     * @param fair
     */
    public ReentrantLock(boolean fair) {
        sync = fair ? new FairSync() : new NonfairSync();
    }

    /**
     * 获取锁
     * 获取不到就一直处于等待状态
     */
    public void lock() {
        sync.lock();
    }

    /**
     * 获取锁
     * 在获取锁时,如果处于等待状态可以将线程中断,结束等待状态
     * @throws InterruptedException
     */
    public void lockInterruptibly() throws InterruptedException {
        sync.acquireInterruptibly(1);
    }

    /**
     * 获取锁
     * 如果获取成功则返回true,如果获取失败则返回false
     * @return
     */
    public boolean tryLock() {
        return sync.nonfairTryAcquire(1);
    }

    /**
     * 获取锁,如果在指定时间内获取成功则返回true,否则返回false
     * @param timeout
     * @param unit
     * @return
     * @throws InterruptedException
     */
    public boolean tryLock(long timeout, TimeUnit unit)
            throws InterruptedException {
        return sync.tryAcquireNanos(1, unit.toNanos(timeout));
    }


    /**
     * 释放锁
     */
    public void unlock() {
        sync.release(1);
    }


    public Condition newCondition() {
        return sync.newCondition();
    }


    public int getHoldCount() {
        return sync.getHoldCount();
    }

    /**
     * 判断是否被当前线程获取了
     * @return
     */
    public boolean isHeldByCurrentThread() {
        return sync.isHeldExclusively();
    }

    /**
     * 判断是否被任何线程获取了
     * @return
     */
    public boolean isLocked() {
        return sync.isLocked();
    }

    /**
     * 判断是否是公平锁
     * @return
     */
    public final boolean isFair() {
        return sync instanceof FairSync;
    }


    protected Thread getOwner() {
        return sync.getOwner();
    }

    /**
     * 判断是否有线程在等待当前锁
     * @return
     */
    public final boolean hasQueuedThreads() {
        return sync.hasQueuedThreads();
    }

    /**
     * 判断thread是否在等待当前锁
     * @param thread
     * @return
     */
    public final boolean hasQueuedThread(Thread thread) {
        return sync.isQueued(thread);
    }

    /**
     * 获取等待队列的长度
     * @return
     */
    public final int getQueueLength() {
        return sync.getQueueLength();
    }


    protected Collection<Thread> getQueuedThreads() {
        return sync.getQueuedThreads();
    }


    public boolean hasWaiters(Condition condition) {
        if (condition == null)
            throw new NullPointerException();
        if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
            throw new IllegalArgumentException("not owner");
        return sync.hasWaiters((AbstractQueuedSynchronizer.ConditionObject) condition);
    }


    public int getWaitQueueLength(Condition condition) {
        if (condition == null)
            throw new NullPointerException();
        if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
            throw new IllegalArgumentException("not owner");
        return sync.getWaitQueueLength((AbstractQueuedSynchronizer.ConditionObject) condition);
    }


    protected Collection<Thread> getWaitingThreads(Condition condition) {
        if (condition == null)
            throw new NullPointerException();
        if (!(condition instanceof AbstractQueuedSynchronizer.ConditionObject))
            throw new IllegalArgumentException("not owner");
        return sync.getWaitingThreads((AbstractQueuedSynchronizer.ConditionObject) condition);
    }


    public String toString() {
        Thread o = sync.getOwner();
        return super.toString() + ((o == null) ?
                "[Unlocked]" :
                "[Locked by thread " + o.getName() + "]");
    }
}

　　

ReadWriteLock.java

public interface ReadWriteLock {
    /**
     * 获取写锁
     */
    Lock readLock();

    /**
     * 获取读锁
     */
    Lock writeLock();
}

 

ReentrantReadWriteLock.java 源码太长,关于ReentrantReadWriteLock类感兴趣的朋友可以自行查阅API文档。

ReentrantReadWriteLock允许多线程读,而只允许一个线程写
但是synchronized只允许获取当前锁的线程读,所以对于多线程读而言,没有ReentrantReadWriteLock性能好

   不过要注意的是，如果有一个线程已经占用了读锁，则此时其他线程如果要申请写锁，则申请写锁的线程会一直等待释放读锁。

　如果有一个线程已经占用了写锁，则此时其他线程如果申请写锁或者读锁，则申请的线程会一直等待释放写锁。

 

synchronized 和 Lock区别

1.Lock是一个接口，而synchronized是Java中的关键字，synchronized是内置的语言实现；

2.synchronized在发生异常时，会自动释放线程占有的锁，因此不会导致死锁现象发生；而Lock在发生异常时，如果没有主动通过unLock()去释放锁，则很可能造成死锁现象，因此使用Lock时需要在finally块中释放锁；

3.Lock可以让等待锁的线程响应中断，而synchronized却不行，使用synchronized时，等待的线程会一直等待下去，不能够响应中断；

4.通过Lock可以知道有没有成功获取锁，而synchronized却无法办到。

5.Lock可以提高多个线程进行读操作的效率。

6.锁的种类

	可重入锁	可中断锁	公平锁	读写锁
Lock	true	true	true	true
Synchronized	true	false	false	false