什么是java公平锁

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1、公平锁是指线程获取锁的顺序按照加锁的顺序分配，先来先得，先进先出。

2、公平锁可以保证线程按时间顺序执行，避免饥饿。但是公平锁的效率很低，因为要保证顺序执行，就要保持有序的队列。

实例

   /**     * Base of synchronization control for this lock. Subclassed     * into fair and nonfair versions below. Uses AQS state to     * represent the number of holds on the lock.     */    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        }    }

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