public class SemaphoreExample1 { private final static int threadCount = 20; public static void main(String[] args) throws Exception { ExecutorService exec = Executors1.newCachedThreadPool(); final Semaphore1 semaphore = new Semaphore1(1); //并发数是3,每3个3个打印 for (int i = 0; i < 4; i++) { exec.execute(() -> { try { semaphore.acquire(); // 获取一个许可,多线程访问 System.out.println("咔咔咔咔咔咔扩扩扩扩扩扩扩"); semaphore.release(); // 释放一个许可,多线程访问 } catch (Exception e) { } }); } exec.shutdown(); } }
//共享锁,跟读写锁的读锁是一样的逻辑。 public class Semaphore1 implements java.io.Serializable { private static final long serialVersionUID = -3222578661600680210L; private final Sync sync; abstract static class Sync extends AbstractQueuedSynchronizer1 { private static final long serialVersionUID = 1192457210091910933L; Sync(int permits) { setState(permits);//state } final int getPermits() { return getState(); } final int nonfairTryAcquireShared(int acquires) {//获取许可,多线程 for (;;) {//死循环,直到获取一个许可。只有在许可没了才去排队 int available = getState(); int remaining = available - acquires;//state减1 if (remaining < 0 || compareAndSetState(available, remaining))//,多线程 return remaining; } } protected final boolean tryReleaseShared(int releases) {//释放许可,多线程访问 for (;;) { int current = getState(); int next = current + releases; if (next < current) // overflow throw new Error("Maximum permit count exceeded"); if (compareAndSetState(current, next))//减一成功,别的线程就可以获取了。 return true; } } final void reducePermits(int reductions) { for (;;) { int current = getState(); int next = current - reductions; if (next > current) // underflow throw new Error("Permit count underflow"); if (compareAndSetState(current, next)) return; } } final int drainPermits() { for (;;) { int current = getState(); if (current == 0 || compareAndSetState(current, 0)) return current; } } } static final class NonfairSync extends Sync { private static final long serialVersionUID = -2694183684443567898L; NonfairSync(int permits) { super(permits); } protected int tryAcquireShared(int acquires) {//获取许可,多线程 return nonfairTryAcquireShared(acquires); } } static final class FairSync extends Sync { private static final long serialVersionUID = 2014338818796000944L; FairSync(int permits) { super(permits); } protected int tryAcquireShared(int acquires) {//获取许可 for (;;) {//死循环,直到获取一个许可。只有在许可没了才去排队 if (hasQueuedPredecessors())//要不要去排队 return -1; int available = getState(); int remaining = available - acquires; if (remaining < 0 || compareAndSetState(available, remaining)) return remaining; } } } public Semaphore1(int permits) { sync = new NonfairSync(permits); } public Semaphore1(int permits, boolean fair) { sync = fair ? new FairSync(permits) : new NonfairSync(permits); } public void acquire() throws InterruptedException {//多线程访问 sync.acquireSharedInterruptibly(1);//获取共享锁 } public void acquireUninterruptibly() throws InterruptedException { sync.acquireShared(1); } public boolean tryAcquire() {//没有获取许可的,就丢弃,不排队 return sync.nonfairTryAcquireShared(1) >= 0; } public boolean tryAcquire(long timeout, TimeUnit unit) throws InterruptedException { return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout)); } public void release() {//多线程访问 sync.releaseShared(1); } public void acquire(int permits) throws InterruptedException { if (permits < 0) throw new IllegalArgumentException(); sync.acquireSharedInterruptibly(permits); } public void acquireUninterruptibly(int permits) throws InterruptedException { if (permits < 0) throw new IllegalArgumentException(); sync.acquireShared(permits); } public boolean tryAcquire(int permits) { if (permits < 0) throw new IllegalArgumentException(); return sync.nonfairTryAcquireShared(permits) >= 0; } public boolean tryAcquire(int permits, long timeout, TimeUnit unit) throws InterruptedException { if (permits < 0) throw new IllegalArgumentException(); return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout)); } public void release(int permits) { if (permits < 0) throw new IllegalArgumentException(); sync.releaseShared(permits); } public int availablePermits() { return sync.getPermits(); } public int drainPermits() { return sync.drainPermits(); } protected void reducePermits(int reduction) { if (reduction < 0) throw new IllegalArgumentException(); sync.reducePermits(reduction); } public boolean isFair() { return sync instanceof FairSync; } public final boolean hasQueuedThreads() { return sync.hasQueuedThreads(); } public final int getQueueLength() { return sync.getQueueLength(); } protected Collection<Thread> getQueuedThreads() { return sync.getQueuedThreads(); } public String toString() { return super.toString() + "[Permits = " + sync.getPermits() + "]"; } }