Semaphore（信号量），内部维护了一组许可证，通过acquire、tryAcquire方法获取许可证，acquire获取不到许可时，会阻塞线程。tryAcquire获取不到许可时，会返回获取结果。通过release释放许可，释放许可会添加可用许可证数量。Semaphore借助AQS实现的共享锁，通过构造参数可以给状态变量赋值，用来控制对资源访问的并发度。
演示示例：
package com.securitit.serialize.juc;
import java.util.Date;
import java.util.concurrent.Semaphore;
import org.assertj.core.util.DateUtil;
public class SemaphoreTester {
// Semaphore实例.
private static final Semaphore semaphore = new Semaphore(2);
public static void main(String[] args) {
// 模拟线程.
new Thread(() -> {
try {
System.out.println(
DateUtil.formatAsDatetime(new Date()) + " " + Thread.currentThread().getName() + " 获取许可开始.");
semaphore.acquire();
System.out.println(
DateUtil.formatAsDatetime(new Date()) + " " + Thread.currentThread().getName() + " 获取许可成功.");
Thread.sleep(5000);
} catch (InterruptedException ex) {
ex.printStackTrace();
} finally {
semaphore.release();
System.out.println(
DateUtil.formatAsDatetime(new Date()) + " " + Thread.currentThread().getName() + " 释放许可成功.");
}
}).start();
// 模拟线程.
new Thread(() -> {
try {
System.out.println(
DateUtil.formatAsDatetime(new Date()) + " " + Thread.currentThread().getName() + " 获取许可开始.");
semaphore.acquire();
System.out.println(
DateUtil.formatAsDatetime(new Date()) + " " + Thread.currentThread().getName() + " 获取许可成功.");
Thread.sleep(5000);
} catch (InterruptedException ex) {
ex.printStackTrace();
} finally {
semaphore.release();
System.out.println(
DateUtil.formatAsDatetime(new Date()) + " " + Thread.currentThread().getName() + " 释放许可成功.");
}
}).start();
// 模拟线程.
new Thread(() -> {
try {
System.out.println(
DateUtil.formatAsDatetime(new Date()) + " " + Thread.currentThread().getName() + " 获取许可开始.");
semaphore.acquire();
System.out.println(
DateUtil.formatAsDatetime(new Date()) + " " + Thread.currentThread().getName() + " 获取许可成功.");
Thread.sleep(2000);
} catch (InterruptedException ex) {
ex.printStackTrace();
} finally {
semaphore.release();
System.out.println(
DateUtil.formatAsDatetime(new Date()) + " " + Thread.currentThread().getName() + " 释放许可成功.");
}
}).start();
}
}
输出结果：
2020-06-26T23:19:28 Thread-0 获取许可开始.
2020-06-26T23:19:28 Thread-1 获取许可开始.
2020-06-26T23:19:28 Thread-2 获取许可开始.
2020-06-26T23:19:28 Thread-0 获取许可成功.
2020-06-26T23:19:28 Thread-1 获取许可成功.
2020-06-26T23:19:33 Thread-1 释放许可成功.
2020-06-26T23:19:33 Thread-2 获取许可成功.
2020-06-26T23:19:33 Thread-0 释放许可成功.
2020-06-26T23:19:35 Thread-2 释放许可成功.
线程Thread-0和Thread-1首先获取许可成功，由于许可数量只有两个，线程Thread-2进入阻塞状态，等待Thread-0和Thread-1释放许可。然后，Thread-1释放许可，Thread-2获取许可成功，继而Thread-0和Thread-2释放许可。
源码分析：
package java.util.concurrent;
import java.util.Collection;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;
public class Semaphore implements java.io.Serializable {
// 序列化版本号.
private static final long serialVersionUID = -3222578661600680210L;
// 同步类实例.
private final Sync sync;
// 同步类.内部类.扩展AQS实现.
abstract static class Sync extends AbstractQueuedSynchronizer {
// 序列化版本号.
private static final long serialVersionUID = 1192457210091910933L;
// Constructor.
Sync(int permits) {
setState(permits);
}
// 获取许可数量.
final int getPermits() {
return getState();
}
// 尝试获取非公平共享锁.
final int nonfairTryAcquireShared(int acquires) {
for (;;) {
// 可用许可数量.
int available = getState();
// 可用许可数量是否够用.
int remaining = available - acquires;
if (remaining < 0 ||
// 以CAS方式设置剩余许可数.
compareAndSetState(available, remaining))
return remaining;
}
}
// 尝试释放共享锁.
protected final boolean tryReleaseShared(int releases) {
for (;;) {
int current = getState();
int next = current + releases;
if (next < current) // overflow
throw new Error("Maximum permit count exceeded");
if (compareAndSetState(current, next))
return true;
}
}
// 减少许可数量.
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;
}
}
// 设置许可数量.
final int drainPermits() {
for (;;) {
int current = getState();
if (current == 0 || compareAndSetState(current, 0))
return current;
}
}
}
// 非公平内部类实现.继承自Sync.
static final class NonfairSync extends Sync {
// 序列化版本号.
private static final long serialVersionUID = -2694183684443567898L;
// 非公平Constructor.
NonfairSync(int permits) {
super(permits);
}
// 尝试获取共享锁.
protected int tryAcquireShared(int acquires) {
return nonfairTryAcquireShared(acquires);
}
}
// 公平内部类实现，继承自Sync.
static final class FairSync extends Sync {
// 序列化版本号.
private static final long serialVersionUID = 2014338818796000944L;
// 公平Constructor.
FairSync(int permits) {
super(permits);
}
// 尝试释放共享锁.
protected int tryAcquireShared(int acquires) {
for (;;) {
if (hasQueuedPredecessors())
return -1;
int available = getState();
int remaining = available - acquires;
if (remaining < 0 ||
compareAndSetState(available, remaining))
return remaining;
}
}
}
// Constructor.
public Semaphore(int permits) {
sync = new NonfairSync(permits);
}
// Constructor.提供额外的公平性参数.
public Semaphore(int permits, boolean fair) {
sync = fair ? new FairSync(permits) : new NonfairSync(permits);
}
// 以阻塞的方式获取许可.
public void acquire() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
// 以可中断的方式获取许可.
public void acquireUninterruptibly() {
sync.acquireShared(1);
}
// 尝试获取许可，快速成功失败.
public boolean tryAcquire() {
return sync.nonfairTryAcquireShared(1) >= 0;
}
// 尝试获取许可,快速成功失败.可指定超时时间.
public boolean tryAcquire(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
}
// 释放资源.
public void release() {
sync.releaseShared(1);
}
// 指定释放资源数.
public void acquire(int permits) throws InterruptedException {
if (permits < 0) throw new IllegalArgumentException();
sync.acquireSharedInterruptibly(permits);
}
// 尝试获取许可,快速成功失败.可响应中断.
public void acquireUninterruptibly(int permits) {
if (permits < 0) throw new IllegalArgumentException();
sync.acquireShared(permits);
}
// 尝试获取许可,快速成功失败.
public boolean tryAcquire(int permits) {
if (permits < 0) throw new IllegalArgumentException();
return sync.nonfairTryAcquireShared(permits) >= 0;
}
// 尝试获取许可,快速成功失败.可指定超时时间.
public boolean tryAcquire(int permits, long timeout, TimeUnit unit)
throws InterruptedException {
if (permits < 0) throw new IllegalArgumentException();
return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout));
}
// 释放指定数量的许可.
public void release(int permits) {
if (permits < 0) throw new IllegalArgumentException();
sync.releaseShared(permits);
}
// 可用许可数量.
public int availablePermits() {
return sync.getPermits();
}
// 已用许可数量.
public int drainPermits() {
return sync.drainPermits();
}
// 减少许可数量.
protected void reducePermits(int reduction) {
if (reduction < 0) throw new IllegalArgumentException();
sync.reducePermits(reduction);
}
// 是否公平.
public boolean isFair() {
return sync instanceof FairSync;
}
// 同步队列中是否有线程.
public final boolean hasQueuedThreads() {
return sync.hasQueuedThreads();
}
// 获取同步队列长度.
public final int getQueueLength() {
return sync.getQueueLength();
}
// 获取队列线程集合.
protected Collection<Thread> getQueuedThreads() {
return sync.getQueuedThreads();
}
// 重写toString方法.
public String toString() {
return super.toString() + "[Permits = " + sync.getPermits() + "]";
}
}
Semaphore源码结构很简单，Sync这个内部类是最主要的。Semaphore完全依赖于AQS的共享锁禁止来实现许可的发放和回收。
注：文中源码均来自于JDK1.8版本，不同版本间可能存在差异。
如果有哪里有不明白或不清楚的内容，欢迎留言哦！