JUC并发工具类--Semaphore
简介
Semaphore（信号量）是一种用于多线程编程的同步工具，用于控制同时访问某个资源的线程数量。
Semaphore维护了一个计数器，线程可以通过调用acquire()方法来获取Semaphore中的许可证，当计数器为0时，调用acquire()的线程将被阻塞，直到有其他线程释放许可证；线程可以通过调用release()方法来释放Semaphore中的许可证，这会使Semaphore中的计数器增加，从而允许被阻塞或其他的线程访问共享资源。
常用API
构造方法
/**
* Creates a {@code Semaphore} with the given number of
* permits and nonfair fairness setting.
*
* @param permits the initial number of permits available.
* This value may be negative, in which case releases
* must occur before any acquires will be granted.
*/
public Semaphore(int permits) {
sync = new NonfairSync(permits);
}
/**
* Creates a {@code Semaphore} with the given number of
* permits and the given fairness setting.
*
* @param permits the initial number of permits available.
* This value may be negative, in which case releases
* must occur before any acquires will be granted.
* @param fair {@code true} if this semaphore will guarantee
* first-in first-out granting of permits under contention,
* else {@code false}
*/
public Semaphore(int permits, boolean fair) {
sync = fair ? new FairSync(permits) : new NonfairSync(permits);
}
参数：permits
表示许可证的数量（资源数）。
参数：fair
是否指定锁的公平性。公平锁是指按照申请锁的顺序获取锁，非公平锁是指当所资源被释放时，正在申请此资源的多个线程争抢锁，谁抢到归谁。
acquire
阻塞并获取许可。通过参数permits，指定一次获取许可的数量。
/**
* Acquires a permit from this semaphore, blocking until one is
* available, or the thread is {@linkplain Thread#interrupt interrupted}.
*
* <p>Acquires a permit, if one is available and returns immediately,
* reducing the number of available permits by one.
*
* <p>If no permit is available then the current thread becomes
* disabled for thread scheduling purposes and lies dormant until
* one of two things happens:
* <ul>
* <li>Some other thread invokes the {@link #release} method for this
* semaphore and the current thread is next to be assigned a permit; or
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread.
* </ul>
*
* <p>If the current thread:
* <ul>
* <li>has its interrupted status set on entry to this method; or
* <li>is {@linkplain Thread#interrupt interrupted} while waiting
* for a permit,
* </ul>
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
* @throws InterruptedException if the current thread is interrupted
*/
public void acquire() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
/**
* Acquires the given number of permits from this semaphore,
* blocking until all are available,
* or the thread is {@linkplain Thread#interrupt interrupted}.
*
* <p>Acquires the given number of permits, if they are available,
* and returns immediately, reducing the number of available permits
* by the given amount.
*
* <p>If insufficient permits are available then the current thread becomes
* disabled for thread scheduling purposes and lies dormant until
* one of two things happens:
* <ul>
* <li>Some other thread invokes one of the {@link #release() release}
* methods for this semaphore, the current thread is next to be assigned
* permits and the number of available permits satisfies this request; or
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread.
* </ul>
*
* <p>If the current thread:
* <ul>
* <li>has its interrupted status set on entry to this method; or
* <li>is {@linkplain Thread#interrupt interrupted} while waiting
* for a permit,
* </ul>
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
* Any permits that were to be assigned to this thread are instead
* assigned to other threads trying to acquire permits, as if
* permits had been made available by a call to {@link #release()}.
*
* @param permits the number of permits to acquire
* @throws InterruptedException if the current thread is interrupted
* @throws IllegalArgumentException if {@code permits} is negative
*/
public void acquire(int permits) throws InterruptedException {
if (permits < 0) throw new IllegalArgumentException();
sync.acquireSharedInterruptibly(permits);
}
参数：permits
一次获取许可证的数量。
acquireUninterruptibly()
获取一个令牌，在获取成功之前，当前线程会被阻塞（中断被忽略）。通过参数permits，指定一次获取许可的数量。
/**
* Acquires a permit from this semaphore, blocking until one is
* available.
*
* <p>Acquires a permit, if one is available and returns immediately,
* reducing the number of available permits by one.
*
* <p>If no permit is available then the current thread becomes
* disabled for thread scheduling purposes and lies dormant until
* some other thread invokes the {@link #release} method for this
* semaphore and the current thread is next to be assigned a permit.
*
* <p>If the current thread is {@linkplain Thread#interrupt interrupted}
* while waiting for a permit then it will continue to wait, but the
* time at which the thread is assigned a permit may change compared to
* the time it would have received the permit had no interruption
* occurred. When the thread does return from this method its interrupt
* status will be set.
*/
public void acquireUninterruptibly() {
sync.acquireShared(1);
}
/**
* Acquires the given number of permits from this semaphore,
* blocking until all are available.
*
* <p>Acquires the given number of permits, if they are available,
* and returns immediately, reducing the number of available permits
* by the given amount.
*
* <p>If insufficient permits are available then the current thread becomes
* disabled for thread scheduling purposes and lies dormant until
* some other thread invokes one of the {@link #release() release}
* methods for this semaphore, the current thread is next to be assigned
* permits and the number of available permits satisfies this request.
*
* <p>If the current thread is {@linkplain Thread#interrupt interrupted}
* while waiting for permits then it will continue to wait and its
* position in the queue is not affected. When the thread does return
* from this method its interrupt status will be set.
*
* @param permits the number of permits to acquire
* @throws IllegalArgumentException if {@code permits} is negative
*/
public void acquireUninterruptibly(int permits) {
if (permits < 0) throw new IllegalArgumentException();
sync.acquireShared(permits);
}
参数：permits
一次获取许可证的数量。
tryAcquire()
尝试获取许可，通过参数permits，指定一次获取许可的数量。如果不指定参数timeout、unit则为非阻塞，指定参数timeout、unit时，会在指定时间内进行阻塞。
/**
* Acquires a permit from this semaphore, only if one is available at the
* time of invocation.
*
* <p>Acquires a permit, if one is available and returns immediately,
* with the value {@code true},
* reducing the number of available permits by one.
*
* <p>If no permit is available then this method will return
* immediately with the value {@code false}.
*
* <p>Even when this semaphore has been set to use a
* fair ordering policy, a call to {@code tryAcquire()} <em>will</em>
* immediately acquire a permit if one is available, whether or not
* other threads are currently waiting.
* This quot;barging" behavior can be useful in certain
* circumstances, even though it breaks fairness. If you want to honor
* the fairness setting, then use
* {@link #tryAcquire(long, TimeUnit) tryAcquire(0, TimeUnit.SECONDS) }
* which is almost equivalent (it also detects interruption).
*
* @return {@code true} if a permit was acquired and {@code false}
* otherwise
*/
public boolean tryAcquire() {
return sync.nonfairTryAcquireShared(1) >= 0;
}
/**
* Acquires the given number of permits from this semaphore, only
* if all are available at the time of invocation.
*
* <p>Acquires the given number of permits, if they are available, and
* returns immediately, with the value {@code true},
* reducing the number of available permits by the given amount.
*
* <p>If insufficient permits are available then this method will return
* immediately with the value {@code false} and the number of available
* permits is unchanged.
*
* <p>Even when this semaphore has been set to use a fair ordering
* policy, a call to {@code tryAcquire} <em>will</em>
* immediately acquire a permit if one is available, whether or
* not other threads are currently waiting. This
* "barging" behavior can be useful in certain
* circumstances, even though it breaks fairness. If you want to
* honor the fairness setting, then use {@link #tryAcquire(int,
* long, TimeUnit) tryAcquire(permits, 0, TimeUnit.SECONDS) }
* which is almost equivalent (it also detects interruption).
*
* @param permits the number of permits to acquire
* @return {@code true} if the permits were acquired and
* {@code false} otherwise
* @throws IllegalArgumentException if {@code permits} is negative
*/
public boolean tryAcquire(int permits) {
if (permits < 0) throw new IllegalArgumentException();
return sync.nonfairTryAcquireShared(permits) >= 0;
}
/**
* Acquires a permit from this semaphore, if one becomes available
* within the given waiting time and the current thread has not
* been {@linkplain Thread#interrupt interrupted}.
*
* <p>Acquires a permit, if one is available and returns immediately,
* with the value {@code true},
* reducing the number of available permits by one.
*
* <p>If no permit is available then the current thread becomes
* disabled for thread scheduling purposes and lies dormant until
* one of three things happens:
* <ul>
* <li>Some other thread invokes the {@link #release} method for this
* semaphore and the current thread is next to be assigned a permit; or
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread; or
* <li>The specified waiting time elapses.
* </ul>
*
* <p>If a permit is acquired then the value {@code true} is returned.
*
* <p>If the current thread:
* <ul>
* <li>has its interrupted status set on entry to this method; or
* <li>is {@linkplain Thread#interrupt interrupted} while waiting
* to acquire a permit,
* </ul>
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
* <p>If the specified waiting time elapses then the value {@code false}
* is returned. If the time is less than or equal to zero, the method
* will not wait at all.
*
* @param timeout the maximum time to wait for a permit
* @param unit the time unit of the {@code timeout} argument
* @return {@code true} if a permit was acquired and {@code false}
* if the waiting time elapsed before a permit was acquired
* @throws InterruptedException if the current thread is interrupted
*/
public boolean tryAcquire(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
}
/**
* Acquires the given number of permits from this semaphore, if all
* become available within the given waiting time and the current
* thread has not been {@linkplain Thread#interrupt interrupted}.
*
* <p>Acquires the given number of permits, if they are available and
* returns immediately, with the value {@code true},
* reducing the number of available permits by the given amount.
*
* <p>If insufficient permits are available then
* the current thread becomes disabled for thread scheduling
* purposes and lies dormant until one of three things happens:
* <ul>
* <li>Some other thread invokes one of the {@link #release() release}
* methods for this semaphore, the current thread is next to be assigned
* permits and the number of available permits satisfies this request; or
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread; or
* <li>The specified waiting time elapses.
* </ul>
*
* <p>If the permits are acquired then the value {@code true} is returned.
*
* <p>If the current thread:
* <ul>
* <li>has its interrupted status set on entry to this method; or
* <li>is {@linkplain Thread#interrupt interrupted} while waiting
* to acquire the permits,
* </ul>
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
* Any permits that were to be assigned to this thread, are instead
* assigned to other threads trying to acquire permits, as if
* the permits had been made available by a call to {@link #release()}.
*
* <p>If the specified waiting time elapses then the value {@code false}
* is returned. If the time is less than or equal to zero, the method
* will not wait at all. Any permits that were to be assigned to this
* thread, are instead assigned to other threads trying to acquire
* permits, as if the permits had been made available by a call to
* {@link #release()}.
*
* @param permits the number of permits to acquire
* @param timeout the maximum time to wait for the permits
* @param unit the time unit of the {@code timeout} argument
* @return {@code true} if all permits were acquired and {@code false}
* if the waiting time elapsed before all permits were acquired
* @throws InterruptedException if the current thread is interrupted
* @throws IllegalArgumentException if {@code permits} is negative
*/
public boolean tryAcquire(int permits, long timeout, TimeUnit unit)
throws InterruptedException {
if (permits < 0) throw new IllegalArgumentException();
return sync.tryAcquireSharedNanos(permits, unit.toNanos(timeout));
}
参数：permits
一次获取许可证的数量。
参数：timeout
阻塞等待时间。
参数：unit
阻塞等待时间单位。
release()
释放许可。通过参数permits，指定一次释放许可的数量。
/**
* Releases a permit, returning it to the semaphore.
*
* <p>Releases a permit, increasing the number of available permits by
* one. If any threads are trying to acquire a permit, then one is
* selected and given the permit that was just released. That thread
* is (re)enabled for thread scheduling purposes.
*
* <p>There is no requirement that a thread that releases a permit must
* have acquired that permit by calling {@link #acquire}.
* Correct usage of a semaphore is established by programming convention
* in the application.
*/
public void release() {
sync.releaseShared(1);
}
/**
* Releases the given number of permits, returning them to the semaphore.
*
* <p>Releases the given number of permits, increasing the number of
* available permits by that amount.
* If any threads are trying to acquire permits, then one
* is selected and given the permits that were just released.
* If the number of available permits satisfies that thread's request
* then that thread is (re)enabled for thread scheduling purposes;
* otherwise the thread will wait until sufficient permits are available.
* If there are still permits available
* after this thread's request has been satisfied, then those permits
* are assigned in turn to other threads trying to acquire permits.
*
* <p>There is no requirement that a thread that releases a permit must
* have acquired that permit by calling {@link Semaphore#acquire acquire}.
* Correct usage of a semaphore is established by programming convention
* in the application.
*
* @param permits the number of permits to release
* @throws IllegalArgumentException if {@code permits} is negative
*/
public void release(int permits) {
if (permits < 0) throw new IllegalArgumentException();
sync.releaseShared(permits);
}
参数：permits
一次释放许可证的数量。
availablePermits()
当前可用的许可。
/**
* Returns the current number of permits available in this semaphore.
*
* <p>This method is typically used for debugging and testing purposes.
*
* @return the number of permits available in this semaphore
*/
public int availablePermits() {
return sync.getPermits();
}
使用场景
资源池
Semaphore可以用于实现资源池，以维护一组有限的共享资源。
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
/**
* 数据库连接池实现
*/
public class SemaphoreDemo2 {
final static ExecutorService executorService = Executors.newCachedThreadPool();
public static void main(String[] args) {
final ConnectPool pool = new ConnectPool(2);
//5个线程并发来争抢连接资源
for (int i = 0; i < 5; i++) {
final int id = i + 1;
executorService.execute(new Runnable() {
@Override
public void run() {
Connect connect = null;
try {
System.out.println("线程" + id + "等待获取数据库连接");
connect = pool.openConnect();
System.out.println("线程" + id + "已拿到数据库连接:" + connect);
//进行数据库操作2秒...然后释放连接
Thread.sleep(2000);
System.out.println("线程" + id + "释放数据库连接:" + connect);
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
pool.releaseConnect(connect);
}
}
});
}
}
}
//数据库连接池
class ConnectPool {
private int size;
private Connect[] connects;
//记录对应下标的Connect是否已被使用
private boolean[] connectFlag;
//信号量对象
private Semaphore semaphore;
/**
* size:初始化连接池大小
*/
public ConnectPool(int size) {
this.size = size;
semaphore = new Semaphore(size, true);
connects = new Connect[size];
connectFlag = new boolean[size];
initConnects();//初始化连接池
}
private void initConnects() {
for (int i = 0; i < this.size; i++) {
connects[i] = new Connect();
}
}
/**
* 获取数据库连接
*
* @return
* @throws InterruptedException
*/
public Connect openConnect() throws InterruptedException {
//得先获得使用许可证，如果信号量为0，则拿不到许可证，一直阻塞直到能获得
semaphore.acquire();
return getConnect();
}
private synchronized Connect getConnect() {
for (int i = 0; i < connectFlag.length; i++) {
if (!connectFlag[i]) {
//标记该连接已被使用
connectFlag[i] = true;
return connects[i];
}
}
return null;
}
/**
* 释放某个数据库连接
*/
public synchronized void releaseConnect(Connect connect) {
for (int i = 0; i < this.size; i++) {
if (connect == connects[i]) {
connectFlag[i] = false;
semaphore.release();
}
}
}
}
/**
* 数据库连接
*/
class Connect {
private static int count = 1;
private int id = count++;
public Connect() {
//假设打开一个连接很耗费资源，需要等待1秒
try {
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
}
System.out.println("连接#" + id + "#已与数据库建立通道！");
}
@Override
public String toString() {
return "#" + id + "#";
}
}
限流
Semaphore可以用于限制对共享资源的并发访问数量，以控制系统的流量。
import java.util.concurrent.Executor;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;
import java.util.concurrent.TimeUnit;
/**
* 模拟限流场景
*/
public class SemaphoreDemo {
/**
* 同一时刻最多只允许有两个并发
*/
private static Semaphore semaphore = new Semaphore(5, true);
private static Executor executor = Executors.newFixedThreadPool(6);
public static void main(String[] args) {
for (int i = 0; i < 10; i++) {
executor.execute(() -> getProductInfo2());
}
}
public static String getProductInfo() {
try {
//申请许可
semaphore.acquire();
System.out.println("请求服务");
TimeUnit.SECONDS.sleep(1);
} catch (InterruptedException e) {
throw new RuntimeException(e);
} finally {
//释放许可
semaphore.release();
}
return "返回商品详情信息";
}
public static String getProductInfo2() {
if (!semaphore.tryAcquire()) {
System.out.println(Thread.currentThread().getName() + "请求被流控了");
return "请求被流控了";
}
try {
System.out.println(Thread.currentThread().getName() + "请求服务");
Thread.sleep(2000);
} catch (InterruptedException e) {
throw new RuntimeException(e);
} finally {
semaphore.release();
}
return "返回商品详情信息";
}
}