在Java并發編程中，JUC（java.util.concurrent）包提供了許多強大的工具類，用于處理多線程并發問題。其中，CountDownLatch、CyclicBarrier和Semaphore是三個常用的同步工具類。本文將詳細解析這三個類的使用場景、源碼實現以及示例代碼，幫助讀者更好地理解和使用它們。
CountDownLatch是一個同步輔助類，允許一個或多個線程等待其他線程完成操作。它通過一個計數器來實現，計數器的初始值由構造函數指定。每當一個線程完成了自己的任務后，計數器的值就會減1。當計數器的值減到0時，等待的線程就會被喚醒。
CountDownLatch通常用于以下場景：
CountDownLatch的核心實現依賴于AQS（AbstractQueuedSynchronizer）。AQS是一個用于構建鎖和同步器的框架，CountDownLatch通過繼承AQS來實現同步功能。
public class CountDownLatch {
private final Sync sync;
public CountDownLatch(int count) {
if (count < 0) throw new IllegalArgumentException("count < 0");
this.sync = new Sync(count);
}
public void await() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
public boolean await(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
}
public void countDown() {
sync.releaseShared(1);
}
public long getCount() {
return sync.getCount();
}
private static final class Sync extends AbstractQueuedSynchronizer {
Sync(int count) {
setState(count);
}
int getCount() {
return getState();
}
protected int tryAcquireShared(int acquires) {
return (getState() == 0) ? 1 : -1;
}
protected boolean tryReleaseShared(int releases) {
for (;;) {
int c = getState();
if (c == 0)
return false;
int nextc = c-1;
if (compareAndSetState(c, nextc))
return nextc == 0;
}
}
}
}
public class CountDownLatchExample {
public static void main(String[] args) throws InterruptedException {
int threadCount = 5;
CountDownLatch latch = new CountDownLatch(threadCount);
for (int i = 0; i < threadCount; i++) {
new Thread(() -> {
System.out.println(Thread.currentThread().getName() + " is running");
latch.countDown();
}).start();
}
latch.await();
System.out.println("All threads have finished");
}
}
CyclicBarrier是一個同步輔助類，允許一組線程互相等待，直到所有線程都到達某個屏障點（barrier point）。與CountDownLatch不同，CyclicBarrier可以重復使用，即當所有線程都到達屏障點后，屏障會被重置，可以再次使用。
CyclicBarrier通常用于以下場景：
CyclicBarrier的核心實現依賴于ReentrantLock和Condition。ReentrantLock用于實現線程間的互斥，Condition用于實現線程間的等待和喚醒。
public class CyclicBarrier {
private final ReentrantLock lock = new ReentrantLock();
private final Condition trip = lock.newCondition();
private final int parties;
private final Runnable barrierCommand;
private int count;
public CyclicBarrier(int parties, Runnable barrierAction) {
if (parties <= 0) throw new IllegalArgumentException();
this.parties = parties;
this.count = parties;
this.barrierCommand = barrierAction;
}
public CyclicBarrier(int parties) {
this(parties, null);
}
public int await() throws InterruptedException, BrokenBarrierException {
try {
return dowait(false, 0L);
} catch (TimeoutException toe) {
throw new Error(toe); // cannot happen
}
}
public int await(long timeout, TimeUnit unit)
throws InterruptedException, BrokenBarrierException, TimeoutException {
return dowait(true, unit.toNanos(timeout));
}
private int dowait(boolean timed, long nanos)
throws InterruptedException, BrokenBarrierException, TimeoutException {
final ReentrantLock lock = this.lock;
lock.lock();
try {
final Generation g = generation;
if (g.broken)
throw new BrokenBarrierException();
if (Thread.interrupted()) {
breakBarrier();
throw new InterruptedException();
}
int index = --count;
if (index == 0) { // tripped
boolean ranAction = false;
try {
final Runnable command = barrierCommand;
if (command != null)
command.run();
ranAction = true;
nextGeneration();
return 0;
} finally {
if (!ranAction)
breakBarrier();
}
}
for (;;) {
try {
if (!timed)
trip.await();
else if (nanos > 0L)
nanos = trip.awaitNanos(nanos);
} catch (InterruptedException ie) {
if (g == generation && ! g.broken) {
breakBarrier();
throw ie;
} else {
Thread.currentThread().interrupt();
}
}
if (g.broken)
throw new BrokenBarrierException();
if (g != generation)
return index;
if (timed && nanos <= 0L) {
breakBarrier();
throw new TimeoutException();
}
}
} finally {
lock.unlock();
}
}
private void nextGeneration() {
trip.signalAll();
count = parties;
generation = new Generation();
}
private void breakBarrier() {
generation.broken = true;
count = parties;
trip.signalAll();
}
private static class Generation {
boolean broken = false;
}
}
public class CyclicBarrierExample {
public static void main(String[] args) {
int threadCount = 5;
CyclicBarrier barrier = new CyclicBarrier(threadCount, () -> {
System.out.println("All threads have reached the barrier");
});
for (int i = 0; i < threadCount; i++) {
new Thread(() -> {
System.out.println(Thread.currentThread().getName() + " is waiting at the barrier");
try {
barrier.await();
} catch (InterruptedException | BrokenBarrierException e) {
e.printStackTrace();
}
System.out.println(Thread.currentThread().getName() + " has passed the barrier");
}).start();
}
}
}
Semaphore是一個計數信號量，用于控制同時訪問某個資源的線程數量。它通過一個計數器來實現，計數器的初始值由構造函數指定。每當一個線程訪問資源時，計數器的值就會減1；當線程釋放資源時，計數器的值就會加1。如果計數器的值為0，則線程必須等待，直到有其他線程釋放資源。
Semaphore通常用于以下場景：
Semaphore來控制同時訪問資源的線程數量。Semaphore的核心實現依賴于AQS（AbstractQueuedSynchronizer）。AQS是一個用于構建鎖和同步器的框架，Semaphore通過繼承AQS來實現同步功能。
public class Semaphore implements java.io.Serializable {
private final Sync sync;
public Semaphore(int permits) {
sync = new NonfairSync(permits);
}
public Semaphore(int permits, boolean fair) {
sync = fair ? new FairSync(permits) : new NonfairSync(permits);
}
public void acquire() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
public void acquireUninterruptibly() {
sync.acquireShared(1);
}
public boolean tryAcquire() {
return sync.nonfairTryAcquireShared(1) >= 0;
}
public boolean tryAcquire(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
}
public void release() {
sync.releaseShared(1);
}
public void acquire(int permits) throws InterruptedException {
if (permits < 0) throw new IllegalArgumentException();
sync.acquireSharedInterruptibly(permits);
}
public void acquireUninterruptibly(int permits) {
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() + "]";
}
abstract static class Sync extends AbstractQueuedSynchronizer {
Sync(int permits) {
setState(permits);
}
final int getPermits() {
return getState();
}
final int nonfairTryAcquireShared(int acquires) {
for (;;) {
int available = getState();
int remaining = available - acquires;
if (remaining < 0 ||
compareAndSetState(available, remaining))
return remaining;
}
}
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;
}
}
final void reducePermits(int reductions) {
for (;;) {
int current = getState();
int next = current - reductions;
if (next > current) // underflow
throw new Error("Permit count under zero");
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 {
NonfairSync(int permits) {
super(permits);
}
protected int tryAcquireShared(int acquires) {
return nonfairTryAcquireShared(acquires);
}
}
static final class FairSync extends Sync {
FairSync(int permits) {
super(permits);
}
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;
}
}
}
}
public class SemaphoreExample {
public static void main(String[] args) {
int permits = 3;
Semaphore semaphore = new Semaphore(permits);
for (int i = 0; i < 10; i++) {
new Thread(() -> {
try {
semaphore.acquire();
System.out.println(Thread.currentThread().getName() + " is running");
Thread.sleep(1000);
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
semaphore.release();
}
}).start();
}
}
}
CountDownLatch、CyclicBarrier和Semaphore是JUC包中常用的同步工具類，它們分別適用于不同的并發場景。CountDownLatch用于等待多個線程完成任務，CyclicBarrier用于多個線程互相等待，Semaphore用于控制同時訪問某個資源的線程數量。通過理解它們的實現原理和使用場景，可以更好地處理多線程并發問題。
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