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【1】CountDownLatch是什么
CountDownLatch，英文翻译为倒计时锁存器。是一个同步辅助类，在完成一组正在其他线程中执行的操作之前，它允许一个或多个线程一直等待。
闭锁可以延迟线程的进度直到其到达终止状态，闭锁可以用来确保某些活动直到其他活动都完成才继续执行：
- 确保某个计算在其需要的所有资源都被初始化之后才继续执行;
- 确保某个服务在其依赖的所有其他服务都已经启动之后才启动;
- 等待直到某个操作所有参与者都准备就绪再继续执行。
CountDownLatch有一个正数计数器，countDown方法对计数器做减操作，await方法等待计数器达到0。所有await的线程都会阻塞直到计数器为0或者等待线程中断或者超时。
如下示例，在多线程运行情况下，计算多线程耗费时间：
public class TestCountDownLatch {
public static void main(String[] args){
LatchDemo latchDemo = new LatchDemo();
long begin = System.currentTimeMillis();
//多线程
for (int i = 0; i <5 ; i++) {
new Thread(latchDemo).start();
}
//主线程
long end = System.currentTimeMillis();
System.out.println("耗费时间："+(end-begin));
}
}
class LatchDemo implements Runnable{
@Override
public void run() {
for (int i = 0; i < 50000; i++) {
if (i%2==0){
System.out.println(i);
}
}
}
}
如上示例，很显然让不能计算出多线程运行的时间！！这时，就可以使用 闭锁解决这个问题。
【2】CountDownLatch源码分析与使用
CountDownLatch源码如下：
package java.util.concurrent;
import java.util.concurrent.locks.AbstractQueuedSynchronizer;
/**
* A synchronization aid that allows one or more threads to wait until
* a set of operations being performed in other threads completes.
*//一种同步辅助工具，允许一个或多个线程等待，直到在其它线程中执行的一组操作完成。
* <p>A {@code CountDownLatch} is initialized with a given <em>count</em>.
* // 一个CountDownLatch对象通常使用一个计数器(count)进行初始化。
*
* The {@link #await await} methods block until the current count reaches
* zero due to invocations of the {@link #countDown} method, after which
* all waiting threads are released and any subsequent invocations of
* {@link #await await} return immediately.
* //await方法将会阻塞直到由于countDown方法执行将count (计数器)归零，
* //随后将释放所有等待的线程，await后面的方法将会立即返回。
*
* This is a one-shot phenomenon-- the count cannot be reset.
* /*这是一次性的现象，不能重置计数。*/
* If you need a version that resets the count, consider using a {@link CyclicBarrier}.
*/*如果你需要重置计数器，考虑使用CyclicBarrier--栅栏*/
* <p>A {@code CountDownLatch} is a versatile synchronization tool
* and can be used for a number of purposes.
* /*这个多功能的同步工具有许多作用*/
*
* A {@code CountDownLatch} initialized with a count of one serves as a
* simple on/off latch, or gate: all threads invoking {@link #await await}
* wait at the gate until it is opened by a thread invoking {@link
* #countDown}.
* //以计数1初始化的{@code CountDownLatch}用作简单的开/关锁存器或闸门：
* //调用{@linka wait a wait}的所有线程都在闸门等待，
* //直到它被调用{@linkcountDown}的线程打开为止。
*
* A {@code CountDownLatch} initialized to <em>N</em>
* can be used to make one thread wait until <em>N</em> threads have
* completed some action, or some action has been completed N times.
* /*初始化为N的{@code CountDownLatch}可用于使一个线程等待，*/
/* 直到<em>N</em>线程已经完成某个操作，或者某个操作已经完成N次。*/
* <p>A useful property of a {@code CountDownLatch} is that it
* doesn't require that threads calling {@code countDown} wait for
* the count to reach zero before proceeding, it simply prevents any
* thread from proceeding past an {@link #await await} until all
* threads could pass.
*/*一个有用特性是，它不要求调用{@code countDown}的线程在继续之前等待计数达到零，*/
/*它只是防止任何线程通过{@linka wait a wait}直到所有线程都可以通过。*/
//下面是使用开关锁的一个简单例子
* <p><b>Sample usage:</b> Here is a pair of classes in which a group
* of worker threads use two countdown latches:
* <ul>
* <li>The first is a start signal that prevents any worker from proceeding
* until the driver is ready for them to proceed;
* <li>The second is a completion signal that allows the driver to wait
* until all workers have completed.
* </ul>
*
* <pre> {@code
* class Driver { // ...
* void main() throws InterruptedException {
* CountDownLatch startSignal = new CountDownLatch(1);
* CountDownLatch doneSignal = new CountDownLatch(N);
*
* for (int i = 0; i < N; ++i) // create and start threads
* new Thread(new Worker(startSignal, doneSignal)).start();
*
* doSomethingElse(); // don't let run yet
* startSignal.countDown(); // let all threads proceed
* doSomethingElse();
* doneSignal.await(); // wait for all to finish
* }
* }
*
* class Worker implements Runnable {
* private final CountDownLatch startSignal;
* private final CountDownLatch doneSignal;
* Worker(CountDownLatch startSignal, CountDownLatch doneSignal) {
* this.startSignal = startSignal;
* this.doneSignal = doneSignal;
* }
* public void run() {
* try {
* startSignal.await();
* doWork();
* doneSignal.countDown();
* } catch (InterruptedException ex) {} // return;
* }
*
* void doWork() { ... }
* }}</pre>
*
//另外一个典型应用
* <p>Another typical usage would be to divide a problem into N parts,
* describe each part with a Runnable that executes that portion and
* counts down on the latch, and queue all the Runnables to an
* Executor. When all sub-parts are complete, the coordinating thread
* will be able to pass through await. (When threads must repeatedly
* count down in this way, instead use a {@link CyclicBarrier}.)
*
* <pre> {@code
* class Driver2 { // ...
* void main() throws InterruptedException {
* CountDownLatch doneSignal = new CountDownLatch(N);
* Executor e = ...
*
* for (int i = 0; i < N; ++i) // create and start threads
* e.execute(new WorkerRunnable(doneSignal, i));
*
* doneSignal.await(); // wait for all to finish
* }
* }
*
* class WorkerRunnable implements Runnable {
* private final CountDownLatch doneSignal;
* private final int i;
* WorkerRunnable(CountDownLatch doneSignal, int i) {
* this.doneSignal = doneSignal;
* this.i = i;
* }
* public void run() {
* try {
* doWork(i);
* doneSignal.countDown();
* } catch (InterruptedException ex) {} // return;
* }
*
* void doWork() { ... }
* }}</pre>
*
* <p>Memory consistency effects: Until the count reaches
* zero, actions in a thread prior to calling
* {@code countDown()}
* <a href="package-summary.html#MemoryVisibility"><i>happen-before</i></a>
* actions following a successful return from a corresponding
* {@code await()} in another thread.
*
* @since 1.5
* @author Doug Lea
*/
public class CountDownLatch {
/**
* CountDownLatch的同步控制，使用AQS状态呈现Count
* Synchronization control For CountDownLatch.
* Uses AQS state to represent count.
*/
private static final class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = 4982264981922014374L;
Sync(int count) {
setState(count);
}
int getCount() {
return getState();
}
protected int tryAcquireShared(int acquires) {
return (getState() == 0) ? 1 : -1;
}
protected boolean tryReleaseShared(int releases) {
// Decrement count; signal when transition to zero
for (;;) {
int c = getState();
if (c == 0)
return false;
int nextc = c-1;
if (compareAndSetState(c, nextc))
return nextc == 0;
}
}
}
private final Sync sync;
/**
* Constructs a {@code CountDownLatch} initialized with the given count.
*
* @param count the number of times {@link #countDown} must be invoked
* before threads can pass through {@link #await}
* @throws IllegalArgumentException if {@code count} is negative
*/
public CountDownLatch(int count) {
if (count < 0) throw new IllegalArgumentException("count < 0");
this.sync = new Sync(count);
}
/**
* Causes the current thread to wait until the latch has counted down to
* zero, unless the thread is {@linkplain Thread#interrupt interrupted}.
*
* <p>If the current count is zero then this method returns immediately.
*
* <p>If the current count is greater than zero then the current
* thread becomes disabled for thread scheduling purposes and lies
* dormant until one of two things happen:
* <ul>
* <li>The count reaches zero due to invocations of the
* {@link #countDown} method; 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,
* </ul>
* then {@link InterruptedException} is thrown and the current thread's
* interrupted status is cleared.
*
* @throws InterruptedException if the current thread is interrupted
* while waiting
*/
public void await() throws InterruptedException {
sync.acquireSharedInterruptibly(1);
}
/**
* Causes the current thread to wait until the latch has counted down to
* zero, unless the thread is {@linkplain Thread#interrupt interrupted},
* or the specified waiting time elapses.
*
* <p>If the current count is zero then this method returns immediately
* with the value {@code true}.
*
* <p>If the current count is greater than zero then the current
* thread becomes disabled for thread scheduling purposes and lies
* dormant until one of three things happen:
* <ul>
* <li>The count reaches zero due to invocations of the
* {@link #countDown} method; or
* <li>Some other thread {@linkplain Thread#interrupt interrupts}
* the current thread; or
* <li>The specified waiting time elapses.
* </ul>
*
* <p>If the count reaches zero then the method returns with the
* value {@code true}.
*
* <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,
* </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
* @param unit the time unit of the {@code timeout} argument
* @return {@code true} if the count reached zero and {@code false}
* if the waiting time elapsed before the count reached zero
* @throws InterruptedException if the current thread is interrupted
* while waiting
*/
public boolean await(long timeout, TimeUnit unit)
throws InterruptedException {
return sync.tryAcquireSharedNanos(1, unit.toNanos(timeout));
}
/**
* 将count-1 ，如果count减一后为0 ，则释放所有等待的线程
*/
public void countDown() {
sync.releaseShared(1);
}
/**
返回当前的count，该方法通常用在debug或者测试中
*/
public long getCount() {
return sync.getCount();
}
public String toString() {
return super.toString() + "[Count = " + sync.getCount() + "]";
}
}
简单总结如下：
- 初始化使用计数器count；
- count代表多个线程执行或者某个操作执行次数；
- countDown()方法将会将count-1；
- count为0将会释放所有等待线程；
- await方法将会阻塞直到count为0；
- 开关锁应用；
- 问题分解应用。
修改【1】中代码如下：
public class TestCountDownLatch {
public static void main(String[] args){
final CountDownLatch latch = new CountDownLatch(5);
LatchDemo latchDemo = new LatchDemo(latch);
long begin = System.currentTimeMillis();
for (int i = 0; i <5 ; i++) {
new Thread(latchDemo).start();
}
try {
//多线程运行结束前一直等待
latch.await();
} catch (InterruptedException e) {
e.printStackTrace();
}
long end = System.currentTimeMillis();
System.out.println("耗费时间："+(end-begin));
}
}
class LatchDemo implements Runnable{
private CountDownLatch latch;
public LatchDemo(CountDownLatch latch){
this.latch=latch;
}
public LatchDemo(){
super();
}
@Override
public void run() {
//当前对象唯一，使用当前对象加锁，避免多线程问题
synchronized (this){
try {
for (int i = 0; i < 50000; i++) {
if (i%2==0){
System.out.println(i);
}
}
}finally {
//保证肯定执行
latch.countDown();
}
}
}
}
测试结果如下图：
