聊聊并发（一）——深入分析Volatile的实现原理： http://www.infoq.com/cn/articles/ftf-java-volatile
深入理解Java内存模型（四）——volatile： http://www.infoq.com/cn/articles/java-memory-model-4/
Java的多线程机制系列：(四)不得不提的volatile及指令重排序(happen-before)：
http://www.cnblogs.com/mengheng/p/3495379.html
CountDownLatch同步化的一个辅助工具，允许一个或多个线程等待，直到所有线程中执行完成
比如主线程计算一个复杂的计算表达式，将表达式分为多个子表达式在线程中去计算，
主线程要计算表达式的最后值，必须等所有的线程计算完子表达式计算，方可计算表达式的值；
再比如一个团队赛跑游戏，最后要计算团队赛跑的成绩，主线程计算最后成绩，要等到所有
团队成员跑完，方可计算总成绩。使用情况两种：第一种，所有线程等待一个开始信息号，当开始信息号启动时，所有线程执行，等待所有线程执行完；第二种，所有线程放在线程池中，执行，等待所有线程执行完，方可执行主线程任务方可执行主线程任务。
测试有统一开始信号的情况：
package juc;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;
public class RunnerGames {
public static void main(String[] args) {
CountDownLatch startSignal = new CountDownLatch(1);
CountDownLatch doneSignal = new CountDownLatch(3);
ExecutorService exec = Executors.newCachedThreadPool();
RunnableMan rm1 = new RunnableMan(startSignal,doneSignal, 1000);
RunnableMan rm2 = new RunnableMan(startSignal,doneSignal, 2000);
RunnableMan rm3 = new RunnableMan(startSignal,doneSignal, 3000);
Future<Integer> score1 = exec.submit(rm1);
Future<Integer> score2 = exec.submit(rm2);
Future<Integer> score3 = exec.submit(rm3);
System.out.println("开始赛跑......");
startSignal.countDown();
try {
doneSignal.await();
} catch (InterruptedException e1) {
e1.printStackTrace();
}
int sumScores =0;
try {
try {
sumScores = score1.get()+score2.get()+score3.get();
} catch (InterruptedException e) {
e.printStackTrace();
}
} catch (ExecutionException e) {
e.printStackTrace();
}
System.out.println("团队赛跑结束,最后成绩为："+sumScores);
exec.shutdown();
}
}
package juc;
import java.util.concurrent.Callable;
import java.util.concurrent.CountDownLatch;
class RunnableMan implements Callable<Integer> {
private final CountDownLatch startSignal;
private final CountDownLatch doneSignal;
private final int i;
RunnableMan(CountDownLatch startSignal,CountDownLatch doneSignal, int i) {
this.startSignal = startSignal;
this.doneSignal = doneSignal;
this.i = i;
}
public Integer call() {
try {
startSignal.await();
doRun(i);
} catch (InterruptedException e) {
e.printStackTrace();
}
doneSignal.countDown();
return new Integer(i);
}
void doRun(int i) throws InterruptedException {
System.out.println("选手"+i/1000+"正在赛跑中........");
Thread.sleep(i*2);
}
}
测试结果：
开始赛跑......
选手3正在赛跑中........
选手2正在赛跑中........
选手1正在赛跑中........
团队赛跑结束,最后成绩为：6000
测试第二种情况，没有开始信号
package juc;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.Executor;
import java.util.concurrent.Executors;
import java.util.concurrent.atomic.AtomicInteger;
/**AtomicInteger，一个提供原子操作的Integer的类。
在Java语言中，++i和i++操作并不是线程安全的，在使用的时候，
不可避免的会用到synchronized关键字。而AtomicInteger则通过一种线程安全的加减操作接口。*/
public class ShopTickets {
private static volatile AtomicInteger tickets = new AtomicInteger(10);
public static void main(String[] args) throws InterruptedException {
CountDownLatch doneSignal = new CountDownLatch(3);
Executor exec = Executors.newCachedThreadPool();
exec.execute(new TicketSales(doneSignal,"售票员1",tickets));
exec.execute(new TicketSales(doneSignal,"售票员2",tickets));
exec.execute(new TicketSales(doneSignal,"售票员3",tickets));
doneSignal.await();
System.out.println("票已售完,所有售票员，停止售票");
}
}
package juc;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.atomic.AtomicInteger;
class TicketSales implements Runnable {
private final CountDownLatch doneSignal;
private final AtomicInteger tickets;
private String saleName;
TicketSales(CountDownLatch doneSignal, String saleName,AtomicInteger tickets) {
this.doneSignal = doneSignal;
this.saleName = saleName;
this.tickets = tickets;
}
public void run() {
doSales(tickets);
}
public void doSales(AtomicInteger tickets) {
try {
while(tickets.get()>0){
System.out.println(saleName+"卖完一张票，还有："+tickets.decrementAndGet()+"张");
Thread.sleep(1000);
}
doneSignal.countDown();
}catch (InterruptedException e) {
e.printStackTrace();
}
}
}
测试结果:
售票员1卖完一张票，还有：8张
售票员3卖完一张票，还有：7张
售票员2卖完一张票，还有：9张
售票员3卖完一张票，还有：5张
售票员1卖完一张票，还有：6张
售票员3卖完一张票，还有：3张
售票员2卖完一张票，还有：4张
售票员3卖完一张票，还有：1张
售票员1卖完一张票，还有：2张
售票员2卖完一张票，还有：0张
票已售完,所有售票员，停止售票
有时测试结果为：
售票员1卖完一张票，还有：9张
售票员2卖完一张票，还有：8张
售票员3卖完一张票，还有：7张
售票员2卖完一张票，还有：6张
售票员1卖完一张票，还有：5张
售票员3卖完一张票，还有：4张
售票员3卖完一张票，还有：2张
售票员2卖完一张票，还有：1张
售票员1卖完一张票，还有：3张
售票员1卖完一张票，还有：0张
售票员2卖完一张票，还有：-1张
票已售完,所有售票员，停止售票
把Thread.sleep(1000)这句去掉，票数为负数的概率较小，但还是会出现，
我测试了3分钟，出现2次。
关注这一句：
while(tickets.get()>0){
问题可能在tickets.get()的时候，票数不为零，在卖票的时候，其他线程已经在其基础上，修改
tickets，而当前线程tickets.get()已经做过判断，不为零，导致的错误。
再次修改：
package juc;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.Executor;
import java.util.concurrent.Executors;
import java.util.concurrent.atomic.AtomicInteger;
public class ShopTickets2 {
/**
* AtomicInteger，一个提供原子操作的Integer的类。 在Java语言中，++i和i++操作并不是线程安全的，在使用的时候，
* 不可避免的会用到synchronized关键字。而AtomicInteger则通过一种线程安全的加减操作接口。
*/
private static volatile AtomicInteger tickets = new AtomicInteger(10);
public static void main(String[] args) throws InterruptedException {
CountDownLatch doneSignal = new CountDownLatch(3);
Executor exec = Executors.newCachedThreadPool();
exec.execute(new TicketSales2(doneSignal, "售票员1", tickets));
exec.execute(new TicketSales2(doneSignal, "售票员2", tickets));
exec.execute(new TicketSales2(doneSignal, "售票员3", tickets));
doneSignal.await();
System.out.println("票已售完,所有售票员，停止售票");
}
}
package juc;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.atomic.AtomicBoolean;
import java.util.concurrent.atomic.AtomicInteger;
class TicketSales2 implements Runnable {
private static volatile AtomicBoolean isDoned = new AtomicBoolean(Boolean.TRUE);
private final CountDownLatch doneSignal;
private final AtomicInteger tickets;
private String saleName;
TicketSales2(CountDownLatch doneSignal, String saleName,AtomicInteger tickets) {
this.doneSignal = doneSignal;
this.saleName = saleName;
this.tickets = tickets;
}
public void run() {
doSales(tickets);
}
public void doSales(AtomicInteger tickets) {
while(isDoned.get()){
System.out.println(saleName+"卖完一张票，还有："+tickets.decrementAndGet()+"张");
if(tickets.get()==0){
isDoned.compareAndSet(true, false);
}
}
doneSignal.countDown();
}
}
测试：
售票员1卖完一张票，还有：9张
售票员3卖完一张票，还有：7张
售票员2卖完一张票，还有：8张
售票员3卖完一张票，还有：5张
售票员1卖完一张票，还有：6张
售票员3卖完一张票，还有：3张
售票员2卖完一张票，还有：4张
售票员2卖完一张票，还有：0张
售票员3卖完一张票，还有：1张
售票员1卖完一张票，还有：2张
票已售完,所有售票员，停止售票
不在出现上述票数为负的情况，关键是添加了所有线程都可见，且线程安全的余票状态
private static volatile AtomicBoolean isDoned = new AtomicBoolean(Boolean.TRUE);
下面来看使用同步锁的场景：
package juc;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.Executor;
import java.util.concurrent.Executors;
import java.util.concurrent.atomic.AtomicInteger;
public class ShopTickets3 {
/**
* AtomicInteger，一个提供原子操作的Integer的类。 在Java语言中，++i和i++操作并不是线程安全的，在使用的时候，
* 不可避免的会用到synchronized关键字。而AtomicInteger则通过一种线程安全的加减操作接口。
*/
private static volatile AtomicInteger tickets = new AtomicInteger(10);
private static volatile Object saleLock = new Object();
public static void main(String[] args) throws InterruptedException {
CountDownLatch doneSignal = new CountDownLatch(3);
Executor exec = Executors.newCachedThreadPool();
exec.execute(new TicketSales3(doneSignal, "售票员1", tickets,saleLock));
exec.execute(new TicketSales3(doneSignal, "售票员2", tickets,saleLock));
exec.execute(new TicketSales3(doneSignal, "售票员3", tickets,saleLock));
doneSignal.await();
System.out.println("票已售完,所有售票员，停止售票");
}
}
package juc;
import java.util.concurrent.CountDownLatch;
import java.util.concurrent.atomic.AtomicInteger;
class TicketSales3 implements Runnable {
private final CountDownLatch doneSignal;
private final AtomicInteger tickets;
private final Object saleLock;
private String saleName;
TicketSales3(CountDownLatch doneSignal, String saleName,AtomicInteger tickets, Object saleLock) {
this.doneSignal = doneSignal;
this.saleName = saleName;
this.tickets = tickets;
this.saleLock = saleLock;
}
public void run() {
doSales(tickets);
}
public void doSales(AtomicInteger tickets) {
boolean flag = true;
while(flag){
synchronized(saleLock){
if(tickets.get()>0){
System.out.println(saleName+"卖完一张票，还有："+tickets.decrementAndGet()+"张");
saleLock.notifyAll();
}
else{
flag= false;
}
}
}
doneSignal.countDown();
}
}
控制台输出：
售票员1卖完一张票，还有：9张
售票员1卖完一张票，还有：8张
售票员1卖完一张票，还有：7张
售票员1卖完一张票，还有：6张
售票员1卖完一张票，还有：5张
售票员1卖完一张票，还有：4张
售票员1卖完一张票，还有：3张
售票员1卖完一张票，还有：2张
售票员3卖完一张票，还有：1张
售票员3卖完一张票，还有：0张
票已售完,所有售票员，停止售票
虽然也可以控制卖票，但只有两个售票员在售票，不能有效的利用资源，达到最大性能。
从上面可以看出，最有效方式为第二种。
/*
* ORACLE PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
* Written by Doug Lea with assistance from members of JCP JSR-166
* Expert Group and released to the public domain, as explained at
* http://creativecommons.org/publicdomain/zero/1.0/
*/
package java.util.concurrent;
import java.util.concurrent.locks.*;
import java.util.concurrent.atomic.*;
/**
* A synchronization aid that allows one or more threads to wait until
* a set of operations being performed in other threads completes.
* 同步化的一个辅助工具，允许一个或多个线程等待，直到所有线程中执行完成
*比如主线程计算一个复杂的计算表达式，将表达式分为多个子表达式在线程中去计算，
*主线程要计算表达式的最后值，必须等所有的线程计算完子表达式计算，方可计算表达式的值；
*再比如一个团队赛跑游戏，最后要计算团队赛跑的成绩，主线程计算最后成绩，要等到所有
*团队成员跑完，方可计算总成绩。
* <p>A {@code CountDownLatch} is initialized with a given [i]count[/i].
* 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. This is a one-shot phenomenon
* -- the count cannot be reset. If you need a version that resets the
* count, consider using a {@link CyclicBarrier}.
*CountDownLatch有一个初始值，调用await的线程，要等待CountDownLatch值为0时，方可
*往下执行
* <p>A {@code CountDownLatch} is a versatile synchronization tool
* and can be used for a number of purposes. 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}. A {@code CountDownLatch} initialized to [i]N[/i]
* can be used to make one thread wait until [i]N[/i] threads have
* completed some action, or some action has been completed N times.
*
* <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.
*
* <p><b>Sample usage:</b> Here is a pair of classes in which a group
* of worker threads use two countdown latches:
* [list]
* <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.
* [/list]
*第一种，所有线程等待一个开始信息号，当开始信息号启动时，所有线程执行，等待所有线程执行完
*方可执行主线程任务
* <pre>
* 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
//给所有线程一个开始信号，比如上面的团队赛发令枪
* startSignal.countDown(); // let all threads proceed
* doSomethingElse();
//等待所有团队赛跑选手，释放跑完信号
* 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 {
//等待开始信息号
* startSignal.await();
//赛跑
* doWork();
//跑完，计时员收到跑完信号通知
* doneSignal.countDown();
* } catch (InterruptedException ex) {} // return;
* }
*
* void doWork() { ... }
* }
*
* </pre>
*
* <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}.)
*所有线程放在线程池中，执行，等待所有线程执行完，方可执行主线程任务
* <pre>
* 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()}
* [url=package-summary.html#MemoryVisibility]<i>happen-before</i>[/url]
* actions following a successful return from a corresponding
* {@code await()} in another thread.
*
* @since 1.5
* @author Doug Lea
*/
public class CountDownLatch {
/**
* Synchronization control For CountDownLatch.
* Uses AQS state to represent count.
*用一个 AQS状态代表信号量数
*/
private static final class Sync extends AbstractQueuedSynchronizer {
private static final long serialVersionUID = 4982264981922014374L;
Sync(int count) {
setState(count);
}
int getCount() {
return getState();
}
//获取信号量
protected int tryAcquireShared(int acquires) {
return (getState() == 0) ? 1 : -1;
}
//释放信号量
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:
* [list]
* <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.
* [/list]
*
* <p>If the current thread:
* [list]
* <li>has its interrupted status set on entry to this method; or
* <li>is {@linkplain Thread#interrupt interrupted} while waiting,
* [/list]
* 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:
* [list]
* <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.
* [/list]
*
* <p>If the count reaches zero then the method returns with the
* value {@code true}.
*
* <p>If the current thread:
* [list]
* <li>has its interrupted status set on entry to this method; or
* <li>is {@linkplain Thread#interrupt interrupted} while waiting,
* [/list]
* 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));
}
/**
* Decrements the count of the latch, releasing all waiting threads if
* the count reaches zero.
*
* <p>If the current count is greater than zero then it is decremented.
* If the new count is zero then all waiting threads are re-enabled for
* thread scheduling purposes.
*
* <p>If the current count equals zero then nothing happens.
*/
public void countDown() {
sync.releaseShared(1);
}
/**
* Returns the current count.
*
* <p>This method is typically used for debugging and testing purposes.
*
* @return the current count
*/
public long getCount() {
return sync.getCount();
}
/**
* Returns a string identifying this latch, as well as its state.
* The state, in brackets, includes the String {@code "Count ="}
* followed by the current count.
*
* @return a string identifying this latch, as well as its state
*/
public String toString() {
return super.toString() + "[Count = " + sync.getCount() + "]";
}
}