JAVA线程间通信的几种方式

今天在群里面看到一个很有意思的面试题:
“编写两个线程,一个线程打印1~25,另一个线程打印字母A~Z,打印顺序为12A34B56C……5152Z,要求使用线程间的通信。”
这是一道非常好的面试题,非常能彰显被面者关于多线程的功力,一下子就勾起了我的兴趣。这里抛砖引玉,给出7种想到的解法。

1. 第一种解法,包含多种小的不同实现方式,但一个共同点就是靠一个共享变量来做控制

a. 利用最基本的synchronizednotifywait

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  public class MethodOne {

private final ThreadToGo threadToGo = new ThreadToGo();

public Runnable newThreadOne() {
final String[] inputArr = Helper.buildNoArr(52);
return new Runnable() {
private String[] arr = inputArr;
public void run() {
try {
for (int i = 0; i < arr.length; i=i+2) {
synchronized (threadToGo) {
while (threadToGo.value == 2)
threadToGo.wait();
Helper.print(arr[i], arr[i + 1]);
threadToGo.value = 2;
threadToGo.notify();
}
}
} catch (InterruptedException e) {
System.out.println("Oops...");
}
}
};
}

public Runnable newThreadTwo() {
final String[] inputArr = Helper.buildCharArr(26);
return new Runnable() {
private String[] arr = inputArr;

public void run() {
try {
for (int i = 0; i < arr.length; i++) {
synchronized (threadToGo) {
while (threadToGo.value == 1)
threadToGo.wait();
Helper.print(arr[i]);
threadToGo.value = 1;
threadToGo.notify();
}
}
} catch (InterruptedException e) {
System.out.println("Oops...");
}
}
};
}

class ThreadToGo {
int value = 1;
}

public static void main(String args[]) throws InterruptedException {
MethodOne one = new MethodOne();
Helper.instance.run(one.newThreadOne());
Helper.instance.run(one.newThreadTwo());
Helper.instance.shutdown();
}
}

b. 利用LockCondition

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public class MethodTwo {

private Lock lock = new ReentrantLock(true);
private Condition condition = lock.newCondition();
private final ThreadToGo threadToGo = new ThreadToGo();

public Runnable newThreadOne() {
final String[] inputArr = Helper.buildNoArr(52);
return new Runnable() {
private String[] arr = inputArr;
public void run() {
for (int i = 0; i < arr.length; i=i+2) {
try {
lock.lock();
while(threadToGo.value == 2)
condition.await();
Helper.print(arr[i], arr[i + 1]);
threadToGo.value = 2;
condition.signal();
} catch (InterruptedException e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
}
};
}

public Runnable newThreadTwo() {
final String[] inputArr = Helper.buildCharArr(26);
return new Runnable() {
private String[] arr = inputArr;

public void run() {
for (int i = 0; i < arr.length; i++) {
try {
lock.lock();
while(threadToGo.value == 1)
condition.await();
Helper.print(arr[i]);
threadToGo.value = 1;
condition.signal();
} catch (Exception e) {
e.printStackTrace();
} finally {
lock.unlock();
}
}
}
};
}

class ThreadToGo {
int value = 1;
}

public static void main(String args[]) throws InterruptedException {
MethodTwo two = new MethodTwo();
Helper.instance.run(two.newThreadOne());
Helper.instance.run(two.newThreadTwo());
Helper.instance.shutdown();
}
}

c. 利用volatile

volatile修饰的变量值直接存在main memory里面,子线程对该变量的读写直接写入main memory,而不是像其它变量一样在local thread里面产生一份copy。volatile能保证所修饰的变量对于多个线程可见性,即只要被修改,其它线程读到的一定是最新的值。

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public class MethodThree {

private volatile ThreadToGo threadToGo = new ThreadToGo();

class ThreadToGo {
int value = 1;
}

public Runnable newThreadOne() {
final String[] inputArr = Helper.buildNoArr(52);
return new Runnable() {
private String[] arr = inputArr;
public void run() {
for (int i = 0; i < arr.length; i=i+2) {
while(threadToGo.value==2){}
Helper.print(arr[i], arr[i + 1]);
threadToGo.value=2;
}
}
};
}

public Runnable newThreadTwo() {
final String[] inputArr = Helper.buildCharArr(26);
return new Runnable() {
private String[] arr = inputArr;

public void run() {
for (int i = 0; i < arr.length; i++) {
while(threadToGo.value==1){}
Helper.print(arr[i]);
threadToGo.value=1;
}
}
};
}

public static void main(String args[]) throws InterruptedException {
MethodThree three = new MethodThree();
Helper.instance.run(three.newThreadOne());
Helper.instance.run(three.newThreadTwo());
Helper.instance.shutdown();
}
}

d. 利用AtomicInteger

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public class MethodFive {

private AtomicInteger threadToGo = new AtomicInteger(1);

public Runnable newThreadOne() {
final String[] inputArr = Helper.buildNoArr(52);
return new Runnable() {
private String[] arr = inputArr;
public void run() {
for (int i = 0; i < arr.length; i=i+2) {
while(threadToGo.get()==2){}
Helper.print(arr[i], arr[i + 1]);
threadToGo.set(2);
}
}
};
}

public Runnable newThreadTwo() {
final String[] inputArr = Helper.buildCharArr(26);
return new Runnable() {
private String[] arr = inputArr;

public void run() {
for (int i = 0; i < arr.length; i++) {
while(threadToGo.get()==1){}
Helper.print(arr[i]);
threadToGo.set(1);
}
}
};
}

public static void main(String args[]) throws InterruptedException {
MethodFive five = new MethodFive();
Helper.instance.run(five.newThreadOne());
Helper.instance.run(five.newThreadTwo());
Helper.instance.shutdown();
}
}

2. 第二种解法,是利用CyclicBarrierAPI;

CyclicBarrier可以实现让一组线程在全部到达Barrier时(执行await()),再一起同时执行,并且所有线程释放后,还能复用它,即为Cyclic。
CyclicBarrier类提供两个构造器:

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public CyclicBarrier(int parties, Runnable barrierAction) {
}

public CyclicBarrier(int parties) {
}

这里是利用它到达Barrier后去执行barrierAction。

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public class MethodFour{

private final CyclicBarrier barrier;
private final List<String> list;

public MethodFour() {
list = Collections.synchronizedList(new ArrayList<String>());
barrier = new CyclicBarrier(2,newBarrierAction());
}

public Runnable newThreadOne() {
final String[] inputArr = Helper.buildNoArr(52);
return new Runnable() {
private String[] arr = inputArr;

public void run() {
for (int i = 0, j=0; i < arr.length; i=i+2,j++) {
try {
list.add(arr[i]);
list.add(arr[i+1]);
barrier.await();
} catch (InterruptedException | BrokenBarrierException e) {
e.printStackTrace();
}
}
}
};
}

public Runnable newThreadTwo() {
final String[] inputArr = Helper.buildCharArr(26);
return new Runnable() {

private String[] arr = inputArr;
public void run() {
for (int i = 0; i < arr.length; i++) {
try {
list.add(arr[i]);
barrier.await();
} catch (InterruptedException | BrokenBarrierException e) {
e.printStackTrace();
}
}
}
};
}

private Runnable newBarrierAction(){
return new Runnable() {
@Override
public void run() {
Collections.sort(list);
list.forEach(c->System.out.print(c));
list.clear();
}
};
}

public static void main(String args[]){
MethodFour four = new MethodFour();
Helper.instance.run(four.newThreadOne());
Helper.instance.run(four.newThreadTwo());
Helper.instance.shutdown();
}
}

这里多说一点,这个API其实还是利用lockcondition,无非是多个线程去争抢CyclicBarrier的instance的lock罢了,最终barrierAction执行时,是在抢到CyclicBarrierinstance的那个线程上执行的。

3. 第三种解法,是利用PipedInputStreamAPI;

这里用流在两个线程间通信,但是Java中的Stream是单向的,所以在两个线程中分别建了一个input和output。这显然是一种很搓的方式,不过也算是一种通信方式吧……-_-T,执行的时候那种速度简直。。。请不要BS我。

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public class MethodSix {

private final PipedInputStream inputStream1;
private final PipedOutputStream outputStream1;
private final PipedInputStream inputStream2;
private final PipedOutputStream outputStream2;
private final byte[] MSG;

public MethodSix() {
inputStream1 = new PipedInputStream();
outputStream1 = new PipedOutputStream();
inputStream2 = new PipedInputStream();
outputStream2 = new PipedOutputStream();
MSG = "Go".getBytes();

try {
inputStream1.connect(outputStream2);
inputStream2.connect(outputStream1);
} catch (IOException e) {
e.printStackTrace();
}
}

public void shutdown() throws IOException {
inputStream1.close();
inputStream2.close();
outputStream1.close();
outputStream2.close();
}

public Runnable newThreadOne() {
final String[] inputArr = Helper.buildNoArr(52);
return new Runnable() {
private String[] arr = inputArr;
private PipedInputStream in = inputStream1;
private PipedOutputStream out = outputStream1;

public void run() {
for (int i = 0; i < arr.length; i=i+2) {
Helper.print(arr[i], arr[i + 1]);
try {
out.write(MSG);
byte[] inArr = new byte[2];
in.read(inArr);
while(true){
if("Go".equals(new String(inArr)))
break;
}
} catch (IOException e) {
e.printStackTrace();
}
}
}
};
}

public Runnable newThreadTwo() {
final String[] inputArr = Helper.buildCharArr(26);
return new Runnable() {
private String[] arr = inputArr;
private PipedInputStream in = inputStream2;
private PipedOutputStream out = outputStream2;

public void run() {
for (int i = 0; i < arr.length; i++) {
try {
byte[] inArr = new byte[2];
in.read(inArr);
while(true){
if("Go".equals(new String(inArr)))
break;
}
Helper.print(arr[i]);
out.write(MSG);
} catch (IOException e) {
e.printStackTrace();
}
}
}
};
}

public static void main(String args[]) throws IOException {
MethodSix six = new MethodSix();
Helper.instance.run(six.newThreadOne());
Helper.instance.run(six.newThreadTwo());
Helper.instance.shutdown();
six.shutdown();
}

4. 第四种解法,是利用BlockingQueue

顺便总结下BlockingQueue的一些内容。
BlockingQueue定义的常用方法如下:

  • add(Object):把Object加到BlockingQueue里,如果BlockingQueue可以容纳,则返回true,否则抛出异常。
  • offer(Object):表示如果可能的话,将Object加到BlockingQueue里,即如果BlockingQueue可以容纳,则返回true,否则返回false。
  • put(Object):把Object加到BlockingQueue里,如果BlockingQueue没有空间,则调用此方法的线程被阻断直到BlockingQueue里有空间再继续。
  • poll(time):获取并删除BlockingQueue里排在首位的对象,若不能立即取出,则可以等time参数规定的时间,取不到时返回null。当不传入time值时,立刻返回。
  • peek():立刻获取BlockingQueue里排在首位的对象,但不从队列里删除,如果队列为空,则返回null。
  • take():获取并删除BlockingQueue里排在首位的对象,若BlockingQueue为空,阻断进入等待状态直到BlockingQueue有新的对象被加入为止。

BlockingQueue有四个具体的实现类:

  • ArrayBlockingQueue:规定大小的BlockingQueue,其构造函数必须带一个int参数来指明其大小。其所含的对象是以FIFO(先入先出)顺序排序的。
  • LinkedBlockingQueue:大小不定的BlockingQueue,若其构造函数带一个规定大小的参数,生成的BlockingQueue有大小限制,若不带大小参数,所生成的BlockingQueue的大小由Integer.MAX_VALUE来决定。其所含的对象是以FIFO顺序排序的。
  • PriorityBlockingQueue:类似于LinkedBlockingQueue,但其所含对象的排序不是FIFO,而是依据对象的自然排序顺序或者是构造函数所带的Comparator决定的顺序。
  • SynchronousQueue:特殊的BlockingQueue,对其的操作必须是放和取交替完成的。

这里我用了两种玩法:

  • 一种是共享一个queue,根据peekpoll的不同来实现;
  • 第二种是两个queue,利用take()会自动阻塞来实现。
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public class MethodSeven {

private final LinkedBlockingQueue<String> queue = new LinkedBlockingQueue<>();

public Runnable newThreadOne() {
final String[] inputArr = Helper.buildNoArr(52);
return new Runnable() {
private String[] arr = inputArr;
public void run() {
for (int i = 0; i < arr.length; i=i+2) {
Helper.print(arr[i], arr[i + 1]);
queue.offer("TwoToGo");
while(!"OneToGo".equals(queue.peek())){}
queue.poll();
}
}
};
}

public Runnable newThreadTwo() {
final String[] inputArr = Helper.buildCharArr(26);
return new Runnable() {
private String[] arr = inputArr;

public void run() {
for (int i = 0; i < arr.length; i++) {
while(!"TwoToGo".equals(queue.peek())){}
queue.poll();
Helper.print(arr[i]);
queue.offer("OneToGo");
}
}
};
}

private final LinkedBlockingQueue<String> queue1 = new LinkedBlockingQueue<>();
private final LinkedBlockingQueue<String> queue2 = new LinkedBlockingQueue<>();

public Runnable newThreadThree() {
final String[] inputArr = Helper.buildNoArr(52);
return new Runnable() {
private String[] arr = inputArr;
public void run() {
for (int i = 0; i < arr.length; i=i+2) {
Helper.print(arr[i], arr[i + 1]);
try {
queue2.put("TwoToGo");
queue1.take();
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
};
}

public Runnable newThreadFour() {
final String[] inputArr = Helper.buildCharArr(26);
return new Runnable() {
private String[] arr = inputArr;

public void run() {
for (int i = 0; i < arr.length; i++) {
try {
queue2.take();
Helper.print(arr[i]);
queue1.put("OneToGo");
} catch (InterruptedException e) {
e.printStackTrace();
}
}
}
};
}

public static void main(String args[]) throws InterruptedException {
MethodSeven seven = new MethodSeven();
Helper.instance.run(seven.newThreadOne());
Helper.instance.run(seven.newThreadTwo());
Thread.sleep(2000);
System.out.println("");
Helper.instance.run(seven.newThreadThree());
Helper.instance.run(seven.newThreadFour());
Helper.instance.shutdown();
}

本文所有代码已上传至GitHub:https://github.com/EdisonXu/POC/tree/master/concurrent-test

本文由 EdisonXu - 徐焱飞 创作,采用 CC BY 4.0 CN协议 进行许可。 可自由转载、引用,但需署名作者且注明文章出处。本位链接为http://edisonxu.com/2017/03/02/java-thread-communication.html
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