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。”
a. 利用最基本的
b. 利用
d. 利用
2. 第二种解法，是利用
3. 第三种解法，是利用
4. 第四种解法，是利用
原文链接： http://edisonxu.org/2017/03/02/java-thread-communication.html
“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。
通用代码：
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
/**
* Created by Edison Xu on 2017/3/2.
*/
public enum Helper {
instance;
private static final ExecutorService tPool = Executors.newFixedThreadPool(2);
public static String[] buildNoArr(int max) {
String[] noArr = new String[max];
for(int i=0;i
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synchronized、notify、wait：
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Lock和Condition：
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();
}
}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。
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：
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：
public CyclicBarrier(int parties, Runnable barrierAction) {
}
public CyclicBarrier(int parties) {
}public class MethodFour{
private final CyclicBarrier barrier;
private final List list;
public MethodFour() {
list = Collections.synchronizedList(new ArrayList());
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();
}
} lock和condition，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。
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
peek和poll的不同来实现； - 第二种是两个queue，利用
take()会自动阻塞来实现。
public class MethodSeven {
private final LinkedBlockingQueue 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 queue1 = new LinkedBlockingQueue<>();
private final LinkedBlockingQueue 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();
} 原文链接： http://edisonxu.org/2017/03/02/java-thread-communication.html