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<max;i++){ noArr[i] = Integer.toString(i+1); } return noArr; } public static String[] buildCharArr(int max) { String[] charArr = new String[max]; int tmp = 65; for(int i=0;i<max;i++){ charArr[i] = String.valueOf((char)(tmp+i)); } return charArr; } public static void print(String... input){ if(input==null) return; for(String each:input){ System.out.print(each); } } public void run(Runnable r){ tPool.submit(r); } public void shutdown(){ tPool.shutdown(); }}
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<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(); }}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<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(); }原文链接： http://edisonxu.org/2017/03/02/java-thread-communication.html
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