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JAVA线程间通信的几种方式

时间:2019-07-15 23:12:12来源:IT技术作者:seo实验室小编阅读:88次「手机版」
 

线程间通信

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。

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synchronizednotifywait

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

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。

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();
      }
}
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。

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。
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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