使用Lock，wait/notify，Semaphore三種方式實現多線程通信

Java的多線程通信有Lock，wait/notify，Semaphore三種方式，以一道常見面試題來簡單演示這三種多線程通信方式。

兩個線程循環間隔打印指定內容，一個打印從1到52的數字，一個打印從A到Z的字母，打印輸出如下：

1
2
A
3
4

B

......

51
52
Z

使用Lock實現代碼如下：

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.locks.Condition;
import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class ThreadCommunicationTest {

    private final Lock lock = new ReentrantLock();

    private final Condition conditionA = lock.newCondition();
    private final Condition conditionB = lock.newCondition();

    private static char currentThread = 'A';

    public static void main(String[] args) {

        ThreadCommunicationTest test = new ThreadCommunicationTest();

        ExecutorService service = Executors.newCachedThreadPool();

        service.execute(test.new RunnableA());
        service.execute(test.new RunnableB());

        service.shutdown();

    }

    private class RunnableA implements Runnable {
        public void run() {
            for (int i = 1; i <= 52; i++) {
                lock.lock();

                try {
                    while (currentThread != 'A') {
                        try {
                            conditionA.await();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }

                    System.out.println(i);
                    if (i % 2 == 0) {
                        currentThread = 'B';
                        conditionB.signal();
                    }
                } finally {
                    lock.unlock();
                }
            }

        }

    }

    private class RunnableB implements Runnable {
        public void run() {
            for (char c = 'A'; c <= 'Z'; c++) {
                lock.lock();
                try {
                    while (currentThread != 'B') {
                        try {
                            conditionB.await();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }

                    System.out.println(c);
                    currentThread = 'A';
                    conditionA.signal();
                } finally {
                    lock.unlock();
                }
            }

        }

    }
}

使用wait/notify實現代碼如下：

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;


public class ThreadCommunicationTest2 {

    private static char currentThread = 'A';
    private final Object t = new Object(); //使用一個同步對象保證兩個線程之間每一時刻只有一個線程工作

    public static void main(String[] args) {
        ThreadCommunicationTest2 test = new ThreadCommunicationTest2();
        ExecutorService service = Executors.newCachedThreadPool();

        service.execute(test.new RunnableA());
        service.execute(test.new RunnableB());

        service.shutdown();
    }

    private class RunnableA implements Runnable {
        public void run() {
            for (int i = 1; i <= 52; i++) {
                synchronized (t) {
                    if(currentThread != 'A'){
                        try {
                            t.wait();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                    System.out.println(i);
                    if (i % 2 == 0) {
                        currentThread = 'B';
                        t.notifyAll();
                    }
                }
            }
        }
    }

    private class RunnableB implements Runnable {
        public void run() {
            for (char c = 'A'; c <= 'Z'; c++) {
                synchronized (t) {
                    if(currentThread != 'B'){
                        try {
                            t.wait();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                    System.out.println(c);
                    currentThread = 'A';
                    t.notifyAll();
                }
            }
        }
    }

}

使用Semaphore信號量的代碼如下：

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Semaphore;

public class ThreadCommunicationTest3 {

    private final Semaphore semap = new Semaphore(1);//創建一個只有1個許可的信號量，保證兩個線程間每一時刻只有一個在工作

    private static char currentThread = 'A';

    public static void main(String[] args) {

        ThreadCommunicationTest3 test = new ThreadCommunicationTest3();

        ExecutorService service = Executors.newCachedThreadPool();

        service.execute(test.new RunnableA());
        service.execute(test.new RunnableB());

        service.shutdown();

    }

    private class RunnableA implements Runnable {
        public void run() {
            for (int i = 1; i <= 52; i++) {
                try {
                    semap.acquire();
                    while (currentThread != 'A') {
                        semap.release();
                    }
                    System.out.println(i);
                    if (i % 2 == 0) {
                        currentThread = 'B';
                    }
                } catch (Exception e) {
                    e.printStackTrace();
                } finally {
                    semap.release();
                }
            }
        }
    }

    private class RunnableB implements Runnable {
        public void run() {
            for (char c = 'A'; c <= 'Z'; c++) {
                try {
                    semap.acquire();
                    while (currentThread != 'B') {
                        semap.release();
                    }
                    System.out.println(c);
                    currentThread = 'A';
                } catch (Exception e) {
                    e.printStackTrace();
                } finally {
                    semap.release();
                }
            }
        }

    }
}