Java 高频面试：2个线程分别依次输出数字和字母

Posted 2021-10-15 小猪快跑22

如题：假设有A、B 2个线程，分别依次输出1a2b3c4d，A线程输出数字，B线程输出字母。

下面有几种方法，感觉方法1是最容易想到的。

方法1：synchronized 和 wait/notify

/**
     * 使用 synchronized 和 wait notify
     */
    private static void func1() {
        char[] num = new char[]{'1', '2', '3', '4'};
        char[] str = new char[]{'A', 'B', 'C', 'D'};
        Object lock = new Object();
        AtomicBoolean printChar = new AtomicBoolean(true);

        new Thread("T1") {
            @Override
            public void run() {
                synchronized (lock) {
                    for (char c : num) {
                        try {
                            while (printChar.get()) {
                                printChar.set(false);
                                System.out.println(getName() + " >>> " + c);
                                lock.notify();
                            }
                            lock.wait();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                    lock.notify(); // 这行一定要加，不然会在所有数据遍历完成后T2线程还是wait状态
                }
            }
        }.start();

        // 防止有的虚拟机上不是先输出数字的
        try {
            Thread.sleep(20);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }

        new Thread("T2") {
            @Override
            public void run() {
                synchronized (lock) {
                    for (char c : str) {
                        try {
                            while (!printChar.get()) {
                                printChar.set(true);
                                System.out.println(getName() + " >>> " + c);
                                lock.notify();
                            }
                            lock.wait();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                    // 这一行是可以去掉的，因为T2线程遍历完最后一个数据会通过notify唤醒T1线程
//                    lock.notify();
                }
            }
        }.start();

    }

方法2：ReentrantLock 加 Condition

/**
     * 使用 ReentrantLock 加 Condition
     */
    private static void func2() {
        char[] num = new char[]{'1', '2', '3', '4'};
        char[] str = new char[]{'A', 'B', 'C', 'D'};
        ReentrantLock reentrantLock = new ReentrantLock(true);
        Condition printCharCondition = reentrantLock.newCondition();
        Condition printNumCondition = reentrantLock.newCondition();

        new Thread("T1") {
            @Override
            public void run() {
                try {
                    reentrantLock.lock();
                    for (char c : num) {
                        System.out.println(getName() + " >>> " + c);
                        printCharCondition.signal();
                        printNumCondition.await();
                    }
                    printCharCondition.signal();
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } finally {
                    reentrantLock.unlock();
                }
            }
        }.start();

        new Thread("T2") {
            @Override
            public void run() {
                try {
                    reentrantLock.lock();
                    for (char c : str) {
                        System.out.println(getName() + " >>> " + c);
                        printNumCondition.signal();
                        printCharCondition.await();
                    }
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } finally {
                    reentrantLock.unlock();
                }
            }
        }.start();
    }

方法3：使用信号量 Semaphore

  /**
     * 使用 Semaphore
     */
    private static void func3() {
        char[] num = new char[]{'1', '2', '3', '4'};
        char[] str = new char[]{'A', 'B', 'C', 'D'};
        Semaphore printNum = new Semaphore(1);
        Semaphore printChar = new Semaphore(0);

        new Thread("T1") {
            @Override
            public void run() {
                for (char c : num) {
                    try {
                        printNum.acquire();
                        System.out.println(getName() + " >>> " + c);
                        printChar.release();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        }.start();

        new Thread("T2") {
            @Override
            public void run() {
                for (char c : str) {
                    try {
                        printChar.acquire();
                        System.out.println(getName() + " >>> " + c);
                        printNum.release();
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                }
            }
        }.start();
    }

方法4：使用自旋

 /**
     * 使用自旋
     */
    private static void func4() {
        char[] num = new char[]{'1', '2', '3', '4'};
        char[] str = new char[]{'A', 'B', 'C', 'D'};

        AtomicBoolean printNum = new AtomicBoolean(true);
        new Thread("T1") {
            @Override
            public void run() {
                for (char c : num) {
                    while (!printNum.get()) {}
                    System.out.println(getName() + " >>> " + c);
                    printNum.set(false);

                    /**
                     * 注意，上面的while循环的判断一定要加 !，否则后续的都可能无法输出了，如下写法就可能后续的无法输出
                     */

//                    while (printNum.get()) {
//                        System.out.println(getName() + " >>> " + c);
//                        printNum.set(false);
//                    }
                }
            }
        }.start();

        new Thread(("T2")) {
            @Override
            public void run() {
                for (char c : str) {
                    while (printNum.get()) {}
                    System.out.println(getName() + " >>> " + c);
                    printNum.set(true);
//                    while (!printNum.get()) {
//                        System.out.println(getName() + " >>> " + c);
//                        printNum.set(true);
//                    }
                }
            }
        }.start();

    }

注意点都已经在代码里面注释啦。