死鎖

多線程死鎖：同步中嵌套同步,致使鎖沒法釋放

模擬死鎖的狀況：

 

public class TrainThread implements Runnable {

    // 定義初始化票100張
    private int ticketCount = 100;

    private Object object = new Object();

    public boolean flag = true;

    public void run() {

        if (flag) {
            while (true) {
                synchronized (object) { // 對象鎖
                    sale();
                }
            }

        } else {
            while (true) {
                sale();
            }

        }

    }

    private synchronized void sale() { // this鎖
        synchronized (object) {
            if (ticketCount > 0) {
                try {
                    Thread.sleep(40);
                } catch (Exception e) {

                }
                System.out.println(Thread.currentThread().getName() + "賣出了第" + (100 - ticketCount + 1 + "張票"));
                ticketCount--;
            }
        }
    }

}

 

public class App {
    public static void main(String[] args) throws InterruptedException {
        TrainThread trainThread = new TrainThread();
        
        //模擬了兩個線程，必須都用同一個Runnable 的子類對象，才能叫 實現資源的共享
        Thread thread1 = new Thread(trainThread);
        Thread thread2 = new Thread(trainThread);
        thread1.start();
        thread1.sleep(40);
        trainThread.flag = false;
        thread2.start();
    }

}

兩個線程，線程1 進入 對象鎖，而後開始sale(),線程1 佔據了this 鎖（會放開），佔據了對象鎖（長期）

線程2 進入了sale(),佔據了this 鎖，由於對象鎖被線程1 佔據，因此不能繼續執行，同時由於佔據了this 鎖，線程1 也進入不了，產生死鎖