【解決線程安全問題：通過Lock鎖對象】

雖然我們可以理解同步代碼塊和同步方法的鎖對象問題，但是我們並沒有直接看到在哪里加上了鎖，在哪里釋放了鎖.

為了更清晰的表達如何加鎖和釋放鎖，JDK5以后提供了一個新的鎖對象Lock,而ReentrantLock是Lock的實現類.

package com.test;

import java.util.concurrent.locks.Lock;
import java.util.concurrent.locks.ReentrantLock;

public class SellTicket implements Runnable {

    private int tickets = 100;
    //定義鎖對象
    private Lock lock = new ReentrantLock();

    @Override
    public void run() {
        while (true) {
            try {
                /**加鎖*/
                lock.lock();
                if (tickets > 0) {
                    try {
                        Thread.sleep(10);
                    } catch (InterruptedException e) {
                        e.printStackTrace();
                    }
                    System.out.println(Thread.currentThread().getName()+"正在出售第"+tickets+"張票");
                    tickets--;
                }
            } finally {
                /**無論代碼執行是否異常，都會到這來釋放鎖*/
                lock.unlock();
            }
        }
    }
    
}

package com.test;

public class SellTicketTest {

    public static void main(String[] args){

        SellTicket sellTicket = new SellTicket();

        Thread t1 = new Thread(sellTicket,"窗口一");
        Thread t2 = new Thread(sellTicket,"窗口二");
        Thread t3 = new Thread(sellTicket,"窗口三");

        t1.start();
        t2.start();
        t3.start();

    }

}