简单了解一下多线程

傷城~ 2022-05-30 23:27 217阅读 0赞

##### 线程安全问题 #####

经典的买票案例:

public class TicketNum implements Runnable { 
        int tickNum=100;
        @Override
        public void run() {
            // TODO Auto-generated method stub
            while(true){
                if(tickNum>0){
                    try {
                        Thread.sleep(100);
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                    System.out.println(Thread.currentThread().getName()+"还有"+(tickNum--)+"张票");
                }
            }
        }
    }
    
    public class ThreadDemo { 
        public static void main(String[] args) {
            TicketNum tn=new TicketNum();
            Thread t1=new Thread(tn,"窗口一");
            Thread t2=new Thread(tn,"窗口二");
            Thread t3=new Thread(tn,"窗口三");
    
            t1.start();
            t2.start();
            t3.start();
        }
    }

结果  
![这里写图片描述][SouthEast]  
这就是线程安全问题,产生原因:  
A：是否是多线程环境  
B：是否有共享数据  
C：是否有多条语句操作共享数据

##### 解决 #####

第一种:同步

public class TicketNum implements Runnable { 
        int tickNum=100;
        @Override
        //方法上加synchronized 
        public synchronized void run() {
            // TODO Auto-generated method stub
            while(true){
                if(tickNum>0){
                    try {
                        Thread.sleep(100);
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    }
                    System.out.println(Thread.currentThread().getName()+"还有"+(tickNum--)+"张票");
                }
            }
        }
    }
    
    public class ThreadDemo { 
        public static void main(String[] args) {
            TicketNum tn=new TicketNum();
            Thread t1=new Thread(tn,"窗口一");
            Thread t2=new Thread(tn,"窗口二");
            Thread t3=new Thread(tn,"窗口三");
    
            t1.start();
            t2.start();
            t3.start();
        }
    }

结果:  
![这里写图片描述][SouthEast 1]

第二种:加锁lock

public class TicketNum2 implements Runnable { 
        int tickNum = 100;
        private static Lock lock = new ReentrantLock();
        @Override
        public void run() {
            while (true) {
                lock.lock();// 锁定 
                if (tickNum > 0) {
                    try {
                        Thread.sleep(100);                  System.out.println(Thread.currentThread().getName() + "上锁");                    System.out.println(Thread.currentThread().getName() + "还有" + (tickNum--) + "张票");
                    } catch (InterruptedException e) {
                        // TODO Auto-generated catch block
                        e.printStackTrace();
                    } finally {                 System.out.println(Thread.currentThread().getName() + "解锁");
                        lock.unlock();// 解锁
                    }
                }
            }
        }
    }
    
    public class LockDemo { 
    public static void main(String[] args) {
        TicketNum2 tn=new TicketNum2();
        Thread t1=new Thread(tn,"窗口一");
        Thread t2=new Thread(tn,"窗口二");
        Thread t3=new Thread(tn,"窗口三"); 
        t1.start();
        t2.start();
        t3.start();
    }
    }

结果:  
![这里写图片描述][SouthEast 2]

##### sleep()与wait() #####

对于sleep()方法，我们首先要知道该方法是属于Thread类中的。而wait()方法，则是属于Object类中的。

sleep()方法导致了程序暂停执行指定的时间，让出cpu该其他线程，但是他的监控状态依然保持者，当指定的时间到了又会自动恢复运行状态。

在调用sleep()方法的过程中，线程不会释放对象锁。

而当调用wait()方法的时候，线程会放弃对象锁，进入等待此对象的等待锁定池，只有针对此对象调用notify()方法后本线程才进入对象锁定池准备

获取对象锁进入运行状态。

##### 创建线程的方式 #####

1.继承Thread类,不推荐  
2.实现Runnable 接口  
3.实现java.util.concurrent下的Callable接口

[SouthEast]: /images/20220531/dac3fb902a19410fb24d4738b514dd27.png
[SouthEast 1]: /images/20220531/237cc19235274196a01485bbef7642d8.png
[SouthEast 2]: /images/20220531/513f6a6b98a04b368aab6c937791877f.png