iOS多线程之2.NSThread的加锁@synchronized

我在上一篇文章讲了线程的生命周期，这篇文章来讲讲线程加锁的注意事项与@synchronized关键字。

那什么时候需要加锁呢，就是当多条线程同时操作一个变量时，就需要加锁了。至于为什么要加锁，可以看看文顶顶的这篇文章：http://www.cnblogs.com/wendingding/p/3805841.html， 写的非常明白。读本篇文章之前建议读一下。

上代码
声明变量

@interface ViewController ()
@property (strong, nonatomic)NSThread *thread1;
@property (strong, nonatomic)NSThread *thread2;
@property (strong, nonatomic)NSThread *thread3;
@property (assign, nonatomic)int leftTickets;
@end

实现代码

- (void)viewDidLoad {
    [super viewDidLoad];
    
    self.thread1 = [[NSThread alloc] initWithTarget:self selector:@selector(sellTickets) object:nil];
    self.thread2 = [[NSThread alloc] initWithTarget:self selector:@selector(sellTickets) object:nil];
    self.thread3 = [[NSThread alloc] initWithTarget:self selector:@selector(sellTickets) object:nil];
    self.thread1.name = @"thread1";
    self.thread2.name = @"thread2";
    self.thread3.name = @"thread3";
    // 总票数
    self.leftTickets = 10;
}
// 点击屏幕开启线程
- (void)touchesBegan:(NSSet<UITouch *> *)touches withEvent:(UIEvent *)event {
    [self.thread1 start];
    [self.thread2 start];
    [self.thread3 start];
}
- (void)sellTickets {
    while (1) {
        @synchronized (self) {
            if (self.leftTickets > 0) {
                [NSThread sleepForTimeInterval:0.2];
                int count = self.leftTickets;
                self.leftTickets = count - 1;
                NSLog(@"剩余的票数%d",self.leftTickets);
                NSLog(@"当前线程=%@", [NSThread currentThread]);
            }else {
                NSLog(@"票卖完了");
                NSLog(@"退出线程%@",[NSThread currentThread]);
                [NSThread exit];
            }
        }
    }
}

打印日志

2016-11-04 11:52:25.117 TTTTTTTTTT[6753:74162] 剩余的票数9
2016-11-04 11:52:25.117 TTTTTTTTTT[6753:74162] 当前线程=<NSThread: 0x608000073880>{number = 3, name = thread1}
2016-11-04 11:52:25.393 TTTTTTTTTT[6753:74163] 剩余的票数8
2016-11-04 11:52:25.393 TTTTTTTTTT[6753:74163] 当前线程=<NSThread: 0x608000074540>{number = 4, name = thread2}
2016-11-04 11:52:25.661 TTTTTTTTTT[6753:74164] 剩余的票数7
2016-11-04 11:52:25.661 TTTTTTTTTT[6753:74164] 当前线程=<NSThread: 0x608000074580>{number = 5, name = thread3}
2016-11-04 11:52:25.932 TTTTTTTTTT[6753:74162] 剩余的票数6
2016-11-04 11:52:25.933 TTTTTTTTTT[6753:74162] 当前线程=<NSThread: 0x608000073880>{number = 3, name = thread1}
2016-11-04 11:52:26.164 TTTTTTTTTT[6753:74163] 剩余的票数5
2016-11-04 11:52:26.165 TTTTTTTTTT[6753:74163] 当前线程=<NSThread: 0x608000074540>{number = 4, name = thread2}
2016-11-04 11:52:26.438 TTTTTTTTTT[6753:74164] 剩余的票数4
2016-11-04 11:52:26.439 TTTTTTTTTT[6753:74164] 当前线程=<NSThread: 0x608000074580>{number = 5, name = thread3}
2016-11-04 11:52:26.704 TTTTTTTTTT[6753:74162] 剩余的票数3
2016-11-04 11:52:26.705 TTTTTTTTTT[6753:74162] 当前线程=<NSThread: 0x608000073880>{number = 3, name = thread1}
2016-11-04 11:52:26.975 TTTTTTTTTT[6753:74163] 剩余的票数2
2016-11-04 11:52:26.976 TTTTTTTTTT[6753:74163] 当前线程=<NSThread: 0x608000074540>{number = 4, name = thread2}
2016-11-04 11:52:27.232 TTTTTTTTTT[6753:74164] 剩余的票数1
2016-11-04 11:52:27.233 TTTTTTTTTT[6753:74164] 当前线程=<NSThread: 0x608000074580>{number = 5, name = thread3}
2016-11-04 11:52:27.505 TTTTTTTTTT[6753:74162] 剩余的票数0
2016-11-04 11:52:27.505 TTTTTTTTTT[6753:74162] 当前线程=<NSThread: 0x608000073880>{number = 3, name = thread1}
2016-11-04 11:52:27.505 TTTTTTTTTT[6753:74163] 票卖完了
2016-11-04 11:52:27.506 TTTTTTTTTT[6753:74163] 退出线程<NSThread: 0x608000074540>{number = 4, name = thread2}

我们一般用@synchronized来给线程加锁。它有什么用呢：
（1） 堵塞所在线程，线程里面剩下的任务只有当@synchronized里面的代码执行完毕才能继续往下执行，和队列的同步差不多是一个意思。
​（2）当执行@synchronized里面的代码之前，所在线程要先检查是否有其他的线程执行里面的代码。如果没有，才继续往下执行。
再看打印日志里面最后一条，说明了只有线程“thread3”退出了，其他的线程没有退出。

我上篇文章讲，不用管线程的退出，任务执行完线程会自动退出。但是这是一个while循环啊！如果不退出线程，线程会一直执行。
代码

- (void)sellTickets {
    while (1) {
        @synchronized (self) {
            if (self.leftTickets > 0) {
                [NSThread sleepForTimeInterval:0.2];
                int count = self.leftTickets;
                self.leftTickets = count - 1;
                NSLog(@"剩余的票数%d",self.leftTickets);
                NSLog(@"当前线程=%@", [NSThread currentThread]);
            }else {
                NSLog(@"票卖完了");
                NSLog(@"退出线程%@",[NSThread currentThread]);
                // 不让线程退出
                //[NSThread exit];
            }
        }
    }
}

打印的日志

2016-11-04 12:01:53.309 TTTTTTTTTT[7110:78974] 当前线程=<NSThread: 0x600000076f40>{number = 4, name = thread2}
2016-11-04 12:01:53.556 TTTTTTTTTT[7110:78973] 剩余的票数0
2016-11-04 12:01:53.556 TTTTTTTTTT[7110:78973] 当前线程=<NSThread: 0x600000076fc0>{number = 3, name = thread1}
2016-11-04 12:01:53.556 TTTTTTTTTT[7110:78975] 票卖完了
2016-11-04 12:01:53.557 TTTTTTTTTT[7110:78975] 退出线程<NSThread: 0x600000077240>{number = 5, name = thread3}
2016-11-04 12:01:53.558 TTTTTTTTTT[7110:78974] 票卖完了
2016-11-04 12:01:53.559 TTTTTTTTTT[7110:78974] 退出线程<NSThread: 0x600000076f40>{number = 4, name = thread2}
2016-11-04 12:01:53.560 TTTTTTTTTT[7110:78973] 票卖完了

那又为什么只有线程thread2退出呢？（注：每次退出的线程是不确定的）因为当线程thread2退出了，并没有执行完@synchronized里的方法，线程thread1和线程thread3还在等thread2执行完了，它们好去执行呢。但是线程thread2已经死了，不可能再执行了。这就造成线程thread1和线程thread3一直都在内存里，没有被退出，造成了CPU不必要的开销，所以我们最好不要在@synchronized里面退出线程。

- (void)sellTickets {
    while (1) {
        @synchronized (self) {
            if (self.leftTickets > 0) {
                [NSThread sleepForTimeInterval:0.2];
                int count = self.leftTickets;
                self.leftTickets = count - 1;
                NSLog(@"剩余的票数%d",self.leftTickets);
                NSLog(@"当前线程=%@", [NSThread currentThread]);
            }
        }
        if (self.leftTickets == 0) {
            NSLog(@"票卖完了");
            NSLog(@"退出线程%@",[NSThread currentThread]);
            [NSThread exit];
        }
    }
}

2016-11-04 12:06:51.795 TTTTTTTTTT[7295:81206] 票卖完了
2016-11-04 12:06:51.795 TTTTTTTTTT[7295:81207] 票卖完了
2016-11-04 12:06:51.795 TTTTTTTTTT[7295:81208] 票卖完了
2016-11-04 12:06:51.796 TTTTTTTTTT[7295:81206] 退出线程<NSThread: 0x60000026a3c0>{number = 3, name = thread1}
2016-11-04 12:06:51.796 TTTTTTTTTT[7295:81207] 退出线程<NSThread: 0x60000026a380>{number = 4, name = thread2}
2016-11-04 12:06:51.796 TTTTTTTTTT[7295:81208] 退出线程<NSThread: 0x60000026a740>{number = 5, name = thread3}

这就是NSThread加锁以及加锁的一些注意事项。如果感觉对你有用，记得关注啊，我会每周分享一些技术心得。