抛砖引玉
说到锁不得不提线程安全,说到线程安全,作为iOS程序员又不得不提 nonatomic 与 atomic
nonatomic 不会对生成的 getter、setter 方法加同步锁(非原子性)
atomic 会对生成的 getter 、setter 加同步锁(原子性)
setter / getter 被 atomic 修饰的属性时,该属性是读写安全的。然而读写安全并不代表线程安全。
线程安全概念(thread safety)
线程安全就是多线程访问时,采用了加锁机制,当一个线程访问该类的某个数据时,进行保护,其他线程不能进行访问直到该线程读取完,其他线程才可使用。不会出现数据不一致或者数据污染。
线程不安全就是不提供数据访问保护,有可能出现多个线程先后更改数据造成所得到的数据是脏数据。
验证 atomic 非线程安全
验证代码
#import "ViewController.h" @interface ViewController () @property (strong, atomic) NSString *name; @end @implementation ViewController - (void)viewDidLoad { [super viewDidLoad]; //atomic非线程安全验证 //Jack dispatch_async(dispatch_get_global_queue(0, 0), ^{ while (1) { self.name = @"Jack"; NSLog(@"Jack is %@", self.name); } }); //Rose dispatch_async(dispatch_get_global_queue(0, 0), ^{ while (1) { self.name = @"Rose"; NSLog(@"Rose is %@", self.name); } }); }
验证结果
2017-11-29 11:21:27.713446+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.713487+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.713638+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.713659+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.713840+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.714050+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.714205+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.718069+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.718069+0800 LockDemo[42637:1199500] Jack is Rose 2017-11-29 11:21:27.718199+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.718199+0800 LockDemo[42637:1199499] Rose is Jack
最后一行和倒数第三行可以看到,atomic 非线程安全验证完毕。
也就是说 atomic 只能做到读写安全并不能做到线程安全,若要实现线程安全还需要采用更为深层的锁定机制才行。
iOS开发时一般都会使用 nonatomic 属性,因为在iOS中使用同步锁的开销较大,这会带来性能问题,但是在Mac OS X程序时,使用 atomic 属性通常都不会有性能瓶颈。
锁的概念
在计算机科学中,锁是一种同步机制,用于在存在多线程的环境中实施对资源的访问限制。
锁的作用
通俗来讲:就是为了防止在多线程的情况下对共享资源的脏读或者脏写。
也可以理解为:执行多线程时用于强行限制资源访问的同步机制,即并发控制中保证互斥的要求。
iOS开发中常用的锁
@synchronized
NSLock 对象锁
NSRecursiveLock 递归锁
NSConditionLock 条件锁
pthread_mutex 互斥锁(C语言)
dispatch_semaphore 信号量实现加锁(GCD)
OSSpinLock 自旋锁
性能图来源:ibireme
@synchronized
@synchronized 其实是一个 OC 层面的锁, 主要是通过牺牲性能换来语法上的简洁与可读性。
@synchronized 是我们平常使用最多的但是性能最差的。
OC写法:
@synchronized(self) { //需要执行的代码块 }
swift写法:
objc_sync_enter(self) //需要执行的代码块 objc_sync_exit(self)
代码示例:
//线程1 dispatch_async(dispatch_get_global_queue(0, 0), ^{ @synchronized(self) { NSLog(@"第一个线程同步操作开始"); sleep(3); NSLog(@"第一个线程同步操作结束"); } }); //线程2 dispatch_async(dispatch_get_global_queue(0, 0), ^{ sleep(1); @synchronized(self) { NSLog(@"第二个线程同步操作"); } });
结果:
2017-11-29 14:36:52.056457+0800 LockDemo[46145:1306472] 第一个线程同步操作开始 2017-11-29 14:36:55.056868+0800 LockDemo[46145:1306472] 第一个线程同步操作结束 2017-11-29 14:36:55.057261+0800 LockDemo[46145:1306473] 第二个线程同步操作
@synchronized(self) 指令使用的 self 为该锁的唯一标识,只有当标识相同时,才为满足互斥,如果线程2中的 self 改成其它对象,线程2就不会被阻塞。
NSString *s = [NSString string]; //线程1 dispatch_async(dispatch_get_global_queue(0, 0), ^{ @synchronized(self) { NSLog(@"第一个线程同步操作开始"); sleep(3); NSLog(@"第一个线程同步操作结束"); } }); //线程2 dispatch_async(dispatch_get_global_queue(0, 0), ^{ sleep(1); @synchronized(s) { NSLog(@"第二个线程同步操作"); } });
2017-11-29 14:43:54.930414+0800 LockDemo[46287:1312173] 第一个线程同步操作开始 2017-11-29 14:43:55.930761+0800 LockDemo[46287:1312158] 第二个线程同步操作 2017-11-29 14:43:57.932287+0800 LockDemo[46287:1312173] 第一个线程同步操作结束
@synchronized 指令实现锁的优点就是我们不需要在代码中显式的创建锁对象,便可以实现锁的机制,但作为一种预防措施,@synchronized 块会隐式的添加一个异常处理来保护代码,该处理会在异常抛出的时候自动的释放互斥锁。所以如果不想让隐式的异常处理例程带来额外的开销,你可以考虑使用锁对象。
NSLock
NSLock 中实现了一个简单的互斥锁。通过 NSLocking 协议定义了 lock 和 unlock 方法。
@protocol NSLocking - (void)lock; - (void)unlock; @end
举个栗子卖冰棍儿
- (void)nslockTest { //设置冰棍儿的数量为5 _count = 5; //创建锁 _lock = [[NSLock alloc] init]; //线程1 dispatch_async(dispatch_get_global_queue(0, 0), ^{ [self saleIceCream]; }); //线程2 dispatch_async(dispatch_get_global_queue(0, 0), ^{ [self saleIceCream]; }); } - (void)saleIceCream { while (1) { sleep(1); //加锁 [_lock lock]; if (_count > 0) { _count--; NSLog(@"剩余冰棍儿数= %ld, Thread - %@", _count, [NSThread currentThread]); } else { NSLog(@"冰棍儿卖光光 Thread - %@",[NSThread currentThread]); break; } //解锁 [_lock unlock]; } }
加锁结果:
2017-11-29 11:21:27.713446+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.713487+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.713638+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.713659+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.713840+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.714050+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.714205+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.718069+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.718069+0800 LockDemo[42637:1199500] Jack is Rose 2017-11-29 11:21:27.718199+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.718199+0800 LockDemo[42637:1199499] Rose is Jack
0
不加锁结果:
2017-11-29 11:21:27.713446+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.713487+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.713638+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.713659+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.713840+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.714050+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.714205+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.718069+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.718069+0800 LockDemo[42637:1199500] Jack is Rose 2017-11-29 11:21:27.718199+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.718199+0800 LockDemo[42637:1199499] Rose is Jack
1
NSLock 类还增加了 tryLock 和 lockBeforeDate: 方法
2017-11-29 11:21:27.713446+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.713487+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.713638+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.713659+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.713840+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.714050+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.714205+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.718069+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.718069+0800 LockDemo[42637:1199500] Jack is Rose 2017-11-29 11:21:27.718199+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.718199+0800 LockDemo[42637:1199499] Rose is Jack
2
tryLock 试图获取一个锁,但是如果锁不可用的时候,它不会阻塞线程,相反,它只是返回NO。
lockBeforeDate: 方法试图获取一个锁,但是如果锁没有在规定的时间内被获得,它会让线程从阻塞状态变为非阻塞状态(或者返回NO)。
NSRecursiveLock 递归锁
有时候“加锁代码”中存在递归调用,递归开始前加锁,递归调用开始后会重复执行此方法以至于反复执行加锁代码最终造成死锁。
2017-11-29 11:21:27.713446+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.713487+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.713638+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.713659+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.713840+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.714050+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.714205+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.718069+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.718069+0800 LockDemo[42637:1199500] Jack is Rose 2017-11-29 11:21:27.718199+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.718199+0800 LockDemo[42637:1199499] Rose is Jack
3
我们发现 "结束" 永远不会被打印出来,这个时候可以使用递归锁来解决。使用递归锁可以在一个线程中反复获取锁而不造成死锁,这个过程中会记录获取锁和释放锁的次数,只有最后两者平衡锁才被最终释放。
2017-11-29 11:21:27.713446+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.713487+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.713638+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.713659+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.713840+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.714050+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.714205+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.718069+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.718069+0800 LockDemo[42637:1199500] Jack is Rose 2017-11-29 11:21:27.718199+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.718199+0800 LockDemo[42637:1199499] Rose is Jack
4
此时 "结束" 5秒后会被打印出来。
NSConditionLock 条件锁
NSCoditionLock 做多线程之间的任务等待调用,而且是线程安全的。
2017-11-29 11:21:27.713446+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.713487+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.713638+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.713659+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.713840+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.714050+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.714205+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.718069+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.718069+0800 LockDemo[42637:1199500] Jack is Rose 2017-11-29 11:21:27.718199+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.718199+0800 LockDemo[42637:1199499] Rose is Jack
5
NSConditionLock 也跟其它的锁一样,是需要 lock 与 unlock 对应的,只是 lock , lockWhenCondition: 与 unlock ,unlockWithCondition: 是可以随意组合的,当然这是与需求相关的。
POSIX(pthread_mutex)
C语言定义下多线程加锁方式。 pthread_mutex 和 dispatch_semaphore_t 很像,但是完全不同。pthread_mutex 是Unix/Linux平台上提供的一套条件互斥锁的API。
新建一个简单的 pthread_mutex 互斥锁,引入头文件 #import
使用:
#import
2017-11-29 11:21:27.713446+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.713487+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.713638+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.713659+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.713840+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.714050+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.714205+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.718069+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.718069+0800 LockDemo[42637:1199500] Jack is Rose 2017-11-29 11:21:27.718199+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.718199+0800 LockDemo[42637:1199499] Rose is Jack
6
执行结果:
2017-11-29 11:21:27.713446+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.713487+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.713638+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.713659+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.713840+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.714050+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.714205+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.718069+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.718069+0800 LockDemo[42637:1199500] Jack is Rose 2017-11-29 11:21:27.718199+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.718199+0800 LockDemo[42637:1199499] Rose is Jack
7
pthread_mutex 还可以创建条件锁,提供了和 NSCondition 一样的条件控制,初始化互斥锁同时使用 pthread_cond_init 来初始化条件数据结构
2017-11-29 11:21:27.713446+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.713487+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.713638+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.713659+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.713840+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.714050+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.714205+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.718069+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.718069+0800 LockDemo[42637:1199500] Jack is Rose 2017-11-29 11:21:27.718199+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.718199+0800 LockDemo[42637:1199499] Rose is Jack
8
pthread_mutex 还提供了很多函数,有一套完整的API,包含 Pthreads 线程的创建控制等等,非常底层,可以手动处理线程的各个状态的转换即管理生命周期,甚至可以实现一套自己的多线程,感兴趣的可以继续深入了解。
dispatch_semaphore_t
dispatch_semaphore_t GCD中信号量,也可以解决资源抢占问题,支持信号通知和信号等待。每当发送一个信号通知,则信号量 +1;每当发送一个等待信号时信号量 -1,;如果信号量为 0 则信号会处于等待状态,直到信号量大于 0 开始执行。
api注释:
2017-11-29 11:21:27.713446+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.713487+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.713638+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.713659+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.713840+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.714050+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.714205+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.718069+0800 LockDemo[42637:1199499] Rose is Rose 2017-11-29 11:21:27.718069+0800 LockDemo[42637:1199500] Jack is Rose 2017-11-29 11:21:27.718199+0800 LockDemo[42637:1199500] Jack is Jack 2017-11-29 11:21:27.718199+0800 LockDemo[42637:1199499] Rose is Jack
9
使用:
@synchronized(self) { //需要执行的代码块 }
0
执行结果:
@synchronized(self) { //需要执行的代码块 }
1
OSSpinLock 自旋锁
使用:
#import
@synchronized(self) { //需要执行的代码块 }
2
执行结果:
@synchronized(self) { //需要执行的代码块 }
3
OSSpinLock 自旋锁,性能最高的锁。它的缺点是当等待时会消耗大量 CPU 资源,不太适用于较长时间的任务。 YY大神在博客 不再安全的 OSSpinLock 中说明了OSSpinLock已经不再安全,暂不建议使用。
iOS 10 之后,苹果给出了解决方案,就是用 os_unfair_lock 代替 OSSpinLock。
@synchronized(self) { //需要执行的代码块 }
4
#import
@synchronized(self) { //需要执行的代码块 }
5
执行结果:
@synchronized(self) { //需要执行的代码块 }
6
总结
@synchronized:适用线程不多,任务量不大的多线程加锁
NSLock:性能不算差,但感觉用的人不多。
dispatch_semaphore_t:使用信号来做加锁,性能很高和 OSSpinLock 差不多。
NSConditionLock:多线程处理不同任务的通信建议时用, 只加锁的话性能很低。
NSRecursiveLock:性能不错,使用场景限制于递归。
POSIX(pthread_mutex):C语言的底层api,复杂的多线程处理建议使用,也可以封装自己的多线程。
OSSpinLock:性能非常高,可惜不安全了,使用 os_unfair_lock 来代替。
作者:Inlight先森
链接:https://www.jianshu.com/p/bd25479b7cd1
本文由 投稿者 创作,文章地址:https://blog.isoyu.com/archives/ioszhongdesuoshi.html
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