IOS中的线程同步方案

线程同步方案

  • OSSpinLock 自旋锁
  • os_unfair_lock
  • pthread_mutex
  • dispatch_semaphore 信号量
  • dispatch_queue(DISPATCH_QUEUE_SERIAL) 串行队列
  • NSLock
  • NSRecursiveLock
  • NSCondition
  • NSConditionLock 条件锁
  • @synchronized
卖票方法
#import "ViewController.h"
#import <libkern/OSAtomic.h> //导入头文件 自旋锁
@interface ViewController ()
@property (assign, nonatomic) OSSpinLock lock;
@end

/**
 卖票演示
 */
- (void)ticketTest
{
    self.ticketsCount = 15;
    
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            [self saleTicket];
        }
    });
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            [self saleTicket];
        }
    });
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 5; i++) {
            [self saleTicket];
        }
    });
}


/**
 卖1张票
 */
- (void)saleTicket
{
        // 加锁
    OSSpinLockLock(&_lock);

    int oldTicketsCount = self.ticketsCount;
    sleep(.2);
    oldTicketsCount--;
    self.ticketsCount = oldTicketsCount;
    NSLog(@"还剩%d张票 - %@", oldTicketsCount, [NSThread currentThread]);
    
    // 解锁
    OSSpinLockUnlock(&_lock);

//尝试加锁 更加安全
 // if (OSSpinLockTry(&_lock)) {
//        int oldTicketsCount = self.ticketsCount;
//        sleep(.2);
//        oldTicketsCount--;
//        self.ticketsCount = oldTicketsCount;
//        NSLog(@"还剩%d张票 - %@", oldTicketsCount, [NSThread currentThread]);
//
//        OSSpinLockUnlock(&_lock);
//    }
    
}


2018-09-12 09:49:28.048945+0800 Interview04-线程同步[986:58920] 还剩14张票 - <NSThread: 0x60400027d200>{number = 3, name = (null)}
2018-09-12 09:49:28.049229+0800 Interview04-线程同步[986:58920] 还剩13张票 - <NSThread: 0x60400027d200>{number = 3, name = (null)}
2018-09-12 09:49:28.049457+0800 Interview04-线程同步[986:58920] 还剩12张票 - <NSThread: 0x60400027d200>{number = 3, name = (null)}
2018-09-12 09:49:28.050802+0800 Interview04-线程同步[986:58920] 还剩11张票 - <NSThread: 0x60400027d200>{number = 3, name = (null)}
2018-09-12 09:49:28.050978+0800 Interview04-线程同步[986:58920] 还剩10张票 - <NSThread: 0x60400027d200>{number = 3, name = (null)}
2018-09-12 09:49:28.051562+0800 Interview04-线程同步[986:58917] 还剩9张票 - <NSThread: 0x60400027d780>{number = 4, name = (null)}
2018-09-12 09:49:28.051847+0800 Interview04-线程同步[986:58917] 还剩8张票 - <NSThread: 0x60400027d780>{number = 4, name = (null)}
2018-09-12 09:49:28.052481+0800 Interview04-线程同步[986:58917] 还剩7张票 - <NSThread: 0x60400027d780>{number = 4, name = (null)}
2018-09-12 09:49:28.052850+0800 Interview04-线程同步[986:58917] 还剩6张票 - <NSThread: 0x60400027d780>{number = 4, name = (null)}
2018-09-12 09:49:28.053069+0800 Interview04-线程同步[986:58917] 还剩5张票 - <NSThread: 0x60400027d780>{number = 4, name = (null)}
2018-09-12 09:49:28.194761+0800 Interview04-线程同步[986:58919] 还剩4张票 - <NSThread: 0x604000276140>{number = 5, name = (null)}
2018-09-12 09:49:28.195136+0800 Interview04-线程同步[986:58919] 还剩3张票 - <NSThread: 0x604000276140>{number = 5, name = (null)}
2018-09-12 09:49:28.195393+0800 Interview04-线程同步[986:58919] 还剩2张票 - <NSThread: 0x604000276140>{number = 5, name = (null)}
2018-09-12 09:49:28.195626+0800 Interview04-线程同步[986:58919] 还剩1张票 - <NSThread: 0x604000276140>{number = 5, name = (null)}
2018-09-12 09:49:28.195803+0800 Interview04-线程同步[986:58919] 还剩0张票 - <NSThread: 0x604000276140>{number = 5, name = (null)}

OSSpinLock总结
  • OSSpinLock叫做”自旋锁”,等待锁的线程会处于忙等(busy-wait)状态,一直占用着CPU资源

  • 目前已经不再安全,可能会出现优先级反转问题

  • 不安全,会根据优先级反转,优先级高的会先加锁!

  • 优先级反转 如果线程2先进去,线程1的优先级比较高,那么CPU会更线程1分配大量资源,导致线程2资源很好,然后线程2就无法解锁,会导致死锁现象

os_unfair_lock
  • os_unfair_lock用于取代不安全的OSSpinLock ,从iOS10开始才支持
  • 从底层调用看,等待os_unfair_lock锁的线程会处于休眠状态,并非忙等
  • 需要导入头文件#import <os/lock.h>
 os_unfair_lock lock = OS_UNFAIR_LOCK_INIT
 os_unfair_lock_trylock(&lock);//尝试加锁
 os_unfair_lock_lock(& lock);//加锁
 os_unfair_lock_unlock(& lock); //解锁

os_unfair_lock的特点等不到锁就休眠

pthread_mutex
  • pthread开头的是垮平台的
  • mutex叫做”互斥锁”,等待锁的线程会处于休眠状态
  • 需要导入头文件#import <pthread.h>
  pthread_mutex_t mutex = PTHREAD_MUTEX_DEFAULT;
 // #define PTHREAD_MUTEX_ERRORCHECK        1 //排错锁
 // #define PTHREAD_MUTEX_RECURSIVE     2 //递归锁
// #define PTHREAD_MUTEX_DEFAULT         //默认 PTHREAD_MUTEX_NORMAL

    // 初始化属性
    pthread_mutexattr_t attr;
    pthread_mutexattr_init(&attr);
    pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_DEFAULT);
    // 初始化锁
    pthread_mutex_init(mutex, &attr);
    // 销毁属性
    pthread_mutexattr_destroy(&attr);


- (void)ticket
{
    pthread_mutex_lock(&_ticketMutex); //加锁
        执行的代码
     pthread_mutex_unlock(&_ticketMutex); //解锁
}


  • 默认锁只能加一次锁
pthread_mutex 递归锁
  pthread_mutex_t mutex = PTHREAD_MUTEX_RECURSIVE;
  // 初始化属性
    pthread_mutexattr_t attr;
    pthread_mutexattr_init(&attr);
    pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_DEFAULT);
    // 初始化锁
    pthread_mutex_init(mutex, &attr);
    // 销毁属性
    pthread_mutexattr_destroy(&attr);
//比如调用频繁这样调用递归
- (void)otherTest
{
    pthread_mutex_lock(&_mutex);
    
    NSLog(@"%s", __func__);
    
    static int count = 0;
    if (count < 10) {
        count++;
        [self otherTest];
    }
    
    pthread_mutex_unlock(&_mutex);
}
  • 递归锁 允许同一线程可以多次加锁(不同线程 不允许这样就可以保护线程)
NSLock ,NSRecursiveLock
  • NSLock是对mutex普通锁的封装
  • NSRecursiveLock也是对mutex递归锁的封装,API跟NSLock基本一致
@property (strong, nonatomic) NSLock *ticketLock;

- (instancetype)init
{
    if (self = [super init]) {
        self.ticketLock = [[NSLock alloc] init];
    }
    return self;
}

- (void)saleTicket
{
    [self.ticketLock lock];  加锁
    
    加锁的内容代码
    
    [self.ticketLock unlock];  解锁
}
NSCondition
  • NSCondition是对mutex和cond的封装

/**  系统提供的方法*/
- (void)wait;
- (BOOL)waitUntilDate:(NSDate *)limit;
- (void)signal;
- (void)broadcast;

@property (strong, nonatomic) NSCondition *condition;
@property (strong, nonatomic) NSMutableArray *data;
- (instancetype)init
{
    if (self = [super init]) {
        self.condition = [[NSCondition alloc] init];
        self.data = [NSMutableArray array];
    }
    return self;
}

// 生产者-消费者模式

// 线程1
// 删除数组中的元素
- (void)__remove
{
    [self.condition lock];
    NSLog(@"__remove - begin");
    
    if (self.data.count == 0) {
        // 等待
        [self.condition wait];
    }
    
    [self.data removeLastObject];
    NSLog(@"删除了元素");
    
    [self.condition unlock];
}

// 线程2
// 往数组中添加元素
- (void)__add
{
    [self.condition lock];
    
    sleep(1);
    
    [self.data addObject:@"Test"];
    NSLog(@"添加了元素");
    // 信号
    [self.condition signal];
    
    // 广播
//    [self.condition broadcast];
    [self.condition unlock];
    
}

NSConditionLock
  • NSConditionLock是对NSCondition的进一步封装,可以设置具体的条件值
  • 条件锁
/** 系统提供方法 */
- (void)lockWhenCondition:(NSInteger)condition;
- (BOOL)tryLock;
- (BOOL)tryLockWhenCondition:(NSInteger)condition;
- (void)unlockWithCondition:(NSInteger)condition;
- (BOOL)lockBeforeDate:(NSDate *)limit;
- (BOOL)lockWhenCondition:(NSInteger)condition beforeDate:(NSDate *)limit;


现在我想执行__one 执行线程 __two  __three

@property (strong, nonatomic) NSConditionLock *conditionLock;
- (instancetype)init
{
    if (self = [super init]) {
        self.conditionLock = [[NSConditionLock alloc] initWithCondition:1];
    }
    return self;
}

- (void)otherTest
{
    [[[NSThread alloc] initWithTarget:self selector:@selector(__one) object:nil] start];
    
    [[[NSThread alloc] initWithTarget:self selector:@selector(__two) object:nil] start];
    
    [[[NSThread alloc] initWithTarget:self selector:@selector(__three) object:nil] start];
}

- (void)__one
{
    [self.conditionLock lock];
    
    NSLog(@"__one");
    sleep(1);
    
    [self.conditionLock unlockWithCondition:2];
}

- (void)__two
{
    [self.conditionLock lockWhenCondition:2];
    
    NSLog(@"__two");
    sleep(1);
    
    [self.conditionLock unlockWithCondition:3];
}

- (void)__three
{
    [self.conditionLock lockWhenCondition:3];
    
    NSLog(@"__three");
    
    [self.conditionLock unlock];
}

dispatch_queue(DISPATCH_QUEUE_SERIAL)
@property (strong, nonatomic) dispatch_queue_t ticketQueue;

- (instancetype)init
{
    if (self = [super init]) {
        self.ticketQueue = dispatch_queue_create("ticketQueue", DISPATCH_QUEUE_SERIAL);
    }
    return self;
}

- (void)__saleTicket
{
    dispatch_sync(self.ticketQueue, ^{
       代码
    });
}

dispatch_semaphore_t 信号量
  • semaphore叫做”信号量”
  • 信号量的初始值,可以用来控制线程并发访问的最大数量
  • 信号量的初始值为1,代表同时只允许1条线程访问资源,保证线程同步

// 信号量初始值
 int value = 1;
//初始化信号量
@property (strong, nonatomic) dispatch_semaphore_t ticketSemaphore;

- (instancetype)init
{
    if (self = [super init]) {
        self.ticketSemaphore = dispatch_semaphore_create(1);
   }
    return self;
}

// 线程10、7、6、9、8
- (void)test
{
    // 如果信号量的值 > 0,就让信号量的值减1,然后继续往下执行代码
    // 如果信号量的值 <= 0,就会休眠等待,直到信号量的值变成>0,就让信号量的值减1,然后继续往下执行代码
    dispatch_semaphore_wait(self.semaphore, DISPATCH_TIME_FOREVER);
    
    sleep(2);
    NSLog(@"test - %@", [NSThread currentThread]);
    
    // 让信号量的值+1
    dispatch_semaphore_signal(self.semaphore);
}

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