多線程-鎖

dispatch_sync:立馬在當(dāng)前線程同步執(zhí)行任務(wù)。
dispatch_async:不要求立馬在當(dāng)前線程同步執(zhí)行任務(wù)。
使用sync函數(shù)往當(dāng)前串行隊(duì)列中添加任務(wù)，會(huì)卡主當(dāng)前的串行隊(duì)列。

  • 死鎖：使用sync函數(shù)往當(dāng)前串行隊(duì)列中添加任務(wù)，會(huì)卡主當(dāng)前的串行隊(duì)列
  • 多線程安全隱患

1、一塊資源可能會(huì)被多個(gè)線程共享，也就是多個(gè)線程可能會(huì)訪問(wèn)同一塊資源。比如多個(gè)線程訪問(wèn)同一個(gè)對(duì)象、同一個(gè)變量、同一個(gè)文件
2、當(dāng)多個(gè)線程訪問(wèn)同一塊資源時(shí)，很容易引發(fā)數(shù)據(jù)錯(cuò)亂和數(shù)據(jù)安全問(wèn)題

?? 解決方案：使用線程同步技術(shù)

線程同步，就是協(xié)調(diào)步調(diào)，按照預(yù)定的先后次序進(jìn)行運(yùn)行。常見(jiàn)的線程同步技術(shù)：加鎖。

  • 遞歸鎖：允許同一個(gè)線程對(duì)一把鎖進(jìn)行重復(fù)加鎖

  • 1、OSSpinLock叫做”自旋鎖”，等待鎖的線程會(huì)處于忙等（busy-wait）狀態(tài)，一直占用著CPU資源
    目前已經(jīng)不再安全，可能會(huì)出現(xiàn)優(yōu)先級(jí)反轉(zhuǎn)問(wèn)題：CPU分配時(shí)間給優(yōu)先級(jí)高的
    如果等待鎖的線 程優(yōu)先級(jí)較高，它會(huì)一直占用CPU資源，優(yōu)先級(jí)低的線程就無(wú)法釋放鎖
    需要導(dǎo)入頭文件#import <libkern/OSAtomic.h>

#import "OSSpinLockDemo.h"
#import <libkern/OSAtomic.h>

@interface OSSpinLockDemo ()
@property(nonatomic, assign) OSSpinLock moneyLock;
@property(nonatomic, assign) OSSpinLock ticketLock;
@end

@implementation OSSpinLockDemo
- (instancetype)init{
    if (self = [super init]) {
        self.moneyLock = OS_SPINLOCK_INIT;
        self.ticketLock = OS_SPINLOCK_INIT;
    }
    return self;
}

- (void)__saveMoney{
    OSSpinLockLock(&_moneyLock);
    [super __saveMoney];
    OSSpinLockUnlock(&_moneyLock);
}

- (void)__drawMoney{
    OSSpinLockLock(&_moneyLock);
    [super __drawMoney];
    OSSpinLockUnlock(&_moneyLock);
}

- (void)__saleTicket{
    OSSpinLockLock(&_moneyLock);
    [super __saleTicket];
    OSSpinLockUnlock(&_moneyLock);
}
@end
  • 2、os_unfair_lock用于取代不安全的OSSpinLock ，從iOS10開(kāi)始才支持
    從底層調(diào)用看，等待os_unfair_lock鎖的線程會(huì)處于休眠狀態(tài)，并非忙等
    需要導(dǎo)入頭文件#import <os/lock.h>
#import "OSUnfairLockDemo.h"
#import <libkern/OSAtomic.h>
#import <os/lock.h>

@interface OSUnfairLockDemo ()
@property (nonatomic,  assign) os_unfair_lock moneyLock;
@property (nonatomic,  assign) os_unfair_lock ticketLock;
@end

@implementation OSUnfairLockDemo

- (instancetype)init{
    if (self == [super init]) {
        self.moneyLock = OS_UNFAIR_LOCK_INIT;
    }
    return self;
}
- (void)__saveMoney{
    os_unfair_lock_lock(&_moneyLock);
    [super __saveMoney];
    os_unfair_lock_unlock(&_moneyLock);
}

- (void)__saleTicket{
    os_unfair_lock_lock(&_ticketLock);
    [super __saleTicket];
    os_unfair_lock_unlock(&_ticketLock);
}

- (void)__drawMoney{
    os_unfair_lock_lock(&_ticketLock);
    [super __drawMoney];
    os_unfair_lock_unlock(&_ticketLock);
}
@end
  • 3、mutex叫做”互斥鎖”，等待鎖的線程會(huì)處于休眠狀態(tài)
    需要導(dǎo)入頭文件#import <pthread.h>
#import "MutexDemo.h"
#import <pthread.h>

@interface MutexDemo ()
@property (nonatomic, assign) pthread_mutex_t moneyMutex;
@property (nonatomic, assign) pthread_mutex_t ticketMutex;
@end

@implementation MutexDemo

- (void)__initMutex:(pthread_mutex_t *)mutex{
    pthread_mutexattr_t attr;
    pthread_mutexattr_init(&attr);
    pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_DEFAULT);
    pthread_mutex_init(mutex, &attr);
    pthread_mutexattr_destroy(&attr);
}

- (instancetype)init{
    if (self = [super init]) {
//        self.mutex = PTHREAD_MUTEX_INITIALIZER;
        [self __initMutex:&_moneyMutex];
        [self __initMutex:&_ticketMutex];
    }
    return self;
}

- (void)__saveMoney{
    pthread_mutex_lock(&_moneyMutex);
    [super __saveMoney];
    pthread_mutex_unlock(&_moneyMutex);
}

- (void)__drawMoney{
    pthread_mutex_lock(&_moneyMutex);
    [super __drawMoney];
    pthread_mutex_unlock(&_moneyMutex);
}

- (void)__saleTicket{
    pthread_mutex_lock(&_ticketMutex);
    [super __saleTicket];
    pthread_mutex_unlock(&_ticketMutex);
}
@end
  • 4、NSLock是對(duì)mutex普通鎖的封裝。
#import "NSLockDemo.h"
#import <pthread.h>

@interface NSLockDemo ()
@property (nonatomic, strong) NSLock *moneyLock;
@property (nonatomic, strong) NSLock *ticketLock;
@end

@implementation NSLockDemo


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

- (void)otherTest{
    NSLog(@"%s",__func__);
    [self otherTest];
}

- (void)otherTest2{

}

- (void)__saveMoney{
    [self.moneyLock lock];
    [super __saveMoney];
    [self.moneyLock unlock];
}

- (void)__drawMoney{
    [self.ticketLock lock];
    [super __drawMoney];
    [self.ticketLock unlock];
}

- (void)__saleTicket{
    [self.ticketLock lock];
    [super __saleTicket];
    [self.ticketLock unlock];
}
@end
  • 5、NSRecursiveLock也是對(duì)mutex遞歸鎖的封裝，API跟NSLock基本一致
  • 6、NSCondition是對(duì)mutex和cond的封裝
#import "NSConditionDemo.h"
#import <pthread.h>

@interface NSConditionDemo ()
@property (nonatomic, strong) NSCondition *conditionLock;
@property (nonatomic, strong) NSMutableArray *dataArray;
@end

@implementation NSConditionDemo

- (NSMutableArray *)dataArray{
    if (!_dataArray) {
        _dataArray = [NSMutableArray array];
    }
    return _dataArray;
}

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

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

- (void)__add{
    [self.conditionLock lock];
    [self.dataArray addObject:@"test"];
    NSLog(@"添加了元素");
    
    [self.conditionLock unlock];
}

- (void)__remove{
    [self.conditionLock lock];
    if (self.dataArray.count == 0) {
        [self.conditionLock wait];
    }
    [self.dataArray removeLastObject];
    NSLog(@"刪除了元素");
    [self.conditionLock signal];
    [self.conditionLock unlock];
}
@end
  • 7、NSConditionLock是對(duì)NSCondition的進(jìn)一步封裝，可以設(shè)置具體的條件值
#import "NSConditionLockDemo.h"
#import <pthread.h>

@interface NSConditionLockDemo ()
@property (nonatomic, strong) NSConditionLock *conditionLock;
@end

@implementation NSConditionLockDemo

- (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];
}

- (void)__one{
    [self.conditionLock lockWhenCondition:1];
    NSLog(@"添加了元素");
    [self.conditionLock unlockWithCondition:2];
}

- (void)__two{
    [self.conditionLock lockWhenCondition:2];
    NSLog(@"刪除了元素");
    [self.conditionLock unlock];
}
@end
  • 8、semaphore叫做”信號(hào)量”
    信號(hào)量的初始值，可以用來(lái)控制線程并發(fā)訪問(wèn)的最大數(shù)量
    信號(hào)量的初始值為1，代表同時(shí)只允許1條線程訪問(wèn)資源，保證線程同步
#import "SemaphoreDemo.h"

@interface SemaphoreDemo ()
@property (nonatomic, strong) dispatch_semaphore_t  semaphore;
@end

@implementation SemaphoreDemo

- (instancetype)init{
    if (self = [super init]) {
        self.semaphore = dispatch_semaphore_create(2);
    }
    return self;
}

- (void)otherTest{
    for (int i=0; i<20; i++) {
        [[[NSThread alloc] initWithTarget:self selector:@selector(test) object:nil] start];
    }
}

- (void)test{
    dispatch_semaphore_wait(self.semaphore, DISPATCH_TIME_FOREVER);
    sleep(2);
    NSLog(@"%@",[NSThread currentThread]);
    dispatch_semaphore_signal(self.semaphore);
}
@end
  • 9、直接使用GCD的串行隊(duì)列，也是可以實(shí)現(xiàn)線程同步的
#import "SerialQueueDemo.h"
#import <pthread.h>

@interface SerialQueueDemo ()
@property (nonatomic, strong) dispatch_queue_t ticketQueue;
@property (nonatomic, strong) dispatch_queue_t moneyQueue;
@end

@implementation SerialQueueDemo

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

- (void)__saveMoney{
    dispatch_sync(self.ticketQueue, ^{
        [super __saveMoney];
    });
}

- (void)__drawMoney{
    dispatch_sync(self.ticketQueue, ^{
        [super __drawMoney];
    });
}

- (void)__saleTicket{
    dispatch_sync(self.ticketQueue, ^{
        [super __saleTicket];
    });
}
@end
  • 10、@synchronized是對(duì)mutex遞歸鎖的封裝
    源碼查看：objc4中的objc-sync.mm文件
    @synchronized(obj)內(nèi)部會(huì)生成obj對(duì)應(yīng)的遞歸鎖，然后進(jìn)行加鎖、解鎖操作
#import "SynchronizedDemo.h"

@implementation SynchronizedDemo
- (void)__saveMoney{
    @synchronized (self) {        
        [super __saveMoney];
    }
}

- (void)__drawMoney{
    @synchronized (self) {
        [super __drawMoney];
    }
}

- (void)__saleTicket{
    @synchronized (self) {
        [super __saleTicket];
    }
}
@end

  • 11、pthread_rwlock：讀寫(xiě)鎖
    等待鎖的線程會(huì)進(jìn)入休眠
    ?? dispatch_barrier_async：異步柵欄調(diào)用
    這個(gè)函數(shù)傳入的并發(fā)隊(duì)列必須是自己通過(guò)dispatch_queue_cretate創(chuàng)建的
    如果傳入的是一個(gè)串行或是一個(gè)全局的并發(fā)隊(duì)列，那這個(gè)函數(shù)便等同于
    dispatch_async函數(shù)的效果

  • atomic用于保證屬性setter、getter的原子性操作，相當(dāng)于在getter和setter內(nèi)部加了線程同步的鎖
    可以參考源碼objc4的objc-accessors.mm
    它并不能保證使用屬性的過(guò)程是線程安全的

Demo地址

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