多线程的安全隐患

一块资源可能会被多个线程共享，也就是多个线程可能会访问同一块资源；当多个线程访问同一块资源时，很容易引发数据错乱和数据安全问题### 问题案例
卖票和存钱取钱的两个案例，具体见下面代码

@interface BaseDemo: NSObject

- (void)moneyTest;
- (void)ticketTest;

#pragma mark - 暴露给子类去使用
- (void)__saveMoney;
- (void)__drawMoney;
- (void)__saleTicket;
@end

@interface BaseDemo()
@property (assign, nonatomic) int money;
@property (assign, nonatomic) int ticketsCount;
@end

@implementation BaseDemo

/**
 存钱、取钱演示
 */
- (void)moneyTest
{
    self.money = 100;
    
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 10; i++) {
            [self __saveMoney];
        }
    });
    
    dispatch_async(queue, ^{
        for (int i = 0; i < 10; i++) {
            [self __drawMoney];
        }
    });
}

/**
 存钱
 */
- (void)__saveMoney
{
    int oldMoney = self.money;
    sleep(.2);
    oldMoney += 50;
    self.money = oldMoney;
    
    NSLog(@"存50，还剩%d元 - %@", oldMoney, [NSThread currentThread]);
}

/**
 取钱
 */
- (void)__drawMoney
{
    int oldMoney = self.money;
    sleep(.2);
    oldMoney -= 20;
    self.money = oldMoney;
    
    NSLog(@"取20，还剩%d元 - %@", oldMoney, [NSThread currentThread]);
}

/**
 卖1张票
 */
- (void)__saleTicket
{
    int oldTicketsCount = self.ticketsCount;
    sleep(.2);
    oldTicketsCount--;
    self.ticketsCount = oldTicketsCount;
    NSLog(@"还剩%d张票 - %@", oldTicketsCount, [NSThread currentThread]);
}

/**
 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];
        }
    });
}

@end


@interface ViewController ()
@property (strong, nonatomic) BaseDemo *demo;
@end

@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    
    BaseDemo *demo = [[BaseDemo alloc] init];
    [demo ticketTest];
    [demo moneyTest];
}
@end

解决方案是使用线程同步技术（同步，就是协同步调，按预定的先后次序进行，常见的方案就是加锁

线程同步方案

iOS中的线程同步方案有以下这些

• OSSpinLock- os_unfair_lock- pthread_mutex- dispatch_semaphore- dispatch_queue(DISPATCH_QUEUE_SERIAL)- NSLock- NSRecursiveLock- NSCondition- NSConditionLock- @synchronized#### OSSpinLock
  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资源用OSSpinLock来解决上述示例问题

@interface OSSpinLockDemo: BaseDemo

@end

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

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

@implementation OSSpinLockDemo

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

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

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

- (void)__saleTicket
{
	// 不用属性，用一个静态变量也可以
	static OSSpinLock ticketLock = OS_SPINLOCK_INIT;
    OSSpinLockLock(&ticketLock);
    
    [super __saleTicket];
    
    OSSpinLockUnlock(&ticketLock);
}

@end

static的问题

上面的ticketLock也可以用static来修饰作为内部静态变量来使用

#define	OS_SPINLOCK_INIT    0

由于OS_SPINLOCK_INIT就是0，所以才可以用static来修饰；static只能在编译时赋值一个确定值，不能动态赋予一个函数值

// 这样赋值一个函数返回值是会报错的
static OSSpinLock ticketLock = [NSString stringWithFormat:@"haha"];

OSSpinLock的问题

OSSpinLock现在已经不再安全，可能会出现优先级反转问题

由于多线程的本质是在不同线程之间进行来回的调度，每个线程可能对应分配的资源优先级不同；如果优先级低的线程先进行了加锁并准备执行代码，这时优先级高的线程就会在外面循环等待加锁；但因为其优先级高，所以CPU可能会大量的给其分配任务，那么就没办法处理优先级低的线程；优先级低的线程就无法继续往下执行代码，那么也就没办法解锁，所以又会变成了互相等待的局面，造成死锁。这也是苹果现在废弃了OSSpinLock的原因

解决办法

用尝试加锁OSSpinLockTry来替换OSSpinLockLock，如果没有加锁才会进到判断里执行代码并加锁，避免了因上锁了一直在循环等待的问题

// 用卖票的函数来举例，其他几个加锁的方法也是同样
- (void)__saleTicket
{
    if (OSSpinLockTry(&_ticketLock)) {
        [super __saleTicket];
        
        OSSpinLockUnlock(&_ticketLock);
    }
}

通过汇编来分析

我们通过断点来分析加锁之后做了什么

我们在卖票的加锁代码处打上断点，并通过转汇编的方式一步步调用分析

转成汇编后调用OSSpinLockLock

内部会调用_OSSpinLockLockSlow

核心部分，在_OSSpinLockLockSlow会进行比较，然后执行到断点处又会再次跳回0x7fff5e73326f再次执行代码

所以通过汇编底层执行逻辑，我们能看出OSSpinLock是会不断循环去调用判断的，只有解锁之后才会往下执行代码

锁的等级

OSSpinLock自旋锁是高等级的锁（High-level lock），因为会一直循环调用

os_unfair_lock

苹果现在用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

修改示例代码如下

#import "BaseDemo.h"

@interface OSUnfairLockDemo: BaseDemo

@end

#import "OSUnfairLockDemo.h"
#import <os/lock.h>

@interface OSUnfairLockDemo()

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

@implementation OSUnfairLockDemo

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

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

- (void)__saveMoney
{
    os_unfair_lock_lock(&_moneyLock);
    
    [super __saveMoney];
    
    os_unfair_lock_unlock(&_moneyLock);
}

- (void)__drawMoney
{
    os_unfair_lock_lock(&_moneyLock);
    
    [super __drawMoney];
    
    os_unfair_lock_unlock(&_moneyLock);
}

@end

如果不写os_unfair_lock_unlock，那么所有的线程都会卡在os_unfair_lock_lock进入睡眠，不会再继续执行代码，这种情况叫做死锁

通过汇编来分析

我们也通过断点来分析加锁之后做了什么

首先会调用os_unfair_lock_lock

然后会调用os_unfair_lock_lock_slow

然后在os_unfair_lock_lock_slow中会执行__ulock_wait

核心部分，代码执行到syscall会直接跳出断点，不再执行代码，也就是进入了睡眠

所以通过汇编底层执行逻辑，我们能看出os_unfair_lock一旦进行了加锁，就会直接进入休眠，等待解锁后唤醒再继续执行代码，也由此可以认为os_unfair_lock是互斥锁

syscall的调用可以理解为系统级别的调用进入睡眠，会直接卡住线程，不再执行代码

锁的等级

我们进到os_unfair_lock的头文件lock.h，可以看到注释说明os_unfair_lock是一个低等级的锁（Low-level lock），因为一旦发现加锁后就会自动进入睡眠

pthread_mutex

互斥锁

mutex叫做”互斥锁”，等待锁的线程会处于休眠状态

使用代码如下

@interface MutexDemo: BaseDemo

@end

#import "MutexDemo.h"
#import <pthread.h>

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

@implementation MutexDemo

- (void)__initMutex:(pthread_mutex_t *)mutex
{
    // 静态初始化
    // 需要在定义这个变量时给予值才可以这么写
    //        pthread_mutex_t mutex = PTHREAD_MUTEX_INITIALIZER;
    
//    // 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);
    
    // &attr传NULL默认就是PTHREAD_MUTEX_DEFAULT
    pthread_mutex_init(mutex, NULL);
}

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

- (void)__saleTicket
{
    pthread_mutex_lock(&_ticketMutex);
    
    [super __saleTicket];
    
    pthread_mutex_unlock(&_ticketMutex);
}

- (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)dealloc
{
  
  // 对象销毁时调用
  	pthread_mutex_destroy(&_moneyMutex);
    pthread_mutex_destroy(&_ticketMutex);
}

pthread_mutex_t实际就是pthread_mutex *类型

递归锁

当属性设置为PTHREAD_MUTEX_RECURSIVE时，就可以作为递归锁来使用

递归锁允许同一个线程对一把锁进行重复加锁；多个线程不可以用递归锁

- (void)__initMutex:(pthread_mutex_t *)mutex
{
    // 初始化属性
    pthread_mutexattr_t attr;
    pthread_mutexattr_init(&attr);
    pthread_mutexattr_settype(&attr, PTHREAD_MUTEX_RECURSIVE);
    // 初始化锁
    pthread_mutex_init(mutex, &attr);
}

- (void)otherTest
{
    pthread_mutex_lock(&_mutex);
    
    NSLog(@"%s", __func__);
    
    static int count = 0;
    if (count < 10) {
        count++;
        [self otherTest];
    }
    
    pthread_mutex_unlock(&_mutex);
}

根据条件来进行加锁

我们可以设定一定的条件来选择线程之间的调用进行加锁解锁，示例如下

@interface MutexDemo()
@property (assign, nonatomic) pthread_mutex_t mutex;
@property (assign, nonatomic) pthread_cond_t cond;
@property (strong, nonatomic) NSMutableArray *data;
@end

@implementation MutexDemo

- (instancetype)init
{
    if (self = [super init]) {
        // 初始化锁
        pthread_mutex_init(&_mutex, NULL);        
        // 初始化条件
        pthread_cond_init(&_cond, NULL);
        
        self.data = [NSMutableArray array];
    }
    return self;
}

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

// 线程1
// 删除数组中的元素
- (void)__remove
{
    pthread_mutex_lock(&_mutex);
    NSLog(@"__remove - begin");
    
    // 如果数据为空，那么设置条件等待唤醒
    // 等待期间会先解锁，让其他线程执行代码
    if (self.data.count == 0) {
        pthread_cond_wait(&_cond, &_mutex);
    }
    
    [self.data removeLastObject];
    NSLog(@"删除了元素");
    
    pthread_mutex_unlock(&_mutex);
}

// 线程2
// 往数组中添加元素
- (void)__add
{
    pthread_mutex_lock(&_mutex);
    
    sleep(1);
    
    [self.data addObject:@"Test"];
    NSLog(@"添加了元素");
    
    // 一旦添加了元素，变发送条件信号，让等待删除的条件继续执行代码，并再次加锁
    // 信号（通知一个条件）
    pthread_cond_signal(&_cond);
    // 广播（通知所有条件）
//    pthread_cond_broadcast(&_cond);
    
    pthread_mutex_unlock(&_mutex);
}

- (void)dealloc
{
    pthread_mutex_destroy(&_mutex);
    pthread_cond_destroy(&_cond);
}

@end

通过汇编来分析

我们通过断点来分析加锁之后做了什么

首先会执行pthread_mutex_lock

然后会执行pthread_mutex_firstfit_lock_slow

然后会执行pthread_mutex_firstfit_lock_wait

然后会执行__psynch_mutexwait

核心部分，在__psynch_mutexwait里，代码执行到syscall会直接跳出断点，不再执行代码，也就是进入了睡眠

所以pthread_mutex和os_unfair_lock一样，都是在加锁之后会进入到睡眠

锁的等级

pthread_mutex和os_unfair_lock一样，都是低等级的锁（Low-level lock）

NSLock

NSLock是对mutex普通锁的封装

NSLock遵守了<NSLocking>协议，支持以下两个方法

@protocol NSLocking

- (void)lock;
- (void)unlock;

@end

其他常用方法

// 尝试解锁
- (BOOL)tryLock;

// 设定一个时间等待加锁，时间到了如果还不能成功加锁就返回NO
- (BOOL)lockBeforeDate:(NSDate *)limit;

具体使用看下面代码

@interface NSLockDemo: BaseDemo

@end

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

@implementation NSLockDemo


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

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

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

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

@end

分析底层实现

由于NSLock是不开源的，我们可以通过GNUstep Base来分析具体实现

找到NSLock.m可以看到initialize初始化方法里是创建的pthread_mutex_t对象，所以可以确定NSLock是对pthread_mutex的面向对象的封装

@implementation NSLock

+ (id) allocWithZone: (NSZone*)z
{
  if (self == baseLockClass && YES == traceLocks)
    {
      return class_createInstance(tracedLockClass, 0);
    }
  return class_createInstance(self, 0);
}

+ (void) initialize
{
  static BOOL	beenHere = NO;

  if (beenHere == NO)
    {
      beenHere = YES;

      /* Initialise attributes for the different types of mutex.
       * We do it once, since attributes can be shared between multiple
       * mutexes.
       * If we had a pthread_mutexattr_t instance for each mutex, we would
       * either have to store it as an ivar of our NSLock (or similar), or
       * we would potentially leak instances as we couldn't destroy them
       * when destroying the NSLock.  I don't know if any implementation
       * of pthreads actually allocates memory when you call the
       * pthread_mutexattr_init function, but they are allowed to do so
       * (and deallocate the memory in pthread_mutexattr_destroy).
       */
      pthread_mutexattr_init(&attr_normal);
      pthread_mutexattr_settype(&attr_normal, PTHREAD_MUTEX_NORMAL);
      pthread_mutexattr_init(&attr_reporting);
      pthread_mutexattr_settype(&attr_reporting, PTHREAD_MUTEX_ERRORCHECK);
      pthread_mutexattr_init(&attr_recursive);
      pthread_mutexattr_settype(&attr_recursive, PTHREAD_MUTEX_RECURSIVE);

      /* To emulate OSX behavior, we need to be able both to detect deadlocks
       * (so we can log them), and also hang the thread when one occurs.
       * the simple way to do that is to set up a locked mutex we can
       * force a deadlock on.
       */
      pthread_mutex_init(&deadlock, &attr_normal);
      pthread_mutex_lock(&deadlock);

      baseConditionClass = [NSCondition class];
      baseConditionLockClass = [NSConditionLock class];
      baseLockClass = [NSLock class];
      baseRecursiveLockClass = [NSRecursiveLock class];

      tracedConditionClass = [GSTracedCondition class];
      tracedConditionLockClass = [GSTracedConditionLock class];
      tracedLockClass = [GSTracedLock class];
      tracedRecursiveLockClass = [GSTracedRecursiveLock class];

      untracedConditionClass = [GSUntracedCondition class];
      untracedConditionLockClass = [GSUntracedConditionLock class];
      untracedLockClass = [GSUntracedLock class];
      untracedRecursiveLockClass = [GSUntracedRecursiveLock class];
    }
}

NSRecursiveLock

NSRecursiveLock也是对mutex递归锁的封装，API跟NSLock基本一致

NSCondition

NSCondition是对mutex和cond的封装

具体使用代码如下

@interface NSConditionDemo()
@property (strong, nonatomic) NSCondition *condition;
@property (strong, nonatomic) NSMutableArray *data;
@end

@implementation NSConditionDemo

- (instancetype)init
{
    if (self = [super init]) {
        self.condition = [[NSCondition alloc] init];
        self.data = [NSMutableArray array];
    }
    return self;
}

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

// 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];
    
}
@end
```##### 分析底层实现`NSCondition`也遵守了`NSLocking`协议，说明其内部已经封装了锁的相关代码

@interface NSCondition : NSObject {
@private
void *_priv;
}

• (void)wait;
• (BOOL)waitUntilDate:(NSDate *)limit;
• (void)signal;
• (void)broadcast;

@property (nullable, copy) NSString *name API_AVAILABLE(macos(10.5), ios(2.0), watchos(2.0), tvos(9.0));

@end

我们通过`GNUstep Base`也可以看到其初始化方法里对`pthread_mutex_t`进行了封装

@implementation NSCondition

• (id) allocWithZone: (NSZone*)z
  {
  if (self == baseConditionClass && YES == traceLocks)
  {
  return class_createInstance(tracedConditionClass, 0);
  }
  return class_createInstance(self, 0);
  }
• (void) initialize
  {
  [NSLock class]; // Ensure mutex attributes are set up.
  }

• (id) init
  {
  if (nil != (self = [super init]))
  {
  if (0 != pthread_cond_init(&_condition, NULL))
  {
  DESTROY(self);
  }
  else if (0 != pthread_mutex_init(&_mutex, &attr_reporting))
  {
  pthread_cond_destroy(&_condition);
  DESTROY(self);
  }
  }
  return self;
  }

#### NSConditionLock
`NSConditionLock`是对`NSCondition`的进一步封装，可以设置具体的条件值

通过设置条件值可以对线程做依赖控制执行顺序，具体使用见示例代码

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

@implementation NSConditionLockDemo

• (instancetype)init
  {
  // 创建的时候可以设置一个条件
  // 如果不设置，默认就是0
  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
  {
  // 不需要任何条件，只有没有锁就加锁
  [self.conditionLock lock];
• NSLog(@"__one");
  sleep(1);
• [self.conditionLock unlockWithCondition:2];
  }
• (void)__two
  {
  // 根据对应条件来加锁
  [self.conditionLock lockWhenCondition:2];
• NSLog(@"__two");
  sleep(1);
• [self.conditionLock unlockWithCondition:3];
  }
• (void)__three
  {
  [self.conditionLock lockWhenCondition:3];
• NSLog(@"__three");
• [self.conditionLock unlock];
  }

@end

// 打印的先后顺序为：1、2、3

#### dispatch\_queue\_t 
我们可以直接使用`GCD`的串行队列，也是可以实现线程同步的，具体代码可以参考`GCD`部分的示例代码

#### dispatch_semaphore
`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

示例代码如下

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

@implementation SemaphoreDemo

• (instancetype)init
  {
  if (self = [super init]) {
  // 初始化信号量
  // 最多只开5条线程，也就是可以5条线程同时访问同一块空间，然后加锁，其他线程再进来就只能等待了
  self.semaphore = dispatch_semaphore_create(5);
  // 最多只开1条线程
  self.ticketSemaphore = dispatch_semaphore_create(1);
  self.moneySemaphore = dispatch_semaphore_create(1);
  }
  return self;
  }
• (void)__drawMoney
  {
  dispatch_semaphore_wait(self.moneySemaphore, DISPATCH_TIME_FOREVER);
• [super __drawMoney];
• dispatch_semaphore_signal(self.moneySemaphore);
  }
• (void)__saveMoney
  {
  dispatch_semaphore_wait(self.moneySemaphore, DISPATCH_TIME_FOREVER);
• [super __saveMoney];
• dispatch_semaphore_signal(self.moneySemaphore);
  }
• (void)__saleTicket
  {
  // 如果信号量的值>0就减1，然后往下执行代码
  // 当信号量的值<=0时，当前线程就会进入休眠等待（直到信号量的值>0）
  dispatch_semaphore_wait(self.ticketSemaphore, DISPATCH_TIME_FOREVER);
• [super __saleTicket];
• // 让信号量的值加1
  dispatch_semaphore_signal(self.ticketSemaphore);
  }

@end

#### @synchronized

`@synchronized`是对`mutex`递归锁的封装

示例代码如下

@interface SynchronizedDemo: BaseDemo

@end

@implementation SynchronizedDemo

• (void)__drawMoney
  {
  // @synchronized需要加锁的是同一个对象才行
  @synchronized([self class]) {
  [super __drawMoney];
  }
  }
• (void)__saveMoney
  {
  @synchronized([self class]) { // objc_sync_enter
  [super __saveMoney];
  } // objc_sync_exit
  }
• (void)__saleTicket
  {
  static NSObject *lock;
  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
  lock = [[NSObject alloc] init];
  });
• @synchronized(lock) {
  [super __saleTicket];
  }
  }
  @end

##### 源码分析
我们可以通过程序运行中转汇编看到，最终都会调用`objc_sync_enter`

![](https://img2020.cnblogs.com/blog/2320802/202104/2320802-20210408144446882-975064667.jpg)

我们可以通过`objc4`中`objc-sync.mm`来分析对应的源码实现

int objc_sync_enter(id obj)
{
int result = OBJC_SYNC_SUCCESS;

if (obj) {
    SyncData* data = id2data(obj, ACQUIRE);
    ASSERT(data);
    data->mutex.lock();
} else {
    // @synchronized(nil) does nothing
    if (DebugNilSync) {
        _objc_inform("NIL SYNC DEBUG: @synchronized(nil); set a breakpoint on objc_sync_nil to debug");
    }
    objc_sync_nil();
}

return result;

}

可以看到会根据传进来的`obj`找到对应的`SyncData`

typedef struct alignas(CacheLineSize) SyncData {
struct SyncData* nextData;
DisguisedPtr<objc_object> object;
int32_t threadCount; // number of THREADS using this block
recursive_mutex_t mutex;
} SyncData;

在找到`SyncData`里面的成员变量`recursive_mutex_t`的真实类型，里面有一个递归锁

using recursive_mutex_t = recursive_mutex_tt;

class recursive_mutex_tt : nocopy_t {
os_unfair_recursive_lock mLock; // 递归锁

public:
constexpr recursive_mutex_tt() : mLock(OS_UNFAIR_RECURSIVE_LOCK_INIT) {
lockdebug_remember_recursive_mutex(this);
}

constexpr recursive_mutex_tt(__unused const fork_unsafe_lock_t unsafe)
    : mLock(OS_UNFAIR_RECURSIVE_LOCK_INIT)
{ }

void lock()
{
    lockdebug_recursive_mutex_lock(this);
    os_unfair_recursive_lock_lock(&mLock);
}

void unlock()
{
    lockdebug_recursive_mutex_unlock(this);

    os_unfair_recursive_lock_unlock(&mLock);
}

void forceReset()
{
    lockdebug_recursive_mutex_unlock(this);

    bzero(&mLock, sizeof(mLock));
    mLock = os_unfair_recursive_lock OS_UNFAIR_RECURSIVE_LOCK_INIT;
}

bool tryLock()
{
    if (os_unfair_recursive_lock_trylock(&mLock)) {
        lockdebug_recursive_mutex_lock(this);
        return true;
    }
    return false;
}

bool tryUnlock()
{
    if (os_unfair_recursive_lock_tryunlock4objc(&mLock)) {
        lockdebug_recursive_mutex_unlock(this);
        return true;
    }
    return false;
}

void assertLocked() {
    lockdebug_recursive_mutex_assert_locked(this);
}

void assertUnlocked() {
    lockdebug_recursive_mutex_assert_unlocked(this);
}

};

然后我们分析获取`SyncData`的实现方法`id2data`，通过`obj`从`LIST_FOR_OBJ`真正取出数据

static SyncData* id2data(id object, enum usage why)
{
spinlock_t *lockp = &LOCK_FOR_OBJ(object);
SyncData *listp = &LIST_FOR_OBJ(object);
SyncData result = NULL;

if SUPPORT_DIRECT_THREAD_KEYS

// Check per-thread single-entry fast cache for matching object
bool fastCacheOccupied = NO;
SyncData *data = (SyncData *)tls_get_direct(SYNC_DATA_DIRECT_KEY);
if (data) {
    fastCacheOccupied = YES;

    if (data->object == object) {
        // Found a match in fast cache.
        uintptr_t lockCount;

        result = data;
        lockCount = (uintptr_t)tls_get_direct(SYNC_COUNT_DIRECT_KEY);
        if (result->threadCount <= 0  ||  lockCount <= 0) {
            _objc_fatal("id2data fastcache is buggy");
        }

        switch(why) {
        case ACQUIRE: {
            lockCount++;
            tls_set_direct(SYNC_COUNT_DIRECT_KEY, (void*)lockCount);
            break;
        }
        case RELEASE:
            lockCount--;
            tls_set_direct(SYNC_COUNT_DIRECT_KEY, (void*)lockCount);
            if (lockCount == 0) {
                // remove from fast cache
                tls_set_direct(SYNC_DATA_DIRECT_KEY, NULL);
                // atomic because may collide with concurrent ACQUIRE
                OSAtomicDecrement32Barrier(&result->threadCount);
            }
            break;
        case CHECK:
            // do nothing
            break;
        }

        return result;
    }
}

endif

// Check per-thread cache of already-owned locks for matching object
SyncCache *cache = fetch_cache(NO);
if (cache) {
    unsigned int i;
    for (i = 0; i < cache->used; i++) {
        SyncCacheItem *item = &cache->list[i];
        if (item->data->object != object) continue;

        // Found a match.
        result = item->data;
        if (result->threadCount <= 0  ||  item->lockCount <= 0) {
            _objc_fatal("id2data cache is buggy");
        }
            
        switch(why) {
        case ACQUIRE:
            item->lockCount++;
            break;
        case RELEASE:
            item->lockCount--;
            if (item->lockCount == 0) {
                // remove from per-thread cache
                cache->list[i] = cache->list[--cache->used];
                // atomic because may collide with concurrent ACQUIRE
                OSAtomicDecrement32Barrier(&result->threadCount);
            }
            break;
        case CHECK:
            // do nothing
            break;
        }

        return result;
    }
}

// Thread cache didn't find anything.
// Walk in-use list looking for matching object
// Spinlock prevents multiple threads from creating multiple 
// locks for the same new object.
// We could keep the nodes in some hash table if we find that there are
// more than 20 or so distinct locks active, but we don't do that now.

lockp->lock();

{
    SyncData* p;
    SyncData* firstUnused = NULL;
    for (p = *listp; p != NULL; p = p->nextData) {
        if ( p->object == object ) {
            result = p;
            // atomic because may collide with concurrent RELEASE
            OSAtomicIncrement32Barrier(&result->threadCount);
            goto done;
        }
        if ( (firstUnused == NULL) && (p->threadCount == 0) )
            firstUnused = p;
    }

    // no SyncData currently associated with object
    if ( (why == RELEASE) || (why == CHECK) )
        goto done;

    // an unused one was found, use it
    if ( firstUnused != NULL ) {
        result = firstUnused;
        result->object = (objc_object *)object;
        result->threadCount = 1;
        goto done;
    }
}

// Allocate a new SyncData and add to list.
// XXX allocating memory with a global lock held is bad practice,
// might be worth releasing the lock, allocating, and searching again.
// But since we never free these guys we won't be stuck in allocation very often.
posix_memalign((void **)&result, alignof(SyncData), sizeof(SyncData));
result->object = (objc_object *)object;
result->threadCount = 1;
new (&result->mutex) recursive_mutex_t(fork_unsafe_lock);
result->nextData = *listp;
*listp = result;

done:
lockp->unlock();
if (result) {
// Only new ACQUIRE should get here.
// All RELEASE and CHECK and recursive ACQUIRE are
// handled by the per-thread caches above.
if (why == RELEASE) {
// Probably some thread is incorrectly exiting
// while the object is held by another thread.
return nil;
}
if (why != ACQUIRE) _objc_fatal("id2data is buggy");
if (result->object != object) _objc_fatal("id2data is buggy");

if SUPPORT_DIRECT_THREAD_KEYS

    if (!fastCacheOccupied) {
        // Save in fast thread cache
        tls_set_direct(SYNC_DATA_DIRECT_KEY, result);
        tls_set_direct(SYNC_COUNT_DIRECT_KEY, (void*)1);
    } else 

endif

    {
        // Save in thread cache
        if (!cache) cache = fetch_cache(YES);
        cache->list[cache->used].data = result;
        cache->list[cache->used].lockCount = 1;
        cache->used++;
    }
}

return result;

}

`LIST_FOR_OBJ`是一个哈希表，哈希表的实现就是将传进来的`obj`作为`key`，然后对应的锁为`value`

define LIST_FOR_OBJ(obj) sDataLists[obj].data // 哈希表

static StripedMap sDataLists;
// 哈希表的实现就是将传进来的对象作为key，然后对应的锁为value

通过源码分析我们也可以看出，`@synchronized`内部的锁是递归锁

### 锁的比较
#### 性能比较排序
下面是每个锁按性能从高到低排序

- os\_unfair\_lock- OSSpinLock- dispatch\_semaphore- pthread\_mutex- dispatch\_queue(DISPATCH\_QUEUE\_SERIAL)- NSLock- NSCondition- pthread_mutex(recursive)- NSRecursiveLock- NSConditionLock- @synchronized

选择性最高的锁

- dispatch\_semaphore- pthread\_mutex

#### 互斥锁、自旋锁的比较
##### 什么情况使用自旋锁
- 预计线程等待锁的时间很短- 加锁的代码（临界区）经常被调用，但竞争情况很少发生- CPU资源不紧张- 多核处理器

##### 什么情况使用互斥锁
- 预计线程等待锁的时间较长- 单核处理器（尽量减少CPU的消耗）- 临界区有IO操作（IO操作比较占用CPU资源）- 临界区代码复杂或者循环量大- 临界区竞争非常激烈