@synchronized的使用

参考文档：
[https://developer.apple.com/library/content/documentation/Cocoa/Conceptual/Multithreading/ThreadSafety/ThreadSafety.html#//apple_ref/doc/uid/10000057i-CH8-SW1]
官方文档：

The @synchronized directive is a convenient way to create mutex locks
on the fly in Objective-C code. The @synchronized directive does what
any other mutex lock would do—it prevents different threads from
acquiring the same lock at the same time. In this case, however, you
do not have to create the mutex or lock object directly. Instead, you
simply use any Objective-C object as a lock token, as shown in the
following example:
@synchronized可以非常方便的创建互斥锁，@synchronized和其他所是一样的。@synchronized可以阻止多线程在同一时间获得相同的锁。
- (void)myMethod:(id)anObj
{
@synchronized(anObj)
{
// Everything between the braces is protected by the @synchronized directive.
}
}
The object passed to the @synchronized directive is a unique identifier used to distinguish the protected block. If you execute the preceding method in two different threads, passing a different object for the anObj parameter on each thread, each would take its lock and continue processing without being blocked by the other. If you pass the same object in both cases, however, one of the threads would acquire the lock first and the other would block until the first thread completed the critical section.
这个对象是唯一的标识符来保护这个代码块，如果你在两个线程执行这个方法，传入不同的对象，那么每个对象将会保留自己的锁继续执行，不会被另一个对象堵塞。如果你在两个两种情况下传递相同的对象，那么一个线程将会首先获得锁，另一个线程将会阻塞直到对个线程完成这个代码块。
As a precautionary measure, the @synchronized block implicitly adds an exception handler to the protected code. This handler automatically releases the mutex in the event that an exception is thrown. This means that in order to use the @synchronized directive, you must also enable Objective-C exception handling in your code. If you do not want the additional overhead caused by the implicit exception handler, you should consider using the lock classes.
以为一个预防措施，@synchronized代码块隐式的添加了异常处理来保护代码块，handler自动释放互斥锁，当异常抛出的时候。这意味着为了直接使用@synchronized，你必须可以自己捕获异常。如果你不想要@synchronized带来的额外开销，你需要使用锁对象。

从上边文档可以得出以下结论：
1.锁对象要比@synchronized效率高，因为其他锁对象没有异常处理机制。
2.@synchronized锁定的是单个对象对应的代码块。

扩展：
atomic属性锁定的一个对象的访问，包括读和写。@synchronized锁定的是代码块。这也是我们有时候必须使用@synchronized的原因。
因为：
如果我们在一个代码块中有多个原子属性的读写，而这几个原子属性如果是一体的（例如：一个属性是用户的名，一个属性对应的是姓，那么在多线程中就可能出现姓和名对应不上），那么可能就会出现数据错乱。这时候就必须使用@synchronized对代码块进行锁定，保证代码块内同时只有一个线程进行访问。