Cocoa thread synchronization synchronized

Synchronized keyword

It indicates that this method is locked. It is equivalent to checking whether thread a is using thread B (or c d) every time it runs into this method ), if yes, wait until thread B (or C D) that is using this method runs this method and then run thread A. If no, directly run this method in two ways: synchronized Method and synchronized block.

1. Synchronized Method:

The synchronized method is declared by adding the synchronized keyword to the method declaration. For example:

Public synchronized void accessval (INT newval );

The synchronized method controls access to class member variables: each class instance corresponds to a lock, and each synchronized method must obtain the lock of the class instance that calls the method before execution, otherwise the thread is blocked, once the method is executed, the lock is exclusive until the lock is released when the method is returned. Then, the blocked thread can obtain the lock and re-enter the executable status. This mechanism ensures that, at the same time, only one of all the member functions declared as synchronized is in the executable state (because at most only one member function can obtain the lock corresponding to this type of instance ), this effectively avoids access conflicts between class member variables (as long as all methods that may be member variables of the category class are declared
Synchronized ).

In Java, not only are class instances, but each class also corresponds to a lock. In this way, we can declare the static member function of the class as synchronized, to control its access to static member variables of the class.

Synchronized: if a large method is declared as synchronized, the efficiency will be greatly affected. Typically, if you declare the thread-class method run () as synchronized, since the thread is always running throughout its life cycle, it will never succeed in calling any Synchronized Method of this class. Of course, we can put the code of the member variable of the category class into a special method, declare it as synchronized, and call it in the main method to solve this problem, but Java provides us with a better solution, that is, the synchronized block.

2. Synchronized block:

Use the synchronized keyword to declare the synchronized block. Syntax:

Synchronized (syncobject ){

// Code that allows access control

}

The synchronized block is a code block in which the Code must obtain the lock of the object syncobject (as described earlier, it can be a class instance or class) before execution. The specific mechanism is described earlier. It is highly flexible because it can target any code block and can specify any locked object.

Some Understanding of synchronized (this)

1. When two concurrent threads access the synchronized (this) synchronization code block of the same object, only one thread can be executed within a time period. The other thread must wait until the current thread finishes executing this code block before executing this code block.

2. When a thread accesses a synchronized (this) synchronization code block of the object, access by other threads to all other synchronized (this) synchronization code blocks of the object will be blocked.

3. However, when a thread accesses a synchronized (this) synchronization code block of an object, the other thread can still access parts of the object except the synchronized (this) synchronization code block.

4. The third example also applies to other synchronous code blocks. That is to say, when a thread accesses a synchronized (this) synchronization code block of an object, it obtains the object lock of this object. As a result, access by other threads to all the synchronized code parts of the object is temporarily blocked.

5. The above rules apply to other Object locks.