19) Java concurrency, 19java concurrency
> Synchronized
Synchronized keyword,
It includes two usage methods: synchronized Method and synchronized Block.
1> synchronized Method: Declare the synchronized Method by adding the synchronized keyword to the method declaration. 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 as 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. However, when a thread accesses a synchronized (this) synchronization code block of the object, the other thread can still access the non-synchronized (this) synchronization code block of the object.
3. When a thread accesses a synchronized (this) synchronization code block of the object, other threads perform synchronization on all other synchronized (this) access to the synchronization code block will be blocked.
