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This article mainly explains "what are the three elements of concurrent programming". Interested friends may wish to take a look. The method introduced in this paper is simple, fast and practical. Let Xiaobian take you to learn "What are the three elements of concurrent programming"!

Three Elements of Concurrent Programming

** Atomicity:** A particle that cannot be divided. Atomicity means that one or more operations either all succeed or all fail.

Order: The order in which programs are executed in the order in which they are executed. (The processor may reorder instructions)

Visibility: Changes made to shared variables by one thread are immediately visible to another thread.

I. Atomicity

Thread switching causes atomicity problems

int i = 1; //atomic operation i++; //non-atomic operation, read i from main memory to thread working memory, perform +1, and write i to main memory.

Although reading and writing are atomic operations, they are not atomic operations together. We call this "compound operation."

We can use synchronized or Lock to "turn" this compound operation into an atomic operation.

Examples:

//use synchronizedprivate synchronized void increase(){ i++; }//use Lockprivate int i = 0; Lock mLock = new ReentrantLock(); private void increase() { mLock.lock(); try { i++; } finally{ mLock.unlock(); } }

So we can think of this method as a whole, as an indivisible whole.

In addition to this, we can also use the atomic variable class in java.util.concurrent.atomic to ensure that all operations accessing the counter state are atomic.

Examples:

AtomicInteger mAtomicInteger = new AtomicInteger(0); private void increase(){ mAtomicInteger.incrementAndGet(); } II. Visibility

Caching causes visibility problems

int v = 0;//Thread A executes v++; //Thread B executes System.out.print("v=" + v);

Even if thread B is executed after i++ in thread B is executed, there are two cases where the input result of thread B is 0 and 1.

Because i++ has completed the operation in thread A (CPU-1) and has not been updated to main memory immediately, thread B (CPU-2) reads and prints from main memory, and the print is 0.

Disabling caching guarantees visibility, and the volatile keyword disables caching

Synchronized and Lock guarantee visibility.

III. Orderliness

The reason for orderliness is compilation optimization

We all know that the processor will automatically optimize and sort the code we write in order to have better computing efficiency, but it will ensure that the execution results will not change.

Examples:

int a = 0; //statement 1int b = 0; //statement 2i++; //statement 3b++; //statement 4

The execution order of this piece of code is likely not to be executed in the order of 1, 2, 3, and 4 above, because 1 and 2 have no data dependencies, and 3 and 4 have no data dependencies. Can 2, 1, 4, and 3 be executed in this way? No problem at all, the processor will sort us automatically.

There is nothing wrong with single-threading, but it is easy to cause problems in multithreading.

Another example:

//Thread 1init(); initialized = true; //Thread 2while(initialized){ work();}

init(); and inited = true; There is no data dependency, and from a single-threaded point of view, it seems that there is no problem if the code of two sentences is reversed.

But at this time in a multithreaded environment, and the processor really re-order these two sentences of code, then the problem arises, if thread 1 first executes inited = true; at this time, init() is not executed, thread 2 has already started calling the work() method, this time is likely to cause some crashes or other BUG appearance.

Synchronized and Lock ensure atomicity, allowing multiple threads to execute code in sequence, naturally in order.

The volatile keyword also solves this problem. The volatile keyword guarantees order so that the processor does not sort this line of code optimally.

At this point, I believe that everyone has a deeper understanding of "what are the three elements of concurrent programming". Let's actually operate it! Here is the website, more related content can enter the relevant channels for inquiry, pay attention to us, continue to learn!

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