How to implement the memory model and instruction reordering of Java's underlying technology

How to implement memory model and instruction reordering of Java underlying technology

Overview:
In Java underlying technology, memory model and instruction reordering are two Important concept. The memory model controls how shared variables are accessed, while instruction reordering affects the order in which instructions are executed in the program. This article will introduce the basic principles of Java memory model and instruction reordering, and give specific code examples.

1. Memory model:
   Java Memory Model (JMM) defines the behavior rules when multiple threads concurrently access shared data. By using a memory model, we can guarantee data visibility, atomicity, and ordering across multiple threads.

The main concepts in the Java memory model are:

• Main Memory: A memory area shared by all threads that stores the values ​​of shared variables.
• Working Memory: A memory area exclusive to each thread that stores a copy of shared variables.

The rules of the Java memory model are as follows:

• All operations by threads on shared variables must be performed in working memory, rather than directly on main memory.
• Threads cannot directly access each other's working memory, and communication between threads must be completed through main memory.

Code example:

public class MemoryModelDemo {
    private static volatile boolean flag = false;

    public static void main(String[] args) {
        new Thread(() -> {
            while (!flag) {
                // do something
            }
            System.out.println("Thread 1: flag is true");
        }).start();

        new Thread(() -> {
            try {
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
            flag = true;
            System.out.println("Thread 2: flag is changed to true");
        }).start();
    }
}

In the above example, we implement threads through a shared variable flag modified by a volatile modifier communication between. Among them, the first thread continuously checks whether flag is true, and if it is true, it outputs the corresponding information; and the second thread waits for 1 second. Set flag to true. By using the volatile keyword, we ensure the visibility of flag, that is, thread 1 can see the modification of flag by thread 2 in time.

2. Instruction reordering:
   Instruction reordering is an optimization technique in which the compiler or processor reorders the instruction sequence in order to improve program performance. In a single-threaded environment, instruction reordering will not affect the running results of the program. However, in a multi-threaded environment, the execution order of instructions may change due to instruction reordering, thus affecting the correctness of the program.

Instruction reordering in Java is mainly divided into the following three types:

• Compiler reordering: The compiler reorders instructions during the compilation phase.
• Processor reordering: The processor reorders instructions during the execution phase.
• Memory reordering: The memory system reorders read/write operations.

In order to avoid problems caused by instruction reordering, Java provides some keywords to prohibit or limit instruction reordering:

• volatile: Modified shared variables prohibit reordering, ensuring that the read and write operations of variables are sequential.
• synchronized: For the locked code block, it is guaranteed that the instructions inside it will not be reordered with instructions outside the lock code.
• final: Once the modified variable is initialized, it is not allowed to be modified again.

Code example:

public class ReorderingDemo {
    private static int x = 0;
    private static int y = 0;
    private static volatile boolean flag = false;

    public static void main(String[] args) throws InterruptedException {
        new Thread(() -> {
            x = 1;
            flag = true;
        }).start();

        new Thread(() -> {
            if (flag) {
                y = x;
            }
            System.out.println("y = " + y);
        }).start();
    }
}

In the above example, we use the volatile keyword to prohibit the reordering of flag. In the main thread, we start two sub-threads, the first of which sets x to 1 and flag to true. The second sub-thread checks flag, and if it is true, y is assigned the value of x. Due to the use of the volatile keyword, we ensure that all threads' read and write operations on flag are sequential, thus avoiding problems caused by instruction reordering.

Conclusion:
Through the introduction of this article, we have understood the concepts and principles of the memory model and instruction reordering of Java's underlying technology, and given specific code examples. In multithreaded programming, understanding these concepts and principles is very important to write efficient and correct programs. At the same time, we also learned how to use the volatile keyword to implement communication between multiple threads and prohibit instruction reordering.

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