Java Memory Model and Ordering: Uncovering Instruction Reordering Behavior in Multithreaded Programming

---

1. Java Memory Model (JMM)

The article brought by php editor Banana will delve into the Java memory model and orderliness, and reveal the instruction reordering behavior in multi-threaded programming. In multi-threaded programming, instruction reordering may lead to unexpected results in the program. Understanding the Java memory model and ordering is crucial to avoid these problems. This article will explain in detail the principles and effects of instruction reordering to help readers better understand the hidden dangers and solutions in multi-threaded programming.

2. Orderliness

JMM defines the execution order of instructions in the program. Orderliness means that the order of execution of instructions in a program is consistent with the order of the source code of the program. JMM guarantees ordering of the following types:

• Program orderliness: The execution order of instructions in the program is consistent with the order of the source code of the program.
• Statement orderliness: The execution order of the instructions in the statement is consistent with the source code order of the statement.
• Synchronization orderliness: The execution order of instructions in a synchronized block or method is consistent with the source code order of the synchronized block or method.

3. Command reorderingSort

In order to improve performance, the processor may reorder the order of instruction execution. This reordering does not change the final results of the program, but may cause the multithreaded program to behave differently than expected.

Instruction reordering may cause the following problems:

• Visibility problem: Thread A writes to a shared variable, but thread B does not see this write operation.
• Atomicity problem: Thread A performed an atomic operation on a shared variable, but the operation result seen by thread B was not atomic.

4. How to avoid instruction reordering issues

To avoid instruction reordering issues, you can use the following methods:

• Use the volatile keyword: The volatile keyword prevents instructions from reordering access to shared variables.
• Use the synchronized keyword: The synchronized keyword can force threads to execute code blocks in order.
• Use atomic operations: Atomic operations can ensure that operations on shared variables are atomic.

5. Demo code

The following code demonstrates the problems that may result from instruction reordering:

public class ReorderingDemo {

private static int x = 0;
private static int y = 0;

public static void main(String[] args) throws InterruptedException {
Thread thread1 = new Thread(() -> {
x = 1;
y = 1;
});

Thread thread2 = new Thread(() -> {
if (y == 1) {
System.out.println("x is " + x);
}
});

thread1.start();
thread2.start();
thread1.join();
thread2.join();
}
}

In this code, thread 1 first sets the values ​​​​of x and y to 1, then thread 2 checks whether the value of y is 1, and if so, prints the value of x. If the processor reorders the instructions in thread 1, thread 2 may see that y has a value of 1 before x is set to 1, thus printing 0.

6 Conclusion

The Java memory model defines visibility and atomicity between variables in multi-threaded programming. Orderliness means that the order of execution of instructions in a program is consistent with the order of the source code of the program. Instruction reordering can cause multi-threaded programs to behave differently than expected. To avoid instruction reordering issues, you can use the volatile keyword, synchronized keyword, and atomic operations.

The above is the detailed content of Java Memory Model and Ordering: Uncovering Instruction Reordering Behavior in Multithreaded Programming. For more information, please follow other related articles on the PHP Chinese website!