A practical guide to the Java memory model: How to avoid common pitfalls in concurrent programming

• Visibility: Threads can only see their own modifications to shared variables, while modifications to shared variables by other threads require some kind of synchronization mechanism to be visible. arrive.
• Atomicity: An operation is either completely executed or not executed at all, with no intermediate state.
• Ordering: Thread operations on shared variables must be performed in a certain order, even in different threads.

2. happens-before principle

php editor Xigua has carefully written a practical guide to help readers avoid common pitfalls in Java concurrent programming. The article introduces the Java memory model in a simple and easy-to-understand manner, discusses how to effectively solve problems that may be encountered in concurrent programming, and provides readers with practical tips and suggestions. Whether you are a beginner or an experienced developer, this guide will provide you with valuable reference and guidance to help you better meet the challenges of concurrent programming.

Common situations for the happens-before principle include:

• Sequential consistency: If an operation A is executed after another operation B, then A happens-before B.
• Tube lock: If a thread obtains a tube lock, then the thread's unlocking of the tube lock happens-before other threads' unlocking of the tube lock Get operations.
• volatile variables: Write operations to a volatile variable happen-before read operations to the volatile variable.

3. Avoid Common traps in concurrent programming

Understanding the basic concepts of JMM, you can avoid Concurrencycommon pitfalls in programming. These traps include:

• Memory visibility issues: Because threads can only see their own modifications to shared variables, memory visibility issues may occur. For example, if one thread modifies a shared variable and another thread reads the shared variable at the same time, the other thread may read the value before modification. To solve this problem, you can use volatile variables, synchronization mechanisms, or atomic operations to ensure memory visibility.
• Atomicity issues: Atomicity issues can occur because an operation is either completely executed or not executed at all. For example, if one thread is incrementing a shared variable and another thread is decrementing the shared variable at the same time, the value of the shared variable may become incorrect. To solve this problem, atomic operations or synchronization mechanisms can be used to ensure atomicity.
• Ordering issues: Since thread operations on shared variables must be performed in a certain order, ordering issues may occur. For example, if one thread is modifying a shared variable and another thread reads the shared variable at the same time, the other thread may read the modified value before the modification operation is actually completed. To solve this problem, a synchronization mechanism can be used to ensure orderliness.

IV. Summary

Java memory model is a set of rules for Java virtual machine to execute multi-threaded programs. It ensures threadsecurity## by stipulating access rules for shared memory between threads. #Consistency with the program. Understanding the JMM is critical to writing correct concurrent programs. This article uses practical cases to help you deeply understand JMM and master the skills to avoid concurrent programming pitfalls, so as to write more reliable and efficient concurrent programs.

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