What is the memory consistency model in Java concurrent programming?

Java's Memory Consistency Model (MCM) uses the "happen first principle" to ensure consistent access to shared memory in multi-threaded programs. The precedence principle defines the sequential relationship between operations, including program order, locking, volatile variables, final variables, and transitivity. In practice, synchronization methods ensure that writes to shared memory are visible to other threads, but they do not guarantee atomicity.

Memory consistency model in Java concurrent programming

Definition

Memory The consistency model (MCM) defines how shared memory is accessed in multi-threaded programs to ensure that all threads see a consistent state in memory.

Java's MCM

Java uses an MCM called "happens-before", which regulates memory operations between threads relative order. The precedence principle defines the following precedence relationships:

• Program sequence: Operations performed in program order in a thread have precedence relationships.
• Lock: The operation of acquiring the lock has a precedence relationship with the subsequent unlocking operation.
• volatile variables: The write operation to a volatile variable has a precedence relationship with the subsequent read operation to the volatile variable.
• Final variable: The initialization of the final variable has a precedence relationship with the subsequent read operation of the final variable.
• Transitivity: If A happens before B, and B happens before C, then A happens before C.

Practical case

Consider the following code segment:

int x = 0;

public synchronized void incrementX() {
    x++;
}

public int getX() {
    return x;
}

In this example, write the x field The operation occurs in the synchronized method incrementX(). This means that a read operation on the x field (performed in the getX() method) will always see the latest value of x, since synchronization guarantees that any other Threads cannot execute the incrementX() method simultaneously.

Important Notes

• Visibility: precedence-happens-before Ensure that modifications to shared memory by one thread will be visible to other threads visible.
• Atomicity: precedence-happens-before There is no guarantee that operations on shared memory are atomic. If atomic operations are required, mechanisms such as synchronized or atomic variables should be used.

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