Introduction

The Java Memory Model (JMM) is a fundamental but often misunderstood aspect of concurrent programming in Java. Introduced with Java 5, the JMM defines how threads interact with memory, ensuring consistency and predictability in multi-threaded programs. In this article, we'll dive into the depths of JMM, explore its key concepts, and examine how it affects concurrent Java application development.

JMM Fundamental Concepts

1. Visibility

Visibility concerns ensuring that a change made by one thread is visible to other threads. Without proper mechanisms, a thread can hide its changes from other threads indefinitely due to compiler or CPU optimizations.

2. Scheduling

Scheduling refers to the order in which instructions are executed. The JMM allows certain reorderings for performance reasons, but also guarantees certain orders to maintain program semantics.

3. Atomicity

Atomicity guarantees that an operation is performed in a single indivisible step, without possible interference from other threads.

Key mechanisms of the JMM

1. Happens-Before Relationship

This is the basis of the JMM. If an action A "happens-before" an action B, then the effects of A are guaranteed to be visible to B. This relationship is transitive and forms the basis of synchronization in Java.

2. Volatile

The volatile keyword ensures that changes are visible between threads. A read of a volatile variable will always see the last write performed to that variable.

3. Synchronized

The synchronized blocks and methods establish happens-before relationships between threads that acquire and release the same monitor.

4. Final

Properly initialized final fields are guaranteed to be visible to all threads without additional synchronization.

Practical implications

1. Double-Checked Locking

The double-checked locking pattern was broken before Java 5 due to visibility issues. The JMM fixed this issue, allowing its correct use with volatile.

class Singleton {
    private static volatile Singleton instance;

    public static Singleton getInstance() {
        if (instance == null) {
            synchronized (Singleton.class) {
                if (instance == null) {
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

2. Publishing objects

Safe object publishing is crucial to avoid partial visibility issues. The JMM guarantees that if an object is correctly published (for example, via a volatile field or a thread-safe class), all of its fields will be visible.

3. Reorganization of instructions

The JMM allows certain reorganizations which may surprise developers.
For example:

int a, b;
a = 1;
b = 2;

Can be rearranged to:

int a, b;
b = 2;
a = 1;

Unless these instructions are surrounded by appropriate timing barriers.

Conclusion

The Java Memory Model is a crucial aspect of concurrent programming in Java. Although complex, understanding it is essential for writing correct and efficient concurrent code. By mastering the concepts of visibility, scheduling, and atomicity, as well as mechanisms like happens-before, volatile, and synchronized, developers can create robust and efficient multi-threaded applications.
However, it is important to note that even with a good understanding of JMM, concurrent programming remains a challenge. Using high-level abstractions like those provided by the java.util.concurrent package can often simplify development while still taking advantage of JMM guarantees.