Java Concurrency: Volatile vs. Synchronized: When to Use Which?

Difference between volatile and synchronized in Java

Synchronization and volatile are two memory synchronization mechanisms in Java that play crucial roles in multi-threaded programming. While they both serve to control access to shared resources, they have distinct characteristics and use cases.

Volatile:

A volatile variable guarantees that any changes made to it are immediately visible to all threads. Volatile operates at the level of memory visibility, preventing the compiler and runtime from caching or reordering volatile accesses. This ensures that every read of a volatile variable always reflects the most up-to-date value from shared memory.

Synchronized:

Unlike volatile, synchronized provides an exclusive lock on a code block, ensuring that only one thread can execute that block at any given time. Synchronization prevents multiple threads from simultaneously modifying shared variables, ensuring atomic access to critical sections.

"Read-Update-Write" Explanation:

The term "read-update-write" refers to a scenario where a variable's value is read, updated, and then written back. In multi-threaded contexts, this operation must be atomic to maintain data consistency. Volatile variables cannot guarantee atomicity, as they only ensure memory visibility. Synchronized blocks, on the other hand, guarantee atomicity for read-update-write operations.

When to Use Volatile:

Volatile variables are suitable when memory visibility is crucial but synchronization is not required. For instance, variables that are frequently modified and accessed by multiple threads without concurrent updates. Volatile can prevent issues related to caching and reordering of memory accesses.

When to Use Synchronized:

Synchronized blocks are used when mutual exclusion and atomicity are essential. They are ideal for protecting critical sections of code or shared variables that require exclusive access for modifications.

Usage Scenario for Volatile and Synchronized:

Consider the example of a variable called "render" that is read in a rendering loop and set by a keypress event. Since the value of "render" is constantly changing, it is unsuitable for use with synchronized blocks due to excessive overhead. Volatile would be a suitable option in this case, as it ensures memory visibility without blocking threads.

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