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What problem does volatile solve?
The semantics of volatile are not just visibility
Home Java javaTutorial Understanding Java Volatile Keyword Semantics

Understanding Java Volatile Keyword Semantics

Jul 19, 2025 am 04:34 AM

The volatile keyword solves variable visibility and directive reordering problems in Java multithreading. 1. It ensures that all threads can be seen immediately after the variable is modified, and avoids threads from using cached old values; 2. It prevents the compiler and processor from reordering the operations involving volatile variables to ensure the order of operations; 3. It is suitable for scenarios such as status flags, one-time safe release, independent variable assignment, etc. without atomicity; 4. Unlike synchronized, the volatile lock-free mechanism does not guarantee the atomicity of composite operations, but is lighter and more efficient.

Understanding Java Volatile Keyword Semantics

In Java, volatile keyword is often used in multi-threaded environments to ensure the "visibility" of variables. Simply put, when a variable is declared as volatile , the JVM ensures that all threads can see the latest value of the variable instead of the old value in their respective caches. This is very useful in some concurrent scenarios, but it does much more than that.

Understanding Java Volatile Keyword Semantics

What problem does volatile solve?

In a multithreaded program, each thread may cache the variable in its own working memory (such as the CPU cache) rather than reading it from the main memory every time. This may lead to a problem: one thread modifies the value of the variable, and other threads cannot see the change.

For example:

Understanding Java Volatile Keyword Semantics
 boolean flag = true;

// Thread 1:
while (flag) {
    // do something
}

// Thread 2:
flag = false;

If flag is not declared as volatile , thread 1 may never see flag being changed false , because it has been using the value in its own cache. After adding volatile , you can force each access to read from the main memory, thereby avoiding this problem.


The semantics of volatile are not just visibility

In addition to ensuring the visibility of variables, volatile also prevents instructions from reordering. Java compilers and processors may reorder the code in order to optimize performance, as long as it does not affect the single-threaded execution results. But in multithreaded environments, this reordering can lead to unexpected behavior.

Understanding Java Volatile Keyword Semantics

When using a volatile variable, neither the compiler nor the runtime reorders the operations involving the variable. That is to say:

  • All operations before writing a volatile variable will be completed before that write;
  • All operations after reading a volatile variable will be executed only after the read is completed.

This is equivalent to adding a lightweight memory barrier.


Applicable scenarios for using volatile

Although volatile is useful, it does not replace locks (such as synchronized or ReentrantLock ). It is suitable for the following situations:

  • Status flag : For example, flag mentioned above is used to control whether the loop continues.
  • One-time safe release : for example, object references that will no longer be changed after initialization.
  • Operation of independent variables : If the read and write of the variable itself is atomic and does not depend on the current value (such as direct assignment rather than self-increment).

But if your operation involves multiple steps, such as read-modify-write (such as count ), then volatile is not enough, and a stricter synchronization mechanism is needed at this time.


The difference between synchronized

  • synchronized guarantees atomicity and visibility, while volatile only guarantees visibility and orderliness.
  • synchronized blocks the thread until the lock is acquired, and volatile does not cause thread blocking.
  • synchronized is used on code blocks or methods, and volatile is used on variables.

So, if you just need to ensure that the read and write of a certain variable is "latest" and there are no complex composite operations, then volatile is a lightweight option.


Basically that's it. Understanding the semantics of volatile helps write more efficient and safer concurrent code. But remember that it is not omnipotent, and you must be clear about what you are doing when using it.

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