What behavior does the 'volatile' keyword in Java ensure?

The volatile keyword in Java guarantees that reads and writes to a variable are directly from main memory, not from the CPU cache or registers. This ensures visibility of changes to the variable across threads: if one thread modifies a volatile variable, other threads immediately see the new value.

volatile also prevents instruction reordering by the compiler and processor concerning that variable, which is important for correct synchronization.

Example:

volatile boolean flag = false;

// Thread 1
flag = true; // change is immediately visible to other threads

// Thread 2
if (flag) {
    // will definitely see the updated value of flag
}

However, volatile does not guarantee atomicity of complex operations (like increment), for which other synchronization mechanisms are needed.