The principle of volatile in java

The volatile keyword forces variables to be visible across multiple threads and prohibits instruction rearrangement optimization. Its principles include: Visibility: The modified value is immediately visible to all threads and flushed to main memory through the memory barrier. Disable reordering: A memory barrier prevents the compiler and CPU from reordering statements containing volatile variables, ensuring execution in program order.

The principle of volatile keyword in Java

What is volatile keyword?

The volatile keyword is a Java language modifier that declares a variable to be visible across multiple threads and disables instruction reordering optimization.

The principle of volatile keyword

When a variable is declared volatile, it will have the following characteristics:

• Visibility: The modified value of a volatile variable is immediately visible to all threads.
• Reordering prohibited: The compiler and CPU cannot perform reordering optimization on statements containing volatile variables.

Visibility

volatile ensures visibility by inserting a memory barrier at the variable's memory location. When a thread modifies the value of a volatile variable, the memory barrier forces the modified value to be flushed to main memory. Other threads can then get the updated value from main memory.

Disable reordering

Compilers and CPUs often optimize code, such as instruction reordering. This optimization can cause unexpected behavior in multi-threaded programs. The volatile keyword disables this optimization by inserting a memory barrier, ensuring that statements containing volatile variables are executed in program order.

Usage scenarios

volatile keyword is usually used in the following scenarios:

• Multi-thread shared variables:When When multiple threads access the same variable, use the volatile keyword to ensure that the value of the variable is consistent among all threads.
• Interrupt handler: Interrupt handler code usually executes in parallel with the main thread. The volatile keyword can be used to ensure that variables modified in the interrupt handler are visible to the main thread.
• Memory mapped I/O: The volatile keyword can be used to ensure that modifications to memory mapped I/O registers are immediately visible to the hardware.

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