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Why does volatile exist?
I have never used it but I wonder why people use it? What does it exactly do? I searched the forum, I found it only C# or Java topics.
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The volatile keyword is intended to prevent the compiler from applying any optimizations on objects that can change in ways that cannot be determined by the compiler. Objects declared as volatile are omitted from optimization because their values can be changed by code outside the scope of current code at any time.
Yes, volatile must be used whenever you want a mutable variable to be accessed by multiple threads. It is not very common usecase because typically you need to perform more than a single atomic operation (e.g. check the variable state before modifying it), in which case you would use a synchronized block instead.
The volatile modifier is used to let the JVM understand that a thread accessing the variable should always merge its own personal copy of the variable with the original in the memory. Accessing a volatile variable synchronizes all the cached copy of the variables in the main memory.
The volatile keyword flushes the changes directly to the main memory instead of the CPU cache. On the other hand, the transient keyword is used during serialization. Fields that are marked as transient can not be part of the serialization and deserialization.
Consider this code,
int some_int = 100; while(some_int == 100) { //your code } When this program gets compiled, the compiler may optimize this code, if it finds that the program never ever makes any attempt to change the value of some_int, so it may be tempted to optimize the while loop by changing it from while(some_int == 100) to something which is equivalent to while(true) so that the execution could be fast (since the condition in while loop appears to be true always). (if the compiler doesn't optimize it, then it has to fetch the value of some_int and compare it with 100, in each iteration which obviously is a little bit slow.)
However, sometimes, optimization (of some parts of your program) may be undesirable, because it may be that someone else is changing the value of some_int from outside the program which compiler is not aware of, since it can't see it; but it's how you've designed it. In that case, compiler's optimization would not produce the desired result!
So, to ensure the desired result, you need to somehow stop the compiler from optimizing the while loop. That is where the volatile keyword plays its role. All you need to do is this,
volatile int some_int = 100; //note the 'volatile' qualifier now! In other words, I would explain this as follows:
volatile tells the compiler that,
"Hey compiler, I'm volatile and, you know, I can be changed by some XYZ that you're not even aware of. That XYZ could be anything. Maybe some alien outside this planet called program. Maybe some lightning, some form of interrupt, volcanoes, etc can mutate me. Maybe. You never know who is going to change me! So O you ignorant, stop playing an all-knowing god, and don't dare touch the code where I'm present. Okay?"
Well, that is how volatile prevents the compiler from optimizing code. Now search the web to see some sample examples.
Quoting from the C++ Standard ($7.1.5.1/8)
[..] volatile is a hint to the implementation to avoid aggressive optimization involving the object because the value of the object might be changed by means undetectable by an implementation.[...]
Related topic:
Does making a struct volatile make all its members volatile?
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In computer programming, particularly in the C, C++, and C# programming languages, a variable or object declared with the volatile keyword usually has special properties related to optimization and/or threading. Generally speaking, the volatile keyword is intended to prevent the (pseudo)compiler from applying any optimizations on the code that assume values of variables cannot change "on their own." (c) Wikipedia
http://en.wikipedia.org/wiki/Volatile_variable
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