This question is in reference to memory visibility only, not happens-before and happens-after. There are four ways in Java that guarantees changes to memory in one thread to be made visible to another thread. (reference http://gee.cs.oswego.edu/dl/cpj/jmm.html)
According to Java Concurrency in Practice, the bible on such questions:
The visibility effects of volatile variables extend beyond the value of the volatile variable itself. When thread A writes to a volatile variable and subsequently thread B reads that same variable, the values of all variables that were visible to A prior to writing to the volatile variable become visible to B after readin the volatile variable.
Does this mean that the JVM actually keeps track of volatile variable reads and writes, in order to know how to flush memory from A to B and not A to C? So A writes to the variable, and later C reads from the variable, and then later B reads from the variable, the flushing is done on a per-thread basis between A and B and A and C, but not B and C? Or, does it imply that all cached memory is flushed, regardless of threads? Are only the volatile variables flushed, or all cached memory?
For the synchronized
keyword flushing, it says that only memory updated inside the lock is guaranteed to be published to other threads. That implies that in the following code, two threads running method()
, leaving the synchronized block will flush staticVar2
to the other thread, but not staticVar1
, is that correct?
Also, in method2()
, synchronizing over differentLock
can cause happens-before happens-after problems if another thread is executing method()
. However, the question is in terms of visibility. If thread A executes method
, then later thread B executes method2()
, is the value of staticVar2
published from A to B, even though the two threads don't synchronize over the same lock?
static int staticVar1, staticVar2;
void method() {
staticVar1++;
synchronized (lock) {
staticVar2++;
}
}
void method2() {
synchronized (differentLock) {
staticVar2++;
}
}
It appears to me that if staticVar1
is never updated to other threads, then all static variables in any program require a volatile
declaration, or should only be accessed in synchronized
blocks. That seems rather harsh, but is it correct? I've sure seen a whole lot of static variables in my time that aren't synchronized.
For Java, “volatile” tells the compiler that the value of a variable must never be cached as its value may change outside of the scope of the program itself.
If you write volatile variable from multiple threads without using any synchronized constructs, you are bound to get data inconsistency errors.
Marking a variable as volatile basically tells all threads to do read and write operations on main memory only. synchronized tells every thread to go update their value from main memory when they enter the block, and flush the result back to main memory when they exit the block.
Volatile keyword is used to modify the value of a variable by different threads. It is also used to make classes thread safe. It means that multiple threads can use a method and instance of the classes at the same time without any problem.
There is no scope limitation in terms of memory. When you have a read or write barrier it applies for all memory reads/writes.
Where I have seen a limitation is in memory mappings. When you memory map a file you have to be careful how you make this available to another threads as this new memory mapping might not be visible in another thread immediately resulting in a BUS error (and a crash of the JVM) This appears to be a OS bug as the newest versions of Linux and Windows don't appear to have this problem.
That implies that in the following code, two threads running method(), leaving the synchronized block will flush staticVar2 to the other thread, but not staticVar1, is that correct?
statixVar1 will always be flushed when staticVar2 is, perhaps sooner. No guarantee as to when, but the order is guaranteed.
If thread A executes method, then later thread B executes method2(), is the value of staticVar2 published from A to B, even though the two threads don't synchronize over the same lock?
Yes, the lock used doesn't matter for the happen-before guarantees.
Do volatile read-writes flush all memory to all threads, or only between the two accessing threads? Whichever the answer, is all memory flushed or only the volatile variables?
All dirty memory is flushed on a write barrier and all reads will be order consistent on a read barrier. volatile
performs both a write barrier on a write and a read barrier on a read.
Is all changed memory flushed when exiting a synchronized block, or just the memory that was changed within the block?
All of the memory changed by that thread.
Do all static variables accessed by two threads have to be synchronized?
Only if one thread modifies the variable. Any number of threads can read static values without synchronization.
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