Does the JVM create a mutex for every object in order to implement the 'synchronized' keyword? If not, how?

Question

As a C++ programmer becoming more familiar with Java, it's a little odd to me to see language level support for locking on arbitrary objects without any kind of declaration that the object supports such locking. Creating mutexes for every object seems like a heavy cost to be automatically opted into. Besides memory usage, mutexes are an OS limited resource on some platforms. You could spin lock if mutexes aren't available but the performance characteristics of that are significantly different, which I would expect to hurt predictability.

Is the JVM smart enough in all cases to recognize that a particular object will never be the target of the synchronized keyword and thus avoid creating the mutex? The mutexes could be created lazily, but that poses a bootstrapping problem that itself necessitates a mutex, and even if that were worked around I assume there's still going to be some overhead for tracking whether a mutex has already been created or not. So I assume if such an optimization is possible, it must be done at compile time or startup. In C++ such an optimization would not be possible due to the compilation model (you couldn't know if the lock for an object was going to be used across library boundaries), but I don't know enough about Java's compilation and linking to know if the same limitations apply.

Answer 1

Speaking as someone who has looked at the way that some JVMs implement locks ...

The normal approach is to start out with a couple of reserved bits in the object's header word. If the object is never locked, or if it is locked but there is no contention it stays that way. If and when contention occurs on a locked object, the JVM inflates the lock into a full-blown mutex data structure, and it stays that way for the lifetime of the object.

EDIT - I just noticed that the OP was talking about OS-supported mutexes. In the examples that I've looked at, the uninflated mutexes were implemented directly using CAS instructions and the like, rather than using pthread library functions, etc.

Answer 2

This is really an implementation detail of the JVM, and different JVMs may implement it differently. However, it is definitely not something that can be optimized at compile time, since Java links at runtime, and this it is possible for previously unknown code to get a hold of an object created in older code and start synchronizing on it.

Note that in Java lingo, the synchronization primitive is called "monitor" rather than mutex, and it is supported by special bytecode operations. There's a rather detailed explanation here.

Answer 3

You can never be sure that an object will never be used as a lock (consider reflection). Typically every object has a header with some bits dedicated to the lock. It is possible to implement it such that the header is only added as needed, but that gets a bit complicated and you probably need some header anyway (class (equivalent of "vtbl" and allocation size in C++), hash code and garbage collection).

Here's a wiki page on the implementation of synchronisation in the OpenJDK.

(In my opinion, adding a lock to every object was a mistake.)

Answer 4

can't JVM use compare-and-swap instruction directly? let's say each object has a field lockingThreadId storing the id of the thread that is locking it,

while( compare_and_swap (obj.lockingThreadId, null, thisThreadId) != thisTheadId )
    // failed, someone else got it
    mark this thread as waiting on obj.
    shelf this thead

//out of loop. now this thread locked the object

do the work

obj.lockingThreadId = null;
wake up threads waiting on the obj

this is a toy model, but it doesn't seem too expensive, and does no rely on OS.