CODEWITHSUNDEEP
javajakarta-eethread-safety
VJS
VJS

Reputation: 2951

Java Threading Model

I have a question on normal java threading theory.

Thread A has taken a lock on some object, let say xObj. Now how does Thread B know that Thread A has already taken a lock on object xObj.

How this has been implemented in java.

Thanks in advance.

Thanks for your answers....Here i would like to clear that...I don't have to implement this.My concern is How Java has implemented that.How Thread B will get to know that someone has already taken a lock on Object.May be Object class or some other class has implemented this.

Upvotes: 1

Views: 1118

Answers (4)

Amit Deshpande
Amit Deshpande

Reputation: 19185

You have below options

  1. Thread.holdsLock(Object object) in this case you will need thread instance of B in A.
  2. ReentrantLock.getOwner(), ReentrantLock.hasQueuedThread(Thread thread)

But there is no need that B should know that A has the lock , once lock is released it will be available for B to use if there aren't any threads waiting on it.

Upvotes: 1

Stephen C
Stephen C

Reputation: 718826

Thread A has taken a lock on some object, let say xObj. Now how does Thread B know that Thread A has already taken a lock on object xObj.

It depends on the kind of lock you are talking about.

  • For a primitive mutex, one thread cannot test if another thread holds the mutex, let alone find out which thread it is. (It can test if holds the lock however ...)

  • The Lock interface similarly doesn't support this.

  • The ReentrantLock class does provide a method to find this out: see ReentrantLock.getOwner(). Note however that this is a protected method, so you'd need to create a subclass of ReentrantLock if you wanted to make the method generally available.

I would also question the value of such a method. It can only tell you which (if any) thread owns the lock at the instant that the call was made. An instant later, the owner could have changed.

By constrast, Thread.holdsLock() gives you information that you can rely on ... albeit that it is information that isn't useful under most circumstances.

Upvotes: 1

UmNyobe
UmNyobe

Reputation: 22890

Which lock are you talking about? If it is a ReentrantLock for instance, you can just call 

 lock.getOwner()

Edit:

synchronized(xObj) { }

Is not a lock as you are implying. It is conceptually closer to a monitor lock. You have limited functionality using synchronized blocks, and that's exactly why classes like ReentrantLock exists.From the doc

A reentrant mutual exclusion Lock with the same basic behavior and semantics as the implicit monitor lock accessed using synchronized methods and statements, but with extended capabilities.

Upvotes: 0

Kumar Vivek Mitra
Kumar Vivek Mitra

Reputation: 33534

- When a thread gets the lock of an object, then all the synchronized methods can only be accessed through that thread and none, till it leaves the lock.

- If Thread A has the lock of an object and Thread B tries to access the lock on the same object, it can't access it. As the lock is with A, its Not important for Thread B ,that lock is with whom, but is the lock available. If not Thread B moves into blocked state till the lock is released by Thread A.

Upvotes: 0

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