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db4o |

# ExtObjectContainer.setSemaphore Method

attempts to set a semaphore.

bool setSemaphore(
string name,
int waitForAvailability
);

#### Parameters

name
  the name of the semaphore to be set
waitForAvailability
  the time in milliseconds to wait for other transactions to release the semaphore. The parameter may be zero, if the method is to return immediately.

#### Return Value

boolean flag

    true

, if the semaphore could be set or if the calling transaction already owned the semaphore.

    false

, if the semaphore is owned by another transaction.

#### Remarks

attempts to set a semaphore.
Semaphores are transient multi-purpose named flags for ObjectContainers .
A transaction that successfully sets a semaphore becomes the owner of the semaphore. Semaphores can only be owned by a single transaction at one point in time.
This method returns true, if the transaction already owned the semaphore before the method call or if it successfully acquires ownership of the semaphore.
The waitForAvailability parameter allows to specify a time in milliseconds to wait for other transactions to release the semaphore, in case the semaphore is already owned by another transaction.
Semaphores are released by the first occurence of one of the following:
- the transaction releases the semaphore with releaseSemaphore
- the transaction is closed with com.db4o.ObjectContainer.close
- C/S only: the corresponding com.db4o.ObjectServer is closed.
- C/S only: the client com.db4o.ObjectContainer looses the connection and is timed out.
Semaphores are set immediately. They are independant of calling com.db4o.ObjectContainer.commit or com.db4o.ObjectContainer.rollback .
Possible usecases for semaphores:
- prevent other clients from inserting a singleton at the same time. A suggested name for the semaphore: "SINGLETON_" + Object#getClass().getName().
- lock objects. A suggested name: "LOCK_" + getID
- generate a unique client ID. A suggested name: "CLIENT_" + System.currentTimeMillis().

#### See Also

ExtObjectContainer Interface | com.db4o.ext Namespace