Lockable

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org.apache.derby.iapi.services.locks
Interface Lockable

All Known Implementing Classes:: ShExLockable

public interface Lockable

Any object that needs to be locked must implement Lockable. This allows a generic lock manager that can have locking policies defined on a per-object basis. A request to lock the object takes a qualifier, this qualifier may be used the object to implement a complex locking policy, e.g. traditional database shared, update and exclusive locks.

The lock manager uses this ordered protocol to determine if a lock request on a Lockable L with qualifier Q1 in compatiblity space CS1 can be granted:

If no locks are held on L in any compatability space then the request is granted.
If L.requestCompatible(Q1) returns true then the lock is granted.
Otherwise the request is granted if the following expression evaluates to true for every other lock { CSn, Qn} held on L
- ```
    ( ( CSn == CS1 ) && L.lockerAlwaysCompatible() ) 
```
- ```
 || (L.reqestCompatible(Q1, Qn)) 
```

If the request is granted then a call is made to L.lockEvent(CS1, Q1) .
When the lock is released a call is made to L.unlockEvent(CS1, Q1) .

The lock manager uses equals() and hashCode() to identify unique Lockables.
If the class implementing Lockable requires that each instance of class correspond to a different locked object then the equals() method must test equality via the reference equality test (==), this is the default behaviour for equality.
If the class implementing Lockable require that each instance of the class that has the same value (as defined by the class) corresponds to a locked object then its equals() method must reflect that, e.g. by testing equality of its fields. In this case the first Lockable to be locked will be kept by lock manager as the key for the lock. Thus even after the first caller unlocks the obejct, its reference will still be kept by the lock manager. Thus Lockable's that per value equality must be designed so that they are never re-used for different lockable concepts.
In either case the equals() method must accept a reference to an object of a different type.
As per standard hashtable rules the value returned by hashCode() must be in sync with the equals() method.
MT - Mutable - : single thread required, synchronization is provided by the lock manager. If the class implementing Lockable uses value equality then it must have an immutable identity.

Method Summary

boolean lockAttributes(int flag, java.util.Hashtable attributes)
          If this lockable object wants to participate in a diagnostic virtual lock table, then put any relavent attributes of this lock into the attributes list (the attribute must be an immutable object).

boolean lockerAlwaysCompatible()
          Returns true if any lock request on a Lockable L in a compatibility space CS1 is compatible with any other lock held on L in CS1.

void lockEvent(Latch lockInfo)
          Note the fact the object is locked.

boolean requestCompatible(java.lang.Object requestedQualifier, java.lang.Object grantedQualifier)
          Return true if the requested qualifier is compatible with the already granted qualifier.

void unlockEvent(Latch lockInfo)
          Note that the object has been unlocked

Method Detail

lockEvent

public void lockEvent(Latch lockInfo)

Note the fact the object is locked. Performs required actions to ensure that unlockEvent() work correctly. This method does not actually perform any locking of the object, the locking mechanism is provided by the lock manager.

If the class supports multiple lockers of the object then this method will be called once per locker, each with their own qualifier.

Must only be called by the lock manager. Synchronization will be handled by the lock manager.

requestCompatible

public boolean requestCompatible(java.lang.Object requestedQualifier,
                                 java.lang.Object grantedQualifier)

Return true if the requested qualifier is compatible with the already granted qualifier.

lockerAlwaysCompatible

public boolean lockerAlwaysCompatible()

Returns true if any lock request on a Lockable L in a compatibility space CS1 is compatible with any other lock held on L in CS1.

unlockEvent

public void unlockEvent(Latch lockInfo)

Note that the object has been unlocked