Class LockControl

java.lang.Object
org.apache.derby.impl.services.locks.LockControl
All Implemented Interfaces:
Control
final class LockControl extends Object implements Control
A LockControl contains a reference to the item being locked and doubly linked lists for the granted locks and the waiting locks.

MT - Mutable - Container object : single thread required

  • Field Details

    • ref

      private final Lockable ref

    • firstGrant

      private Lock firstGrant
      This lock control uses an optimistic locking scheme. When the first lock on an object is granted it simply sets firstGrant to be that object, removing the need to allocate a list if no other locks are granted before the first lock is release. If a second lock is granted then a list is allocated and the firstGrant lock is moved into the list. Once a list has been created it is always used.

    • granted

      private List<Lock> granted

    • waiting

      private List<Lock> waiting

    • lastPossibleSkip

      private Lock lastPossibleSkip

  • Constructor Details

    • LockControl

      protected LockControl(Lock firstLock, Lockable ref)

    • LockControl

      private LockControl(LockControl copyFrom)

  • Method Details

    • getLockControl

      public LockControl getLockControl()
      Specified by:
      getLockControl in interface Control

    • isEmpty

      public boolean isEmpty()
      Specified by:
      isEmpty in interface Control

    • grant

      void grant(Lock lockItem)
      Grant this lock.

    • unlock

      public boolean unlock(Latch lockInGroup, int unlockCount)
      Specified by:
      unlock in interface Control

    • isGrantable

      public boolean isGrantable(boolean noWaitersBeforeMe, CompatibilitySpace compatibilitySpace, Object qualifier)
      This routine can be called to see if a lock currently on the wait list could be granted. If this lock has waiters ahead of it then we do not jump over the waiter(s) even if we can be granted. This avoids the first waiter being starved out.
      Specified by:
      isGrantable in interface Control

    • addLock

      public Lock addLock(LockTable ls, CompatibilitySpace compatibilitySpace, Object qualifier)
      Add a lock into this control, granted it if possible. This can be entered in several states.
    • The Lockable is locked (granted queue not empty), and there are no waiters (waiting queue is empty)
    • The Lockable is locked and there are waiters
    • The Lockable is locked and there are waiters and the first is potentially granted
    • The Lockable is unlocked and there are waiters and the first is potentially granted. Logically the item is still locked, it's just that the lock has just been released and the first waker has not woken up yet. This call is never entered when the object is unlocked and there are no waiters. 1) The Lockable has just been unlocked,

    • isUnlocked

      protected boolean isUnlocked()

    • firstWaiter

      public ActiveLock firstWaiter()
      Return the first lock in the wait line, null if the line is empty.
      Specified by:
      firstWaiter in interface Control

    • getNextWaiter

      ActiveLock getNextWaiter(ActiveLock item, boolean remove, LockTable ls)
      Get the next waiting lock (if any).

    • getLockable

      public Lockable getLockable()
      Return the lockable object controlled by me.
      Specified by:
      getLockable in interface Control

    • getFirstGrant

      public Lock getFirstGrant()
      Specified by:
      getFirstGrant in interface Control

    • getGranted

      public List<Lock> getGranted()
      Specified by:
      getGranted in interface Control

    • getWaiting

      public List<Lock> getWaiting()
      Specified by:
      getWaiting in interface Control

    • giveUpWait

      protected void giveUpWait(Object item, LockTable ls)
      Give up waiting up on a lock

    • addWaiters

      public void addWaiters(Map<Object,Object> waiters)
      Add the waiters of this lock into this Map object.
      Each waiting thread gets two entries in the hashtable
      1. key=compatibility space - value=ActiveLock
      2. key=ActiveLock - value={LockControl for first waiter|ActiveLock of previosue waiter}
      Specified by:
      addWaiters in interface Control

    • getGrants

      List<Lock> getGrants()
      Return a Stack of the held locks (Lock objects) on this Lockable.

    • getLock

      public final Lock getLock(CompatibilitySpace compatibilitySpace, Object qualifier)
      Find a granted lock matching this space and qualifier
      Specified by:
      getLock in interface Control

    • blockedByParent

      public boolean blockedByParent(Lock childLock)

      Returns true if the childLock is blocked because its parent owns a conficting lock. This code was written to support the fix to DERBY-6554. The only known way that this condition arises is when a write attempt by a nested user transaction is blocked by a read lock held by the main transaction. This only happens while trying to write to SYS.SYSSEQUENCES while managing sequence generators.

    • shallowClone

      public Control shallowClone()
      make a shallow clone of myself
      Specified by:
      shallowClone in interface Control

    • addWaiter

      private void addWaiter(Lock lockItem, LockTable ls)
      Add a lock request to a list of waiters.
      Parameters:
      lockItem - The lock request
      ls - The lock table

    • popFrontWaiter

      private Object popFrontWaiter(LockTable ls)
      Remove and return the first lock request from a list of waiters.
      Parameters:
      ls - The lock table
      Returns:
      The removed lock request

    • removeWaiter

      private Object removeWaiter(int index, LockTable ls)
      Remove and return the lock request at the given index from a list of waiters.
      Parameters:
      index - The index at which to remove the lock request
      ls - The lock table
      Returns:
      The removed lock request

    • removeWaiter

      private int removeWaiter(Object item, LockTable ls)
      Remove and return the given lock request from a list of waiters.
      Parameters:
      item - The item to remove
      ls - The lock table
      Returns:
      The number of items removed