Mercurial > hg > blitz_condensed
annotate src/org/dancres/blitz/txnlock/LockCache.java @ 0:3dc0c5604566
Initial checkin of blitz 2.0 fcs - no installer yet.
author | Dan Creswell <dan.creswell@gmail.com> |
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date | Sat, 21 Mar 2009 11:00:06 +0000 |
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Initial checkin of blitz 2.0 fcs - no installer yet.
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1 package org.dancres.blitz.txnlock; |
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2 |
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3 import java.util.HashMap; |
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4 |
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5 import java.lang.ref.ReferenceQueue; |
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6 import java.lang.ref.WeakReference; |
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7 |
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8 import org.dancres.blitz.oid.OID; |
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9 |
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10 class LockCache { |
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11 private HashMap theLocks = new HashMap(); |
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12 |
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13 private ReferenceQueue theDeadLocks = new ReferenceQueue(); |
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14 |
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15 LockCache() { |
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16 } |
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17 |
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18 TxnLock getOrInsert(OID aOID) { |
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19 cleanQueue(); |
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20 |
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21 synchronized(this) { |
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22 TxnLock myLock = get(aOID); |
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23 |
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24 if (myLock == null) { |
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25 myLock = new TxnLock(); |
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26 put(aOID, myLock); |
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27 } |
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28 |
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29 return myLock; |
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30 } |
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31 } |
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32 |
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33 TxnLock get(OID aOID) { |
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34 cleanQueue(); |
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35 |
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36 synchronized(this) { |
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37 LockHolder myHolder = (LockHolder) theLocks.get(aOID); |
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38 |
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39 return (myHolder == null) ? null : (TxnLock) myHolder.get(); |
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40 } |
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41 } |
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42 |
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43 void put(OID aOID, TxnLock aLock) { |
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44 cleanQueue(); |
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45 |
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46 synchronized(this) { |
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47 LockHolder myHolder = new LockHolder(aOID, aLock, theDeadLocks); |
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48 |
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49 theLocks.put(aOID, myHolder); |
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50 } |
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51 } |
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52 |
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53 private void cleanQueue() { |
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54 LockHolder myRef; |
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55 |
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56 while ((myRef = (LockHolder) theDeadLocks.poll()) != null) { |
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57 synchronized(this) { |
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58 LockHolder myOther = (LockHolder) theLocks.remove(myRef.getOID()); |
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59 |
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60 /* |
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61 Check that the reference we're releasing is the same as the |
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62 one we currently have in the table. Otherwise: |
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63 |
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64 It could be that get(OID) was called above and the holder was |
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65 recovered but it's reference had been cleared resulting in |
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66 allocation of a new lock BEFORE we've processed the reference |
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67 from the queue. Thus we allocate the new lock, the old |
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68 reference (from get(OID)) is enqueued and we then delete the |
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69 new lock - oops! |
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70 */ |
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71 if ((myOther != null) &&(! myOther.equals(myRef))) { |
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72 theLocks.put(myOther.getOID(), myOther); |
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73 } |
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74 } |
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75 } |
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76 } |
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77 |
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78 private class LockHolder extends WeakReference { |
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79 private OID theOID; |
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80 |
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81 LockHolder(OID aOID, TxnLock aLock, ReferenceQueue aQueue) { |
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82 super(aLock, aQueue); |
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83 theOID = aOID; |
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84 } |
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85 |
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86 OID getOID() { |
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87 return theOID; |
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88 } |
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90 } |