Mercurial > hg > blitz_stable

annotate src/org/dancres/blitz/txnlock/LockCache.java @ 32:243c74d599bf

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author Dan Creswell <dan.creswell@gmail.com>
date Sat, 12 Jun 2010 10:41:47 +0100
parents 3dc0c5604566
children
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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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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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89 }
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90 }