ProxyObjectGate


1   /* ProxyObjectGate.java
2   */
3   
4   package org.ozoneDB.core.DbRemote;
5   
6   import java.util.HashMap  ;
7   import java.util.Iterator  ;
8   
9   import org.ozoneDB.OzoneProxy;
10  import org.ozoneDB.core.GarbageCollector;
11  import org.ozoneDB.core.ObjectID;
12  
13  /**
14      A gate for Proxy objects. Every Proxy which leaves the Database should wander
15      through a ProxyObjectGate to be registered (so that the objects referenced by such a Proxy
16      are known to be referenced and thus reachable).
17  
18      Every Proxy which left the Database, was registered and is now known to be unreachable should
19      sign itself off exactly one time.
20  
21      @author <A HREF="http://www.medium.net/">Medium.net</A>
22  */
23  public class ProxyObjectGate {
24      /**
25          This is a Mapping from {@link ObjectID} to Integer. It represents the
26          count of references this client holds to the database object represented by the objectID.
27  
28          Entries have to be added when returning {@link OzoneProxy}s directly or indirectly.
29          Entries have to be removed if a {#link OzoneProxy} is finalize()d on the client side.
30          This way, the ozoneDB always know which objects are referenced and thus have to be
31          considered reachable, even if they were not reachable internally.
32      */
33      protected   HashMap               objectsReferencesByClient;
34  
35      protected   GarbageCollector    garbageCollectorToBeNotifiedOfExportedReferences;
36  
37      /**
38          This is an Integer which represents the number "1".
39      */
40      protected final static Integer   one = new Integer  (1);
41  
42      /**
43          Creates a new ProxyObjectGate.
44      */
45      protected ProxyObjectGate() {
46          objectsReferencesByClient   =   new HashMap  ();
47      }
48  
49      public void addObjectReferencedByClient(OzoneProxy proxy) {
50          if (proxy!=null) {
51              synchronized (objectsReferencesByClient) {
52                  ObjectID id = proxy.remoteID();
53  
54                  if (garbageCollectorToBeNotifiedOfExportedReferences!=null) {
55                      garbageCollectorToBeNotifiedOfExportedReferences.notifyDatabaseObjectIsAboutToBeExported(id);
56                  }
57  
58                  Object   oldEntry = objectsReferencesByClient.put(id,one);
59  
60                  if (oldEntry!=null) {
61                      objectsReferencesByClient.put(id,new Integer  (((Integer  ) oldEntry).intValue()+1));
62                  }
63              }
64          }
65      }
66  
67      protected void removeObjectReferencedByClient(OzoneProxy proxy) {
68          removeObjectReferencedByClient(proxy.remoteID());
69      }
70  
71      protected void removeObjectReferencedByClient(ObjectID id) {
72          synchronized (objectsReferencesByClient) {
73              Object   oldEntry = objectsReferencesByClient.remove(id);
74  
75              if (oldEntry!=null) {
76                  if (oldEntry!=one) {
77                      int count = ((Integer  ) oldEntry).intValue();
78  
79                      count--;
80  
81                      if (count>0) {
82                          if (count>1) {
83                              objectsReferencesByClient.put(id,new Integer  (count));
84                          } else {
85                              objectsReferencesByClient.put(id,one);
86                          }
87                      }
88                  } else {
89                      // The reference count was exactly one, now the reference is removed from the map.
90                  }
91              } else {
92                  // oldEntry never may be null. What should we do here?
93              }
94          }
95      }
96  
97      /**
98          Starts filtering references to database objects ({@link OzoneProxy}s) which
99          are exported to the client.
100         Every reference which is exported will be notified to the given GarbageCollector.
101         Additionally, references which are known to be used by the client are notified to the
102         given GarbageCollector within this call.
103     */
104     public void startFilterDatabaseObjectReferencesExports(GarbageCollector garbageCollector) {
105 
106         synchronized (objectsReferencesByClient) {
107             this.garbageCollectorToBeNotifiedOfExportedReferences   =   garbageCollector;
108 
109             Iterator   i = objectsReferencesByClient.keySet().iterator();
110 
111             while (i.hasNext()) {
112                 garbageCollector.notifyDatabaseObjectIsExported((ObjectID) i.next());
113             }
114         }
115     }
116 }
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