ConnectedUser


1   /* Copyright (C) 2004 - 2006 db4objects Inc. http://www.db4o.com */
2   
3   package com.db4odoc.f1.semaphores;
4   
5   
6   import java.util.*;
7   import com.db4o.*;
8   import com.db4o.config.*;
9   import com.db4o.ext.*;
10  import com.db4o.query.*;
11  
12  /**
13   * This class demonstrates how semaphores can be used 
14   * to rule out race conditions when providing exact and
15   * up-to-date information about all connected clients 
16   * on a server. The class also can be used to make sure
17   * that only one login is possible with a give user name
18   * and ipAddress combination.
19   */
20  public class ConnectedUser {
21      
22      static final String   SEMAPHORE_CONNECTED = "ConnectedUser_";
23      static final String   SEMAPHORE_LOCK_ACCESS = "ConnectedUser_Lock_";
24      
25      static final int TIMEOUT = 10000;  // concurrent access timeout 10 seconds
26      
27      String   userName;
28      String   ipAddress;
29      
30      public ConnectedUser(String   userName, String   ipAddress){
31          this.userName = userName;
32          this.ipAddress = ipAddress;
33      }
34      
35      // make sure to call this on the server before opening the database
36      // to improve querying speed 
37      public static void configure(){
38          ObjectClass objectClass = Db4o.configure().objectClass(ConnectedUser.class); 
39          objectClass.objectField("userName").indexed(true);
40          objectClass.objectField("ipAddress").indexed(true);
41      }
42      
43      // call this on the client to ensure to have a ConnectedUser record 
44      // in the database file and the semaphore set
45      public static void login(ObjectContainer client, String   userName, String   ipAddress){
46          if(! client.ext().setSemaphore(SEMAPHORE_LOCK_ACCESS, TIMEOUT)){
47              throw new RuntimeException  ("Timeout trying to get access to ConnectedUser lock");
48          }
49          Query q = client.query();
50          q.constrain(ConnectedUser.class);
51          q.descend("userName").constrain(userName);
52          q.descend("ipAddress").constrain(ipAddress);
53          if(q.execute().size() == 0){
54              client.set(new ConnectedUser(userName, ipAddress));
55              client.commit();
56          }
57          String   connectedSemaphoreName = SEMAPHORE_CONNECTED + userName + ipAddress;
58          boolean unique = client.ext().setSemaphore(connectedSemaphoreName, 0);
59          client.ext().releaseSemaphore(SEMAPHORE_LOCK_ACCESS);
60          if(! unique){
61              throw new RuntimeException  ("Two clients with same userName and ipAddress");
62          }
63      }
64      
65      // here is your list of all connected users, callable on the server
66      public static List connectedUsers(ObjectServer server){
67          ExtObjectContainer serverObjectContainer = server.ext().objectContainer().ext();
68          if(serverObjectContainer.setSemaphore(SEMAPHORE_LOCK_ACCESS, TIMEOUT)){
69              throw new RuntimeException  ("Timeout trying to get access to ConnectedUser lock");
70          }
71          List list = new ArrayList();
72          Query q = serverObjectContainer.query();
73          q.constrain(ConnectedUser.class);
74          ObjectSet objectSet = q.execute();
75          while(objectSet.hasNext()){
76              ConnectedUser connectedUser = (ConnectedUser)objectSet.next();
77              String   connectedSemaphoreName = 
78                  SEMAPHORE_CONNECTED + 
79                  connectedUser.userName + 
80                  connectedUser.ipAddress;
81              if(serverObjectContainer.setSemaphore(connectedSemaphoreName, TIMEOUT)){
82                  serverObjectContainer.delete(connectedUser);
83              }else{
84                  list.add(connectedUser);
85              }
86          }
87          serverObjectContainer.commit();
88          serverObjectContainer.releaseSemaphore(SEMAPHORE_LOCK_ACCESS);
89          return list;
90      }
91  }
92
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