| ConnectionPool.java |
1 /*
2 * Copyright (c) 1998-2005, The University of Sheffield.
3 *
4 * This file is part of GATE (see http://gate.ac.uk/), and is free
5 * software, licenced under the GNU Library General Public License,
6 * Version 2, June 1991 (in the distribution as file licence.html,
7 * and also available at http://gate.ac.uk/gate/licence.html).
8 *
9 * Atanas Kiryakov, 01/02/2002
10 *
11 * $Id: ConnectionPool.java,v 1.4 2005/01/11 13:51:36 ian Exp $
12 */
13 package gate.persist;
14
15
16 import java.sql.Connection;
17 import java.sql.SQLException;
18 import java.util.Vector;
19
20 /**
21 A generic implementation of pool of references to objects of any kind.
22 It is thread-safe, so, allows multiple users to get and release objects
23 "simultaneously". In fact, the standard Java synchronization is used.
24 <BR><BR>
25 The base idea is that, a calling routine will try to get
26 an object from the pool with method Get. On success, it will use the
27 object and return it in the pool with the method Put.
28 <BR><BR>
29 If there ares no available objects in the pool, Get will return <B>null</B>.
30 Then the calling routine should create a new object. Further, scenario goes
31 in the same way - when finished using the object, calling routine shoud Put
32 it in the pool for future use, instead of leaving it to be garbage-collected.
33 <BR><BR>
34 The pool initially is empty. The only way to increase the number of objects
35 managed by the pool, is some external process to Put an object, that was
36 created, instead of previously Get from the pool.
37 <BR><BR>
38 Pool stores only references to currently "free" or available objects. When
39 some external routine Gets an object from the pool, its reference is not
40 locked, it is simply removed from the pool.
41 */
42 public class ConnectionPool {
43 private Vector connections;
44 private int size;
45 private String url;
46 private int connCount;
47
48 /**
49 Constructs and object pool with specified size.
50 @param size determines the maximum size of the pool. This is the number
51 free objects that it can manage at the same time
52 */
53 public ConnectionPool(int size, String url) {
54 this.size = size;
55 connections = new Vector(this.size);
56 this.url = url;
57 this.connCount = 0;
58 } // ConnectionPool
59
60 /**
61 Pulls out an object from the pool. The reference to the object is removed
62 from the pool and their is no longer any kind of relation between this
63 object and the pool. It can be returned back (released) by Put method.
64 @return an object from the pool, if available.<BR>
65 Otherwise, returns <B>null</B>
66 */
67 public synchronized Connection get()
68 throws SQLException,ClassNotFoundException{
69 int currAvailable = connections.size();
70 if (currAvailable > 0){
71 Connection conn = (Connection) connections.elementAt(currAvailable-1);
72 connections.removeElementAt(currAvailable-1);
73 return conn;
74 }
75 else {
76 if (connCount < size) {
77 Connection newCon = DBHelper.connect(url);
78 connCount++;
79 return newCon;
80 }
81 else {
82 try {
83 wait();
84 }
85 catch (java.lang.InterruptedException ie) {
86 throw new SQLException(" Thread interrupted while waiting "
87 +"to get Connection from ConnectionPool !");
88 }
89 return get();
90 }
91 }
92 } // Get
93
94 /**
95 Puts an object in the pool, those stating that it's "free" or available
96 for use by another processes and routines. An object can be put in the pool,
97 without need to be get from there before.
98 @return <B>true</B> on success<BR>
99 <B>false</B> when the object was not put in the pool,
100 because the maximal number of references in the pool was riched
101 */
102 public synchronized boolean put(Connection conn){
103 connections.addElement(conn);
104 notify();
105 return true;
106 } // Put
107
108 public void finalize() {
109 for (int i = 0; i<connections.size(); i++){
110 try {
111 DBHelper.disconnect((Connection) connections.elementAt(i));
112 }
113 catch (Exception e){
114 e.printStackTrace();
115 }
116 }
117 }
118 }