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  34   <title>javax.sql.rowset.spi</title>
  35    
  36 </head>
  37   <body bgcolor="#ffffff">
  38 
  39 The standard classes and interfaces that a third party vendor has to
  40 use in its implementation of a synchronization provider. These classes and 
  41 interfaces are referred to as the Service Provider Interface (SPI). A vendor may 
  42 have its implementation included on the JDBC web page that lists available
  43 <code>SyncProvider</code> implementations by sending email to <code>jdbc@sun.com</code>.
  44 Doing this helps make developers aware of the implementation. To make it possible
  45 for a <code>RowSet</code> object to use an implementation, the vendor must register
  46 it with the <code>SyncFactory</code> singleton. (See the class comment for
  47 <code>SyncProvider</code> for a full explanation of the registration process and 
  48 the naming convention to be used.)
  49 <P>
  50 <h2>Table of Contents</h2>
  51 <ul>
  52 <li><a href="#pkgspec">1.0 Package Specification</a>
  53 <li><a href="#arch">2.0 Service Provider Architecture</a>
  54 <li><a href="#impl">3.0 Implementer's Guide</a>
  55 <li><a href="#resolving">4.0 Resolving Synchronization Conflicts</a>
  56 <li><a href="#relspec">5.0 Related Specifications</a>
  57 <li><a href="#reldocs">6.0 Related Documentation</a>
  58 </ul>
  59 
  60 <h3><a name="pkgspec">1.0 Package Specification</h3>
  61 <P>
  62 The following classes and interfaces make up the <code>javax.sql.rowset.spi</code>
  63 package:
  64 <UL>
  65   <LI><code>SyncFactory</code>
  66   <LI><code>SyncProvider</code>
  67   <LI><code>SyncFactoryException</code>
  68   <LI><code>SyncProviderException</code>
  69   <LI><code>SyncResolver</code>
  70   <LI><code>XmlReader</code>
  71   <LI><code>XmlWriter</code>
  72   <LI><code>TransactionalWriter</code>
  73 </UL>
  74 The following interfaces, in the <code>javax.sql</code> package, are also part of the SPI:
  75 <UL>
  76   <LI><code>RowSetReader</code>
  77   <LI><code>RowSetWriter</code>
  78 </UL>
  79 <P>
  80 A <code>SyncProvider</code> implementation provides a disconnected <code>RowSet</code>
  81 object with the mechanisms for reading data into it and for writing data that has been
  82 modified in it
  83 back to the underlying data source.  A <i>reader</i>, a <code>RowSetReader</code> or
  84 <code>XMLReader</code> object, reads data into a <code>RowSet</code> object when the 
  85 <code>CachedRowSet</code> methods <code>execute</code> or <code>populate</code> 
  86 are called.  A <i>writer</i>, a <code>RowSetWriter</code> or <code>XMLWriter</code>
  87 object, writes changes back to the underlying data source when the 
  88 <code>CachedRowSet</code> method <code>acceptChanges</code> is called.
  89 <P>
  90 The process of writing changes in a <code>RowSet</code> object to its data source
  91 is known as <i>synchronization</i>.  The <code>SyncProvider</code> implementation that a
  92 <code>RowSet</code> object is using determines the level of synchronization that the
  93 <code>RowSet</code> object's writer uses. The various levels of synchronization are
  94 referred to as <i>grades</i>.
  95 <P>
  96 The lower grades of synchronization are
  97 known as <i>optimistic</i> concurrency levels because they optimistically
  98 assume that there will be no conflicts or very few conflicts.  A conflict exists when
  99 the same data modified in the <code>RowSet</code> object has also been modified 
 100 in the data source. Using the optimistic concurrency model means that if there
 101 is a conflict, modifications to either the data source or the <code>RowSet</code> 
 102 object will be lost.
 103 <P>
 104 Higher grades of synchronization are called <i>pessimistic</i> because they assume
 105 that others will be accessing the data source and making modifications.  These
 106 grades set varying levels of locks to increase the chances that no conflicts
 107 occur.
 108 <P>
 109 The lowest level of synchronization is simply writing any changes made to the 
 110 <code>RowSet</code> object to its underlying data source.  The writer does
 111 nothing to check for conflicts. 
 112 If there is a conflict and the data
 113 source values are overwritten, the changes other parties have made by to the data
 114 source are lost. 
 115 <P>
 116 The <code>RIXMLProvider</code> implementation uses the lowest level 
 117 of synchronization and just writes <code>RowSet</code> changes to the data source.
 118 This is true because  typically XML data sources do not enable transaction
 119 techniques for maintaining the integrity of data. However, specific standards 
 120 groups have considered offering XML-based synchronization.  For details, see
 121 <PRE>
 122      <a href="http://www.syncml.org">http://www.syncml.org</a>
 123 </PRE>
 124 <P>
 125 For the the next level up, the
 126 writer checks to see if there are any conflicts, and if there are, 
 127 it does not write anything to the data source.  The problem with this concurrency
 128 level is that if another party has modified the corresponding data in the data source 
 129 since the <code>RowSet</code> object got its data,
 130 the changes made to the <code>RowSet</code> object are lost. The
 131 <code>RIOptimisticProvider</code> implementation uses this level of synchronization.
 132 <P>
 133 At higher levels of synchronization, referred to as pessimistic concurrency,
 134 the writer take steps to avoid conflicts by setting locks. Setting locks
 135 can vary from setting a lock on a single row to setting a lock on a table 
 136 or the entire data source. The level of synchronization is therefore a tradeoff 
 137 between the ability of users to access the data source concurrently and the  ability
 138 of the writer to keep the data in the <code>RowSet</code> object and its data source
 139 synchronized.
 140 <P>
 141 It is a requirement that all disconnected <code>RowSet</code> objects 
 142 (<code>CachedRowSet</code>, <code>FilteredRowSet</code>, <code>JoinRowSet</code>, 
 143 and <code>WebRowSet</code> objects) obtain their <code>SyncProvider</code> objects
 144 from the <code>SyncFactory</code> mechanism.  
 145 <P>
 146 The reference implementation (RI) provides two synchronization providers.
 147         <UL>
 148        <LI><b><tt>RIOptimisticProvider</tt></b> <br>
 149            The default provider that the <code>SyncFactory</code> instance will
 150            supply to a disconnected <code>RowSet</code> object when no provider
 151            implementation is specified.<BR>
 152            This synchronization provider uses an optimistic concurrency model,
 153            assuming that there will be few conflicts among users 
 154            who are accessing the same data in a database.  It avoids
 155            using locks; rather, it checks to see if there is a conflict
 156            before trying to synchronize the <code>RowSet</code> object and the
 157            data source. If there is a conflict, it does nothing, meaning that
 158                    changes to the <code>RowSet</code> object are not persisted to the data
 159            source.
 160        <LI><B><tt>RIXMLProvider</tt></B> <BR>
 161             A synchronization provider that can be used with a
 162             <code>WebRowSet</code> object, which is a rowset that can be written 
 163             in XML format or read from XML format. The 
 164             <code>RIXMLProvider</code> implementation does no checking at all for
 165             conflicts and simply writes any updated data in the
 166             <code>WebRowSet</code> object to the underlying data source.
 167             <code>WebRowSet</code> objects use this provider when they are 
 168             dealing with XML data.
 169         </UL>
 170 
 171 These <code>SyncProvider</code> implementations
 172 are bundled with the reference implementation, which makes them always available to
 173 <code>RowSet</code> implementations. 
 174 <code>SyncProvider</code> implementations make themselves available by being
 175 registered with the <code>SyncFactory</code> singleton.  When a <code>RowSet</code> 
 176 object requests a provider, by specifying it in the constructor or as an argument to the
 177 <code>CachedRowSet</code> method <code>setSyncProvider</code>, 
 178 the <code>SyncFactory</code> singleton
 179 checks to see if the requested provider has been registered with it.
 180 If it has, the <code>SyncFactory</code> creates an instance of it and passes it to the
 181 requesting <code>RowSet</code> object.  
 182 If the <code>SyncProvider</code> implementation that is specified has not been registered,
 183 the <code>SyncFactory</code> singleton causes a <code>SyncFactoryException</code> object
 184 to be thrown.  If no provider is specified,
 185 the <code>SyncFactory</code> singleton will create an instance of the default
 186 provider implementation, <code>RIOptimisticProvider</code>,
 187 and pass it to the requesting <code>RowSet</code> object.
 188 
 189 <P>
 190 If a <code>WebRowSet</code> object does not specify a provider in its constructor, the
 191 <code>SyncFactory</code> will give it an instance of <code>RIOptimisticProvider</code>.
 192 However, the constructor for <code>WebRowSet</code> is implemented to set the provider 
 193 to the <code>RIXMLProvider</code>, which reads and writes a <code>RowSet</code> object
 194 in XML format.
 195 <P>
 196 See the <a href="SyncProvider.html">SyncProvider</a> class
 197 specification for further details.
 198 <p>
 199 Vendors may develop a <tt>SyncProvider</tt> implementation with any one of the possible
 200 levels of synchronization, thus giving <code>RowSet</code> objects a choice of
 201 synchronization mechanisms.  A vendor can make its implementation available by 
 202 registering the fully qualified class name with Oracle Corporation at
 203 <code>jdbc@sun.com</code>. This process is discussed in further detail below. 
 204 <P>
 205 
 206 <a name="arch"><h3>2.0 Service Provider Interface Architecture</h3>
 207 <ul>
 208 <b>2.1 Overview</b>
 209 <p>
 210 The Service Provider Interface provides a pluggable mechanism by which
 211 <code>SyncProvider</code> implementations can be registered and then generated when
 212 required. The lazy reference mechanism employed by the <code>SyncFactory</code> limits
 213 unnecessary resource consumption by not creating an instance until it is 
 214 required by a disconnected
 215 <code>RowSet</code> object. The <code>SyncFactory</code> class also provides
 216 a standard API to configure logging options and streams that <b>may</b> be provided
 217 by a particular <code>SyncProvider</code> implementation.
 218 <p>
 219 <b>2.2 Registering with the <code>SyncFactory</code></b>
 220 <p>
 221 A third party <code>SyncProvider</code> implementation must be registered with the 
 222 <code>SyncFactory</code> in order for a disconnected <code>RowSet</code> object 
 223 to obtain it and thereby use its <code>javax.sql.RowSetReader</code> and 
 224 <code>javax.sql.RowSetWriter</code>
 225 implementations. The following registration mechanisms are available to all 
 226 <code>SyncProvider</code> implementations:
 227 <ul>
 228 <li><b>System properties</b> - Properties set at the command line. These
 229 properties are set at run time and apply system-wide per invocation of the Java
 230 application. See the section <a href="#reldocs">"Related Documentation"</a>
 231 further related information.
 232 <p>
 233 <li><b>Property Files</b> - Properties specified in a standard property file.
 234 This can be specified using a System Property or by modifying a standard
 235 property file located in the platform run-time. The
 236 reference implementation of this technology includes a standard property
 237 file than can be edited to add additional <code>SyncProvider</code> objects.
 238 <p> 
 239 <li><b>JNDI Context</b> - Available providers can be registered on a JNDI
 240 context. The <tt>SyncFactory</tt> will attempt to load <tt>SyncProvider</tt>
 241 objects bound to the context and register them with the factory. This
 242 context must be supplied to the <code>SyncFactory</code> for the mechanism to 
 243 function correctly.
 244 </ul>
 245 <p>
 246 Details on how to specify the system properties or properties in a property file
 247 and how to configure the JNDI Context are explained in detail in the
 248 <a href="SyncFactory.html"><code>SyncFactory</code></a> class description.
 249 <p>
 250 <b>2.3 SyncFactory Provider Instance Generation Policies</b>
 251 <p>
 252 The <code>SyncFactory</code> generates a requested <code>SyncProvider</code>
 253 object if the provider has been correctly registered.  The
 254 following policies are adhered to when either a disconnected <code>RowSet</code> object
 255 is instantiated with a specified <code>SyncProvider</code> implementation or is 
 256 reconfigured at runtime with an alternative <code>SyncProvider</code> object.
 257 <ul>
 258 <li> If a <code>SyncProvider</code> object is specified and the <code>SyncFactory</code>
 259 contains <i>no</i> reference to the provider, a <code>SyncFactoryException</code> is 
 260 thrown.
 261 <p>
 262 <li> If a <code>SyncProvider</code> object is specified and the <code>SyncFactory</code>
 263 contains a reference to the provider, the requested provider is supplied.
 264 <p>
 265 <li> If no <code>SyncProvider</code> object is specified, the reference
 266 implementation provider <code>RIOptimisticProvider</code> is supplied.
 267 </ul>
 268 <p>
 269 These policies are explored in more detail in the <a href="SyncFactory.html">
 270 <code>SyncFactory</code></a> class.
 271 </ul>
 272 
 273 <li><a name="impl"><h3>3.0 SyncProvider Implementer's Guide</h3>
 274 <ul>
 275 <b>3.1 Requirements</b>
 276 <p>
 277 A compliant <code>SyncProvider</code> implementation that is fully pluggable
 278 into the <code>SyncFactory</code> <b>must</b> extend and implement all
 279 abstract methods in the <a href="SyncProvider.html"><code>SyncProvider</code></a>
 280 class. In addition, an implementation <b>must</b> determine the 
 281 grade, locking and updatable view capabilities defined in the
 282 <code>SyncProvider</code> class definition. One or more of the
 283 <code>SyncProvider</code> description criteria <b>must</b> be supported. It
 284 is expected that vendor implementations will offer a range of grade, locking, and
 285 updatable view capabilities.
 286 <p>
 287 Furthermore, the <code>SyncProvider</code> naming convention <b>must</b> be followed as
 288 detailed in the <a href="SyncProvider.html"><code>SyncProvider</code></a> class
 289 description.
 290 <p>
 291 <b>3.2 Grades</b>
 292 <p>
 293 JSR 114 defines a set of grades to describe the quality of synchronization
 294 a <code>SyncProvider</code> object can offer a disconnected <code>RowSet</code>
 295 object. These grades are listed from the lowest quality of service to the highest.
 296 <ul>
 297 <li><b>GRADE_NONE</b> - No synchronization with the originating data source is
 298 provided. A <code>SyncProvider</code> implementation returning this grade will simply
 299 attempt to write any data that has changed in the <code>RowSet</code> object to the
 300 underlying data source, overwriting whatever is there. No attempt is made to compare 
 301 original values with current values to see if there is a conflict. The 
 302 <code>RIXMLProvider</code> is implemented with this grade.
 303 <p>
 304 <li><b>GRADE_CHECK_MODIFIED_AT_COMMIT</b> - A low grade of optimistic synchronization.
 305 A <code>SyncProvider</code> implementation returning this grade
 306 will check for conflicts in rows that have changed between the last synchronization 
 307 and the current synchronization under way. Any changes in the originating data source
 308 that have been modified will not be reflected in the disconnected <code>RowSet</code>
 309 object. If there are no conflicts, changes in the <code>RowSet</code> object will be
 310 written to the data source. If there are conflicts, no changes are written.
 311 The <code>RIOptimisticProvider</code> implementation uses this grade.
 312 <p>
 313 <li><b>GRADE_CHECK_ALL_AT_COMMIT</b> - A high grade of optimistic synchronization.
 314 A <code>SyncProvider</code> implementation   returning this grade
 315 will check all rows, including rows that have not changed in the disconnected
 316 <code>RowSet</code> object. In this way, any changes to rows in the underlying
 317 data source will be reflected in the disconnected <code>RowSet</code> object
 318 when the synchronization finishes successfully.
 319 <p>
 320 <li><b>GRADE_LOCK_WHEN_MODIFIED</b> - A pessimistic grade of synchronization.
 321 <code>SyncProvider</code> implementations returning this grade will lock
 322 the row in the originating  data source that corresponds to the row being changed
 323 in the <code>RowSet</code> object to reduce the possibility of other
 324 processes modifying the same data in the data source.
 325 <p>
 326 <li><b>GRADE_LOCK_WHEN_LOADED</b> - A higher pessimistic synchronization grade.
 327 A <code>SyncProvider</code> implementation returning this grade will lock
 328 the entire view and/or  table affected by the original query used to
 329 populate a <code>RowSet</code> object.
 330 </ul>
 331 <p>
 332 <b>3.3 Locks</b>
 333 <p>
 334 JSR 114 defines a set of constants that specify whether any locks have been
 335 placed on a <code>RowSet</code> object's underlying data source and, if so,
 336 on which constructs the locks are placed.  These locks will remain on the data
 337 source while the <code>RowSet</code> object is disconnected from the data source.
 338 <P>
 339 These constants <b>should</b> be considered complementary to the 
 340 grade constants. The default setting for the majority of grade settings requires
 341 that no data source locks remain when a <code>RowSet</code> object is disconnected 
 342 from its data source.
 343 The grades <code>GRADE_LOCK_WHEN_MODIFIED</code> and
 344 <code>GRADE_LOCK_WHEN_LOADED</code> allow a disconnected <code>RowSet</code> object
 345 to have a fine-grained control over the degree of locking.
 346 <ul>
 347 <li><b>DATASOURCE_NO_LOCK</b> - No locks remain on the originating data source. 
 348 This is the default lock setting for all <code>SyncProvider</code> implementations 
 349 unless otherwise directed by a <code>RowSet</code> object.
 350 <p>
 351 <li><b>DATASOURCE_ROW_LOCK</b> - A lock is placed on the rows that are touched by
 352 the original SQL query used to populate the <code>RowSet</code> object.
 353 <p> 
 354 <li><b>DATASOURCE_TABLE_LOCK</b> - A lock is placed on all tables that are touched
 355 by the query that was used to populate the <code>RowSet</code> object.
 356 <p>
 357 <li><b>DATASOURCE_DB_LOCK</b>
 358 A lock is placed on the entire data source that is used by the <code>RowSet</code>
 359 object.
 360 </ul>
 361 <p>
 362 <b>3.4 Updatable Views</b>
 363 <p>
 364 A <code>RowSet</code> object may be populated with data from an SQL <code>VIEW</code>.
 365 The following constants indicate whether a <code>SyncProvider</code> object can
 366 update data in the table or tables from which the <code>VIEW</code> was derived.
 367 <ul>
 368 <li><b>UPDATABLE_VIEW_SYNC</b>
 369 Indicates that a <code>SyncProvider</code> implementation  supports synchronization
 370 to the table or tables from which the SQL <code>VIEW</code> used to populate  a
 371 a <code>RowSet</code> object is derived.
 372 <p>
 373 <li><b>NONUPDATABLE_VIEW_SYNC</b>
 374 Indicates that a <code>SyncProvider</code> implementation  does <b>not</b> support
 375 synchronization to the table or tables from which the SQL <code>VIEW</code> 
 376 used to populate  a <code>RowSet</code> object is derived.
 377 </ul>
 378 <p>
 379 <b>3.5 Usage of <code>SyncProvider</code> Grading and Locking</b>
 380 <p>
 381 In the example below, the reference <tt>CachedRowSetImpl</tt> implementation
 382 reconfigures its current <tt>SyncProvider</tt> object by calling the 
 383 <tt>setSyncProvider</tt> method.<br>
 384        
 385 <PRE>
 386     CachedRowSetImpl crs = new CachedRowSetImpl();
 387     crs.setSyncProvider("com.foo.bar.HASyncProvider");
 388 </PRE>
 389     An application can retrieve the <tt>SyncProvider</tt> object currently in use
 390 by a disconnected <code>RowSet</code> object. It can also retrieve the
 391 grade of synchronization with which the provider was implemented and the degree of
 392 locking currently in use.  In addition, an application has the flexibility to set
 393 the degree of locking to be used, which can increase the possibilities for successful
 394 synchronization.  These operation are shown in the following code fragment.
 395 <PRE>
 396     SyncProvider sync = crs.getSyncProvider();
 397 
 398     switch (sync.getProviderGrade()) {
 399     case: SyncProvider.GRADE_CHECK_ALL_AT_COMMIT
 400          //A high grade of optimistic synchronization
 401     break;
 402     case: SyncProvider.GRADE_CHECK_MODIFIED_AT_COMMIT 
 403          //A low grade of optimistic synchronization 
 404     break;
 405     case: SyncProvider.GRADE_LOCK_WHEN_LOADED 
 406          // A pessimistic synchronization grade 
 407     break;
 408     case: SyncProvider.GRADE_LOCK_WHEN_MODIFIED 
 409          // A pessimistic synchronization grade 
 410     break;
 411     case: SyncProvider.GRADE_NONE 
 412       // No synchronization with the originating data source provided
 413     break;
 414     }
 415           
 416     switch (sync.getDataSourcLock() {
 417       case: SyncProvider.DATASOURCE_DB_LOCK
 418        // A lock is placed on the entire datasource that is used by the
 419        // <code>RowSet</code> object 
 420        break;
 421 
 422       case: SyncProvider.DATASOURCE_NO_LOCK
 423        // No locks remain on the  originating data source.
 424       break;
 425 
 426       case: SyncProvider.DATASOURCE_ROW_LOCK
 427        // A lock is placed on the rows that are  touched by the original 
 428        // SQL statement used to populate
 429        // the RowSet object that is using the SyncProvider
 430        break;
 431 
 432       case: DATASOURCE_TABLE_LOCK
 433        // A lock is placed on  all tables that are touched by the original 
 434        // SQL statement used to populated
 435        // the RowSet object that is using the SyncProvider
 436        break;
 437 
 438 </PRE>
 439     It is also possible using the static utility method in the
 440 <code>SyncFactory</code> class to determine the list of <code>SyncProvider</code>
 441 implementations currently registered with the <code>SyncFactory</code>.
 442        
 443 <pre>
 444         Enumeration e = SyncFactory.getRegisteredProviders();
 445 </pre>    
 446 
 447 </ul>
 448 
 449 <h3><a name="resolving">4.0 Resolving Synchronization Conflicts</h3>
 450 
 451 The interface <code>SyncResolver</code> provides a way for an application to
 452 decide manually what to do when a conflict occurs. When the <code>CachedRowSet</code>
 453 method <code>acceptChanges</code> finishes and has detected one or more conflicts,
 454 it throws a <code>SyncProviderException</code> object.  An application can
 455 catch the exception and
 456 have it retrieve a <code>SyncResolver</code> object by calling the method
 457 <code>SyncProviderException.getSyncResolver()</code>. 
 458 <P>
 459 A <code>SyncResolver</code> object, which is a special kind of 
 460 <code>CachedRowSet</code> object or
 461 a <code>JdbcRowSet</code> object that has implemented the <code>SyncResolver</code> 
 462 interface,  examines the conflicts row by row. It is a duplicate of the 
 463 <code>RowSet</code> object being synchronized except that it contains only the data
 464 from the data source this is causing a conflict. All of the other column values are
 465 set to <code>null</code>. To navigate from one conflict value to another, a
 466 <code>SyncResolver</code> object provides the methods <code>nextConflict</code> and
 467 <code>previousConflict</code>.
 468 <P>
 469 The <code>SyncResolver</code> interface also
 470 provides methods for doing the following:
 471 <UL>
 472  <LI>finding out whether the conflict involved an update, a delete, or an insert
 473  <LI>getting the value in the data source that caused the conflict
 474  <LI>setting the value that should be in the data source if it needs to be changed
 475      or setting the value that should be in the <code>RowSet</code> object if it needs
 476      to be changed
 477 </UL>
 478 <P>
 479 When the <code>CachedRowSet</code> method <code>acceptChanges</code> is called, it 
 480 delegates to the <code>RowSet</code> object's  <code>SyncProvider</code> object.
 481 How the writer provided by that <code>SyncProvider</code> object is implemented
 482 determines what level (grade) of checking for conflicts will be done.  After all 
 483 checking for conflicts is completed and one or more conflicts has been found, the method
 484 <code>acceptChanges</code> throws a <code>SyncProviderException</code> object. The
 485 application can catch the exception and use it to obtain a <code>SyncResolver</code> object.  
 486 <P>
 487 The application can then use <code>SyncResolver</code> methods to get information
 488 about each conflict and decide what to do.  If the application logic or the user
 489 decides that a value in the <code>RowSet</code> object should be the one to
 490 persist, the application or user can overwrite the data source value with it.  
 491 <P>
 492 The comment for the <code>SyncResolver</code> interface has more detail.
 493 
 494 <a name="relspec"><h3>5.0 Related Specifications</h3>
 495 <ul>
 496 <li><a href="http://java.sun.com/products/jndi">JNDI 1.3</a>
 497 <li><a href="{@docRoot}/../technotes/guides/logging/index.html">Java Logging
 498 APIs</a>
 499 </ul>
 500 <a name="reldocs"><h3>6.0 Related Documentation</h3>
 501 <ul>
 502 <li><a href="{@docRoot}/../technotes/tools/index.html#basic">System
 503 properties</a>
 504 <li>Resource Files
 505 <li><a href="http://java.sun.com/tutorial/jdbc">DataSource for JDBC
 506 Connections</a>
 507 </ul>
 508 
 509 </body>
 510 </html>
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