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| <title>javax.sql.rowset.providers Package</title> |
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| Repository for the <code>RowSet</code> reference implementations of the |
| <code>SyncProvider</code> abstract class. These implementations provide a |
| disconnected <code>RowSet</code> |
| object with the ability to synchronize the data in the underlying data |
| source with its data. These implementations are provided as |
| the default <code>SyncProvider</code> implementations and are accessible via the |
| <code>SyncProvider</code> SPI managed by the <code>SyncFactory</code>. |
| |
| <h3>1.0 <code>SyncProvider</code> Reference Implementations</h3> |
| The main job of a <code>SyncProvider</code> implementation is to manage |
| the reader and writer mechanisms. |
| The <code>SyncProvider</code> SPI, as specified in the <code>javax.sql.rowset.spi</code> |
| package, provides a pluggable mechanism by which <code>javax.sql.RowSetReader</code> |
| and <code>javax.sql.RowSetWriter</code> implementations can be supplied to a disconnected |
| <code>RowSet</code> object. |
| <P> |
| A reader, a <code>javax.sql.RowSetReader</code> |
| object, does the work necessary to populate a <code>RowSet</code> object with data. |
| A writer, a <code>javax.sql.RowSetWriter</code> object, does the work necessary for |
| synchronizing a <code>RowSet</code> object's data with the data in the originating |
| source of data. Put another way, a writer writes a <code>RowSet</code> |
| object's data back to the data source. |
| <P> |
| Generally speaking, the course of events is this. The reader makes a connection to |
| the data source and reads the data from a <code>ResultSet</code> object into its |
| <code>RowSet</code> object. Then it closes the connection. While |
| the <code>RowSet</code> object is disconnected, an application makes some modifications |
| to the data and calls the method <code>acceptChanges</code>. At this point, the |
| writer is called to write the changes back to the database table or view |
| from which the original data came. This is called <i>synchronization</i>. |
| <P> |
| If the data in the originating data source has not changed, there is no problem |
| with just writing the <code>RowSet</code> object's new data to the data source. |
| If it has changed, however, there is a conflict that needs to be resolved. One |
| way to solve the problem is not to let the data in the data source be changed in |
| the first place, which can be done by setting locks on a row, a table, or the |
| whole data source. Setting locks is a way to avoid conflicts, but it can be |
| very expensive. Another approach, which is at the other end of the spectrum, |
| is simply to assume that no conflicts will occur and thus do nothing to avoid |
| conflicts. |
| Different <code>SyncProvider</code> implementations may handle synchronization in |
| any of these ways, varying from doing no checking for |
| conflicts, to doing various levels of checking, to guaranteeing that there are no |
| conflicts. |
| <P> |
| The <code>SyncProvider</code> class offers methods to help a <code>RowSet</code> |
| object discover and manage how a provider handles synchronization. |
| The method <code>getProviderGrade</code> returns the |
| grade of synchronization a provider offers. An application can |
| direct the provider to use a particular level of locking by calling |
| the method <code>setDataSourceLock</code> and specifying the level of locking desired. |
| If a <code>RowSet</code> object's data came from an SQL <code>VIEW</code>, an |
| application may call the method <code>supportsUpdatableView</code> to |
| find out whether the <code>VIEW</code> can be updated. |
| <P> |
| Synchronization is done completely behind the scenes, so it is third party vendors of |
| synchronization provider implementations who have to take care of this complex task. |
| Application programmers can decide which provider to use and the level of locking to |
| be done, but they are free from having to worry about the implementation details. |
| <P> |
| The JDBC <code>RowSet</code> Implementations reference implementation provides two |
| implementations of the <code>SyncProvider</code> class: |
| |
| <UL> |
| <LI> |
| <b><code>RIOptimisticProvider</code></b> - provides the <code>javax.sql.RowSetReader</code> |
| and <code>javax.sql.RowSetWriter</code> interface implementations and provides |
| an optimistic concurrency model for synchronization. This model assumes that there |
| will be few conflicts and therefore uses a relatively low grade of synchronization. |
| If no other provider is available, this is the default provider that the |
| <code>SyncFactory</code> will supply to a <code>RowSet</code> object. |
| <br> |
| <LI> |
| <b><code>RIXMLProvider</code></b> - provides the <code>XmlReader</code> (an extension |
| of the <code>javax.sql.RowSetReader</code> interface) and the <code>XmlWriter</code> |
| (an extension of the <code>javax.sql.RowSetWriter</code> interface) to enable |
| <code>WebRowSet</code> objects to write their state to a |
| well formed XML document according to the <code>WebRowSet</code> XML schema |
| definition.<br> |
| </UL> |
| |
| <h3>2.0 Basics in RowSet Population & Synchronization</h3> |
| A rowset's first task is to populate itself with rows of column values. |
| Generally, these rows will come from a relational database, so a rowset |
| has properties that supply what is necessary for making a connection to |
| a database and executing a query. A rowset that does not need to establish |
| a connection and execute a command, such as one that gets its data from |
| a tabular file instead of a relational database, does not need to have these |
| properties set. The vast majority of RowSets, however, do need to set these |
| properties. The general rule is that a RowSet is required to set only the |
| properties that it uses.<br> |
| <br> |
| The <code>command</code> property contains the query that determines what |
| data a <code>RowSet</code> will contain. Rowsets have methods for setting a query's |
| parameter(s), which means that a query can be executed multiple times with |
| different parameters to produce different result sets. Or the query can be |
| changed to something completely new to get a new result set. |
| <p>Once a rowset contains the rows from a <code>ResultSet</code> object or some |
| other data source, its column values can be updated, and its rows can be |
| inserted or deleted. Any method that causes a change in the rowset's values |
| or cursor position also notifies any object that has been registered as |
| a listener with the rowset. So, for example, a table that displays the rowset's |
| data in an applet can be notified of changes and make updates as they |
| occur.<br> |
| <br> |
| The changes made to a rowset can be propagated back to the original data |
| source to keep the rowset and its data source synchronized. Although this |
| involves many operations behind the scenes, it is completely transparent |
| to the application programmer and remains the concern of the RowSet provider |
| developer. All an application has to do is invoke the method <code>acceptChanges</code>, |
| and the data source backing the rowset will be updated to match the current |
| values in the rowset. </p> |
| |
| <p>A disconnected rowset, such as a <code>CachedRowSet</code> or <code>WebRowSet</code> |
| object, establishes a connection to populate itself with data from a database |
| and then closes the connection. The <code>RowSet</code> object will remain |
| disconnected until it wants to propagate changes back to its database table, |
| which is optional. To write its changes back to the database (synchronize with |
| the database), the rowset establishes a connection, write the changes, and then |
| once again disconnects itself.<br> |
| </p> |
| |
| <h3> 3.0 Other Possible Implementations</h3> |
| There are many other possible implementations of the <code>SyncProvider</code> abstract |
| class. One possibility is to employ a more robust synchronization model, which |
| would give a <code>RowSet</code> object increased trust in the provider's |
| ability to get any updates back to the original data source. Another possibility |
| is a more formal synchronization mechanism such as SyncML |
| (<a href="http://www.syncml.org/">http://www.syncml.org/</a>) <br> |
| <br> |
| <br> |
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