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35 <title>javax.sql.rowset.providers Package</title> |
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36 </head> |
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37 <body bgcolor="#ffffff"> |
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38 Repository for the <tt>RowSet</tt> reference implementations of the |
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39 <tt>SyncProvider</tt> abstract class. These implementations provide a |
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40 disconnected <code>RowSet</code> |
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41 object with the ability to synchronize the data in the underlying data |
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42 source with its data. These implementations are provided as |
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43 the default <tt>SyncProvider</tt> implementations and are accessible via the |
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44 <tt>SyncProvider</tt> SPI managed by the <tt>SyncFactory</tt>. |
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45 |
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46 <h3>1.0 <code>SyncProvider</code> Reference Implementations</h3> |
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47 The main job of a <tt>SyncProvider</tt> implementation is to manage |
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48 the reader and writer mechanisms. |
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49 The <tt>SyncProvider</tt> SPI, as specified in the <tt>javax.sql.rowset.spi</tt> |
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50 package, provides a pluggable mechanism by which <tt>javax.sql.RowSetReader</tt> |
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51 and <tt>javax.sql.RowSetWriter</tt> implementations can be supplied to a disconnected |
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52 <tt>RowSet</tt> object. |
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53 <P> |
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54 A reader, a <code>javax.sql.RowSetReader</code> |
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55 object, does the work necessary to populate a <code>RowSet</code> object with data. |
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56 A writer, a <code>javax.sql.RowSetWriter</code> object, does the work necessary for |
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57 synchronizing a <code>RowSet</code> object's data with the data in the originating |
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58 source of data. Put another way, a writer writes a <code>RowSet</code> |
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59 object's data back to the data source. |
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60 <P> |
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61 Generally speaking, the course of events is this. The reader makes a connection to |
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62 the data source and reads the data from a <code>ResultSet</code> object into its |
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63 <code>RowSet</code> object. Then it closes the connection. While |
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64 the <code>RowSet</code> object is disconnected, an application makes some modifications |
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65 to the data and calls the method <code>acceptChanges</code>. At this point, the |
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66 writer is called to write the changes back to the database table or view |
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67 from which the original data came. This is called <i>synchronization</i>. |
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68 <P> |
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69 If the data in the originating data source has not changed, there is no problem |
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70 with just writing the <code>RowSet</code> object's new data to the data source. |
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71 If it has changed, however, there is a conflict that needs to be resolved. One |
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72 way to solve the problem is not to let the data in the data source be changed in |
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73 the first place, which can be done by setting locks on a row, a table, or the |
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74 whole data source. Setting locks is a way to avoid conflicts, but it can be |
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75 very expensive. Another approach, which is at the other end of the spectrum, |
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76 is simply to assume that no conflicts will occur and thus do nothing to avoid |
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77 conflicts. |
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78 Different <code>SyncProvider</code> implementations may handle synchronization in |
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79 any of these ways, varying from doing no checking for |
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80 conflicts, to doing various levels of checking, to guaranteeing that there are no |
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81 conflicts. |
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82 <P> |
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83 The <code>SyncProvider</code> class offers methods to help a <code>RowSet</code> |
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84 object discover and manage how a provider handles synchronization. |
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85 The method <code>getProviderGrade</code> returns the |
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86 grade of synchronization a provider offers. An application can |
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87 direct the provider to use a particular level of locking by calling |
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88 the method <code>setDataSourceLock</code> and specifying the level of locking desired. |
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89 If a <code>RowSet</code> object's data came from an SQL <code>VIEW</code>, an |
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90 application may call the method <code>supportsUpdatableView</code> to |
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91 find out whether the <code>VIEW</code> can be updated. |
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92 <P> |
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93 Synchronization is done completely behind the scenes, so it is third party vendors of |
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94 synchronization provider implementations who have to take care of this complex task. |
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95 Application programmers can decide which provider to use and the level of locking to |
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96 be done, but they are free from having to worry about the implementation details. |
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97 <P> |
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98 The JDBC <code>RowSet</code> Implementations reference implementation provides two |
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99 implementations of the <code>SyncProvider</code> class: |
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100 |
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101 <UL> |
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102 <LI> |
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103 <b><tt>RIOptimisticProvider </tt></b>- provides the <tt>javax.sql.RowSetReader</tt> |
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104 and <tt>javax.sql.RowSetWriter</tt> interface implementations and provides |
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105 an optimistic concurrency model for synchronization. This model assumes that there |
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106 will be few conflicts and therefore uses a relatively low grade of synchronization. |
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107 If no other provider is available, this is the default provider that the |
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108 <code>SyncFactory</code> will supply to a <code>RowSet</code> object. |
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109 <br> |
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110 <LI> |
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111 <b><tt>RIXMLProvider </tt></b>- provides the <tt>XmlReader</tt> (an extension |
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112 of the <tt>javax.sql.RowSetReader</tt> interface) and the <tt>XmlWriter</tt> |
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113 (an extension of the <tt>javax.sql.RowSetWriter</tt> interface) to enable |
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114 <tt>WebRowSet</tt> objects to write their state to a |
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115 well formed XML document according to the <tt>WebRowSet</tt> XML schema |
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116 definition.<br> |
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117 </UL> |
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118 |
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119 <h3>2.0 Basics in RowSet Population & Synchronization</h3> |
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120 A rowset's first task is to populate itself with rows of column values. |
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121 Generally, these rows will come from a relational database, so a rowset |
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122 has properties that supply what is necessary for making a connection to |
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123 a database and executing a query. A rowset that does not need to establish |
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124 a connection and execute a command, such as one that gets its data from |
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125 a tabular file instead of a relational database, does not need to have these |
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126 properties set. The vast majority of RowSets, however, do need to set these |
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127 properties. The general rule is that a RowSet is required to set only the |
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128 properties that it uses.<br> |
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129 <br> |
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130 The <tt>command</tt> property contains the query that determines what |
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131 data a <code>RowSet</code> will contain. Rowsets have methods for setting a query's |
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132 parameter(s), which means that a query can be executed multiple times with |
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133 different parameters to produce different result sets. Or the query can be |
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134 changed to something completely new to get a new result set. |
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135 <p>Once a rowset contains the rows from a <tt>ResultSet</tt> object or some |
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136 other data source, its column values can be updated, and its rows can be |
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137 inserted or deleted. Any method that causes a change in the rowset's values |
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138 or cursor position also notifies any object that has been registered as |
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139 a listener with the rowset. So, for example, a table that displays the rowset's |
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140 data in an applet can can be notified of changes and make updates as they |
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141 occur.<br> |
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142 <br> |
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143 The changes made to a rowset can be propagated back to the original data |
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144 source to keep the rowset and its data source synchronized. Although this |
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145 involves many operations behind the scenes, it is completely transparent |
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146 to the application programmer and remains the concern of the RowSet provider |
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147 developer. All an application has to do is invoke the method <tt>acceptChanges</tt>, |
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148 and the data source backing the rowset will be updated to match the current |
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149 values in the rowset. </p> |
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150 |
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151 <p>A disconnected rowset, such as a <tt>CachedRowSet</tt> or <tt>WebRowSet</tt> |
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152 object, establishes a connection to populate itself with data from a database |
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153 and then closes the connection. The <code>RowSet</code> object will remain |
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154 disconnected until it wants to propagate changes back to its database table, |
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155 which is optional. To write its changes back to the database (synchronize with |
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156 the database), the rowset establishes a connection, write the changes, and then |
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157 once again disconnects itself.<br> |
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158 </p> |
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159 |
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160 <h3> 3.0 Other Possible Implementations</h3> |
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161 There are many other possible implementations of the <tt>SyncProvider</tt> abstract |
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162 class. One possibility is to employ a more robust synchronization model, which |
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163 would give a <code>RowSet</code> object increased trust in the provider's |
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164 ability to get any updates back to the original data source. Another possibility |
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165 is a more formal synchronization mechanism such as SyncML |
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166 (<a href="http://www.syncml.org/">http://www.syncml.org/</a>) <br> |
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167 <br> |
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168 <br> |
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169 </body> |
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170 </html> |