glenn 2002/08/30 06:41:25
Modified: webapps/tomcat-docs jndi-datasource-examples-howto.xml
Log:
Add FAQ for Random Closed Connection Exceptions
Revision Changes Path
1.5 +67 -0
jakarta-tomcat-4.0/webapps/tomcat-docs/jndi-datasource-examples-howto.xml
Index: jndi-datasource-examples-howto.xml
===================================================================
RCS file:
/home/cvs/jakarta-tomcat-4.0/webapps/tomcat-docs/jndi-datasource-examples-howto.xml,v
retrieving revision 1.4
retrieving revision 1.5
diff -u -r1.4 -r1.5
--- jndi-datasource-examples-howto.xml 18 Aug 2002 00:55:25 -0000 1.4
+++ jndi-datasource-examples-howto.xml 30 Aug 2002 13:41:25 -0000 1.5
@@ -749,6 +749,73 @@
</subsection>
+<subsection name="Random Connection Closed Exceptions">
+<p>
+These can occur when one request gets a db connection from the connection
+pool and closes it twice. When using a connection pool, closing the
+connection just returns it to the pool for reuse by another request,
+it doesn't close the connection. And Tomcat uses multiple threads to
+handle concurrent requests. Here is an example of the sequence
+of events which could cause this error in Tomcat:
+<pre>
+ Request 1 running in Thread 1 gets a db connection.
+
+ Request 1 closes the db connection.
+
+ The JVM switches the running thread to Thread 2
+
+ Request 2 running in Thread 2 gets a db connection
+ (the same db connection just closed by Request 1).
+
+ The JVM switches the running thread back to Thread 1
+
+ Request 1 closes the db connection a second time in a finally block.
+
+ The JVM switches the running thread back to Thread 2
+
+ Request 2 Thread 2 tries to use the db connection but fails
+ because Request 1 closed it.
+</pre>
+Here is an example of properly written code to use a db connection
+obtained from a connection pool:
+<pre>
+ Connection conn = null;
+ Statement stmt = null; // Or PreparedStatement if needed
+ ResultSet rs = null;
+ try {
+ conn = ... get connection from connection pool ...
+ stmt = conn.createStatement("select ...");
+ rs = stmt.executeQuery();
+ ... iterate through the result set ...
+ rs.close();
+ rs = null;
+ stmt.close();
+ stmt = null;
+ conn.close(); // Return to connection pool
+ conn = null; // Make sure we don't close it twice
+ } catch (SQLException e) {
+ ... deal with errors ...
+ } finally {
+ // Always make sure result sets and statements are closed,
+ // and the connection is returned to the pool
+ if (rs != null) {
+ try { rs.close(); } catch (SQLException e) { ; }
+ rs = null;
+ }
+ if (stmt != null) {
+ try { stmt.close(); } catch (SQLException e) { ; }
+ stmt = null;
+ }
+ if (conn != null) {
+ try { conn.close(); } catch (SQLException e) { ; }
+ conn = null;
+ }
+ }
+</pre>
+</p>
+
+</subsection>
+
</section>
</body>
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