Oracle themselves acknowledge that this is an issue in their JDBC FAQ and state that in the Oracle 11g JDBC drivers the mapping has reverted back to java.sql.Timestamp and Oracle's DATE being equivalent rather than different. In the meantime, those of us in the real world have to deal with real applications using the Oracle 10g drivers which map java.sql.Timestamp to Oracle's TIMESTAMP data type instead.
As I described before, we came up with a short term fix of creating a different index on the table being queried, with the DATE column as the last column in the index. This did have a major performance benefit (15 hours of the last run down to 2.5 hours with the new index), but was still not the optimal solution. Oracle was still doing an INDEX SKIP SCAN rather than an INDEX UNIQUE SCAN, and so was reading many more index entries than it needed to i.e. more Buffer Gets than it needed to.
I put together a Java program using JDBC to test the effects of using different Java data types (Classes really) and different JDBC settings. The simplest fix is to use the "V8Compatible" property as mentioned in the Oracle FAQ linked to earlier. You create the Property, set the "V8Compatible" value to "true" and then make your normal JDBC connection to Oracle (I've removed all error handling code in the following):
// Create and set V8Compatible propertyWith no other changes than the 3 NEW lines indicated, the application's java.util.Date value now came through as an Oracle DATE after going through the intermediate mapping layers, and the original index was now correctly used in a UNIQUE SCAN.
Properties oraProperties = new Properties () ; // NEW
oraProperties.setProperty("oracle.jdbc.V8Compatible", "true") ; // NEW
// Standard JDBC connection code
OracleDataSource ods = new OracleDataSource();
// Set properties of the connection
ods.setConnectionProperties (connectionProperties) ; // NEW
Connection dbcon = ods.getConnection () ;
This is clearly the easiest and most direct solution, and does what we want - all Java Date / Time values now come through mapped as Oracle DATE values, rather than TIMESTAMP values. Although the "V8Compatible" property is deprecated in Oracle 11g, this will have no effect as the behaviour of the Oracle 11g JDBC drivers has changed to what we want, and it can ignore the unneeded property setting.
The other possible solution, which is being investigated by those who know more about the Java abstraction layer API being used by the application than I do, is to get this abstraction layer to wrap all java.util.Date objects within oracle.sql.DATE objects. Thus the JDBC connection is only ever given oracle.sql.DATE values, which are direct equivalents of DATE values in the database. So again, any index would be used directly, and no data type conversions would take place within the Oracle database server.
However, the details of this look like they would still involve mapping a java.util.Date object to a java.sql.Timestamp first in the abstraction layer, and then wrapping this within an oracle.sql.DATE object. This is because oracle.sql.DATE has a constructor that takes a java.sql.Timestamp argument, allowing easy conversion. But it does not have a constructor that takes a java.util.Date argument, or any superclass of it. java.sql.Timestamp is a subclass of java.util.Date.
So for now we have one guaranteed long term fix with the "V8Compatible" property, and another possible fix by changing the mapping within the abstraction layer. So now it is up to the people that own the Java application code to deliver their preferred change, and then we can remove the extra index that we created as a short term fix.