M03 Q04 What is a callback? What are there three JdbcTemplate callback interfaces that can be used with queries? What is each used for?
A callback is a code or reference to the code that can be passed as an argument to the method. This method will execute passed callback during execution.
On Java level callback can be:
- Class that implements interface
- Anonymous class
- Lambda expression – JDK 8
- Reference Method – JDK 8
Jdbc Template Callbacks that can be used with queries:
RowMapper – interface for processing ResultSet data on per-row basis, implementation should call ResultSet.get*(..) methods, but should not call ResultSet.next(), it should only extract values from the current row and based on those values should create object, which will be returned from mapRow method, implementation is usually stateless
RowCallbackHandler – interface for processing ResultSet data on a per-row basis, implementation should call ResultSet.get*(..) methods, but should not call ResultSet.next(), it should only extract values from the current row, implementation is usually stateful, it keeps accumulated data in some object, processRow method from this class does not return any value, instead, method saves results into, for example, object field that will keep state.
ResultSetExtractor – interface for processing entire ResultSet data, all rows needs to be processed and implementation should call ResultSet.next() method to move between rows, implementation is usually stateless, the implementation should not close ResultSet, it will be closed by Jdbc Template.
Other solutions:
Also, keep in mind that there are more different ways to implement the same things.
1. You can use queryForList() method which basically implements all the mapping for me.
Also, there is a possibility for you to use SQL which is even simpler.
Jdbc Template other Callbacks:
PreparedStatementCreator – should create PreparedStatement based on Connection provided by JdbcTemplate, implementation should provide SQL and parameters
PreparedStatementSetter – should set values on PreparedStatement provided by JdbcTemplate, implementation should only set parameters, SQL will be set by JdbcTemplate
CallableStatementCreator – should create CallableStatement based on Connection provided by JdbcTemplate, implementation should provide SQL and parameters
PreparedStatementCallback – used internally by JdbcTemplate – generic interface allowing number of operations on single PreparedStatement
CallableStatementCallback – used internally by JdbcTemplate – generic interface allowing number of operations on single CallableStatement