Is it must to close all my ResultSets, Statements and Connections?

This is absolutely necessary and critical to close all the JDBC resources.When you are using JDBC API you are not only allocating resources on Java side but in a database server as well. Failure to properly close ResultSets, Statements (and PreparedStatements and CallableStatements) and Connections can cause
-Exhaustion of connections that can be opened or used
- No new result sets creation
- No execution of any queries of any kind
- DB server get very slow
- Your program/database server will crash