Avoid code duplication with static classes in Java

Question

I'm writing a Java application that will have to manage two different databases (let's say Onebase and Twobase ) created using PostgreSQL .

In order to handle connections and queries, I created for the first one a file (named OnebaseUtil ) with JDBC , that more or less looks like this:

public class OnebaseUtil {
    private static final String JDBC_DRIVER = "org.postgresql.Driver";
    private static Connection conn = null;
    private static String url = "jdbc:postgresql://localhost:5432/onebase?user=useradmin&password=123456";

    private static void dbConnect() { ... }

    private static void dbDisconnect() { ... }

    public static ResultSet dbExecuteQuery(String queryStmt) { ... }

    public static void dbExecuteUpdate(String sqlStmt) { ... }
}

Then I started writing the TwobaseUtil , just finding out the code was exactly the same, except for the url ( ...twobase... instead of ...onebase... ).

How can I avoid having a code duplicate for both databases? Notice that as you can see all methods are static and I would like to keep using them as static through all the code. I thought about using a single file (let's say BothbaseUtil ) setting a constructor with the String passed every time I'm using it... but I would have to instantiate it every time! And I really want to avoid this.

Thanks anyone for help!

Answer 1

In fact this database code pattern is seen very often, but has disadvantages.

One better would use a DataSource and store several datasources declaratively in XML or properties. (Different for development/production).

One way to use such a DataSource would be using CDI, dependency injection. Then you would still have the freedom of creating unit tests by test data sources.

The open/close level and execute functions are tempting too, but try-with-resources and PreparedStatement are superior.

try (Connection con = ...) {
}

Closes the connection automatically even with return/exception.

try (PreparedStatement stm = con.prepare...) {
    stm.setString(1, "me");
    try (ResultSet rs = stm.executeQuery()) {
        while (rs.next()) {
            ...
        }
    }
}

Answer 2

Sounds like you're looking for an enum .

enum Db {
    One("jdbc:postgresql://localhost:5432/onebase?user=useradmin&password=123456"),
    Two("Something else");
    private static final String JDBC_DRIVER = "org.postgresql.Driver";
    private static Connection conn = null;
    private final String url;


    Db(String url) {
        this.url = url;
    }

    private static void dbConnect() { ... }

    private static void dbDisconnect() { ... }

    public static ResultSet dbExecuteQuery(String queryStmt) { ... }

    public static void dbExecuteUpdate(String sqlStmt) { ... }

}

Note that any methods that use the url filed will have to be removed from static but that shouldn't be a bad thing. You still only need them once in the code.

Answer 3

Just pass the url to the method which connects to the database. That way you will first connect to the relevant DB and then perform the queries you need.