Selecting on Oracle via JDBC

Once we decided how to establish a connection to the database, performing a SELECT on it is pretty simple. Here I'm going to use OracleDataSource as connection provider, using pure JDBC doesn't add any complexity.

The only problem in this Java code is that is slightly boring. So boring that there are a few wrapper aiming at keeping the advantages of JDBC easing its usage. See for instance the Spring JdbcTemplate.

Anyway, firstly we open a connection, then a statement, on which we execute a query (a select, in this case) that return a resultset. We normally do something on the resultset, and then we can close what we have opened, in reversed order, resultset, statement, connection. The code is made even more boring by the need of try-catching the SQLExceptions that could be thrown. Last straw in this boredom parade, is that before closing the JDBC objects we have to ensure they have been created, checking them for not being null.

So, our typical plain JDBC code could be seen in three steps:

Initialization

// ...
Connection conn = null;
Statement stmt = null;
ResultSet rs = null;
try {
    conn = ods.getConnection();  // 1
    stmt = conn.createStatement();
    rs = stmt.executeQuery("SELECT first_name FROM employees");  // 2

    // ...
1. I'm using the OracleDataSourceConnector I described in the previous post. Nothing fancy, anyway, I simply get a connection to a Oracle database for the well known hr example user.
2. Having extracted a statement from the connection, I execute a query, that is plain SQL SELECT. The resulting table, here consisting of just one column, is returned in a ResultSet.

Operation

Let's do something with the fetched data. Here I copy the first names to a list of string, then I sort them and I ensure the first and last are what I expect. Usually this part of the job has more sense.
List<String> results = new ArrayList<String>();
while (rs.next()) {
    results.add(rs.getString(1));
}

assertEquals(107, results.size());
Collections.sort(results);
assertEquals("Adam", results.get(0));
assertEquals("Winston", results.get(106));
The employees table on HR should have 107 rows, first names should range from Adam to Winston, however, your result may vary.

Cleanup

Finally (actually, the code is normally performed in the finally block following the try one where the initialization and operation happened - meaning that we would always perform this step, whatever happens there), being done with the data, we can close result set, statement, and connection.
assertNotNull(rs);
rs.close();

assertNotNull(stmt);
stmt.close();

assertNotNull(conn);
conn.close();
Notice that we have to strictly follow the inverted order seen in initialization.
Notice also that each close() method could throw and exception so, again, these calls have to be in a try-catch block.

See full code on GitHub, class GettingStartedTest, method testSelect().

Reference: Oracle Database JDBC Developer's Guide 12c Release 2 (12.2) 2.4 Sample: Connecting, Querying, and Processing the Results

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