Calling a Stored Function from JDBC

The code we should write to call a stored procedure or function via JDBC is slightly different from the one we write to perform a SQL SELECT. A CallableStatement is used instead of a Statement plus a ResultSet to set the parameter, execute the call, and extract the result.

To check it, I have firstly created a stored function in my Oracle 12 instance, on the HR schema. It takes a SQL CHAR in input, and returns the same string with 'suffix' appended:

create or replace FUNCTION foo (val CHAR)
RETURN CHAR AS
BEGIN
    RETURN val || 'suffix';
END;

I ensured it works as I expected running this query:

SELECT foo('hello') FROM DUAL;

The I have written a couple of tests to see how to get the same result in Java. Not just one, because we can use either the JDBC escape and the PL/SQL block syntax to achieve the same result, being the first

{? = call foo(?)

and the latter

begin ? := foo(?); end;

The core of both tests is in these few lines:

cs = conn.prepareCall(call);  // 1
cs.registerOutParameter(1, Types.CHAR);  // 2
cs.setString(2, "aa");
cs.execute();
String result = cs.getString(1);  // 3

1. Assuming conn is a good java.sql connection to the HR user on my Oracle database, and call is either the JDBC escape or the PL/SQL block string showed above, the prepareCall() should return a good CallableStatement.
2. The callable statement has to know the type of the output parameter is a character string. Then we set the other parameter, with the string that we want to pass as input.
3. After executing the callable statement, we get the first (and only) result as a string.

The actual code, that you could find on GitHub, class GettingStartedTest methods callFoo(), testFunctionCallJdbcEscape(), and testFunctionCallPlSqlBlock(), is a bit more verbose because I have to provide all the standard boilerplate, initializing, testing, cleaning up.

Reference: Oracle Database JDBC Developer's Guide 12c Release 2 (12.2) 2.8 Stored Procedure Calls in JDBC Programs