How to call a custom Oracle function returning a value from JPA

I used this to execute native functions in oracle:

Query query = em.createNativeQuery("select my_function(:param) from dual");
query.setParameter("param", 4);
return query.getSingleResult();

From https://vladmihalcea.com/how-to-call-oracle-stored-procedures-and-functions-from-hibernate/:

First, we can simply call the Oracle function just like any other SQL query:

BigDecimal commentCount = (BigDecimal) entityManager
    .createNativeQuery(
        "SELECT fn_count_comments(:postId) FROM DUAL"
    )
    .setParameter("postId", 1L)
    .getSingleResult();

Another approach is to call the database function using plain JDBC API:

Session session = entityManager.unwrap( Session.class );

final AtomicReference<Integer> commentCount = 
    new AtomicReference<>();

session.doWork( connection -> {
    try (CallableStatement function = connection
            .prepareCall(
                "{ ? = call fn_count_comments(?) }"
            )
        ) {
        function.registerOutParameter( 1, Types.INTEGER );
        function.setInt( 2, 1 );
        function.execute();
        commentCount.set( function.getInt( 1 ) );
    }
} );

When using Spring Data JPA / Spring JDBC you can use SimpleJdbcCall for calling functions and procedures.

For such function header:

FUNCTION get_text (p_param IN VARCHAR2) RETURN VARCHAR2

Use this call:

import org.springframework.jdbc.core.JdbcTemplate;
import org.springframework.jdbc.core.namedparam.MapSqlParameterSource;
import org.springframework.jdbc.core.namedparam.SqlParameterSource;
import org.springframework.jdbc.core.simple.SimpleJdbcCall;
import org.springframework.stereotype.Repository;

@Repository
public class MyRepository {

    @Autowired
    private JdbcTemplate jdbcTemplate;

    @Override
    public String callGetTextFunction(String param) {
        SimpleJdbcCall jdbcCall = new SimpleJdbcCall(jdbcTemplate)
                .withCatalogName("PKG_PACKAGE") //package name
                .withFunctionName("GET_TEXT"); //function name
        SqlParameterSource paramMap = new MapSqlParameterSource()
                .addValue("p_param", param));
        //First parameter is function output parameter type.
        return jdbcCall.executeFunction(String.class, paramMap));
    }

}

If you are using EclipseLink you can use a StoredFunctionCall object to call a stored function such as this.

You could also define it as a named query using the @NamedStoredFunctionQuery.

It's a quite old question but I have not found here the proper solution... Here is a code that works in my project:

    EntityManager em;
    String functionName = "NameOfStoredFunction";
    Map<String, Object> arguments = new LinkedHashMap();
    arguments.put("nameOfArgument", "value");
    StoredFunctionCall functionCall = new StoredFunctionCall();
    functionCall.setProcedureName(functionName);
    functionCall.setResult("RESULT", String.class);

    for(String key : arguments.keySet()){
        functionCall.addNamedArgumentValue(key, arguments.get(key));
    }

    ValueReadQuery valQuery = new ValueReadQuery();
    valQuery.setCall(functionCall);

    Query query = ((JpaEntityManager)em.getDelegate()).createQuery(valQuery);

    String call_result = (String)query.getSingleResult();