There are some minor potential dangers associated with Groovy's def keyword. The Groovy style and language feature guidelines for Java developers provides some warnings about use of def. In this blog post, I demonstrate an advantage of being more explicit in typing when using Groovy SQL with an Oracle database to avoid a potential "Invalid column index" SQLException because I've run into this issue a few times.

The following Groovy script provides comments on Oracle database tables matching a provided search string. In this case, what the script does is not as important as look at the code that defines the SQL query string (lines 18-21).

searchDbComments.groovy (using def without String typing or as String)

#!/usr/bin/env groovy
// searchDbComments.groovy

this.class.classLoader.rootLoader.addURL(
   new URL("file:///C:/oraclexe/app/oracle/product/11.2.0/server/jdbc/lib/ojdbc6.jar"))

if (args.length < 1)
{
   println "USAGE: searchDbComments.groovy <searchString>"
   System.exit(-1)
}

def searchString = args[0].toUpperCase()

import groovy.sql.Sql
def sql = Sql.newInstance("jdbc:oracle:thin:@localhost:1521:xe", "hr", "hr",
                          "oracle.jdbc.pool.OracleDataSource")
def dbTableCommentsQry = """
SELECT table_name, table_type, comments
  FROM user_tab_comments
 WHERE UPPER(comments) LIKE '%${searchString}%'"""

sql.eachRow(dbTableCommentsQry)
{
   println "${it.table_name} (${it.table_type}): ${it.comments}"
}

When the above code is executed, the following error is generated:

WARNING: Failed to execute: 
SELECT table_name, table_type, comments
  FROM user_tab_comments
 WHERE UPPER(comments) LIKE '%?%' because: Invalid column index

Caught: java.sql.SQLException: Invalid column index

java.sql.SQLException: Invalid column index
    at oracle.jdbc.driver.OraclePreparedStatement.setStringInternal(OraclePreparedStatement.java:5303)
    at oracle.jdbc.driver.OraclePreparedStatement.setObjectCritical(OraclePreparedStatement.java:8323)
    at oracle.jdbc.driver.OraclePreparedStatement.setObjectInternal(OraclePreparedStatement.java:8259)
    at oracle.jdbc.driver.OraclePreparedStatement.setObjectInternal(OraclePreparedStatement.java:9012)
    at oracle.jdbc.driver.OraclePreparedStatement.setObject(OraclePreparedStatement.java:8993)
    at oracle.jdbc.driver.OraclePreparedStatementWrapper.setObject(OraclePreparedStatementWrapper.java:230)
    at searchDbComments.run(searchDbComments.groovy:23)

Addressing the "invalid column index" SQLException is easy. One solution is to change the "def" on lines 18-21 to an explicit "String" type. Another solution, shown in the next code listing, is to use Groovy's "as" coercion keyword to explicitly allow the "def" to be used and have the dbTableCommentsQry variable be typed as a String.

searchDbComments.groovy (using as String)

#!/usr/bin/env groovy
// searchDbComments.groovy

this.class.classLoader.rootLoader.addURL(
   new URL("file:///C:/oraclexe/app/oracle/product/11.2.0/server/jdbc/lib/ojdbc6.jar"))

if (args.length < 1)
{
   println "USAGE: searchDbComments.groovy <searchString>"
   System.exit(-1)
}

def searchString = args[0].toUpperCase()

import groovy.sql.Sql
def sql = Sql.newInstance("jdbc:oracle:thin:@localhost:1521:xe", "hr", "hr",
                          "oracle.jdbc.pool.OracleDataSource")
def dbTableCommentsQry = """
SELECT table_name, table_type, comments
  FROM user_tab_comments
 WHERE UPPER(comments) LIKE '%${searchString}%'""" as String

sql.eachRow(dbTableCommentsQry)
{
   println "${it.table_name} (${it.table_type}): ${it.comments}"
}

Using "def" only or no "def" with no type at all leads to the above error. Explicitly defining the String variable used in the query either via static typing or via use of "as" keyword allows the code to execute properly. One could use a static typing with "def", but that is thought to be redundant.

There is nothing necessarily wrong about using "def," but one does need to be careful with its application. Guillaume Laforge has written that "def is fine in method bodies or for particular dynamic aspects, but for everything that is a 'contract' (method signatures, properties, etc), it's better to use explicit types."