Processing SQL Statements with JDBC
In general, to process any SQL statement with JDBC, you follow these steps:
- Establishing a connection.
- Create a statement.
- Execute the query.
- Process the
ResultSet
object. - Close the connection.
This page uses the following method, CoffeesTables.viewTable
, from the tutorial sample to demonstrate these steps. This method outputs the contents of the table COFFEES
. This method will be discussed in more detail later in this tutorial:
public static void viewTable(Connection con, String dbName) throws SQLException { Statement stmt = null; String query = "select COF_NAME, SUP_ID, PRICE, " + "SALES, TOTAL " + "from " + dbName + ".COFFEES"; try { stmt = con.createStatement(); ResultSet rs = stmt.executeQuery(query); while (rs.next()) { String coffeeName = rs.getString("COF_NAME"); int supplierID = rs.getInt("SUP_ID"); float price = rs.getFloat("PRICE"); int sales = rs.getInt("SALES"); int total = rs.getInt("TOTAL"); System.out.println(coffeeName + "\t" + supplierID + "\t" + price + "\t" + sales + "\t" + total); } } catch (SQLException e ) { JDBCTutorialUtilities.printSQLException(e); } finally { if (stmt != null) { stmt.close(); } } }
Establishing Connections
First, establish a connection with the data source you want to use. A data source can be a DBMS, a legacy file system, or some other source of data with a corresponding JDBC driver. This connection is represented by a Connection
object. See Establishing a Connection for more information.
Creating Statements
A Statement
is an interface that represents a SQL statement. You execute Statement
objects, and they generate ResultSet
objects, which is a table of data representing a database result set. You need a Connection
object to create a Statement
object.
For example, CoffeesTables.viewTable
creates a Statement
object with the following code:
stmt = con.createStatement();
There are three different kinds of statements:
Statement
: Used to implement simple SQL statements with no parameters.PreparedStatement
: (ExtendsStatement
.) Used for precompiling SQL statements that might contain input parameters. See Using Prepared Statements for more information.CallableStatement:
(ExtendsPreparedStatement
.) Used to execute stored procedures that may contain both input and output parameters. See Stored Procedures for more information.
Executing Queries
To execute a query, call an execute
method from Statement
such as the following:
execute
: Returnstrue
if the first object that the query returns is aResultSet
object. Use this method if the query could return one or moreResultSet
objects. Retrieve theResultSet
objects returned from the query by repeatedly callingStatement.getResultSet
.executeQuery
: Returns oneResultSet
object.executeUpdate
: Returns an integer representing the number of rows affected by the SQL statement. Use this method if you are usingINSERT
,DELETE
, orUPDATE
SQL statements.
For example, CoffeesTables.viewTable
executed a Statement
object with the following code:
ResultSet rs = stmt.executeQuery(query);
See Retrieving and Modifying Values from Result Sets for more information.
Processing ResultSet Objects
You access the data in a ResultSet
object through a cursor. Note that this cursor is not a database cursor. This cursor is a pointer that points to one row of data in the ResultSet
object. Initially, the cursor is positioned before the first row. You call various methods defined in the ResultSet
object to move the cursor.
For example, CoffeesTables.viewTable
repeatedly calls the method ResultSet.next
to move the cursor forward by one row. Every time it calls next
, the method outputs the data in the row where the cursor is currently positioned:
try { stmt = con.createStatement(); ResultSet rs = stmt.executeQuery(query); while (rs.next()) { String coffeeName = rs.getString("COF_NAME"); int supplierID = rs.getInt("SUP_ID"); float price = rs.getFloat("PRICE"); int sales = rs.getInt("SALES"); int total = rs.getInt("TOTAL"); System.out.println(coffeeName + "\t" + supplierID + "\t" + price + "\t" + sales + "\t" + total); } } // ...
See Retrieving and Modifying Values from Result Sets for more information.
Closing Connections
When you are finished using a Statement
, call the method Statement.close
to immediately release the resources it is using. When you call this method, its ResultSet
objects are closed.
For example, the method CoffeesTables.viewTable
ensures that the Statement
object is closed at the end of the method, regardless of any SQLException
objects thrown, by wrapping it in a finally
block:
} finally { if (stmt != null) { stmt.close(); } }
JDBC throws an SQLException
when it encounters an error during an interaction with a data source. See Handling SQL Exceptions for more information.
In JDBC 4.1, which is available in Java SE release 7 and later, you can use a try-with-resources statement to automatically close Connection
, Statement
, and ResultSet
objects, regardless of whether an SQLException
has been thrown. An automatic resource statement consists of a try
statement and one or more declared resources. For example, you can modify CoffeesTables.viewTable
so that its Statement
object closes automatically, as follows:
public static void viewTable(Connection con) throws SQLException { String query = "select COF_NAME, SUP_ID, PRICE, " + "SALES, TOTAL " + "from COFFEES"; try (Statement stmt = con.createStatement()) { ResultSet rs = stmt.executeQuery(query); while (rs.next()) { String coffeeName = rs.getString("COF_NAME"); int supplierID = rs.getInt("SUP_ID"); float price = rs.getFloat("PRICE"); int sales = rs.getInt("SALES"); int total = rs.getInt("TOTAL"); System.out.println(coffeeName + ", " + supplierID + ", " + price + ", " + sales + ", " + total); } } catch (SQLException e) { JDBCTutorialUtilities.printSQLException(e); } }
The following statement is an try
-with-resources statement, which declares one resource, stmt
, that will be automatically closed when the try
block terminates:
try (Statement stmt = con.createStatement()) { // ... }
See The try
-with-resources Statement in the Essential Classes trail for more information.