Lesson 13 Building Database Applications with JDBC

layout of the JDBC API
Connect to a database by using

a JDBC driver
Submit queries and get results from the database
Specify JDBC driver information externally
Use transactions with JDBC
Use the JDBC 4.1 RowSetProvider and RowSetFactory
Use a Data Access Object Pattern to decouple data and business methods

get an instance of a Connection object, using the JDBC

driver named in the JDBC URL:
String url = "jdbc:derby://localhost:1527/EmployeeDB";
Connection con = DriverManager.getConnection (url);
The URL syntax for a JDBC driver is:
jdbc::[subsubprotocol:][databaseName][;attribute=value]
Each vendor can implement its own subprotocol.
The URL syntax for an Oracle Thin driver is:
jdbc:oracle:thin:@//[HOST][:PORT]/SERVICE
Example:
jdbc:oracle:thin:@//myhost:1521/orcl

the key API classes that you will use to connect

to the database, execute queries, and manipulate data:
java.sql.Connection: A connection that represents the session between your Java application and the database
Connection con = DriverManager.getConnection(url, username, password);
java.sql.Statement: An object used to execute a static SQL statement and return the result
Statement stmt = con.createStatement();
java.sql.ResultSet: A object representing a database result set
String query = "SELECT * FROM Employee";
ResultSet rs = stmt.executeQuery(query);

Employee";
ResultSet rs = stmt.executeQuery(query);

ResultSet cursor
rs.next()
rs.next()
rs.next()
rs.next()
rs.next()
null
The last next() method

invocation returns false, and the rs instance is now null.

The first next() method invocation returns true, and rs points to the first row of data.

java.sql.ResultSet;
import java.sql.SQLException;
import java.util.Date;

public class SimpleJDBCTest {

public static void main(String[] args) {
String url = "jdbc:derby://localhost:1527/EmployeeDB";
String username = "public";
String password = "tiger";
String query = "SELECT * FROM Employee";
try (Connection con =
DriverManager.getConnection (url, username, password);
Statement stmt = con.createStatement ();
ResultSet rs = stmt.executeQuery (query)) {

The hard-coded JDBC URL, username, and password is just for this simple example.

{
int empID =

rs.getInt("ID");
String first = rs.getString("FirstName");
String last = rs.getString("LastName");
Date birthDate = rs.getDate("BirthDate");
float salary = rs.getFloat("Salary");
System.out.println("Employee ID: " + empID + "\n"
+ "Employee Name: " + first + " " + last + "\n"
+ "Birth Date: " + birthDate + "\n"
+ "Salary: " + salary);
} // end of while
} catch (SQLException e) {
System.out.println("SQL Exception: " + e);
} // end of try-with-resources
}
}

Loop through all of the rows in the ResultSet.

layer between your Java application and the database. However, you

also need to consider SQL syntax and semantics when writing database applications.
Most databases support a standard set of SQL syntax and semantics described by the American National Standards Institute (ANSI) SQL-92 Entry-level specification.
You can programmatically check for support for this specification from your driver:
Connection con = DriverManager.getConnection(url, username, password);
DatabaseMetaData dbm = con.getMetaData();
if (dbm.supportsANSI92EntrySQL()) {
// Support for Entry-level SQL-92 standard
}

resulting database errors. To report all the exceptions thrown, you

can iterate through the SQLExceptions thrown:
catch(SQLException ex) {
while(ex != null) {
System.out.println("SQLState: " + ex.getSQLState());
System.out.println("Error Code:" + ex.getErrorCode());
System.out.println("Message: " + ex.getMessage());
Throwable t = ex.getCause();
while(t != null) {
System.out.println("Cause:" + t);
t = t.getCause();
}
ex = ex.getNextException();
}
}

Vendor-dependent state codes, error codes and messages

released
close()
close()
Call close explicitly or in try-with-resources

DriverManager.getConnection(url, username, password);
Statement stmt

= con.createStatement();
ResultSet rs = stmt.executeQuery (query)){
The compiler checks to see that the object inside the parentheses implements java.lang.AutoCloseable.
This interface includes one method: void close().
The close method is automatically called at the end of the try block in the proper order (last declaration to first).
Multiple closeable resources can be included in the try block, separated by semicolons.

compactly:
try (ResultSet rs = DriverManager.getConnection(url, username, password).createStatement().executeQuery(query)) {
However, only the

close method of ResultSet is called, which is not a good practice.
Always keep in mind which resources you need to close when using try-with-resources.

you must create a SQL query wrapper object, an instance

of the Statement object.
Statement stmt = con.createStatement();
Use the Statement instance to execute a SQL query:
ResultSet rs = stmt.executeQuery (query);
Note that there are three Statement execute methods:

discover the number of columns and their type.
int numCols

= rs.getMetaData().getColumnCount();
String [] colNames = new String[numCols];
String [] colTypes = new String[numCols];
for (int i= 0; i < numCols; i++) {
colNames[i] = rs.getMetaData().getColumnName(i+1);
colTypes[i] = rs.getMetaData().getColumnTypeName(i+1);
}
System.out.println ("Number of columns returned: " + numCols);
System.out.println ("Column names/types returned: ");
for (int i = 0; i < numCols; i++) {
System.out.println (colNames[i] + " : " + colTypes[i]);
}

Note that these methods are indexed from 1, not 0.

is: “How many rows were returned?”

public int rowCount(ResultSet rs)

throws SQLException{
int rowCount = 0;
int currRow = rs.getRow();
// Valid ResultSet?
if (!rs.last()) return -1;
rowCount = rs.getRow();
// Return the cursor to the current position
if (currRow == 0) rs.beforeFirst();
else rs.absolute(currRow);
return rowCount;
}
To use this technique, the ResultSet must be scrollable.

Move the cursor to the last row, this method returns false if the ResultSet is empty.

Returning the row cursor to its original position before the call is a good practice.

at one time by a query is determined by the

JDBC driver. You may wish to control this behavior for large data sets.
For example, if you wanted to limit the number of rows fetched into cache to 25, you could set the fetch size:

rs.setFetchSize(25);

Calls to rs.next() return the data in the cache until the 26th row, at which time the driver will fetch another 25 rows.

to pass arguments to a precompiled SQL statement.
double value

= 100_000.00;
String query = "SELECT * FROM Employee WHERE Salary > ?";
PreparedStatement pStmt = con.prepareStatement(query);
pStmt.setDouble(1, value);
ResultSet rs = pStmt.executeQuery();

In this code fragment, a prepared statement returns all columns of all rows whose salary is greater than $100,000.
PreparedStatement is useful when you have a SQL statements that you are going to execute multiple times.

Substitutes value for the first parameter in the prepared statement.

Parameter for substitution.

to be executed against the database.
CallableStatement cStmt

= con.prepareCall("{CALL EmplAgeCount (?, ?)}");
int age = 50;
cStmt.setInt (1, age);
ResultSet rs = cStmt.executeQuery();
cStmt.registerOutParameter(2, Types.INTEGER);
boolean result = cStmt.execute();
int count = cStmt.getInt(2);
System.out.println("There are " + count +
" Employees over the age of " + age);
Stored procedures are executed on the database.

The IN parameter is passed in to the stored procedure.

The OUT parameter is returned from the stored procedure.

groups of operations as though they were one.
Either all operations

in a transaction occur or none occur at all.
The operations involved in a transaction might rely on one or more databases.

the set of properties that is known by the acronym

ACID.
Atomicity: A transaction is done or undone completely. In the event of a failure, all operations and procedures are undone, and all data rolls back to its previous state.
Consistency: A transaction transforms a system from one consistent state to another consistent state.
Isolation: Each transaction occurs independently of other transactions that occur at the same time.
Durability: Completed transactions remain permanent, even during system failure.

an inconsistent state.) (B)
$1000
+$100
$1100
$500
-$100
$400
2) Deposit: $100
1) Withdraw: $100
Transfer:

$100
From: Acct 1
To: Acct 2

ATM

Account 1

Account 2

$1000

$500
-$100
$400

1) Withdraw: $100

Transfer: $100
From: Acct 1
To: Acct 2

ATM

A

B

Failed
Deposit

Bank

Bank

Account 1

Account 2

a transfer is successful, changes are committed (made permanent). (B)
$1100
$400
Commit
Transfer

Successful

Commit

$1000
+$100
$1100

$500
-$100
$400

2) Deposit: $100

1) Withdraw: $100

Transfer: $100
From: Acct 1
To: Acct 2

ATM

Account 1

Account 2

A

Bank

Transaction Started by Bank

Account 1

Account 2

ATM

B

Bank

Transaction Started by Bank

a problem occurs, the transaction is rolled back to the

previous consistent state. (B)

$500
-$100
$400

1) Withdraw: $100

Transfer: $100
From: Acct 1
To: Acct 2

ATM

Account 1

Account 2

A

Bank

Transaction Started by Bank

$1000

Failed
Deposit

$1000

$500

Rollback

Error Message

Rollback

ATM

B

Bank

Transaction Started by Bank

Account 1

Account 2

in auto-commit mode.
Each individual SQL statement is treated as a

transaction and automatically committed after it is executed.
To group two or more statements together, you must disable auto-commit mode.
con.setAutoCommit (false);
You must explicitly call the commit method to complete the transaction with the database.
con.commit();

You can also programmatically roll back transactions in the event of a failure.
con.rollback();

the new RowSet 1.1 API. One of the new features

of this API is RowSetProvider.
javax.sql.rowset.RowSetProvider is used to create a RowSetFactory object:

myRowSetFactory = RowSetProvider.newFactory();

The default RowSetFactory implementation is:

com.sun.rowset.RowSetFactoryImpl

RowSetFactory is used to create one of the RowSet 1.1 RowSet object types.

RowSet implementations:

RowSetProvider.newFactory().createJdbcRowSet()) {
jdbcRs.setUrl(url);
jdbcRs.setUsername(username);

jdbcRs.setPassword(password);
jdbcRs.setCommand("SELECT * FROM Employee");
jdbcRs.execute();
// Now just treat JDBC Row Set like a ResultSet object
while (jdbcRs.next()) {
int empID = jdbcRs.getInt("ID");
String first = jdbcRs.getString("FirstName");
String last = jdbcRs.getString("LastName");
Date birthDate = jdbcRs.getDate("BirthDate");
float salary = jdbcRs.getFloat("Salary");
}
//... other methods
}

the sample JDBC code.




By combining the code that accesses the

database with the “business” logic, the data access methods and the Employee table are tightly coupled.
Any changes to the table (such as adding a field) will require a complete change to the application.
Employee data is not encapsulated within the example application.

layout of the JDBC API
Connect to a database by using

a JDBC driver
Submit queries and get results from the database
Specify JDBC driver information externally
Use transactions with JDBC
Use the JDBC 4.1 RowSetProvider and RowSetFactory
Use a Data Access Object Pattern to decouple data and business methods

number of rows affected?
stmt.execute(query);
stmt.executeUpdate(query);
stmt.executeQuery(query);
stmt.query(query);

only one record, it is not necessary to use the

ResultSet's next() method.
True
False

try (Statement stmt = con.createStatement();
ResultSet rs =

stmt.executeQuery(query)){
//...
} catch (SQLException s) {
}
True
False

itemCount FROM Customer " +

"WHERE lastName='?' AND firstName='?'";
try (PreparedStatement pStmt = con.prepareStatement(query)) {
for (int i = 0; i < params.length; i++)
pStmt.setObject(i, params[i]);
ResultSet rs = pStmt.executeQuery();
while (rs.next()) System.out.println (rs.getInt("itemCount"));
} catch (SQLException e){ }

Assuming there is a valid Connection object and the SQL query will produce at least one row, what is the result?
Each itemCount value for customer Bob Smith
Compiler error
A run time error
A SQLException