1. CS 160: Software Engineering October 1 Class Meeting Department of Computer Science San Jose State University Fall 2014 Instructor: Ron Mak www.cs.sjsu.edu/~mak

2. Computer Science Dept. Fall 2014: October 1 CS 160: Software Engineering © R. Mak 2 JDBC  Use the JDBC (Java Database Connectivity) API in the java.sql package to make your Java program communicate with a database.  Requires the use of a database driver. Download Connector/J from http://dev.mysql.com/downloads/connector/j/ http://dev.mysql.com/downloads/connector/j/ Jar file: mysql-connector-java-5.1.33-bin.jar

3. Computer Science Dept. Fall 2014: October 1 CS 160: Software Engineering © R. Mak 3 JDBC Connection  Make a connection to the database using a URL, username, and password. import java.sql.*;... private static String DB_URL = "jdbc:mysql://localhost:3306/school"; private static String USERNAME = "root"; private static String PASSWORD = "sesame";... Connection conn = DriverManager.getConnection(DB_URL, USERNAME, PASSWORD);

4. Computer Science Dept. Fall 2014: October 1 CS 160: Software Engineering © R. Mak 4 JDBC Query  Create a statement and then generate a result set by executing a query. String QUERY = "SELECT * FROM teacher"; Statement stmt = conn.createStatement(); ResultSet rs = stmt.executeQuery(QUERY);

5. Computer Science Dept. Fall 2014: October 1 CS 160: Software Engineering © R. Mak 5 Iterate over a JDBC Result Set IdLastFirst 7003RogersTom 7008ThompsonArt 7012LaneJohn 7051FlynnMabel Teacher int id; String lastName; String firstName; while (rs.next()) { id = rs.getInt("id"); lastName = rs.getString("last"); firstName = rs.getString("first");... } Instead of the database field names, you can use 1, 2, 3,...

6. Computer Science Dept. Fall 2014: October 1 CS 160: Software Engineering © R. Mak 6 Object-Relational Mapping  Create Java objects from relational database tables. public class Teacher { int id; String lastName; String firstName; }... while (rs.next()) { Teacher teacher = new Teacher(rs.getInt("id"), rs.getString("last"), rs.getString("first"));... } The Java Persistence Architecture (JPA) and open-source tools such as Hibernate do object-relational mapping between a Java program and a relational database.

7. Computer Science Dept. Fall 2014: October 1 CS 160: Software Engineering © R. Mak 7 SQL Query Example  Who are John Lane’s students? IdLastFirst 1001DoeJohn 1005NovakTim 1009KleinLeslie 1014JaneMary 1021SmithKim CodeTeacher_idSubjectRoom 9087008Data structures114 9267003Java programming101 9317051Compilers222 9517012Software engineering210 9747012Operating systems109 Student_idClass_code 1001926 1001951 1001908 1005974 1005908 1014931 1021926 1021974 1021931 SELECT student.first, student.last, subject FROM student, teacher, class, student_class WHERE teacher.last = 'Lane' AND teacher.first = 'John' AND teacher_id = teacher.id AND code = class_code AND student.id = student_id ORDER BY subject, student.last +-------+-------+----------------------+ | first | last | subject | +-------+-------+----------------------+ | Tim | Novak | Operating systems | | Kim | Smith | Operating systems | | John | Doe | Software engineering | +-------+-------+----------------------+ IdLastFirst 7003RogersTom 7008ThompsonArt 7012LaneJohn 7051FlynnMabel Teacher StudentClass Student_Class

8. Computer Science Dept. Fall 2014: October 1 CS 160: Software Engineering © R. Mak 8 JDBC Prepared Statement String query = "SELECT student.first, student.last, subject " + "FROM student, teacher, class, student_class " + "WHERE teacher.last = ? AND teacher.first = ? " + "AND teacher_id = teacher.id " + "AND code = class_code AND student.id = student_id " + "ORDER BY subject, student.last"; PreparedStatement ps = conn.prepareStatement(query);  A query statement in a loop is inefficient, because the database server has to reparse the statement and build an execution plan each time, even if the statement doesn’t change.  Use a prepared statement instead. Note the two ? parameters.

9. Computer Science Dept. Fall 2014: October 1 CS 160: Software Engineering © R. Mak 9 JDBC Prepared Statement, cont’d  You can do repeated queries on different teachers by using a prepared statement and parameter substitution. for (Teacher teacher : teachers) { String lastName = teacher.getLastName(); String firstName = teacher.getFirstName(); ps.setString(1, lastName); ps.setString(2, firstName); ResultSet rs = ps.executeQuery(); while (rs.next()) {... } Count the ?’s from 1, not 0.

10. Computer Science Dept. Fall 2014: October 1 CS 160: Software Engineering © R. Mak 10 JDBC Result Set Metadata  Each result set has metadata that contains useful information about the query. number of columns column labels etc. ResultSet rs = ps.getResultSet(); ResultSetMetaData rsmd = rs.getMetaData();... int colCount = rsmd.getColumnCount(); String label1 = rsmd.getColumnLabel(1); String label2 = rsmd.getColumnLabel(2);

11. Computer Science Dept. Fall 2014: October 1 CS 160: Software Engineering © R. Mak 11 Database Record Insert, Update, and Delete  There are SQL statements to insert, update, and delete records. See the Servlet/JSP book. INSERT INTO teacher (id, last, first) VALUES (7088, 'Mak', 'Ron'), (7090, 'Wilson', 'Brian') UPDATE teacher SET first = 'Ronald' WHERE first = 'Ron' DELETE FROM teacher WHERE id = 7090 This can update multiple records!

12. Computer Science Dept. Fall 2014: October 1 CS 160: Software Engineering © R. Mak 12 executeUpdate()  JDBC API: Use the executeUpdate() method of a statement or prepared statement object to modify the database (insert, update, or delete).  See the Servlet/JSP book.  The return value is the number of records that were affected. _

13. Computer Science Dept. Fall 2014: October 1 CS 160: Software Engineering © R. Mak 13 Closing JDBC Objects  When you’re done with them, don’t forget to close your JDBC statement, prepared statement, and result set objects, and especially the database connection object.  Note that most JDBC API calls throw an exception if an error occurred, generally SQLException, which you’ll need to catch. A database server can support only a limited number of connections. stmt.close(); ps.close(); rs.close(); conn.close();

14. Computer Science Dept. Fall 2014: October 1 CS 160: Software Engineering © R. Mak 14 JavaBeans  Use object-relational mapping to create JavaBeans.  A JavaBean is an object instantiated from a class that: Has no public fields Has a default (no-argument) constructor Has public getters and setter methods for its private fields Optionally implements java.io.Serializable _

15. Computer Science Dept. Fall 2014: October 1 CS 160: Software Engineering © R. Mak 15 JavaBeans, cont’d  Use JavaBeans to represent: Model objects (in the MVC sense) of your application.  Examples: student, teacher, class, etc. Query results  Not all model objects need to be JavaBeans.  Not all model objects need to be persisted. _

16. Computer Science Dept. Fall 2014: October 1 CS 160: Software Engineering © R. Mak 16 JavaBeans, cont’d  The Teacher class as a JavaBean: public class Teacher implements java.io.Serializable { private int id; private String lastName; private String firstName; public Teacher() { this(0, "", ""); } public Teacher(int id, String lastName, String firstName) { this.id = id; this.lastName = lastName; this.firstName = firstName; } public int getId() { return id; }... public void setId(int id) { this.id = id; }... }

17. Computer Science Dept. Fall 2014: October 1 CS 160: Software Engineering © R. Mak 17 JavaBeans, cont’d  JSP pages have special features for JavaBeans. Example: JSP Expression Language (EL) with a teacher object. Id ${teacher.id} First name ${teacher.firstName} Last name ${teacher.lastName} EL will automatically generate the appropriate getter method calls.

18. Computer Science Dept. Fall 2014: October 1 CS 160: Software Engineering © R. Mak 18 Database Connection Pool  Opening and closing a database connection are slow operations. A running web application may have many simultaneous servlet threads each making data requests, and each request needs a connection.  Solution: Create a pool of open connections. When a data request needs a connection, it gets an open connection from the pool. When the request is done, it returns the open connection to the pool for another request to use. The pool can grow and shrink based on usage.

19. Computer Science Dept. Fall 2014: October 1 CS 160: Software Engineering © R. Mak 19 Database Connection Pool (cont’d)  Tomcat provides tomcat-dbcp.jar that contains an implementation of a database connection pool.  Edit your application’s context.xml file to set parameters for the connection pool. See the Servlet/JSP book. Connection pool Servlet threadsDatabase OPEN CONNECTIONS

20. Computer Science Dept. Fall 2014: October 1 CS 160: Software Engineering © R. Mak 20 Data Access Layer  Databases and SQL are extremely powerful.  Let MySQL do what it’s good at doing, and let Java do what it’s good at doing. For example, don’t write Java code to sort the retrieved records – let the database do that! _

21. Computer Science Dept. Fall 2014: October 1 CS 160: Software Engineering © R. Mak 21 Data Access Layer, cont’d  Add a data access layer to your server-side architecture.  The data access layer contains all the JDBC API calls and manages the database connection pool.  Keep the rest of your application loosely-coupled from the database code.  With object-relational mapping, the rest of your application deals only with objects, not result sets. _

22. Computer Science Dept. Fall 2014: October 1 CS 160: Software Engineering © R. Mak 22 SERVER SIDE Multilayered Server-Side Architecture Presentation Layer View Objects (JSPs) Application Layer Controller Objects (Servlets) Data Access Layer Fetch and Store Model Objects (JavaBeans) Database

23. Computer Science Dept. Fall 2014: October 1 CS 160: Software Engineering © R. Mak 23 Your Initial End-to-End Thread  Make the first development goal of your web application be the initial end-to-end round-trip thread of execution.  This thread doesn’t have to do much. Example: Fetch and display some data from the database based on a user selection. The user makes a selection on a web page  server code  database access to fetch and create a JavaBean  server code  display JavaBean data on a web page _

24. Computer Science Dept. Fall 2014: October 1 CS 160: Software Engineering © R. Mak 24 Your Initial End-to-End Thread, cont’d  Demonstrate that your architectural framework is sound.  Validate that all the framework components work together.  From then on: Always build on code that’s already working. _