Intro to JDBC Joseph Sant Applied Computing and Engineering Sciences Sheridan ITAL

Persistence Retaining information in a database or file after a user request. Could be implemented using  Relational databases.  Files.  Object-Oriented Databases.  Object-Relational Databases.

JDBC Set of classes for interacting with a Relational Database. Implementation Neutral. The API is designed with the aim that code, once written can be run against several different database implementations (e.g. Oracle, PostGresQL) without modification. The reality is that true implementation neutrality is still difficult.

JDBC Abstract SQL in Java Program JDBC Driver Vendor Specific Execution of Java SQL Ansi SQLVendor’s SQL SQL ResultSet Java-Friendly ResultSet

JDBC-Key terms Connection. An authorized connection between a Java Program or servlet and a running database. Driver. Software object to take JDBC API calls from a Java program or servlet, translate it appropriately for the specific database, then pass it on to that database.

JDBC-Key terms Query. A statement that requests a Result Set from a database. It does NOT change the state of the database.  SELECT Update. A statement that potentially changes the state of the database. It may be any of the following:  DELETE  INSERT  UPDATE  DROP  Others

Overview of Database Processing using JDBC 1. Load Database Driver (once). 2. Establish a Connection to database from program (usually once but not always). 3. Create a Statement object to enable actions against the database. 1. Execute Queries using executeQuery() (repetitively). Process the ResultSet for each executeQuery(). 2. AND/OR 3. Execute Updates using executeUpdate() (repetitively). Return result of executeUpdate() should be tested and responded to.

Establishing a Connection: PreRequisites. The database engine you are connecting to is running. The program or servlet connecting to the database has access rights to that database. The driver, which is a java class file or jar file, is accessible to either the application, applet or servlet.

Establishing a Connection: Technique Dynamically Load the driver class using Class.forName. Driver must be available on CLASSPATH or in the case of servlets in the common/lib folder. Attempt to get a connection by calling the getConnection method of the DriverManager class. Always handle a failure to connect.

Establishing a Connection: Technique //Load driver class by package name then create instance Class.forName("com.mysql.jdbc.Driver").newInstance(); String url = "jdbc:mysql://localhost:3306/RentalDatabase"; // Connect using a url, a username without password con = DriverManager.getConnection(url, "RentalApp", "rentcars");

Statements Create a Statement object to execute an SQL command. Statement stmt = con.createStatement(); Use the executeUpdate() to execute a delete, drop, insert, or update (and other SQL commands that change the state of the database). Use the executeQuery() method to execute a select.

Statements ResultSet rs = state.executeQuery(“Select * from TableName”); Create a Statement object to execute an SQL command. Statement stmt = con.createStatement(); Use the executeUpdate() to execute a delete, drop, insert, or update (and other SQL commands that change the state of the database). Use the executeQuery() method to execute a select.

Processing ResultSets A query returns 0 or more rows of into a ResultSet. ResultSet rs = stmt.executeQuery(“Select * from TableName”); You iterate through the results in a result set using the next() method processing the data 1 row at a time. while (rs.next()) { // do something with current row. } Close the result set using close() rs.close();

Processing a Row in a ResultSet Rows returned during an iteration through a ResultSet contain fields. These fields must be converted into Java data types from the field type in the database. You can convert a field into a Java type by specifying either the Field Name or field number (if you know it) and choosing the conversion (getDouble,getInt, getString). while (rs.next()) { System.out.println(rs.getDouble(1)); System.out.println(rs.getString(“LastName”); System.out.println(rs.getInt(3)); } wasNull() tests whether the last conversion resulted in a null.