Exception handling allows you to handle exceptional conditions such as program-defined errors in a controlled fashion.
When an exception condition occurs, an exception is thrown. The term thrown means that current program execution stops, and control is redirected to the nearest applicable catch clause. If no applicable catch clause exists, then the program's execution ends.
JDBC Exception handling is very similar to Java Excpetion handling but for JDBC, the most common exception you'll deal with is java.sql.SQLException.
A SQLException can occur both in the driver and the database. When such an exception occurs, an object of type SQLException will be passed to the catch clause.
The passed SQLException object has the following methods available for retrieving additional information about the exception:
By utilizing the information available from the Exception object, you can catch an exception and continue your program appropriately. Here is the general form of a try block:
try { // Your Code between these braces!!! } catch(Exception ex) { // Your exception handling Code between // braces, similar to the exception clause // in a PL/SQL block. } finally { // Your must-always-be-executed Code between // braces. Like closing database connection. }
Study the following example code to understand the usage of try....catch...finally blocks.
//STEP 1. Import required packages import java.sql.*; public class example { // JDBC driver name and database URL static final String JDBC_DRIVER = "com.mysql.jdbc.Driver"; static final String DB_URL = "jdbc:mysql://localhost/EMP"; // Database credentials static final String USER = "username"; static final String PASS = "password"; public static void main(String[] args) { Connection conn = null; try{ //STEP 2: Register JDBC driver Class.forName("com.mysql.jdbc.Driver"); //STEP 3: Open a connection System.out.println("Connecting to database..."); conn = DriverManager.getConnection(DB_URL,USER,PASS); //STEP 4: Execute a query System.out.println("Creating statement..."); Statement stmt = conn.createStatement(); String sql; sql = "SELECT id, first, last, age FROM Friends"; ResultSet rs = stmt.executeQuery(sql); //STEP 5: Extract data from result set while(rs.next()){ //Retrieve by column name int id = rs.getInt("id"); int age = rs.getInt("age"); String first = rs.getString("first"); String last = rs.getString("last"); //Display values System.out.print("ID: " + id); System.out.print(", Age: " + age); System.out.print(", First: " + first); System.out.println(", Last: " + last); } //STEP 6: Clean-up environment rs.close(); stmt.close(); conn.close(); }catch(SQLException se){ //Handle errors for JDBC se.printStackTrace(); }catch(Exception e){ //Handle errors for Class.forName e.printStackTrace(); }finally{ //finally block used to close resources try{ if(conn!=null) conn.close(); }catch(SQLException se){ se.printStackTrace(); }//end finally try }//end try System.out.println("Have A Nice Day"); }//end main }//end example
Now compile above example:
C:\>javac example.java C:\>
it produces following result if there is no problem, otherwise corresponding error would be caught and error message would be displayed:
C:\>java example Connecting to database... Creating statement... ID: 100, Age: 21, First: Sophia, Last: Lomela ID: 101, Age: 23, First: Katrina, Last: Kane ID: 102, Age: 24, First: Guru, Last: Morkel ID: 103, Age: 22, First: Simone, Last: Washer C:\>
Try above example by passing wrong database name or wrong username or password and check the result.
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