problem using JDBC to connect to mysql

问题:

I'm trying to set up a connection between my applet and my mysql server using jdbc

I added the jar mysql-connector-java-5.1.14-bin.jar to the project

then I used this code

public void databaseTesting(){
        Connection con;
        try {
            Class.forName("com.mysql.jdbc.Driver").newInstance();

            con = DriverManager.getConnection( "jdbc:mysql://localhost:3306/test", "root","");
            System.err.println("connected !");

            Statement stmt = con.createStatement();
            ResultSet rs = stmt.executeQuery("SELECT * FROM `test`.`test`;");
            while (rs.next()) {
                int A = rs.getInt("columnA");
                int B = rs.getInt("columnB");
                System.err.println("A "+A+" B "+B);
            }
        } catch (SQLException e) {
            System.err.println("failed to connect");
        } catch (InstantiationException e) {
            System.err.println("InstantiationException");
        } catch (IllegalAccessException e) {
            System.err.println("IllegalAccessException");
        } catch (ClassNotFoundException e) {
            System.err.println("ClassNotFoundException");
        }
    }

and for some reason I keep getting ClassNotFoundException.

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edit

the jar is added to the build path and appears in the Referenced Libraries.

the exception is thrown by

Class.forName("com.mysql.jdbc.Driver").newInstance();

Anybody got an idea why ?

Thanks in advance jason

回答1:

Do you run this application through eclipse? Is it a Dynamic web project, if so then try adding the jar file to the WEB-INF\lib folder

回答2:

Then the JAR is not in the runtime classpath.

Since you explicitly mentioned "project", I'll assume that you're using an IDE. You have to add the JAR file to the so-called Build Path (which represents both the compiletime and runtime classpath). In Eclipse for example, rightclick the JAR file you dropped in the project folder, choose Build Path > Add to Build Path and that should be it.

See also:

• Mini tutorial with JDBC and MySQL

---

Update: If it keeps complaining, then it is still not in the runtime classpath. Either you did it wrong or the runtime environment didn't use this JAR. Did you run it as a Java Application or as a Java Applet? (even though it's a bad practice to do JDBC inside an applet). If you're actually running this as an applet, it has got to be in the runtime classpath of the applet as well. You can specify it in the archive attribute/parameter of the applet.

回答3:

Your code is OK! It will work when:

1. Your mysql-connector-java-5*-bin.jar is in place.
2. The login & password are correct.
3. Your database exists
4. Your table exists

5. Your ColumnA and ColumnB are integers

   If you are using an IDE just add the jar to the Libraries.