Dyn-loading with newInstance() vs. static ref

Question

Hello.
Here is code that does what I want:
Class.forName("com.mysql.jdbc.Driver").newInstance();  // load mysql/jdbc driver
Connection conn = DriverManager.getConnection("jdbc:mysql://ebi:3306/jlpt", "root", "pwd");
// conn is good-to-go
....Regarding this line:
Class.forName("com.mysql.jdbc.Driver").newInstance();as the returned value is ignored, this is, except for the classloading, a no-op?
Now, my understanding, is that any reference to a class triggers the loading of the class? If so, a static reference to "com.mysql.jdbc.Driver" should also load the jdbc/mysql driver class. However, this code fails with a "driver not found" exception:
String whatever = com.mysql.jdbc.Driver.DBNAME_PROPERTY_KEY;  // why not load mysql/jdbc driver?
Connection conn = DriverManager.getConnection("jdbc:mysql://ebi:3306/jlpt", "root", "pwd");
// java.sql.SQLException: No suitable driver found for jdbc:mysql://ebi:3306/jlpt
//   at java.sql.DriverManager.getConnection(DriverManager.java:602)
//   at java.sql.DriverManager.getConnection(DriverManager.java:185)
....Here is how I tested that a static reference loads a class:
public class Main {
    public static void main(String[] args) {
        System.out.println("start");
        int q = ClassA.i;  // toggle this by commenting out / in
        System.out.println("end");
public class ClassA {
    static {
        System.out.println("ClassA just loaded.");
    static int i = 1;
}Dynamic classloading is difficult for me, and I hope I can learn from this. Thanks.
Edited by: rerf on Jul 9, 2010 9:05 AM

Answer 1

jverd wrote:
rerf wrote:
Hello.
Here is code that does what I want:
Class.forName("com.mysql.jdbc.Driver").newInstance();  // load mysql/jdbc driver
Connection conn = DriverManager.getConnection("jdbc:mysql://ebi:3306/jlpt", "root", "pwd");
// conn is good-to-go
....Regarding this line:
Class.forName("com.mysql.jdbc.Driver").newInstance();as the returned value is ignored, this is, except for the classloading, a no-op?JDBC drivers are supposed to have a static initializer block so that when the class is loaded, it registers itself with DriverManager. That's why we do the explicit Class.forName(). Note that the newInstance() is unnecessary.
Now, my understanding, is that any reference to a class triggers the loading of the class? If so, a static reference to "com.mysql.jdbc.Driver" should also load the jdbc/mysql driver class. However, this code fails with a "driver not found" exception:
String whatever = com.mysql.jdbc.Driver.DBNAME_PROPERTY_KEY;  // why not load mysql/jdbc driver?
Connection conn = DriverManager.getConnection("jdbc:mysql://hiroo:3306/jlpt", "root", "rpp");
// java.sql.SQLException: No suitable driver found for jdbc:mysql://ebi:3306/jlpt
//   at java.sql.DriverManager.getConnection(DriverManager.java:602)
//   at java.sql.DriverManager.getConnection(DriverManager.java:185)
I don't know. Just as a guess, it might be that the "whatever" line is getting optimized away since you never use "whatever." Try printing it out and see if that changes things. I would have expected that to cause the class to load as well.this runs ok:
String whatever = com.mysql.jdbc.Driver.DBNAME_PROPERTY_KEY;
// Connection conn = DriverManager.getConnection("jdbc:mysql://ebi:3306/jlpt", "root", "pwd");
System.out.println("whatever --> " + whatever);
....this still fails with: "java.sql.SQLException: No suitable driver found"
String whatever = com.mysql.jdbc.Driver.DBNAME_PROPERTY_KEY;
Connection conn = DriverManager.getConnection("jdbc:mysql://ebi:3306/jlpt", "root", "pwd");
System.out.println("whatever --> " + whatever);
// java.sql.SQLException: No suitable driver found for jdbc:mysql://ebi:3306/jlpt
//   at java.sql.DriverManager.getConnection(DriverManager.java:602)
//   at java.sql.DriverManager.getConnection(DriverManager.java:185)
//   at kdata.DenJ.xtract(DenJ.java:85)
//   at kdata.Main.main(Main.java:13)
.... Anyway, I am ok. Thanks for your time.