ColdFusion, J2EE, Software Development

Java Class.forName(String className) and JDBC

54 Comments

A reader asked a question via a comment a couple months ago that I didn’t really have an answer for (and had always kind of wondered the same thing). In the original post (which showed how to use JDBC with ColdFusion), I used the following snippet of code:

Class.forName("jdbc.DriverXYZ");
Connection con = DriverManager.getConnection(url,
  "myLogin", "myPassword");

and the reader wanted to know what the Class.forName(..) method did. The most common answer you’ll hear is that it loads the database driver, which, while technically true, is shallow. Where does it get loaded? How does it happen? And why?

To answer the question I started with the JavaDoc for the Class.forName() method. According to the documentation, the method:

… attempts to locate, load, and link the class or interface

I wasn’t perfectly clear on what “locate, load, and link” meant, so I did a little digging through the Java Language Specification. According to chapter 12 of the JLS:

Loading refers to the process of finding the binary form of a class or interface type with a particular name, perhaps by computing it on the fly, but more typically by retrieving a binary representation previously computed from source code by a compiler, and constructing, from that binary form, a Class object to represent the class or interface.

Next, again according to the JLS, it must be transformed from it’s binary representation to something the Java virtual machine can use, this process is called linking. Finally, the class is initialized, which is the process that executes the static initializer and the initializers for static fields declared in the class.

So then back to the original problem, when Class.forName() is called with an argument like this:

Class.forName("org.gjt.mm.mysql.Driver");

the classloader attempts to load and link the Driver class in the “org.gjt.mm.mysql” package and if successful, the static initializer is run. The MySQL Driver (download the source code) static initializer looks like this:

static {
  try {
    java.sql.DriverManager.registerDriver(new Driver());
  } catch (SQLException E) {
    throw new RuntimeException("Can't register driver!");
  }
}

So it calls a static method in the java.sql.DriverManager class which apparently registers a copy of itself when it loads.

So now I understand the where and the how, what about why? To understand the why you have to look at the next line in the initial code example:

Connection con = DriverManager.getConnection(url,
  "myLogin", "myPassword");

The DriverManager class (view DriverManager source here) returns a database connection given a JDBC URL string, a username and a password. In order to create that connection, the DriverManager class has to know which database driver you want to use. It does that by iterating over the array (internally a Vector) of drivers that have registered with it (ie: the registerDriver(Driver driver) method illustrated above) and calls the acceptsURL(url)) method on each driver in the array, effectively asking the driver to tell it whether or not it can handle the JDBC URL.

So there you have it. Class.forName explained.

54 thoughts on “Java Class.forName(String className) and JDBC”

  1. “Class.forName explained.” More like: Hidden-magic antipattern exposed! Or: Thank god for open-source where you can see what’s actually happening under-the-covers.

    Reply

  2. >Thank god for open-source where you can see what’s actually happening under-the-covers.

    A good specification should do it 😉

    Reply

  4. why can’t the following code do the job of “locate, load, and link”?

    org.gjt.mm.mysql.Driver driver = new org.gjt.mm.mysql.Driver();

    This would locate , load and link by executing the static block

    Thanks,

    Reply

  5. Can you please mention how the entry is being made in the DriverManager only when a driver is loaded using Class.forName() instead of a class like java.util.Vector ?What will happen if a class like vector is loaded using Class.forName() ?

    Reply

  8. Never found such explained answer on the forums. I think its our duty as a java developer to work on the common problems like this.

    Thanks for the details.

    Reply

  10. This are some hidden facts that most of the Java Masters might be unaware of. Thanks for sharing this information.

    Reply

  13. Very good explanation… I had encountered that piece of code a lot of times but never understood what was going on under the covers.
    A search on google returned your website and that was an interesting read!

    Reply

  16. Please tell me if we write to class.forName() statements simultaneously trying to load diferent drivers.ir first i load oracle driver and then jdbc which one will it load

    Reply

  23. Nice job! i was googling and googling and found this, after reading through the spec and going through the class Class, makes perfect sense, thanks for making me lazy, hehe!

    Reply

  24. Well written and to the point! Now I understand!
    On the why part – why does the standard way of opening a database be so complex? i.e. Loading the driver in step 1, and passing a URL in the step 2 which (hopefully) identifies the driver in step 1. Why not simply call the DriverManager with both the driver class, and the URL in one call? (The driver manager can do step 1 and 2 internally?).

    Reply

  28. Hatsoff 2 the author.your explanation is a lesson for all of us not only belonging to this subject, But we can follow your way while posting further explanation on the web.

    Reply

  29. Great Explaination of Class.forName(),
    Before this page I had gone through lot of websites but didn’t get the explaination as in this page.

    Thanks a lot Mr. Aaron Johnson.

    Reply

  40. I am getting Class not Found Exception when i pass parameter to the class
    String abc = “ABC”;
    String xyz = “XYZ”;
    String str = “com.app.ProductDet(“+abc+”,”+xyz+”)”;
    Class c = Class.forName(str);
    try{
    AClass ac = (AClass) c.newInstance();
    }catch(AllException e)

    Reply

  45. I understood the concept but when we use new operator for the Driver, Why we need to register again. Does not new operator execute the static block and register the driver.

    Reply

  52. Hi ,Thanks for the insight.
    Something now comes to my mind like can I create my connection object avilable by below..?

    Driver driver = new RespectiveDriverClassVendor();
    Connection con =DriverManager.getConnection();

    Though staticblock is in Driver Implemetation class is taking care of binding it to Drivermanager class.

    Reply

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