Java Glossary : Class.forName

Last updated 2004-06-28 by Roedy Green ©1996-2004 Canadian Mind Products

Java definitions: 0-9 A B C D E F G H I J K L M N O P Q R S T U V W X Y Z

You are here : home : Java Glossary : C words : Class.forName.

Class.forName

Before I leap into explaining to use Class.forName, I want to give you an idea of what it is for.

• You want to write a program that composes Java programs, compiles them, then executes them, on the fly, without human intervention. It might generate thousands of such little programs. You need to be able to execute class files that did not exist when your calling code was written.
• You want to write a program that generates JVM byte codes, class file images in RAM, then executes them, without ever writing the *.class file to disk. For this you also need to write a custom ClassLoader since the standard one does not look in RAM, only on disk in the classpath.
• You want write a program such as the HolidayCalculator which comes with modules for computing a few dozen Holidays. You want to make it possible for users of the program to easily add more holiday calculator plug-ins for obscure holidays without having to look at, modify or recompile any source code.
• You want to write a BeanBox editor that can take any Bean-compliant class and execute it and edit its fields. For this you also need Reflection to find out what get/set methods are available.
• You want to write a spreadsheet amanuensis, a simple, flexible, high performance spreadsheet component.

Now let's look under the hood to understand how the various tools work. There are Class objects (objects of class Class) that represent classes (including arrays), interfaces, and primitives. The Class.newInstance method lets you create new objects of that class without requiring a variable declared specifically of that class. You are using the default public no arg constructor. Your class had better have one. This allows code to be much more open ended than in other languages, with new variants added dynamically. The Class.getName method lets you display the class name. There are also methods to discover the details of the fields and methods associated with the class in the java.lang.reflect.* package.

You can't instantiate Class objects, but most the Class methods are instance methods, not static. So how do you get a Class object:

Class cDog = Dog.class;                // Dog class object.
Class cDog2 = dog.getClass();          // actual class of dog object, not class of dog reference.
Class cCurrent = this.getClass();      // current class.
Class cDog3 = Class.forName( "Dog" )   // Dog class from class name.
Class cDog4 = Class.forName( "com.mindprod.dogs.Dog" ) // Dog class from package name.
Class cString = java.lang.String.class; // gets String class
Class cStringArray = String[].class;   // gets String array class
Class cdouble = double.class;          // gets pseudoclass representing double primitive.
Class cDouble = Double.class;          // gets Double wrapper class
Class cdouble2 = Double.TYPE;          // gets underlying double primitive.

You can then use methods like Class.toString, Class.getName, Class.getLoader and Class.getSuperclass to tell you even more about the class. Once you have the class, you can then play games with java.lang.reflect.*. Given just the class object, you can find out the constructors, methods, parameters to those methods and the members. You can also dynamically construct new objects and calls to methods on those objects.

To keep thing simple, when you have variable classes, they all implement some interface, abstract class or base class, in this case HolInfo:

// variable fully qualified class name
String classname = "com.mindprod.Holidays.Christmas";

// Create a class Christmas object, with an interface HolInfo reference
HolInfo holidayDelegate = ( HolInfo ) ( Class.forName ( classname ).newInstance() );

// Execute methods of the Christmas object.
String rule = holidayDelegate.getRule();

To go the other way use obj.getClass().getName().

For a practical example of the technique,

Holiday Calculator: source also uses Class.forName ¤ Learn To Count: source code to allow you to add new language translators or calculators or classes for new holidays, without modifying or recompiling the program; you just add the class names to a properties file

Here is how you find out where a class file came from on disk or on the net.

// Find out where the source code for for some class came from
CodeSource source = SomeClass.getProtectionDomain().getCodeSource();
if ( source != null )
   {
   URL location = source.getLocation();
  ...
   }

It may not have location, e.g. it was dynamically generated on the fly.

ClassLoader ¤ JASM ¤ javac.exe ¤ spreadsheet amanuensis

home	Canadian Mind Products
	mindprod.com IP:[24.87.56.253]
	Your IP:[80.134.30.163]
	You are visitor number 8978.
	Please send errors, omissions and suggestions
	to improve this page to Roedy Green.
	You can get a fresh copy of this page from:	or possibly from your local J: drive mirror:
	http://mindprod.com/jgloss/classforname.html	J:\mindprod\jgloss\classforname.html