Class.forName : Java Glossary

Uses of classForName

• You want write a program such as the Holiday calculator 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.

  When you want code to work with a class that does not yet exist, or is not available, at the time of compilation, you define an interface. The base code references only that interface. You tell your clients that any new code must implement that interface. Then at runtime you use Class. forName to load the new class, then use its methods via the interface methods. The Holiday calculator works this way. You can plug in you own code for new holidays without recompiling. Other places you see this provider/plug-in approach are JDBC (Java Data Base Connectivity) drivers and JCE providers.

• 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 (Java Virtual Machine) byte codes, class file images in RAM (Random Access Memory), 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 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.

Under the Hood

Class.forName eagerly loads the class if it not already loaded. Inside the JVM there is a HashMap of all the classes that have been previously loaded. So Class. forName takes under a millisecond if the class you have want is already loaded, If not, it might take 15 milliseconds or so to load it. You pay this time penalty only the first time you use the class. Class. forName is still slower than hard coding the name of the class into your code. With hard coding, you avoid repeated HashMap lookups.

Creating Class Objects

classForName is oblivious to any import statements, so you must fully qualify your class names.

The .class syntax is a kludge, especially double.class. It behaves as though it were a read-only static field even though there is no such field, though obviously there must a hidden pointer in the Class object to the class name.

Class Instance Methods

Nested Classes

Class Names

Dynamic Class Loading

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

To go the other way, to get the class name from an object, use obj.getClass().getName().

For a practical example of the technique,

Sample Code to Dynamically Load Classes

Here is some code that dynamically loads classes, given just the unqualified class name. It looks in two different packages. It caches the classes it finds. The public method returns instances of the named class that implement the Macro interface, not Class objects. This code is the guts of the HTML (Hypertext Markup Language) Static Macros engine. The key tools are Class. forName and Class.

Finding Classes

Warning

• Don’t go frivolously using Class.forName since it will allocate the statics and load all the methods for that class.
• Class.forName just loads the class. It does not run the static initialisation code. To trigger the static init code, execute some dummy static method.

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