A ClassLoader
, of course, is a program
that loads classes. There is one class loader built into the Java
Virtual Machine, but your application can create additional ones as
needed. Learning to write and run a working class loader and use it
to load a class and run its methods is a nontrivial exercise. In
fact, you rarely need to write a class loader, but knowing how is
helpful in understanding how the JVM finds classes, creates objects,
and calls methods.
ClassLoader
itself is abstract; you must subclass
it to provide a loadClass( )
method that loads classes as you wish. It can load the bytes from a
network connection, a local disk, RAM, a serial port, or anywhere
else. Or you can construct the class file in memory yourself if you
have access to a compiler.
You must call this class loader’s loadClass( )
method for any classes you wish to load from it. Note
that it will be called to load all classes required for classes you
load (parent classes that aren’t already loaded, for example).
However, the JVM will still load classes that you instantiate with
the new
operator “normally” via
CLASSPATH.
To write a class loader, you need to subclass
ClassLoader
and implement at least
findClass( )
and loadClass( )
. The loadClass( )
method needs to get
the class file into a byte array (typically by reading it), convert
the array into a Class
object, and return the
result.
What? That sounds a bit like “And Then a Miracle Occurs . . .
" And it is. The miracle of class creation, however, happens
down inside the JVM, where you don’t have access to it.
Instead, your ClassLoader
has to call the
final
defineClass( )
method in your superclass (which
is java.lang.ClassLoader
). This is illustrated in
Figure 25-1, where a stream of bytes containing a
hypothetical Chicken
class is converted into a
ready-to-run Chicken
class in the JVM by calling
the defineClass( )
method.
To use your ClassLoader
subclass, you need to
instantiate it and call its loadClass( )
method
with the name of the class you want to load. This gives you a
Class
object for the named class; the
Class
object in turn lets you construct instances,
find and call methods, etc. Refer back to Section 25.3.
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