20.14

java.lang.ClassLoader

THE PACKAGE JAVA.LANG

20.14 The Class

java.lang.ClassLoader

A class loader is an object that is responsible for loading classes. Given the name

of a class, it should attempt to locate or generate data that constitutes a definition

for the class. A typical strategy is to transform the name into a file name and then

read a  class file  of that name from a file system.

Every

Class

 object contains a reference to the

ClassLoader

 that defined it

( 20.3.7). Whenever executable Java code needs to use a class that has not yet

been loaded, the

loadClass

 method is invoked for the class loader of the class

containing the code in question.

Class objects for array classes are not created by class loaders, but are created

automatically as required by the Java runtime. The class loader for an array class,

as returned by the

getClassLoader

 method of class

Class

 ( 20.3.7), is the same

as the class loader for its element type; if the element type is a primitive type, then

the array class has no class loader.

Class loaders may typically be used by security managers ( 20.17) to indicate

security domains: two classes may considered to be  friendly  or  related  to each

other only if they were defined by the same class loader.

public abstract class ClassLoader {

protected ClassLoader() throws SecurityException;

protected abstract Class loadClass(String name,

boolean resolve)

throws ClassNotFoundException;

protected final Class defineClass(byte data[],

int offset, int length)

throws NullPointerException, IndexOutOfBoundsException,

ClassFormatError;

protected final void resolveClass(Class c)

throws NullPointerException;

protected final Class findSystemClass(String name)

throws ClassNotFoundException;

}

20.14.1

protected ClassLoader() throws SecurityException

This constructor is invoked for every newly created class loader. Because the class

ClassLoader

 is

abstract

, it is not possible to create a new instance of the class

ClassLoader

 itself; however, every constructor for a subclass of

ClassLoader

necessarily invokes this constructor, explicitly or implicitly, directly or indirectly.

All this constructor does is to enforce a security check: if there is a security

manager, its

checkCreateClassLoader

 method ( 20.17.10) is called.

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