使用的是双亲委托模式：
jvm启动时，会启动jre/rt.jar里的类加载器：bootstrap classloader，用来加载java核心api；然后启动扩展类加载器ExtClassLoader加载扩展类，并加载用户程序加载器AppClassLoader，并指定ExtClassLoader为他的父类；
当类被加载时，会先检查在内存中是否已经被加载，如果是，则不再加载，如果没有，再由AppClassLoader来加载，先从jar包里找，没有再从classpath里找；
如果自定义loader类，就会存在这命名空间的情况，不同的加载器加载同一个类时，产生的实例其实是不同的；
Java代码
public Class<?> loadClass(String name) throws ClassNotFoundException {
return loadClass(name, false);
}
public Class<?> loadClass(String name) throws ClassNotFoundException {
return loadClass(name, false);
}
loadClass(String name)方法再调用loadClass(String name, boolean resolve)方法:
- name - 类的二进制名称
- resolve - 如果该参数为 true，则分析这个类
Java代码
protected synchronized Class<?> loadClass(String name, boolean resolve)
throws ClassNotFoundException
{
// First, check if the class has already been loaded
//JVM 规范规定ClassLoader可以在缓存保留它所加载的Class，如果一个Class已经被加载过，则直接从缓存中获取
Class c = findLoadedClass(name);
if (c == null) {
try {
if (parent != null) {
c = parent.loadClass(name, false);
} else {
c = findBootstrapClass0(name);
}
} catch (ClassNotFoundException e) {
// If still not found, then invoke findClass in order
// to find the class.
c = findClass(name);
}
}
if (resolve) {
resolveClass(c);
}
return c;
}
protected synchronized Class<?> loadClass(String name, boolean resolve)
throws ClassNotFoundException
{
// First, check if the class has already been loaded
//JVM 规范规定ClassLoader可以在缓存保留它所加载的Class，如果一个Class已经被加载过，则直接从缓存中获取
Class c = findLoadedClass(name);
if (c == null) {
try {
if (parent != null) {
c = parent.loadClass(name, false);
} else {
c = findBootstrapClass0(name);
}
} catch (ClassNotFoundException e) {
// If still not found, then invoke findClass in order
// to find the class.
c = findClass(name);
}
}
if (resolve) {
resolveClass(c);
}
return c;
}
如果ClassLoader并没有加载这个class，则调用findBootstrapClass0:
Java代码
private Class findBootstrapClass0(String name)
throws ClassNotFoundException
{
check();
if (!checkName(name))
throw new ClassNotFoundException(name);
return findBootstrapClass(name);
}
private Class findBootstrapClass0(String name)
throws ClassNotFoundException
{
check();
if (!checkName(name))
throw new ClassNotFoundException(name);
return findBootstrapClass(name);
}
该方法会调用check()方法来判断这个类是否已经初始化，并且通过checkName(name)来判断由name指定的这个类是否存在
最后调用findBootstrapClass(name):
Java代码
private native Class findBootstrapClass(String name)
throws ClassNotFoundException;
private native Class findBootstrapClass(String name)
throws ClassNotFoundException;
而这个findBootstrapClass方法是一个native方法，这是我们的root loader，这个载入方法并非是由JAVA所写，而是C++写的，它会最终调用JVM中的原生findBootstrapClass方法来完成类的加载。
如果上面两个都找不到，则使用findClass(name)来查找指定类名的Class:
Java代码
protected Class<?> findClass(String name) throws ClassNotFoundException {
throw new ClassNotFoundException(name);
}
protected Class<?> findClass(String name) throws ClassNotFoundException {
throw new ClassNotFoundException(name);
}
JDK5.0中的说明：
使用指定的二进制名称查找类。此方法应该被类加载器的实现重写，该实现按照委托模型来加载类。在通过父类加载器检查所请求的类后，此方法将被 loadClass 方法调用。默认实现抛出一个 ClassNotFoundException。
所以，我们在自定义类中，只需要重写findClass()即可。
MyClassLoader类：
Java代码
public class MyClassLoader extends ClassLoader {
private String fileName;
public MyClassLoader(String fileName) {
this.fileName = fileName;
}
protected Class<?> findClass(String className) throws ClassNotFoundException {
Class clazz = this.findLoadedClass(className);
if (null == clazz) {
try {
String classFile = getClassFile(className);
FileInputStream fis = new FileInputStream(classFile);
FileChannel fileC = fis.getChannel();
ByteArrayOutputStream baos = new
ByteArrayOutputStream();
WritableByteChannel outC = Channels.newChannel(baos);
ByteBuffer buffer = ByteBuffer.allocateDirect(1024);
while (true) {
int i = fileC.read(buffer);
if (i == 0 || i == -1) {
break;
}
buffer.flip();
outC.write(buffer);
buffer.clear();
}
fis.close();
byte[] bytes = baos.toByteArray();
clazz = defineClass(className, bytes, 0, bytes.length);
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
return clazz;
}
private byte[] loadClassBytes(String className) throws
ClassNotFoundException {
try {
String classFile = getClassFile(className);
FileInputStream fis = new FileInputStream(classFile);
FileChannel fileC = fis.getChannel();
ByteArrayOutputStream baos = new
ByteArrayOutputStream();
WritableByteChannel outC = Channels.newChannel(baos);
ByteBuffer buffer = ByteBuffer.allocateDirect(1024);
while (true) {
int i = fileC.read(buffer);
if (i == 0 || i == -1) {
break;
}
buffer.flip();
outC.write(buffer);
buffer.clear();
}
fis.close();
return baos.toByteArray();
} catch (IOException fnfe) {
throw new ClassNotFoundException(className);
}
}
private String getClassFile(String name) {
StringBuffer sb = new StringBuffer(fileName);
name = name.replace('.', File.separatorChar) + ".class";
sb.append(File.separator + name);
return sb.toString();
}
}
public class MyClassLoader extends ClassLoader {
private String fileName;
public MyClassLoader(String fileName) {
this.fileName = fileName;
}
protected Class<?> findClass(String className) throws ClassNotFoundException {
Class clazz = this.findLoadedClass(className);
if (null == clazz) {
try {
String classFile = getClassFile(className);
FileInputStream fis = new FileInputStream(classFile);
FileChannel fileC = fis.getChannel();
ByteArrayOutputStream baos = new
ByteArrayOutputStream();
WritableByteChannel outC = Channels.newChannel(baos);
ByteBuffer buffer = ByteBuffer.allocateDirect(1024);
while (true) {
int i = fileC.read(buffer);
if (i == 0 || i == -1) {
break;
}
buffer.flip();
outC.write(buffer);
buffer.clear();
}
fis.close();
byte[] bytes = baos.toByteArray();
clazz = defineClass(className, bytes, 0, bytes.length);
} catch (FileNotFoundException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
return clazz;
}
private byte[] loadClassBytes(String className) throws
ClassNotFoundException {
try {
String classFile = getClassFile(className);
FileInputStream fis = new FileInputStream(classFile);
FileChannel fileC = fis.getChannel();
ByteArrayOutputStream baos = new
ByteArrayOutputStream();
WritableByteChannel outC = Channels.newChannel(baos);
ByteBuffer buffer = ByteBuffer.allocateDirect(1024);
while (true) {
int i = fileC.read(buffer);
if (i == 0 || i == -1) {
break;
}
buffer.flip();
outC.write(buffer);
buffer.clear();
}
fis.close();
return baos.toByteArray();
} catch (IOException fnfe) {
throw new ClassNotFoundException(className);
}
}
private String getClassFile(String name) {
StringBuffer sb = new StringBuffer(fileName);
name = name.replace('.', File.separatorChar) + ".class";
sb.append(File.separator + name);
return sb.toString();
}
}
该类中通过调用defineClass(String name, byte[] b, int off, int len)方法来定义一个类：
Java代码
protected final Class<?> defineClass(String name, byte[] b, int off, int len)
throws ClassFormatError
{
return defineClass(name, b, off, len, null);
}
protected final Class<?> defineClass(String name, byte[] b, int off, int len)
throws ClassFormatError
{
return defineClass(name, b, off, len, null);
}
注：MyClassLoader加载类时有一个局限，必需指定.class文件，而不能指定.jar文件。该类中的大部分代码是从网上搜索到的，是出自一牛人之笔，只是不知道原帖在哪，希望不会被隐藏。
MainClassLoader类：
Java代码
public class MainClassLoader {
public static void main(String[] args) {
try {
MyClassLoader tc = new MyClassLoader("F:\\OpenLib\\");
Class c = tc.findClass("Test");
c.newInstance();
} catch (ClassNotFoundException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
} catch (InstantiationException e) {
e.printStackTrace();
}
}
}
public class MainClassLoader {
public static void main(String[] args) {
try {
MyClassLoader tc = new MyClassLoader("F:\\OpenLib\\");
Class c = tc.findClass("Test");
c.newInstance();
} catch (ClassNotFoundException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
} catch (InstantiationException e) {
e.printStackTrace();
}
}
}
最后是一个简单的Test测试类：
Java代码
public class Test
{
public Test() {
System.out.println("Test");
}
public static void main(String[] args) {
System.out.println("Hello World");
}
}
（转自： http://huangcanqin.iteye.com/blog/1273170 ）