昊天

JVM在加载类的时候，都是通过ClassLoader的loadClass()方法来加载class的，loadClass(String name)方法：

 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

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:

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):

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:

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：

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：

 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。MainClassLoader类：

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：

public class Test
{
	public Test() {
		System.out.println("Test");
	}
	public static void main(String[] args) {
		System.out.println("Hello World");
	}
}