自定义ClassLoader
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。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：
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");}}
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