像tomcat容器那样自定义一个 Classloader

public class AppClassLoader extends ClassLoader {
 private static Map<String, Class> classes = new HashMap<String, Class>();
 private static AppClassLoader loader;
 
 private String jarName;
 /**
  * @param jarName 
  * @throws Exception
  */
 private AppClassLoader(String jarName) throws Exception {
 this.jarName = jarName;
 init();
 }
 /**
  * 生成或取得AppClassLoader实例
  * @return AppClassLoader
  */
 public static synchronized AppClassLoader getInstance(String jarName) {
 if (loader == null)
 try {
 loader = new AppClassLoader(jarName);
 } catch (Exception e) {
 System.err.println(e.getMessage());
 return null;
 }
 return loader;
 }
 private void init() throws Exception {
 JarInputStream jar = new JarInputStream(new FileInputStream(new File(jarName)));
 JarEntry entry;
 while ((entry = jar.getNextJarEntry()) != null) {
 if (entry.getName().toLowerCase().endsWith(".class")) {
 String classname = entry.getName().substring(0, entry.getName().length() - ".class".length())
 .replace('/', '.');
 byte[] data = getResourceData(jar);
 Class clazz = defineClass(classname, data, 0, data.length);
 classes.put(classname, clazz);
 }
 }
 }
 @Override
 public Class findClass(String name) {
 if (classes.get(name) != null) {
 return classes.get(name);
 }
 return null;
 }
 final static private byte[] getResourceData(JarInputStream jar) throws IOException {
 ByteArrayOutputStream data = new ByteArrayOutputStream();
 byte[] buffer = new byte[8192];
 int size;
 while (jar.available() > 0) {
 size = jar.read(buffer);
 if (size > 0) {
 data.write(buffer, 0, size);
 }
 }
 byte[] val = data.toByteArray();
 data.close();
 return val;
 }
}