package com.zzj.classloader;
import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
/** 
 * 双亲委派类加载器，重写findClass(name)方法 
 * 
 * @author Administrator 
 * 
 */
public class ParentsDelegateClassLoader extends ClassLoader {
 private static final String EXT = ".class";
 private String path;
 public ParentsDelegateClassLoader() {
  path = this.getResource("").getPath();
 }
 public ParentsDelegateClassLoader(String path) {
  this.path = path;
 }
 @Override 
   protected Class<?> findClass(String name) throws ClassNotFoundException {
  byte[] b = null;
  try {
   b = loadClassFile(name);
  }
  catch (IOException e) {
   e.printStackTrace();
  }
  return this.defineClass(name, b, 0, b.length);
 }
 private byte[] loadClassFile(String name) throws IOException {
  String classFile = getClassFile(name);
  System.out.println("即将加载class文件" + classFile);
  ByteArrayOutputStream out = new ByteArrayOutputStream();
  InputStream input = new FileInputStream(classFile);
  int count;
  byte[] temp = new byte[1024];
  while ((count = input.read(temp)) > -1) {
   out.write(temp, 0, count);
  }
  out.close();
  input.close();
  return out.toByteArray();
 }
 private String getClassFile(String name) {
  String pathName = name.replace(".", File.separator);
  if (path.endsWith("/") || path.endsWith("\\")) {
   return path + pathName + EXT;
  }
  return path + File.separator + pathName + EXT;
 }
}

package com.zzj.classloader;
public class App {
 private static final String path = "F:\\ClassloaderTest\\bin";
 private static final String classname = "com.zzj.classloader.User";
 public static void main(String[] args) throws Exception {
  ParentsDelegateClassLoader classLoader = new ParentsDelegateClassLoader(path);
  Class<?> clazz = classLoader.loadClass(classname);
  System.out.println(clazz);
  System.out.println(clazz.getClassLoader());
 }
}

class com.zzj.classloader.User 
sun.misc.Launcher$AppClassLoader@19821f 

即将加载class文件F:\ClassloaderTest\bin\com\zzj\classloader\User.class 
class com.zzj.classloader.User 
com.zzj.classloader.ParentsDelegateClassLoader@61de33 

public Class<?> loadClass(String name) throws ClassNotFoundException { 
  return loadClass(name, false); 
  } 

protected synchronized Class<?> loadClass(String name, boolean resolve) 
  throws ClassNotFoundException 
  { 
  // 先判断该类是否已被当前的类加载器加载 
  Class c = findLoadedClass(name); 
  if (c == null) { 
    try { 
    if (parent != null) {// 如果存在父类加载器，则委派给父类加载 
      c = parent.loadClass(name, false); 
    } else {// 如果父类加载为空，则其父类加载器为引导类加载器 
      c = findBootstrapClass0(name); 
    } 
    } catch (ClassNotFoundException e) { 
      // 如果父类加载器加载失败，则自己加载，调用的就是findClass方法！ 
      c = findClass(name); 
    } 
  } 
  if (resolve) { 
    resolveClass(c); 
  } 
  return c; 
  } 

package com.zzj.classloader;
import java.io.ByteArrayOutputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;
/** 
 * 非双亲委派类加载器，重写loadClass(name)方法 
 * 
 * @author Administrator 
 * 
 */
public class NotParentsDelegateClassLoader extends ClassLoader {
 private static final String EXT = ".class";
 private String path;
 public NotParentsDelegateClassLoader() {
  path = this.getResource("").getPath();
 }
 public NotParentsDelegateClassLoader(String path) {
  this.path = path;
 }
 @Override 
   public Class<?> loadClass(String name) throws ClassNotFoundException {
  byte[] b = null;
  try {
   b = loadClassFile(name);
  }
  catch (FileNotFoundException e) {
   System.err.println("加载器" + this.getClass().getName() 
             + "没有找到class文件" + name + "，将委派给父类加载器！");
   // 委派给父类加载器 
   return getClass().getClassLoader().loadClass(name);
  }
  catch (IOException e) {
   System.err.println("加载器" + this.getClass().getName() + "加载class文件" 
             + name + "失败，将委派给父类加载器！");
   // 委派给父类加载器 
   return getClass().getClassLoader().loadClass(name);
  }
  // 检查该类是否被当前类加载器加载过（只检查当前类加载器，不会检查父类加载器） 
  Class<?> clazz = findLoadedClass(name);
  if (clazz != null) {
   System.out.println("类" + name + "已被加载过！");
   return clazz;
  } else {
   System.out.println("类" + name + "尚未被加载！");
  }
  return this.defineClass(name, b, 0, b.length);
 }
 private byte[] loadClassFile(String name) throws IOException, 
       FileNotFoundException {
  String classFile = getClassFile(name);
  System.out.println("即将加载class文件" + classFile);
  ByteArrayOutputStream out = new ByteArrayOutputStream();
  InputStream input = new FileInputStream(classFile);
  int count;
  byte[] temp = new byte[1024];
  while ((count = input.read(temp)) > -1) {
   out.write(temp, 0, count);
  }
  out.close();
  input.close();
  return out.toByteArray();
 }
 private String getClassFile(String name) {
  String pathName = name.replace(".", File.separator);
  if (path.endsWith("/") || path.endsWith("\\")) {
   return path + pathName + EXT;
  }
  return path + File.separator + pathName + EXT;
 }
}

package com.zzj.classloader;
public class App2 {
 private static final String className = "com.zzj.classloader.User";
 public static void main(String[] args) throws Exception {
  NotParentsDelegateClassLoader classLoader = new NotParentsDelegateClassLoader();
  Class<?> clazz = classLoader.loadClass(className);
  System.out.println(clazz);
  System.out.println(clazz.getClassLoader());
 }
}

即将加载class文件/E:/Myeclipse/zzjtest/WebRoot/WEB-INF/classes/com\zzj\classloader\User.class 
类com.zzj.classloader.User尚未被加载！ 
即将加载class文件/E:/Myeclipse/zzjtest/WebRoot/WEB-INF/classes/java\lang\Object.class 
class com.zzj.classloader.User 
com.zzj.classloader.NotParentsDelegateClassLoader@61de33 
加载器com.zzj.classloader.NotParentsDelegateClassLoader没有找到class文件java.lang.Object，将委派给父类加载器！