来源自网络，很喜欢就收藏了；

需要注意的是：如果想要对这个实例进行测试的话，首先需要在c盘建立一个c://myjava的目录。然后将相应的java文件放在这个目录中。并将产生的.clas文件放在c://myjava/com/lg.test目录下，否则是找不到的。这是要注意的。。

class FileClassLoader ：

 
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package com.lg.test;
import java.io.ByteArrayOutputStream;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
/**
 * Created by 鏉庢灉 on 2016/8/6.
 */
public class FileClassLoader extends ClassLoader {
    String rootDir=null;
    public FileClassLoader(String rootDir) {
        this.rootDir = rootDir;
    }
    @Override
      protected Class<?> findClass(String className) throws ClassNotFoundException {
        //首先检查是否已经被加载了。
        Class<?> c = findLoadedClass(className);
        String path = rootDir + "/" + className.replace('.', '/') + ".class";
        if (c != null) {
            return c;
        } else {
            /*双亲委托模式*/
            ClassLoader loaderParent = this.getParent();
            c = loaderParent.loadClass(className);
            if (c != null) {
                return c;
            } else {
                /*如果再不行的话，就再进行加载。因为字节码的本质就是一个字节数组*/
                InputStream is = null;
                ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
                try {
                    is = new FileInputStream(path);
                    byte[] buffer = new byte[1024];
                    int len = 0;
                    while ((len = is.read(buffer)) != -1) {
                        outputStream.write(buffer, 0, len);
                    }
                    c = defineClass(className, buffer, 0, buffer.length);
                }
                catch (Exception e) {
                    e.printStackTrace();
                }
                finally {
                    if (is != null) {
                        try {
                            is.close();
                        }
                        catch (IOException e) {
                            e.printStackTrace();
                        }
                    }
                }
            }
            return c;
        }
    }
}

class Demo ：

?
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package com.lg.test;
/**
 * Created by 鏉庢灉 on 2016/8/6.
 */
/*相同的类加载器对同一个类进行加载，得到的hascode是相同的
 * 不同的类加载器对同一类进行加载，得到的hascode是不一样的*/
public class Demo {
    public static void main(String[] args) {
        FileClassLoader loader = new FileClassLoader("c://myjava");
        FileClassLoader loader2=new FileClassLoader("c://myjava");
        try {
            Class<?> c = loader.findClass("com.lg.test.HelloWorld");
            Class<?> c0=loader.findClass("com.lg.test.HelloWorld");
            Class<?> c1=loader2.findClass("com.lg.test.HelloWorld");
            Class<?> c2=loader.findClass("com.lg.test.Demo01");
            Class<?> c3=loader.findClass("java.lang.String");
            System.out.println(c.hashCode());
            System.out.println(c.getClassLoader());
            System.out.println(c0.hashCode());
            System.out.println(c0.getClassLoader());
            System.out.println(c1.hashCode());
            System.out.println(c1.getClassLoader());
            System.out.println(c2.hashCode());
            System.out.println(c2.getClassLoader());
            System.out.println(c3.hashCode());
            System.out.println(c3.getClassLoader());
        }
        catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }
}

最后运行的结果为：

366712642
sun.misc.Launcher$AppClassLoader@4e0e2f2a
366712642
sun.misc.Launcher$AppClassLoader@4e0e2f2a
366712642
sun.misc.Launcher$AppClassLoader@4e0e2f2a
1829164700
sun.misc.Launcher$AppClassLoader@4e0e2f2a
2018699554
null