自定义类加载器

自定义类加载器其实就是通过继承classLoader类，并重写findClass方法来实现自定义类加载器的效果

import java.io.*;

/**
 * 简单文件系统类加载器
 * 通过输入类的全限定名在指定文件路径下加载class
 */
public class FileSystemClassLoader extends ClassLoader {
    private String root;

    public FileSystemClassLoader(String root) {
        this.root = root;
    }

    /**
     * 重写findClass 依然使用双亲委派机制
     * @param name
     * @return
     * @throws ClassNotFoundException
     */
    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        Class<?> c = findLoadedClass(name);
        if (c != null) {
            return c;
        } else {
            try {
                ClassLoader parent = this.getParent();
                c = parent.loadClass(name);
                System.out.println(name);
            }catch (Exception e){

            }

            if (c != null) {
                System.out.println("Hello");
                return c;
            } else {
                byte[] data = getClassName(name);
                if (data == null) {
                    throw new ClassNotFoundException();
                } else {
                    c = defineClass(name, data, 0, data.length);
                }
            }
        }
        return c;
    }

    /**
     * 通过类名获取到类的绝对路径名称
     * class文件写入byte[]数据中
     * @param name
     * @return 
     */
    private byte[] getClassName(String name) {
        String path = root + "/" + name.replace(".", "/") + ".class";
        System.out.println(path);
        InputStream is = null;
        ByteArrayOutputStream byos = new ByteArrayOutputStream();
        try {
            is = new FileInputStream(path);
            int len = 0;
            byte[] buffer = new byte[1024];
            while((len = is.read(buffer))!=-1){
                    byos.write(buffer,0,len);
               }
            return byos.toByteArray();
        } catch (IOException e) {
            e.printStackTrace();
            return null;
        }finally {
            if(is!=null){
                try {
                    is.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
    }
}

public static void main(String[] args) {
        FileSystemClassLoader fs = new FileSystemClassLoader("C:/Users/Administrator/Desktop/temp");
        try {
            Class<?> c = fs.findClass("com.smy.HelloWorld");
            Class c1 = fs.findClass("java.lang.String");
            System.out.println(c);
            System.out.println(c.getClassLoader());
            System.out.println(c1.getClassLoader());
        } catch (ClassNotFoundException e) {
            e.printStackTrace();
        }
    }

输入结果

class com.smy.HelloWorld
reflect.FileSystemClassLoader@1540e19d
null