继承自系统的ClassLoader

 class FileSystemClassloader extends ClassLoader

实现findClass()

@Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        byte[] data = getClassData(name);
        if (data == null) {
            throw new ClassNotFoundException();
        }else {
            return defineClass(name, data, 0, data.length);
        }
    }

通过文件输入流获取.class文件的byte数组

private byte[] getClassData(String name) {
       //包名转成文件路径
        String path = pathForClassName(name);
        FileInputStream fis = null;
        try {
            fis = new FileInputStream(path);
            ByteArrayOutputStream bos = new ByteArrayOutputStream();
            int len = 0;
            byte[] buffer = new byte[1024];
            while ((len = fis.read(buffer))!= -1) {
                bos.write(buffer, 0, len);
            }
            bos.flush();
            return bos.toByteArray();
        } catch (Exception e) {
            e.printStackTrace();
        }finally {
            if (fis != null) {
                try {
                    fis.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
        return null;
    }

路径转化,将包名转化为实际的硬盘路径

private String pathForClassName(String name) {
        String path = rootDir+File.separator+name.replace(".", File.separator)+".class";
        System.out.println("path-->"+path);
        return path;
        
    }

类 FileSystemClassLoader的 findClass()方法首先根据类的全名在硬盘上查找类的字节代码文件（.class 文件），然后读取该文件内容，最后通过 defineClass()方法来把这些字节代码转换成 java.lang.Class类的实例。