记录我的学习过程：自定义类加载器

public class FileSystemClassLoader extends ClassLoader {

    //com.bjsxt.test.User  --> d:/myjava/  com/bjsxt/test/User.class     

    private String rootDir;

    public FileSystemClassLoader(String rootDir){

    this.rootDir = rootDir;

}

@Override

protected Class findClass(String name) throws ClassNotFoundException {

    Class c = findLoadedClass(name);

    //应该要先查询有没有加载过这个类。如果已经加载，则直接返回加载好的类。如果没有，则加载新的类。

    if(c!=null){

        return c;

    }else{

        ClassLoader parent = this.getParent();

    try {

        c = parent.loadClass(name);   //委派给父类加载

    } catch (Exception e) {

        // e.printStackTrace();

    }

        if(c!=null){

            return c;

        //找不到就自己加载

        }else{

            byte[] classData = getClassData(name);

            if(classData==null){

                throw new ClassNotFoundException();

            }else{

                c = defineClass(name, classData, 0,classData.length);

            }

        }

    }

return c;

}

    private byte[] getClassData(String classname){  //com.bjsxt.test.User  d:/myjava/  com/bjsxt/test/User.class

        String path = rootDir +"/"+ classname.replace('.', '/')+".class";

        // IOUtils,可以使用它将流中的数据转成字节数组

        InputStream is = null;

        ByteArrayOutputStream baos = new ByteArrayOutputStream();

        try{

            is  = new FileInputStream(path);

            byte[] buffer = new byte[1024];

            int temp=0;

            while((temp=is.read(buffer))!=-1){

            baos.write(buffer, 0, temp);

            }

        return baos.toByteArray();

        }catch(Exception e){

            e.printStackTrace();

            return null;

        }finally{

            try {

                if(is!=null){

                    is.close();

                }

            } catch (IOException e) {

                e.printStackTrace();

            }

            try {

                if(baos!=null){

                    baos.close();

                }

            } catch (IOException e) {

                e.printStackTrace();

            }

        }

    }

}