自定义ClassLoader

ClassLoader再java中十分重要，它主要是在Class装载的加载阶段，将系统外部的获得Class二进制数据流，载进系统，交给jvm进行连接、初始化等操作。代码如下

//这是我要加载的外部类
public class Stone {
    static {
        System.out.println("i am a stone and very hard!!!");
    }
}

　　

这是继承ClassLoader类之后重写其findclass,并返回其defineClass，实现自动ClassLoader
public class MyclassLoader extends ClassLoader {
    //读取路径
    private String path;
    //读取名称
    private String classLoaderName;

    public MyclassLoader(String path, String classLoaderName) {
        this.path = path;
        this.classLoaderName = classLoaderName;
    }

    @Override
    protected Class<?> findClass(String name) throws ClassNotFoundException {
        byte[] b = loadClassData(name);
        return defineClass(name, b, 0, b.length);
    }

    private byte[] loadClassData(String name) {
        name = path + name + ".class";
        InputStream in = null;
        ByteArrayOutputStream out = null;
        try {
            //读文件
            in = new FileInputStream(new File(name));
            //接受文件
            out = new ByteArrayOutputStream();
            //设置起始位置
            int i = 0;
            //循环读入
            while ((i = in.read()) != -1) {
                out.write(i);
            }
            return out.toByteArray();
        } catch (Exception e) {
            e.getStackTrace();

        } finally {
            if (out != null) {
                try {
                    out.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if (in != null) {
                try {
                    in.close();
                } catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
        //返回二进制字节流
        return null;
    }
}

　　

这是最终的检测，验证代码
public class ClassLoaderChecker {
    public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException {
        MyclassLoader myclassLoader = new MyclassLoader("D:\\redis\\","ccloader");
        Class<?> stone = myclassLoader.loadClass("Stone");
        System.out.println(stone.getClassLoader()+"- _ -");
        stone.newInstance();
    }
}

　　理解原理，然后注意系统外部文件的存放位置，使用前需要在idea中编译，或者自行手动javac命令编译。