写一个自定义类加载器demo

public class MyTest16 extends ClassLoader {
    private String classLoaderName;
    private String fileExtension = ".class";
    private String path;

    public MyTest16(String classLoaderName) {
        super(); // 将系统类加载器当做该类加载器的父类加载器
        this.classLoaderName = classLoaderName;
    }

    public void setPath(String path){
        this.path = path;
    }

    public MyTest16(ClassLoader parent, String classLoaderName) {
        super(parent);  //显示指定该类的加载器的父类加载器
        this.classLoaderName = classLoaderName;
    }

    private byte[] loadClassData(String className) {
        InputStream is = null;
        byte[] data = null;
        ByteArrayOutputStream baos = null;

        String classNameResult = className.replace(".", "\\");

        System.out.println(classNameResult);
        try {
            //注意win和linux
            this.classLoaderName = this.classLoaderName.replace(".", "\\");
            //指定磁盘全路径
           String fileUrl = classNameResult + this.fileExtension;
            is = new FileInputStream(this.path + new File(fileUrl));
            baos = new ByteArrayOutputStream();
            int ch ;
            while (-1 != (ch = is.read())) {
                baos.write(ch);
            }
            // 字节数组输出流转换成字节数组
            data = baos.toByteArray();
        } catch (Exception e) {
            e.printStackTrace();
        } finally {
            try {
                is.close();
                baos.close();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
        return data;
    }

    @Override
    protected Class<?> findClass(String className) throws ClassNotFoundException {

        System.out.println("findClass invoked:" + className);
        System.out.println("class loader name" + this.classLoaderName);

        byte[] data = this.loadClassData(className);
        //返回Class对象
        return this.defineClass(className, data, 0 , data.length);
    }

    @Override
    public String toString() {
        return "[" + this.classLoaderName + "]";
    }

    public static void main(String[] args) throws IllegalAccessException, InstantiationException, ClassNotFoundException {
        // 创建自定义类加载器 名字“loader1” 父类加载器是系统类加载器
        MyTest16 loader1 = new MyTest16("loader1");
        //此路径为classPath，故 findClass方法不会被调用执行！ 如果换个路径，不是classPath就会去执行了！
        loader1.setPath("D:\\eclipse_pj\\dianshang\\jvmTest\\out\\production\\jvmTest\\");
        Class<?> clazz = loader1.loadClass("com.jvm.t1.MyTest15");
        System.out.println("class:"+ clazz.hashCode());
        Object object = clazz.newInstance();
        System.out.println(object);

        // loader1 指向新的实例
        loader1 = new MyTest16("loader1");
        //此路径为classPath，故 findClass方法不会被调用执行！ 如果换个路径，不是classPath就会去执行了！
        loader1.setPath("D:\\eclipse_pj\\dianshang\\jvmTest\\out\\production\\jvmTest\\");
        //指向新的
        clazz = loader1.loadClass("com.jvm.t1.MyTest15");
        System.out.println("class:"+ clazz.hashCode());
        // 指向新的
        object = clazz.newInstance();
        System.out.println(object);

    }
}

调用：

public class Test17 {
    public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException {
        MyTest16 loader1 = new MyTest16("loader1");
        loader1.setPath("D:\\test\\jvm\\");

        Class<?> clazz = loader1.loadClass("com.jvm.t1.test");
        System.out.println("class"+ clazz.hashCode());

        Object o = clazz.newInstance();
    }
}