Java 自定义ClassLoad-优快云博客

本文链接：https://blog.youkuaiyun.com/tomli2017/article/details/71056060

本文介绍了一个自定义类加载器的实现过程，包括如何加载类文件并创建Class实例，以及如何通过反射调用方法。此外，还展示了如何使用自定义类加载器加载并运行一个简单的对象。

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public class DiyClassLoad extends ClassLoader {

    public Class loadClass(String name, boolean resolve) throws ClassNotFoundException {
        Class clazz = null;
        try {
            clazz = findLoadedClass(name);
            if (clazz != null) {
                return clazz;
            }

            byte[] bs = getClassBytes(name);
            if (bs != null && bs.length > 0) {
                clazz = defineClass(name, bs, 0, bs.length);
            }
            if (clazz == null) { 
                clazz = findSystemClass(name);
            }
            if (resolve && clazz != null) {
                resolveClass(clazz);
            }
        } catch (IOException e) {
            throw new ClassNotFoundException(e.toString());
        }
        System.out.println("calzz == " + clazz);
        return clazz;
    }

    private byte[] getClassBytes(String className) throws IOException {
        String path = System.getProperty("java.class.path") + File.separator;
        path += className.replace('.', File.separatorChar) + ".class";
        System.out.println(path);
        FileInputStream fis = null;
        try {
            fis = new FileInputStream(path);
        } catch (FileNotFoundException e) {
            System.out.println(e);
            return null;   
        }
        byte[] bs = new byte[fis.available()];
        fis.read(bs);
        return bs;
    }

}

public class DiyObject {

    public void print(String str) {
        System.out.println("this is DiyObject！");
        System.out.println(str);
    }

    public String toString() {
        return "this is DiyObject！";
    }

    public static void main(String[] args) throws Exception {
        DiyClassLoad loader = new DiyClassLoad();
        Class c = loader.loadClass("com.***.tom.test.classload.DiyObject", false);
        Object o = c.newInstance();
        Method m = c.getMethod("print", java.lang.String.class);
        m.invoke(o, "just test for calssload !");
    }
}

系统的3个类加载器分别为：
1. Bootstrap CLassloder
2. Extention ClassLoader
3. AppClassLoader

加载器获取的是Class

参考资料：

http://blog.youkuaiyun.com/ggjjzhzz/article/details/321292

http://www.cnblogs.com/editice/p/5420712.html