java类的动态加载

java动态加载的机制：
ClassLoader->SecureClassloader–>URLClassLoader–>AppClassLoader
loadClass–>findClass(重写方法)–>defineClass(从字节码加载类)

初始化的时候会加载静态代码块
实例化的时候会加载构造代码块、无参构造函数

1、URLClassLoader动态加载任意类

测试静态类

package loadClass;

import java.io.IOException;

public class test {
    static {
        try {
            Runtime.getRuntime().exec("calc");
        } catch (IOException e) {
            e.printStackTrace();
        }
    }
}

测试动态加载：

package loadClass;

import com.sun.jndi.toolkit.url.UrlUtil;
import serializes.Person;

import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;

public class loadclass_test {
    public static void main(String[] args) throws ClassNotFoundException, IllegalAccessException, InstantiationException, MalformedURLException {

		//file协议加载
        //URLClassLoader urlClassLoader = new URLClassLoader(new URL[] {new URL("file:///C:\\temp\\")});	//文件所在目录
        //http协议加载
        //URLClassLoader urlClassLoader = new URLClassLoader(new URL[] {new URL("http://127.0.0.1:888/")});
        //jar协议
        URLClassLoader urlClassLoader = new URLClassLoader(new URL[] {new URL("jar:file:///C:\\temp\\test.jar!/")});
        Class<?> loadClass = urlClassLoader.loadClass("loadClass.test");	//全名：包名+类名
        loadClass.newInstance();

    }
}

执行完成截图：
在这里插入图片描述

2、defineClass 动态加载任意类

		ClassLoader systemClassLoader = ClassLoader.getSystemClassLoader();	//获取系统的类加载器（classloader）
        Method defineClass = ClassLoader.class.getDeclaredMethod("defineClass", String.class, byte[].class, int.class, int.class);
        defineClass.setAccessible(true);
        byte[] path = Files.readAllBytes(Paths.get("C:\\temp\\test.class"));
        Class test = (Class)defineClass.invoke(systemClassLoader, "loadClass.test", path, 0, path.length);
        test.newInstance();

3、Unsave动态加载任意类:

		ClassLoader systemClassLoader = ClassLoader.getSystemClassLoader();
		Class c = Unsafe.class;
        Field theUnsafe = c.getDeclaredField("theUnsafe");
        theUnsafe.setAccessible(true);
        Unsafe unsafe = (Unsafe)theUnsafe.get(null);
        byte[] path = Files.readAllBytes(Paths.get("C:\\temp\\test.class"));
        Class<?> test1 = unsafe.defineClass("loadClass.test", path, 0, path.length, systemClassLoader, null);
        test1.newInstance();