Java任意类加载方法

准备要加载的测试类test.java

public class test {

    // 静态变量
    public static int staticVar;

    // 实例变量
    public int instanceVar;

    // 静态代码块
    static
    {
        System.out.println("静态代码块");
    }

    // 构造代码块
    {
        System.out.println("构造代码块");
    }

    // 无参构造器
    public test() {
        System.out.println("无参构造器");
    }

    // 有参构造器
    public test(int instanceVar){
        System.out.println("有参构造器");
    }

    // 静态方法
    public static void staticAction(){
        System.out.println("静态方法");
    }
}

编译为test.class文件

javac -encoding UTF-8 test.java -d E:\tmp

方法一：使用URLClassLoader类

URLClassLoaderDemo.java：

package ThreeClassLoader;

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

public class URLClassLoaderDemo {
    public static void main(String[] args) throws  Exception{
        URLClassLoader urlClassLoader = new URLClassLoader(new URL[]{new URL("file:///E:/tmp/")});
        Class<?> c = urlClassLoader.loadClass("test");
        c.newInstance();
    }
}

也可以使用http://远程加载

可以在kali开个PHP服务，将编译好的文件放到目录下

php -S 0:80

利用http://进行加载

package ThreeClassLoader;

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

public class URLClassLoaderDemo {
    public static void main(String[] args) throws  Exception{
        URLClassLoader urlClassLoader = new URLClassLoader(new URL[]{new URL("http://192.168.41.132/")});
        Class<?> c = urlClassLoader.loadClass("test");
        c.newInstance();
    }
}

方法二：使用ClassLoader.defineClass()方法

ClassLoaderDefineClassDemo.java：

package ThreeClassLoader;

import java.lang.reflect.Method;
import java.nio.file.Files;
import java.nio.file.Paths;

public class ClassLoaderDefineClassDemo {
    public static void main(String[] args) throws  Exception{
        ClassLoader c = ClassLoader.getSystemClassLoader();
        Method cname = ClassLoader.class.getDeclaredMethod("defineClass",String.class,byte[].class,int.class,int.class);
        cname.setAccessible(true);
        byte[] classBytes = Files.readAllBytes(Paths.get("E:/tmp/test.class"));
        Class<?> cc = (Class<?>) cname.invoke(c,"test",classBytes,0,classBytes.length);
        cc.newInstance();
    }
}

方法三：使用Unsafe.defineClass()方法

package ThreeClassLoader;

import sun.misc.Unsafe;

import java.lang.reflect.Field;
import java.nio.file.Files;
import java.nio.file.Paths;

public class UnsafeDefineClassDemo {
    public static void main(String[] args) throws Exception{
        ClassLoader cl = ClassLoader.getSystemClassLoader();
        Class c = cl.getClass();
        Field f = Unsafe.class.getDeclaredField("theUnsafe");
        f.setAccessible(true);
        Unsafe unsafe = (Unsafe) f.get(null);
        byte[] classBytes = Files.readAllBytes(Paths.get("E:/tmp/test.class"));
        Class<?> u = unsafe.defineClass("test",classBytes,0,classBytes.length,cl,null);
        u.newInstance();
    }
}