java自定义ClassLoader加载指定的class文件

Posted 2020-12-15 JLCUI

继承ClassLoader并且重写findClass方法就可以自定义一个类加载器，具体什么是类加载器以及类加载器的加载过程与顺序下次再说，下面给出一个小demo

首先定义一个类，比如MyTest，并且将其编译成class文件，然后放到一个指定的文件夹下面，其中文件夹的最后几层就是它的包名，这里我将这个编译好的类放到 : /Users/allen/Desktop/cn/lijie/bug .class

package cn.lijie;
public class bugnew {
    public void shownew() {
        System.out.println("shownew test!");
    }
}

　　

package com.infosec.manager.controller.login;

import java.lang.reflect.Method;
import java.net.URI;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;

public class bug extends ClassLoader {
	public void show() {
		System.out.println("show test!");
	}

	public void shows() {
		String myPath = "file:///D:/infosec/netauth/accountsync/bugnew.class";
		System.out.println(myPath);
		byte[] cLassBytes = null;
		Path path = null;
		try {
			path = Paths.get(new URI(myPath));
			cLassBytes = Files.readAllBytes(path);
		
		Class clazz = defineClass("cn.lijie.bugnew", cLassBytes, 0, cLassBytes.length);
		Object obj = clazz.newInstance();
		Method method = clazz.getMethod("shownew");
		method.invoke(obj);
		}catch (Exception e) {
		}
	}
}

　　

自定义的类加载器：

 

package com.infosec.manager.controller.login;

import java.io.IOException;
import java.net.URI;
import java.net.URISyntaxException;
import java.nio.file.Files;
import java.nio.file.Path;
import java.nio.file.Paths;

public class MyClassLoader extends ClassLoader {
	
	protected Class<?> findClass() {
		String myPath = "file:///D:/infosec/netauth/accountsync/bug.class";
		byte[] cLassBytes = null;
		Path path = null;
		try {
			path = Paths.get(new URI(myPath));
			cLassBytes = Files.readAllBytes(path);
		} catch (IOException | URISyntaxException e) {
			e.printStackTrace();
		}
		Class clazz = defineClass("com.infosec.manager.controller.login.bug", cLassBytes, 0, cLassBytes.length);
		return clazz;
	}
}

 

　　

 

测试的主函数：

 

package com.infosec.manager.controller.login;

import java.lang.reflect.Method;

public class TestController {
	
	 public static void main(String[] args) throws ClassNotFoundException {
	        MyClassLoader loader = new MyClassLoader();
	        Class<?> aClass = loader.findClass();
	        try {
	            Object obj = aClass.newInstance();
	            Method method = aClass.getMethod("shows");
	            method.invoke(obj);
	        } catch (Exception e) {
	            e.printStackTrace();
	        }
	    }
}

 

　　

 

执行主函数，调用外部class的show方法：

 

 

判断class<?>[] 是否包含某个元素 instanceof 

	private boolean guessInterface(Object object) {
		if (object instanceof SynchAccountApi) {
			return true;
		}
		if (object instanceof SyncUsersFromAppInterface) {
			return true;
		}
		Class<?>[] allInterfaces = ClassUtils.getAllInterfaces(object);
		return Arrays.stream(allInterfaces).anyMatch(aClass -> aClass.getName().contains("SynchAccountApi"));
	}