【JavaSec】Java反射知识点补充

news2025/11/3 23:32:46

0x03反射-补充零散知识点

文章目录

0x03反射-补充零散知识点
- Runtime类
- - setAccessible(true)
  - 三种命令执行的方法
  - static变量赋值前面学过就不多说
  - final变量赋值
  - InDirect final间接赋值
  - static + final

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Runtime类

Runtime 类中有 exec 方法，可以用来命令执行。

setAccessible(true)

一般情况下，我们使用反射机制不能对类的私有 private 字段进行操作，绕过私有权限的访问

三种命令执行的方法

package IOStream;

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.Map;

public class FileStreamTest {
    public static void main(String[] args){
        InputStream inputStream = null;
        try {
            //命令执行方法1
//            inputStream = Runtime.getRuntime().exec("whoami").getInputStream();

            //命令执行方法2
//            inputStream = new ProcessBuilder("calc").start().getInputStream();

            //命令执行方法3 反射间接调用
            String[] cmds = new String[]{"calc"};
            Class clazz = Class.forName("java.lang.ProcessImpl");
            Method method = clazz.getDeclaredMethod("start", String[].class, Map.class, String.class, ProcessBuilder.Redirect[].class, boolean.class);
            method.setAccessible(true);
            Process e = (Process) method.invoke(null, cmds, null, ".", null, true);
            inputStream = e.getInputStream();


        } catch (ClassNotFoundException | NoSuchMethodException e) {
            e.printStackTrace();
        } catch (InvocationTargetException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }
        byte[] cache = new byte[1024];   //cache缓存
        ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
        int readLen = 0;
        while(true){
            try {
                if (!((readLen = inputStream.read(cache)) != -1)) break;
            } catch (IOException e) {
                e.printStackTrace();
            }
            byteArrayOutputStream.write(cache, 0, readLen);
        }
        //输出上面exec命令执行的结果
        System.out.println(byteArrayOutputStream);
    }
}

try catch中的内容是自动生成的

static变量赋值前面学过就不多说
final变量赋值

直接赋值

FinalPerson：

package ReflectPlus.pojo;

public class FinalPerson {
    private final String name = "happySu";
    public final int age = 18;

    public void printInfo(){
        System.out.println(name +" " + age);
    }
}

FinalReflect：

package ReflectPlus.Service;

import java.lang.reflect.Field;
import java.lang.reflect.Method;

public class FinalReflection {
    public static void main(String[] args){
        try{
            //直接引入包的位置  不要再加src了
            Class c = Class.forName("ReflectPlus.pojo.FinalPerson");
            Object m = c.newInstance();
            Method printMethod = c.getDeclaredMethod("printInfo");
            printMethod.invoke(m);

            Field nameField = c.getDeclaredField("name");
            Field ageField = c.getDeclaredField("age");

            nameField.setAccessible(true);
            ageField.setAccessible(true);
            System.out.println("================");
            nameField.set(m,"111");
            ageField.set(m,12);

            printMethod.invoke(m);
        }catch (Exception e){
            e.printStackTrace();
        }
    }
}

但是根本无法修改成功永远18！

InDirect final间接赋值

Person：

package ReflectPlus.pojo;

public class InDirectPerson {
    private final StringBuilder sex = new StringBuilder("male");
    public final int age = (null != null ? 18 : 18);

    //构造函数方法赋值
    private final String name;
    public InDirectPerson(){
        name = "happy";
    }
    public void printInfo(){
        System.out.println(name + " " + age + " " + sex);
    }
}

Reflect：

package ReflectPlus.Service;

import java.lang.reflect.Field;
import java.lang.reflect.Method;

public class InDirectReflect {
    public static void main(String[] args){
        try{
            //调用
            Class c = Class.forName("ReflectPlus.pojo.InDirectPerson");
            Object m = c.newInstance();
            Method printMethod = c.getDeclaredMethod("printInfo");
            printMethod.invoke(m);

            System.out.println("=================");

            //修改
            Field nameField = c.getDeclaredField("name");
            Field ageField = c.getDeclaredField("age");
            Field sexField = c.getDeclaredField("sex");

            nameField.setAccessible(true);
            ageField.setAccessible(true);
            sexField.setAccessible(true);
            nameField.set(m, "newhappy");
            ageField.set(m,180);
            sexField.set(m,new StringBuilder("female"));
            printMethod.invoke(m);

        }catch(Exception e){
            e.printStackTrace();
        }

    }

}

直接成功

一点点想法：

这样看来能否修改成功的关键不在于我们Reflect类是怎么写怎么构造的，而是Person类本身的情况

static + final

Person：

package ReflectPlus.pojo;

public class StaticFinalPerson {
    static final StringBuilder name = new StringBuilder("happySu");

    public void printInfo(){
        System.out.println(name);
    }


}

Reflect：

package ReflectPlus.Service;

import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;

public class StaticFinalReflect {
    public static void main(String[] args){
        try{
            //调用
            Class c = Class.forName("ReflectPlus.pojo.StaticFinalPerson");
            Object m = c.newInstance();
            Method printMethod = c.getDeclaredMethod("printInfo");
            printMethod.invoke(m);

            //修改
            Field nameField = c.getDeclaredField("name");
            nameField.setAccessible(true);
            Field nameModifyField = nameField.getClass().getDeclaredField("modifiers");
            nameModifyField.setAccessible(true);
            nameModifyField.setInt(nameField, nameField.getModifiers() & ~Modifier.FINAL);

            nameField.set(m, new StringBuilder("newhappy"));

            nameModifyField.setInt(nameField, nameField.getModifiers() & ~Modifier.FINAL);
            printMethod.invoke(m);


        }catch(Exception e){
            e.printStackTrace();
        }

    }

}