Frida Labs Java层操作指南<\/h1>

一、Frida基础使用<\/h2>

1. Java.perform函数<\/h3>
Java<\/span>.perform<\/span>(function<\/span>() {
<\/span><\/span>    \/\/ 你的代码
<\/span><\/span><\/span><\/span>});
<\/span><\/span><\/code><\/pre>
固定写法，确保脚本在Java虚拟机准备好后运行<\/li>
避免"Java is not available"错误<\/li>
创建特殊上下文，使脚本能与Android应用中的Java代码交互<\/li>
<\/ul>
2. 获取类引用<\/h3>
var<\/span> classRef<\/span> =<\/span> Java<\/span>.use<\/span>("package.name.class"<\/span>);
<\/span><\/span><\/code><\/pre>
获取要Hook的Java类的引用<\/li>
package.name.class<\/code>是完整类名，如com.example.app.LoginManager<\/code><\/li>
后续可通过该引用访问类中的方法和属性<\/li>
<\/ul>
3. Hook方法实现<\/h3>
classRef<\/span>.methodToHook<\/span>.implementation<\/span> =<\/span> function<\/span>(args<\/span>) {
<\/span><\/span>    \/\/ 自定义逻辑
<\/span><\/span><\/span><\/span>};
<\/span><\/span><\/code><\/pre>
重写类中某个方法的实现<\/li>
methodToHook<\/code>是要Hook的方法名<\/li>
args<\/code>是方法参数列表<\/li>
函数体内的逻辑会替换原方法实现<\/li>
<\/ul>
二、Frida Labs实战案例<\/h2>
0x1 Hook修改方法逻辑\/返回值\/参数<\/h3>
场景<\/strong>：使用随机数判断输入，满足表达式则打印flag<\/p>
两种思路<\/strong>：<\/p>

Hook get_random<\/code>函数，固定返回值<\/li>
Hook check<\/code>函数，直接传入自定义值<\/li>
<\/ol>
常见错误<\/strong>：<\/p>

缺少-U<\/code>：未连接到Android模拟器<\/li>
缺少-f<\/code>：找不到指定进程(Failed to spawn: unable to find process with name...<\/code>)<\/li>
<\/ul>
0x2 调用未被调用的静态方法<\/h3>
问题<\/strong>：setImmediate(main)<\/code>可能导致hook失败<\/p>
解决方案<\/strong>：<\/p>

使用setTimeout(main, 1000)<\/code>延迟注入<\/li>
确保应用完全启动后再注入<\/li>
<\/ol>
注意<\/strong>：直接使用包名注入(frida com.ad2001.frida0x2<\/code>)可能失败，因为：<\/p>

默认是spawn模式，缺少-f<\/code>参数<\/li>
不是attach模式<\/li>
<\/ul>
0x3 修改静态变量<\/h3>
关键概念<\/strong>：<\/p>

a<\/code>：Frida创建的Java类代理对象，用于操作类<\/li>
a.code.value<\/code>：访问并修改类中code<\/code>静态字段的值<\/li>
a.$new()<\/code>：创建类的新实例<\/li>
<\/ul>
示例<\/strong>：<\/p>
var<\/span> a<\/span> =<\/span> Java<\/span>.use<\/span>("com.ad2001.frida0x3.Checker"<\/span>);
<\/span><\/span>a<\/span>.code<\/span>.value<\/span> =<\/span> 123<\/span>; \/\/ 修改静态变量
<\/span><\/span><\/span><\/code><\/pre>0x4 调用非MainActivity的非静态方法<\/h3>
步骤<\/strong>：<\/p>

实例化目标类<\/li>
调用目标方法<\/li>
<\/ol>
示例<\/strong>：<\/p>
Java<\/span>.perform<\/span>(function<\/span>() {
<\/span><\/span>    var<\/span> check<\/span> =<\/span> Java<\/span>.use<\/span>("com.ad2001.frida0x4.Check"<\/span>);
<\/span><\/span>    var<\/span> checker<\/span> =<\/span> check<\/span>.$new<\/span>(); \/\/ 创建实例
<\/span><\/span><\/span><\/span>    console<\/span>.log<\/span>(checker<\/span>.get_flag<\/span>()); \/\/ 调用实例方法
<\/span><\/span><\/span><\/span>});
<\/span><\/span><\/code><\/pre>0x5 调用MainActivity的非静态方法<\/h3>
挑战<\/strong>：直接创建MainActivity实例不可行<\/p>
原因<\/strong>：<\/p>

Activity需要上下文(Context)才能工作<\/li>
UI操作必须在主线程(带Looper)执行<\/li>
Activity生命周期由系统管理<\/li>
<\/ol>
解决方案<\/strong>：获取系统已创建的MainActivity实例<\/p>
模板<\/strong>：<\/p>
Java<\/span>.perform<\/span>(function<\/span>() {
<\/span><\/span>    Java<\/span>.choose<\/span>("com.ad2001.frida0x5.MainActivity"<\/span>, {
<\/span><\/span>        onMatch<\/span>:<\/span> function<\/span>(instance<\/span>) {
<\/span><\/span>            \/\/ instance是已存在的MainActivity实例
<\/span><\/span><\/span><\/span>            console<\/span>.log<\/span>(instance<\/span>.flag<\/span>());
<\/span><\/span>        },
<\/span><\/span>        onComplete<\/span>:<\/span> function<\/span>() {
<\/span><\/span>            console<\/span>.log<\/span>("end"<\/span>);
<\/span><\/span>        }
<\/span><\/span>    });
<\/span><\/span>});
<\/span><\/span><\/code><\/pre>注意<\/strong>：立即注入可能导致hook失败，解决方案同0x2<\/p>
0x6 综合案例<\/h3>
场景<\/strong>：<\/p>

MainActivity中的非静态方法get_flag<\/code><\/li>
参数通过Checker类对象传递<\/li>
<\/ul>
解决方案<\/strong>：结合0x3、0x4、0x5的技术<\/p>

创建Checker类实例<\/li>
修改实例变量<\/li>
获取MainActivity实例<\/li>
调用get_flag<\/code>方法并传入Checker实例<\/li>
<\/ol>
0x7 Hook构造函数<\/h3>
构造函数<\/strong>：Java类中初始化对象属性的特殊方法<\/p>
Frida中使用$init<\/code>关键字hook构造函数<\/strong>：<\/p>
var<\/span> Checker<\/span> =<\/span> Java<\/span>.use<\/span>("com.ad2001.frida0x7.Checker"<\/span>);
<\/span><\/span>Checker<\/span>.$init<\/span>.implementation<\/span> =<\/span> function<\/span>(a<\/span>, b<\/span>) {
<\/span><\/span>    this<\/span>.$init<\/span>(123<\/span>, 321<\/span>); \/\/ 修改构造参数
<\/span><\/span><\/span><\/span>};
<\/span><\/span><\/code><\/pre>常见错误<\/strong>：<\/p>

cannot read property 'overload' of undefined<\/code>：方法未找到<\/li>
Cannot access an instance field without an instance<\/code>：尝试直接访问实例字段<\/li>
<\/ol>
三、最佳实践<\/h2>

注入时机<\/strong>：使用setTimeout<\/code>延迟注入，避免应用未完全启动<\/li>
错误处理<\/strong>：检查类和方法是否存在再操作<\/li>
实例访问<\/strong>：对于实例字段\/方法，必须通过实例访问<\/li>
Android组件<\/strong>：避免直接创建Activity实例，使用Java.choose<\/code>获取已有实例<\/li>
静态成员<\/strong>：直接通过类引用修改静态变量<\/li>
<\/ol>
四、命令总结<\/h2>
# 基本命令<\/span>
<\/span><\/span>frida -U -f com.example.app -l script.js
<\/span><\/span>
<\/span><\/span># 参数说明<\/span>
<\/span><\/span>-U 连接到USB设备
<\/span><\/span>-f 指定包名启动应用
<\/span><\/span>-l 加载脚本
<\/span><\/span><\/code><\/pre>通过掌握这些核心概念和技术，您将能够有效地使用Frida进行Android应用的Java层动态分析和修改。<\/p>