Android类加载机制分析与实现<\/h1>

1. 前言<\/h2>
在Android系统中，通过使用动态加载类可以实现加载插件和第一代整体加壳。本文旨在深入分析动态加载dex的原理及实现方式。<\/p>

2. 基础知识<\/h2>

2.1 ClassLoader体系<\/h3>

Android中的ClassLoader是一个用于加载类文件的对象，主要作用是将Java类的字节码加载到内存中，并创建对应的Class对象。Android系统中有三种基本类加载器：<\/p>

BootClassLoader<\/strong> (启动类加载器)<\/p>

负责加载Android系统启动时所需的核心类库<\/li>
是系统级别的类加载器，开发者无法直接使用<\/li> <\/ul> <\/li>

PathClassLoader<\/strong> (路径类加载器)<\/p>

主要用于加载Android应用程序的本地类和库文件<\/li>
只能加载已经安装到设备上的类文件，不能加载外部存储中的类文件<\/li> <\/ul> <\/li>

DexClassLoader<\/strong> (动态类加载器)<\/p>

可以加载外部存储中的dex文件、jar文件或包含dex文件的zip压缩包<\/li>
相比PathClassLoader更加灵活，可以加载来自不同来源的类文件<\/li> <\/ul> <\/li> <\/ol>
类加载器验证代码<\/h4>
public<\/span> void<\/span> testClassLoader<\/span>()<\/span> {<\/span> <\/span><\/span> \/\/ 获取MainActivity的类加载器 <\/span><\/span><\/span><\/span> ClassLoader pathClassLoader =<\/span> MainActivity.<\/span>class<\/span>.<\/span>getClassLoader<\/span>();<\/span> <\/span><\/span> Log.<\/span>d<\/span>(<\/span>"xz_aliyun"<\/span>,<\/span> pathClassLoader.<\/span>toString<\/span>());<\/span> <\/span><\/span> <\/span><\/span> \/\/ 获取String类的类加载器 <\/span><\/span><\/span><\/span> ClassLoader bootClassLoader =<\/span> String.<\/span>class<\/span>.<\/span>getClassLoader<\/span>();<\/span> <\/span><\/span> Log.<\/span>d<\/span>(<\/span>"xz_aliyun"<\/span>,<\/span> bootClassLoader.<\/span>toString<\/span>());<\/span> <\/span><\/span>}<\/span> <\/span><\/span><\/code><\/pre>运行结果：<\/p> MainActivity的加载器是PathClassLoader<\/li> String类的加载器是BootClassLoader<\/li> <\/ul> 类加载器层级遍历<\/h4> public<\/span> void<\/span> testClassLoader<\/span>()<\/span> {<\/span> <\/span><\/span> ClassLoader classLoader =<\/span> MainActivity.<\/span>class<\/span>.<\/span>getClassLoader<\/span>();<\/span> <\/span><\/span> Log.<\/span>d<\/span>(<\/span>"xz_aliyun"<\/span>,<\/span> classLoader.<\/span>toString<\/span>());<\/span> <\/span><\/span> <\/span><\/span> ClassLoader parent =<\/span> classLoader.<\/span>getParent<\/span>();<\/span> <\/span><\/span> while<\/span> (<\/span>parent !=<\/span> null<\/span>)<\/span> {<\/span> <\/span><\/span> Log.<\/span>d<\/span>(<\/span>"xz_aliyun"<\/span>,<\/span> "parent->"<\/span> +<\/span> parent.<\/span>toString<\/span>());<\/span> <\/span><\/span> parent =<\/span> parent.<\/span>getParent<\/span>();<\/span> <\/span><\/span> }<\/span> <\/span><\/span>}<\/span> <\/span><\/span><\/code><\/pre>DexClassLoader创建<\/h4> DexClassLoader构造函数需要四个参数：<\/p> String dexPath<\/code>: 指定要加载的类所在的dex文件位置<\/li> String optimizedDirectory<\/code>: 指定优化后的dex文件的输出目录路径<\/li> String librarySearchPath<\/code>: 指定要搜索的本地库的路径<\/li> ClassLoader parent<\/code>: 指定DexClassLoader的父类加载器<\/li> <\/ul> 创建示例：<\/p> public<\/span> void<\/span> testClassLoader<\/span>(<\/span>Context context,<\/span> String dexfilepath)<\/span> {<\/span> <\/span><\/span> File optfile =<\/span> context.<\/span>getDir<\/span>(<\/span>"opt_dex"<\/span>,<\/span> Context.<\/span>MODE_PRIVATE<\/span>);<\/span> <\/span><\/span> File libfile =<\/span> context.<\/span>getDir<\/span>(<\/span>"lib_path"<\/span>,<\/span> Context.<\/span>MODE_PRIVATE<\/span>);<\/span> <\/span><\/span> <\/span><\/span> DexClassLoader dexClassLoader =<\/span> new<\/span> DexClassLoader(<\/span> <\/span><\/span> dexfilepath,<\/span> <\/span><\/span> optfile.<\/span>getAbsolutePath<\/span>(),<\/span> <\/span><\/span> libfile.<\/span>getAbsolutePath<\/span>(),<\/span> <\/span><\/span> MainActivity.<\/span>class<\/span>.<\/span>getClassLoader<\/span>()<\/span> <\/span><\/span> );<\/span> <\/span><\/span>}<\/span> <\/span><\/span><\/code><\/pre>2.2 双亲委派机制<\/h3> 双亲委派机制是指当要加载一个类时，先让父类加载器去尝试加载。如果父类加载器找不到这个类，子类加载器才会去加载。这种机制可以保证：<\/p> 核心类优先被系统的类加载器加载<\/li> 避免重复加载和混乱<\/li> 增强系统稳定性和安全性<\/li> <\/ol> 双亲委派验证代码<\/h4> public<\/span> void<\/span> testParentDelegationMechanism<\/span>()<\/span> {<\/span> <\/span><\/span> ClassLoader classloader =<\/span> MainActivity.<\/span>class<\/span>.<\/span>getClassLoader<\/span>();<\/span> <\/span><\/span> try<\/span> {<\/span> <\/span><\/span> Class StringClass =<\/span> classloader.<\/span>loadClass<\/span>(<\/span>"java.lang.String"<\/span>);<\/span> <\/span><\/span> Log.<\/span>d<\/span>(<\/span>"xz_aliyun"<\/span>,<\/span> "load StringClass success!"<\/span> +<\/span> classloader.<\/span>toString<\/span>());<\/span> <\/span><\/span> }<\/span> catch<\/span> (<\/span>ClassNotFoundException e)<\/span> {<\/span> <\/span><\/span> e.<\/span>printStackTrace<\/span>();<\/span> <\/span><\/span> Log.<\/span>d<\/span>(<\/span>"xz_aliyun"<\/span>,<\/span> "load StringClass fail!"<\/span> +<\/span> classloader.<\/span>toString<\/span>());<\/span> <\/span><\/span> }<\/span> <\/span><\/span>}<\/span> <\/span><\/span><\/code><\/pre>运行结果：PathClassLoader可以成功加载String.class类，因为双亲委派机制下，请求会被委派给BootClassLoader处理。<\/p> 3. 动态加载类实现<\/h2> 3.1 动态加载非组件类<\/h3> 插件类示例<\/h4> public<\/span> class<\/span> TestClass<\/span> {<\/span> <\/span><\/span> public<\/span> void<\/span> testClassLoader<\/span>()<\/span> {<\/span> <\/span><\/span> Log.<\/span>d<\/span>(<\/span>"xz_aliyun"<\/span>,<\/span> "=testClassLoader is called="<\/span>);<\/span> <\/span><\/span> }<\/span> <\/span><\/span>}<\/span> <\/span><\/span><\/code><\/pre>动态加载实现<\/h4> public<\/span> void<\/span> testClassLoader<\/span>(<\/span>Context context,<\/span> String dex_file_path)<\/span> {<\/span> <\/span><\/span> File opt_file =<\/span> context.<\/span>getDir<\/span>(<\/span>"opt_dex"<\/span>,<\/span> Context.<\/span>MODE_PRIVATE<\/span>);<\/span> <\/span><\/span> File lib_file =<\/span> context.<\/span>getDir<\/span>(<\/span>"lib_path"<\/span>,<\/span> Context.<\/span>MODE_PRIVATE<\/span>);<\/span> <\/span><\/span> <\/span><\/span> \/\/ 实例化DexClassLoader <\/span><\/span><\/span><\/span> DexClassLoader dexClassLoader =<\/span> new<\/span> DexClassLoader(<\/span> <\/span><\/span> dex_file_path,<\/span> <\/span><\/span> opt_file.<\/span>getAbsolutePath<\/span>(),<\/span> <\/span><\/span> lib_file.<\/span>getAbsolutePath<\/span>(),<\/span> <\/span><\/span> MainActivity.<\/span>class<\/span>.<\/span>getClassLoader<\/span>()<\/span> <\/span><\/span> );<\/span> <\/span><\/span> <\/span><\/span> try<\/span> {<\/span> <\/span><\/span> \/\/ 加载类 <\/span><\/span><\/span><\/span> Class<?><\/span> clazz =<\/span> dexClassLoader.<\/span>loadClass<\/span>(<\/span>"com.demo.testdex.TestClass"<\/span>);<\/span> <\/span><\/span> \/\/ 创建实例 <\/span><\/span><\/span><\/span> Object obj =<\/span> clazz.<\/span>newInstance<\/span>();<\/span> <\/span><\/span> \/\/ 获取方法 <\/span><\/span><\/span><\/span> Method testClassLoader =<\/span> clazz.<\/span>getDeclaredMethod<\/span>(<\/span>"testClassLoader"<\/span>);<\/span> <\/span><\/span> \/\/ 调用方法 <\/span><\/span><\/span><\/span> testClassLoader.<\/span>invoke<\/span>(<\/span>obj);<\/span> <\/span><\/span> }<\/span> catch<\/span> (<\/span>Exception e)<\/span> {<\/span> <\/span><\/span> throw<\/span> new<\/span> RuntimeException(<\/span>e);<\/span> <\/span><\/span> }<\/span> <\/span><\/span>}<\/span> <\/span><\/span><\/code><\/pre>权限处理<\/h4> Android 6.0+需要处理存储权限：<\/p> <uses-permission<\/span> android:name=<\/span>"android.permission.READ_EXTERNAL_STORAGE"<\/span> \/><\/span> <\/span><\/span><\/code><\/pre>private<\/span> boolean<\/span> checkExternalStoragePermission<\/span>()<\/span> {<\/span> <\/span><\/span> if<\/span> (<\/span>Build.<\/span>VERSION<\/span>.<\/span>SDK_INT<\/span> >=<\/span> Build.<\/span>VERSION_CODES<\/span>.<\/span>M<\/span>)<\/span> {<\/span> <\/span><\/span> if<\/span> (<\/span>checkSelfPermission(<\/span>Manifest.<\/span>permission<\/span>.<\/span>READ_EXTERNAL_STORAGE<\/span>)<\/span> <\/span><\/span> !=<\/span> PackageManager.<\/span>PERMISSION_GRANTED<\/span>)<\/span> {<\/span> <\/span><\/span> requestPermissions(<\/span> <\/span><\/span> new<\/span> String[]{<\/span>Manifest.<\/span>permission<\/span>.<\/span>READ_EXTERNAL_STORAGE<\/span>},<\/span> <\/span><\/span> REQUEST_READ_EXTERNAL_STORAGE <\/span><\/span> );<\/span> <\/span><\/span> return<\/span> false<\/span>;<\/span> <\/span><\/span> }<\/span> <\/span><\/span> }<\/span> <\/span><\/span> return<\/span> true<\/span>;<\/span> <\/span><\/span>}<\/span> <\/span><\/span> <\/span><\/span>@Override<\/span> <\/span><\/span>public<\/span> void<\/span> onRequestPermissionsResult<\/span>(<\/span>int<\/span> requestCode,<\/span> String[]<\/span> permissions,<\/span> int<\/span>[]<\/span> grantResults)<\/span> {<\/span> <\/span><\/span> super<\/span>.<\/span>onRequestPermissionsResult<\/span>(<\/span>requestCode,<\/span> permissions,<\/span> grantResults);<\/span> <\/span><\/span> if<\/span> (<\/span>requestCode ==<\/span> REQUEST_READ_EXTERNAL_STORAGE)<\/span> {<\/span> <\/span><\/span> if<\/span> (<\/span>grantResults.<\/span>length<\/span> ><\/span> 0<\/span> &&<\/span> grantResults[<\/span>0<\/span>]<\/span> ==<\/span> PackageManager.<\/span>PERMISSION_GRANTED<\/span>)<\/span> {<\/span> <\/span><\/span> Log.<\/span>d<\/span>(<\/span>TAG,<\/span> "Permission granted."<\/span>);<\/span> <\/span><\/span> Context applicationContext =<\/span> getApplicationContext();<\/span> <\/span><\/span> testClassLoader(<\/span>applicationContext,<\/span> "\/sdcard\/class.dex"<\/span>);<\/span> <\/span><\/span> }<\/span> else<\/span> {<\/span> <\/span><\/span> Log.<\/span>e<\/span>(<\/span>TAG,<\/span> "Permission denied."<\/span>);<\/span> <\/span><\/span> }<\/span> <\/span><\/span> }<\/span> <\/span><\/span>}<\/span> <\/span><\/span><\/code><\/pre>3.2 动态加载组件类<\/h3> 方法一：替换LoadedApk中的mClassLoader<\/h4> public<\/span> void<\/span> replaceClassLoader<\/span>(<\/span>ClassLoader classLoader)<\/span> {<\/span> <\/span><\/span> try<\/span> {<\/span> <\/span><\/span> \/\/ 加载ActivityThread类 <\/span><\/span><\/span><\/span> Class<?><\/span> activityThreadClazz =<\/span> classLoader.<\/span>loadClass<\/span>(<\/span>"android.app.ActivityThread"<\/span>);<\/span> <\/span><\/span> \/\/ 获取currentActivityThread方法 <\/span><\/span><\/span><\/span> Method currentActivityThread =<\/span> activityThreadClazz.<\/span>getDeclaredMethod<\/span>(<\/span>"currentActivityThread"<\/span>);<\/span> <\/span><\/span> currentActivityThread.<\/span>setAccessible<\/span>(<\/span>true<\/span>);<\/span> <\/span><\/span> \/\/ 获取ActivityThread对象 <\/span><\/span><\/span><\/span> Object activityThreadObj =<\/span> currentActivityThread.<\/span>invoke<\/span>(<\/span>null<\/span>);<\/span> <\/span><\/span> \/\/ 获取mPackages字段 <\/span><\/span><\/span><\/span> Field mPackages =<\/span> activityThreadClazz.<\/span>getDeclaredField<\/span>(<\/span>"mPackages"<\/span>);<\/span> <\/span><\/span> mPackages.<\/span>setAccessible<\/span>(<\/span>true<\/span>);<\/span> <\/span><\/span> ArrayMap mPobj =<\/span> (<\/span>ArrayMap)<\/span> mPackages.<\/span>get<\/span>(<\/span>activityThreadObj);<\/span> <\/span><\/span> \/\/ 获取LoadedApk对象 <\/span><\/span><\/span><\/span> WeakReference wr =<\/span> (<\/span>WeakReference)<\/span> mPobj.<\/span>get<\/span>(<\/span>this<\/span>.<\/span>getPackageName<\/span>());<\/span> <\/span><\/span> Object loadedApkobj =<\/span> wr.<\/span>get<\/span>();<\/span> <\/span><\/span> \/\/ 获取LoadedApk类 <\/span><\/span><\/span><\/span> Class<?><\/span> loadedApkClazz =<\/span> classLoader.<\/span>loadClass<\/span>(<\/span>"android.app.LoadedApk"<\/span>);<\/span> <\/span><\/span> \/\/ 获取mClassLoader字段 <\/span><\/span><\/span><\/span> Field mClassLoader =<\/span> loadedApkClazz.<\/span>getDeclaredField<\/span>(<\/span>"mClassLoader"<\/span>);<\/span> <\/span><\/span> mClassLoader.<\/span>setAccessible<\/span>(<\/span>true<\/span>);<\/span> <\/span><\/span> \/\/ 替换类加载器 <\/span><\/span><\/span><\/span> mClassLoader.<\/span>set<\/span>(<\/span>loadedApkobj,<\/span> classLoader);<\/span> <\/span><\/span> }<\/span> catch<\/span> (<\/span>Exception e)<\/span> {<\/span> <\/span><\/span> throw<\/span> new<\/span> RuntimeException(<\/span>e);<\/span> <\/span><\/span> }<\/span> <\/span><\/span>}<\/span> <\/span><\/span><\/code><\/pre>使用方式：<\/p> public<\/span> void<\/span> testClassLoader<\/span>(<\/span>Context context,<\/span> String dex_file_path)<\/span> {<\/span> <\/span><\/span> File opt_file =<\/span> context.<\/span>getDir<\/span>(<\/span>"opt_dex"<\/span>,<\/span> Context.<\/span>MODE_PRIVATE<\/span>);<\/span> <\/span><\/span> File lib_file =<\/span> context.<\/span>getDir<\/span>(<\/span>"lib_path"<\/span>,<\/span> Context.<\/span>MODE_PRIVATE<\/span>);<\/span> <\/span><\/span> <\/span><\/span> \/\/ 实例化DexClassLoader <\/span><\/span><\/span><\/span> DexClassLoader dexClassLoader =<\/span> new<\/span> DexClassLoader(<\/span> <\/span><\/span> dex_file_path,<\/span> <\/span><\/span> opt_file.<\/span>getAbsolutePath<\/span>(),<\/span> <\/span><\/span> lib_file.<\/span>getAbsolutePath<\/span>(),<\/span> <\/span><\/span> MainActivity.<\/span>class<\/span>.<\/span>getClassLoader<\/span>()<\/span> <\/span><\/span> );<\/span> <\/span><\/span> <\/span><\/span> \/\/ 替换类加载器 <\/span><\/span><\/span><\/span> replaceClassLoader(<\/span>dexClassLoader);<\/span> <\/span><\/span> <\/span><\/span> \/\/ 加载并启动Activity <\/span><\/span><\/span><\/span> Class<?><\/span> secondActivityclazz =<\/span> dexClassLoader.<\/span>loadClass<\/span>(<\/span>"com.demo.testdex.SecondActivity"<\/span>);<\/span> <\/span><\/span> Intent intent =<\/span> new<\/span> Intent(<\/span>MainActivity.<\/span>this<\/span>,<\/span> secondActivityclazz);<\/span> <\/span><\/span> startActivity(<\/span>intent);<\/span> <\/span><\/span>}<\/span> <\/span><\/span><\/code><\/pre>方法二：修改类加载器层级<\/h4> public<\/span> void<\/span> startTestActivity<\/span>(<\/span>Context context,<\/span> String dex_file_path)<\/span> {<\/span> <\/span><\/span> File opt_file =<\/span> context.<\/span>getDir<\/span>(<\/span>"opt_dex"<\/span>,<\/span> 0<\/span>);<\/span> <\/span><\/span> File lib_file =<\/span> context.<\/span>getDir<\/span>(<\/span>"lib_path"<\/span>,<\/span> 0<\/span>);<\/span> <\/span><\/span> <\/span><\/span> ClassLoader pathClassLoader =<\/span> MainActivity.<\/span>class<\/span>.<\/span>getClassLoader<\/span>();<\/span> <\/span><\/span> ClassLoader bootClassLoader =<\/span> MainActivity.<\/span>class<\/span>.<\/span>getClassLoader<\/span>().<\/span>getParent<\/span>();<\/span> <\/span><\/span> <\/span><\/span> \/\/ 实例化DexClassLoader，父加载器设为BootClassLoader <\/span><\/span><\/span><\/span> DexClassLoader dexClassLoader =<\/span> new<\/span> DexClassLoader(<\/span> <\/span><\/span> dex_file_path,<\/span> <\/span><\/span> opt_file.<\/span>getAbsolutePath<\/span>(),<\/span> <\/span><\/span> lib_file.<\/span>getAbsolutePath<\/span>(),<\/span> <\/span><\/span> bootClassLoader <\/span><\/span> );<\/span> <\/span><\/span> <\/span><\/span> \/\/ 修改PathClassLoader的父加载器为DexClassLoader <\/span><\/span><\/span><\/span> try<\/span> {<\/span> <\/span><\/span> Field parent =<\/span> ClassLoader.<\/span>class<\/span>.<\/span>getDeclaredField<\/span>(<\/span>"parent"<\/span>);<\/span> <\/span><\/span> parent.<\/span>setAccessible<\/span>(<\/span>true<\/span>);<\/span> <\/span><\/span> parent.<\/span>set<\/span>(<\/span>pathClassLoader,<\/span> dexClassLoader);<\/span> <\/span><\/span> }<\/span> catch<\/span> (<\/span>Exception e)<\/span> {<\/span> <\/span><\/span> throw<\/span> new<\/span> RuntimeException(<\/span>e);<\/span> <\/span><\/span> }<\/span> <\/span><\/span> <\/span><\/span> \/\/ 加载并启动Activity <\/span><\/span><\/span><\/span> Class<?><\/span> clazz =<\/span> dexClassLoader.<\/span>loadClass<\/span>(<\/span>"com.demo.testdex.SecondActivity"<\/span>);<\/span> <\/span><\/span> startActivity(<\/span>new<\/span> Intent(<\/span>MainActivity.<\/span>this<\/span>,<\/span> clazz));<\/span> <\/span><\/span>}<\/span> <\/span><\/span><\/code><\/pre>4. 关键问题解析<\/h2> 为什么直接使用DexClassLoader加载组件类没有生命周期？<\/strong><\/p> Android系统对四大组件有特定的启动流程和生命周期管理机制<\/li> 直接使用DexClassLoader加载的类不会被系统纳入正常管理流程<\/li> 替换LoadedApk中的mClassLoader后，系统会使用自定义加载器加载组件类，从而触发正常生命周期<\/li> <\/ul> <\/li> 父加载类与父类的区别<\/strong><\/p> PathClassLoader和DexClassLoader的父类是BaseDexClassLoader<\/li> PathClassLoader的父加载类是BootClassLoader<\/li> 双亲委派机制中"双亲"指的是父加载器，而非继承关系中的父类<\/li> <\/ul> <\/li> <\/ol> 5. 总结<\/h2> Android类加载机制的核心要点：<\/p> 三种基本类加载器各司其职，分工明确<\/li> 双亲委派机制保障了类加载的安全性和稳定性<\/li> 动态加载非组件类相对简单，直接使用DexClassLoader即可<\/li> 动态加载组件类需要修改系统类加载机制，有两种主要实现方式<\/li> 不同Android版本实现细节可能有所差异，需要注意兼容性<\/li> <\/ol> 通过深入理解Android类加载机制，开发者可以实现插件化、热修复等高级功能，但同时也需要注意安全性和稳定性问题。<\/p>

Android类加载机制分析与实现<\/h1>

1. 前言<\/h2> 在Android系统中，通过使用动态加载类可以实现加载插件和第一代整体加壳。本文旨在深入分析动态加载dex的原理及实现方式。<\/p>

2. 基础知识<\/h2>

3. 动态加载类实现<\/h2>

3.1 动态加载非组件类<\/h3>

3.2 动态加载组件类<\/h3>

1. 前言<\/h2>
在Android系统中，通过使用动态加载类可以实现加载插件和第一代整体加壳。本文旨在深入分析动态加载dex的原理及实现方式。<\/p>