Android 中反射还能这么用？
cnmd9247
9年前
<p>这几天稍微过了一下Weex的源码，可谓是亲眼目睹了它的源码。无意间发现一个类，叫WXHack，搜索一下代码，发现在Weex里用的地方就一处，好奇心驱使下去看了WXHack的源码，好家伙！看完之后总觉得这个类似曾相识，后来昨天在看OpenAtlas的代码的时候又看到了这个类，相关链接如下</p> <ul> <li><a href="/misc/goto?guid=4959671917898715265">Hack.java</a></li> <li><a href="/misc/goto?guid=4959671917990245968">Interception.java</a></li> </ul> <p>后来我断定这个类应该是淘宝Atlas里的类，Weex直接抽出来了。为了验证自己的想法，下了个淘宝客户端反编译了下去找这个类，真的找到了。</p> <p><img alt="这里写图片描述" src="https://simg.open-open.com/show/01501cb471923c4fcbe836f7be57b9db.png"></p> <p>想看源码的可以直接点上面的链接去看OpenAtals里的类，也可以看这个保持了淘宝的包结构，但是反编译并完全的项目，也就是包含了一些字节码的，但是这两个类应该是完整的。</p> <ul> <li><a href="/misc/goto?guid=4959671918073274270">AtlasForAndroid</a></li> </ul> <p>那么携程的动态加载框架也算是有Atlas的成分了，自然也就有这两个类了,不出所料的找到了它们。</p> <ul> <li><a href="/misc/goto?guid=4959671918157718797">Hack.java</a></li> <li><a href="/misc/goto?guid=4959671918242520330">Interception.java</a></li> </ul> <p>其实这个应该也不是淘专的专利，因为后来我在github上搜索了一番，发现了几个类似的类，这几个类在eclipse中竟然出现了。</p> <ul> <li><a href="/misc/goto?guid=4959671918328289146">HackedField.java</a></li> <li><a href="/misc/goto?guid=4959671918407878328">HackedMethod0.java</a></li> <li><a href="/misc/goto?guid=4959671918493365896">HackedMethod1.java</a></li> </ul> <p>那么，这两个类到底有什么作用呢，莫慌，容我慢慢道来。</p> <p>首先，从名字上可以看出这是一个Hack类，它的作用就是辅助反射，但是它们却不是简简单单的辅助反射，而是反射后包装的形式：类，方法，字段 ，也就是HackedClass,HackedConstructor,HackedMethod,HackedField这几个反射的包装形式。并且还有AssertionFailureHandler用于处理反射发生异常时的处理，你可以选择扔出异常或者自己处理掉。</p> <p>在看他的源码之前，我们先来看看传统的反射是怎么玩的。<br> 写了两三个类用来测试。</p> <p>Student实体类，内部有一个IBehavior类，代表学生的行为；此外还有静态变量，非静态变量，静态方法，非静态方法，泛型变量，作用就是等下测试不同场景的反射。</p> <pre> <code class="language-java">public class Student implements Serializable { private static String school = "清华大学"; private String name; private int age; private HashMap<String, Integer> scores = new HashMap<String, Integer>(); private IBehavior behavior = new BehaviorImpl(); public Student() { } public Student(String name, int age) { this.name = name; this.age = age; } public IBehavior getBehavior() { return behavior; } public void setBehavior(IBehavior behavior) { this.behavior = behavior; } public void addScore(String name, int score) { scores.put(name, score); } public HashMap<String, Integer> getScores() { return scores; } public void setScores(HashMap<String, Integer> scores) { this.scores = scores; } public String getName() { return name; } public void setName(String name) { this.name = name; } public int getAge() { return age; } public void setAge(int age) { this.age = age; } public static String getSchool() { return school; } public static void setSchool(String school) { Student.school = school; } public void say(String word) { System.out.println(word); } @Override public String toString() { return "Student{" + "school=" + school + ", age=" + age + ", name='" + name + '\'' + ", scores=" + scores + ", behavior=" + behavior + '}'; } } </code></pre> <p>IBehavior接口以及其实现类的代码如下</p> <pre> <code class="language-java">public interface IBehavior { void perform(String behaiviorName, String behaiviorContent); } public class BehaviorImpl implements IBehavior { @Override public void perform(String behaiviorName, String behaiviorContent) { System.out.println("behaiviorName:" + behaiviorName); System.out.println("behaiviorContent:" + behaiviorContent); } } </code></pre> <p>你要反射，就先得拿到Class对象，两种方式，一直是Class在你的项目中，可以直接引用，第二种就是这个Class可能你不能直接引用，比如Android中的ContextImpl，一般来说，第二种的使用场景会更广泛。</p> <pre> <code class="language-java">//直接通过class Class<?> studentClassByClass = Student.class; //通过class的字符串全类名 Class<?> studentClassByName = Class.forName("cn.edu.zafu.Student");</code></pre> <p>然后你可以直接调用class的newInstance方法新建一个对象，但是对于没有默认构造函数的类来说，你需要拿到构造函数再去新建对象。这里需要注意的是你不知道修饰符是private，protected，public中的哪一个时，建议调用setAccessible设置成true，不然极有可能会发生异常。</p> <pre> <code class="language-java">//构造函数new对象 Constructor<?> constructor = studentClassByName.getConstructor(String.class, int.class); constructor.setAccessible(true); Student student = (Student) constructor.newInstance("区长", 22); System.out.println(student);</code></pre> <p>对于静态变量的访问，可以拿到Field，直接设置即可。set方法的第一个参数传null就是了。</p> <pre> <code class="language-java">//静态变量 Field schoolField = studentClassByName.getDeclaredField("school"); schoolField.setAccessible(true); schoolField.set(null, "浙江大学"); System.out.println(Student.getSchool());</code></pre> <p>但是对于非静态变量，set方法的第一个参数你就需要传递一个实例进去，比如上面通过构造函数new出来的对象。</p> <pre> <code class="language-java">//非静态变量 Field ageFiled = studentClassByName.getDeclaredField("age"); ageFiled.setAccessible(true); Integer age = (Integer) ageFiled.get(student); System.out.println(age); ageFiled.set(student, 100); System.out.println(student.getAge());</code></pre> <p>同理静态方法同静态变量</p> <pre> <code class="language-java">//静态方法调用 Method setSchoolMethod = studentClassByName.getDeclaredMethod("setSchool", String.class); setSchoolMethod.invoke(null, "清华大学"); System.out.println(Student.getSchool());</code></pre> <p>非静态方法同非静态变量</p> <pre> <code class="language-java">//非静态方法调用 Method sayMethod = studentClassByName.getDeclaredMethod("say", String.class); sayMethod.setAccessible(true); sayMethod.invoke(student, "hello world");</code></pre> <p>等等，还有一个泛型变量呢</p> <pre> <code class="language-java"> private HashMap<String, Integer> scores = new HashMap<String, Integer>();</code></pre> <p>懵逼了，不会写。。。。。其实还是一样，强转就ok了，只不过强转前需要对类型进行校验。</p> <p>然后就是异常处理，反射大多数人都是通过try catch直接捕捉异常。。。这也没什么好说的。</p> <p>劫持，什么叫劫持呢，其实就是替换字段，有人说你直接反射拿到变量，直接反射替换就是了，这是一种方式，但是这并不是我想要的，比如我要劫持Student里的behavior变量，但是我不想修改它，我想保留它原来的逻辑，但是我又想加入新的东西，这个就有点类型面向切面编程了，比如日志的注入。怎么做？懵逼了。。。。显然是动态代理啊。使用代理类去完成劫持操作，既可以保留原有操作，又可以增加新的逻辑。</p> <p>那么Hack类是怎么做的呢。首先生成HackedClass类，怎么生成的呢,调用Hack的into方法，入参有两种形式，一种就是直接传Class，另一种就是传递Class的全类名字符串。</p> <pre> <code class="language-java">public static <T> HackedClass<T> into(final Class<T> clazz) { return new HackedClass<T>(clazz); } @SuppressWarnings({"rawtypes", "unchecked"}) public static <T> HackedClass<T> into(final String class_name) throws HackDeclaration.HackAssertionException { try { return new HackedClass(Class.forName(class_name)); } catch (final ClassNotFoundException e) { fail(new HackDeclaration.HackAssertionException(e)); return new HackedClass(null); // TODO: Better solution to avoid null? } }</code></pre> <p>into方法返回的是HackedClass对象，这个对象是对反射的包装类之一。其源码如下</p> <pre> <code class="language-java">public static class HackedClass<C> { protected Class<C> mClass; public HackedClass(final Class<C> clazz) { mClass = clazz; } public HackedField<C, Object> staticField(final String name) throws HackDeclaration.HackAssertionException { return new HackedField<C, Object>(mClass, name, Modifier.STATIC); } public HackedField<C, Object> field(final String name) throws HackDeclaration.HackAssertionException { return new HackedField<C, Object>(mClass, name, 0); } public HackedMethod staticMethod(final String name, final Class<?>... arg_types) throws HackDeclaration.HackAssertionException { return new HackedMethod(mClass, name, arg_types, Modifier.STATIC); } public HackedMethod method(final String name, final Class<?>... arg_types) throws HackDeclaration.HackAssertionException { return new HackedMethod(mClass, name, arg_types, 0); } public HackedConstructor constructor(final Class<?>... arg_types) throws HackDeclaration.HackAssertionException { return new HackedConstructor(mClass, arg_types); } public Class<C> getmClass() { return mClass; } } </code></pre> <p>如果你要获得静态方法，就调用staticMethod，静态变量就调用staticField，非静态方法就调用method，非静态变量就调用field，要获得构造函数就调用constructor，当然你也可以调用getmClass获得Class。最终如果你调用方法相关的函数会得到HackedMethod，调用变量相关的会得到HackedField，调用构造函数相关的会获得HackedConstructor，这三个类的源码如下。自己看源码。。。。不解释了。。。太长了，主要是对反射的封装，并达到面向对象的效果。</p> <pre> <code class="language-java">public static class HackedField<C, T> { private final Field mField; HackedField(final Class<C> clazz, final String name, int modifiers) throws HackDeclaration.HackAssertionException { Field field = null; try { if (clazz == null) { return; } field = clazz.getDeclaredField(name); if (modifiers > 0 && (field.getModifiers() & modifiers) != modifiers) { fail(new HackDeclaration.HackAssertionException(field + " does not match modifiers: " + modifiers)); } field.setAccessible(true); } catch (final NoSuchFieldException e) { HackDeclaration.HackAssertionException hae = new HackDeclaration.HackAssertionException(e); hae.setHackedClass(clazz); hae.setHackedFieldName(name); fail(hae); } finally { mField = field; } } @SuppressWarnings("unchecked") public <T2> HackedField<C, T2> ofGenericType(final Class<?> type) throws HackDeclaration.HackAssertionException { if (mField != null && !type.isAssignableFrom(mField.getType())) { fail(new HackDeclaration.HackAssertionException(new ClassCastException(mField + " is not of type " + type))); } return (HackedField<C, T2>) this; } @SuppressWarnings("unchecked") public HackedField<C, T> ofType(final String type_name) throws HackDeclaration.HackAssertionException { try { return (HackedField<C, T>) ofType(Class.forName(type_name)); } catch (final ClassNotFoundException e) { fail(new HackDeclaration.HackAssertionException(e)); return this; } } @SuppressWarnings("unchecked") public <T2> HackedField<C, T2> ofType(final Class<T2> type) throws HackDeclaration.HackAssertionException { if (mField != null && !type.isAssignableFrom(mField.getType())) { fail(new HackDeclaration.HackAssertionException(new ClassCastException(mField + " is not of type " + type))); } return (HackedField<C, T2>) this; } public void hijack(final C instance, final Interception.InterceptionHandler<?> handler) { final T delegatee = get(instance); if (delegatee == null) { throw new IllegalStateException("Cannot hijack null"); } final Class<?>[] interfaces = delegatee.getClass().getInterfaces(); set(instance, Interception.proxy(delegatee, handler, interfaces)); } public T get(final C instance) { try { @SuppressWarnings("unchecked") final T value = (T) mField.get(instance); return value; } catch (final IllegalAccessException e) { e.printStackTrace(); return null; } } public void set(final C instance, final Object value) { try { mField.set(instance, value); } catch (final IllegalAccessException e) { e.printStackTrace(); } } public Field getField() { return mField; } } public static class HackedMethod { protected final Method mMethod; HackedMethod(final Class<?> clazz, final String name, final Class<?>[] arg_types, int modifiers) throws HackDeclaration.HackAssertionException { Method method = null; try { if (clazz == null) { return; } method = clazz.getDeclaredMethod(name, arg_types); if (modifiers > 0 && (method.getModifiers() & modifiers) != modifiers) { fail(new HackDeclaration.HackAssertionException(method + " does not match modifiers: " + modifiers)); } method.setAccessible(true); } catch (final NoSuchMethodException e) { HackDeclaration.HackAssertionException hae = new HackDeclaration.HackAssertionException(e); hae.setHackedClass(clazz); hae.setHackedMethodName(name); fail(hae); } finally { mMethod = method; } } public Object invoke(final Object receiver, final Object... args) throws IllegalArgumentException, InvocationTargetException { Object obj = null; try { obj = mMethod.invoke(receiver, args); return obj; } catch (final IllegalAccessException e) { /* Should never happen */ e.printStackTrace(); } return obj; } public Method getMethod() { return mMethod; } } public static class HackedConstructor { protected Constructor<?> mConstructor; HackedConstructor(final Class<?> clazz, final Class<?>[] arg_types) throws HackDeclaration.HackAssertionException { try { if (clazz == null) { return; } mConstructor = clazz.getDeclaredConstructor(arg_types); } catch (NoSuchMethodException e) { HackDeclaration.HackAssertionException hae = new HackDeclaration.HackAssertionException(e); hae.setHackedClass(clazz); fail(hae); } } public Object getInstance(final Object... arg_types) throws IllegalArgumentException { Object obj = null; mConstructor.setAccessible(true); try { obj = mConstructor.newInstance(arg_types); } catch (Exception e) { e.printStackTrace(); } return obj; } }</code></pre> <p>注意HackedField中有个方法叫hijack，这个方法的作用就是劫持。</p> <p>最终，如果反射失败的话会进入Hack的fail方法处理，我们可以设置AssertionFailureHandler处理器，返回false会扔出异常，否则不会扔出异常。可以在里面做一些事，比如埋点。这样就可以集中到一个地方处理了。</p> <pre> <code class="language-java">private static void fail(HackDeclaration.HackAssertionException e) throws HackDeclaration.HackAssertionException { if (sFailureHandler == null || !sFailureHandler.onAssertionFailure(e)) { throw e; } } public static void setAssertionFailureHandler(AssertionFailureHandler handler) { sFailureHandler = handler; } public interface AssertionFailureHandler { boolean onAssertionFailure(HackDeclaration.HackAssertionException failure); }</code></pre> <p>瞎说了这么多，来实践一把。</p> <p>获得HackedClass对象，调用Hack.into方法</p> <pre> <code class="language-java">//通过Class来获得一个HackedClass Hack.HackedClass<Student> hackPersonByClass = Hack.into(Student.class); //输出class测试 System.out.println(hackPersonByClass.getmClass()); //通过Class的全类名获得一个HackedClass Hack.HackedClass<Student> hackPersonByName = Hack.into("cn.edu.zafu.Student"); //输出class测试 System.out.println(hackPersonByName.getmClass());</code></pre> <p>创建实例，调用HackedClass的constructor方法，将参数的类型传入，然后调用getInstance方法就可以获得一个实例了。</p> <pre> <code class="language-java">//获得构造函数 Hack.HackedConstructor personConstructor = hackPersonByName.constructor(String.class, int.class); //创建并获得实例 Student student = (Student) personConstructor.getInstance("区长", 121); //输出结果测试 System.out.println(student); //直接调用反射获得的对象的方法 student.say("世界你好,世界再见");</code></pre> <p>非静态变量需要通过field方法获得一个HackedField对象，这时候你获得的field是一个Object类型的，因此如果你想要类型安全，需要调用一下ofType方法，将类型传入。</p> <pre> <code class="language-java">//通过field -> ofType 获得一个HackedField,非静态 Hack.HackedField<Student, String> hackName = hackPersonByName.field("name").ofType(String.class); //反射调用 String name = hackName.get(student); //输出结果测试 System.out.println(name); //通过field -> ofType 获得一个HackedField,非静态 Hack.HackedField<Student, Integer> hackAge = hackPersonByName.field("age").ofType(int.class); //反射设置属性 hackAge.set(student, 16); //反射获得age,进行验证 Integer age = hackPersonByName.field("age").ofType(int.class).get(student); //输出结果测试 System.out.println(age);</code></pre> <p>静态变量直接调用staticField方法即可，除了上面的ofType将类型传入，这个类型可以是Class对象，也可以是Class字符串的全类名。</p> <pre> <code class="language-java">//反射获得静态变量 Hack.HackedField<Student, Object> hackSchool = hackPersonByName.staticField("school").ofType("java.lang.String"); //获得静态变量值 String sSchool = (String) hackSchool.get(null); //输出结果测试 System.out.println(sSchool); //设置值 hackSchool.getField().set(null, "北京大学"); //获得值验证是否设置成功,通过getField()方式 sSchool = (String) hackSchool.getField().get(null); //输出结果测试 System.out.println(sSchool);</code></pre> <p>泛型参数可以调用ofGenericType方法转为泛型，比如下面的Map</p> <pre> <code class="language-java">//泛型参数 Hack.HackedField<Student, Map<String, Integer>> hackScores = hackPersonByName.field("scores").ofGenericType(Map.class); Map<String, Integer> stringIntegerMap = hackScores.get(student); stringIntegerMap.put("语文", 80); stringIntegerMap.put("数学", 90); //泛型参数设置 hackScores.set(student, stringIntegerMap); //输出结果测试 System.out.println(student.getScores());</code></pre> <p>方法的调用需要调用method相关的方法，非静态方法直接调用method方法获得HackedMethod对象，需要将方法的参数类型传入。之后直接invoke调用的时候将调用对象实例和参数传入。</p> <pre> <code class="language-java">//反射非静态方法调用 hackPersonByName.method("say", String.class).invoke(student, "fuck the source code");</code></pre> <p>静态方法和非静态方法相比就是invoke的时候调用对象可以直接传null。</p> <pre> <code class="language-java">//反射静态方法调用 hackPersonByName.staticMethod("setSchool", String.class).invoke(null, "南京大学"); //输出结果测试 System.out.println(Student.getSchool()); //反射静态方法调用 String school = (String) hackPersonByName.staticMethod("getSchool").getMethod().invoke(null); System.out.println(school);</code></pre> <p>以上调用的最终输出如下图所示,代码可能跟图有出入（貌似修改过前后顺序）</p> <p><img alt="这里写图片描述" src="https://simg.open-open.com/show/06bbe823b11c41faf81add8cc07cb365.png"></p> <p>Hack类里的异常处理最终都会走到fail方法中，fail方法中会判断AssertionFailureHandler是否为空，空的情况下会直接扔出异常，否则会看AssertionFailureHandler的处理结果，如果结果返回true，则不扔出异常，返回fasle的情况下也会扔出异常，我们通过setAssertionFailureHandler方法设置一个异常处理器就可以了。下面调用两个不存在的字段，让它进入到这个处理器中，如果字段名是notHandler则扔出异常，否则输出信息并返回true。</p> <pre> <code class="language-java">//异常处理 Hack.setAssertionFailureHandler(new Hack.AssertionFailureHandler() { @Override public boolean onAssertionFailure(Hack.HackDeclaration.HackAssertionException failure) { //如果是notHandler字段,则不处理,即扔出异常,否则打印输出,不扔出异常 if ("notHandler".equals(failure.getHackedFieldName())) { return false; } Class<?> hackedClass = failure.getHackedClass(); String hackedFieldName = failure.getHackedFieldName(); String hackedMethodName = failure.getHackedMethodName(); System.out.println("=====onAssertionFailure start====="); System.out.println("hackedClass:" + hackedClass); System.out.println("hackedFieldName:" + hackedFieldName); System.out.println("hackedMethodName:" + hackedMethodName); System.out.println("=====onAssertionFailure end====="); //返回true不会抛出异常,否则抛出异常 return true; } }); //获得一个不存在的对象,验证onAssertionFailure回调 Hack.HackedField<Student, String> unknownField = hackPersonByName.field("unknownField").ofType(String.class); Hack.HackedField<Student, String> notHandler = hackPersonByName.field("notHandler").ofType(String.class); </code></pre> <p>最终的效果如下</p> <p><img alt="这里写图片描述" src="https://simg.open-open.com/show/d81bd5dda0695d110d30ddae744d17e6.png"></p> <p>劫持字段，字段的劫持就是通过动态代理来实现的，内部的逻辑如下</p> <pre> <code class="language-java">public static abstract class InterceptionHandler<T> implements InvocationHandler { private T mDelegatee; @Override public Object invoke(Object obj, Method method, Object[] args) throws Throwable { Object obj2 = null; try { obj2 = method.invoke(delegatee(), args); } catch (IllegalArgumentException e) { e.printStackTrace(); } catch (IllegalAccessException e2) { e2.printStackTrace(); } catch (InvocationTargetException e3) { throw e3.getTargetException(); } return obj2; } protected T delegatee() { return this.mDelegatee; } void setDelegatee(T t) { this.mDelegatee = t; } }</code></pre> <p>这个字段的方法调用都是调用委托对象，这个对象就是我们原始的字段，这个delegatee方法我们可以重写返回新对象，这样就和反射直接替换没有什么差别了。当然也可以重写invoke方法，在调用前和调用后做一些处理。比如我做一些日志输出。如下代码</p> <pre> <code class="language-java">//字段劫持处理 Interception.InterceptionHandler handler = new Interception.InterceptionHandler<IBehavior>() { @Override public Object invoke(Object obj, Method method, Object[] args) throws Throwable { System.out.println("hijack:[invoke start]"); Object o = super.invoke(obj, method, args); System.out.println("hijack:[invoke end]"); return o; } }; //劫持behavior字段 hackPersonByName.field("behavior").hijack(student, handler); //测试劫持效果 student.getBehavior().perform("sleep", "sleep 10h");</code></pre> <p>最终效果如下<br> <img alt="这里写图片描述" src="https://simg.open-open.com/show/7e299e5db7a72183594385398d1043a3.png"></p> <p>好了说了这么多，还是不明白这个东西有什么软用呢？Android动态加载资源的时候我们需要用到的ContextImpl对象，需要对AssetManager和Resources需要做一些操作，就可以这么来了。</p> <pre> <code class="language-java">import java.lang.reflect.InvocationTargetException; public class Hacks extends HackDeclaration implements AssertionFailureHandler { public static HackedClass<AssetManager> AssetManager; public static HackedMethod AssetManager_addAssetPath; public static HackedClass<Object> ContextImpl; public static HackedField<Object, Resources> ContextImpl_mResources; public static boolean sIsReflectChecked; public static boolean sIsReflectAvailable; public static boolean defineAndVerify() { if (sIsReflectChecked) { return sIsReflectAvailable; } Hacks hacks = new Hacks(); try { Hack.setAssertionFailureHandler(hacks); allClasses(); allConstructors(); allFields(); allMethods(); sIsReflectAvailable = true; return sIsReflectAvailable; } catch (Throwable e) { e.printStackTrace(); } finally { Hack.setAssertionFailureHandler(null); sIsReflectChecked = true; } return false; } private static void allClasses() throws HackAssertionException { AssetManager = Hack.into(AssetManager.class); ContextImpl = Hack.into("android.app.ContextImpl"); } private static void allConstructors() throws HackAssertionException { } private static void allFields() throws HackAssertionException { ContextImpl_mResources = ContextImpl.field("mResources").ofType(Resources.class); } private static void allMethods() throws HackAssertionException { AssetManager_addAssetPath = AssetManager.method("addAssetPath", String.class); } @Override public boolean onAssertionFailure(HackAssertionException hackAssertionException) { // throw it return false; } } </code></pre> <p>调用 Hacks.defineAndVerify反复进行定义和验证，成功后就可以直接调用了。</p> <pre> <code class="language-java">boolean flag = Hacks.defineAndVerify(); if (flag) { AssetManager assetManager = AssetManager.class.newInstance(); Hacks.AssetManager_addAssetPath.invoke(assetManager,"assetPath"); Hacks.ContextImpl_mResources.set(context, resources); }</code></pre> <p>上面的这个例子是伪代码，实际情况自己去把握~~~~写的比较随意，看看就好了~</p> <p>来自： http://blog.csdn.net/sbsujjbcy/article/details/51280274</p>