热修复原理分析
什么是热修复
热修复是指在用户无感知情况下对已安装的app使用补丁进行紧急修复。优点，效率高，用户体验好。
热修复框架
android热修复技术主要可以分为两类：
这两类都有着自己的优缺点，事实上从来都没有最好的方案，只有最适合自己的。
热修复原理
热修复实现的利用了Java的类加载机制，关键点是dexElments,代码如下：
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/**
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* Finds the named class in one of the dex files pointed at by
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* this instance. This will find the one in the earliest listed
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* path element. If the class is found but has not yet been
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* defined, then this method will define it in the defining
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* context that this instance was constructed with.
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*
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* @param name of class to find
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* @param suppressed exceptions encountered whilst finding the class
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* @return the named class or {@code null} if the class is not
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* found in any of the dex files
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*/
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public Class findClass(String name, List<Throwable> suppressed) {
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for (Element element : dexElements) {
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DexFile dex = element.dexFile;
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if (dex != null) {
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Class clazz = dex.loadClassBinaryName(name, definingContext, suppressed);
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if (clazz != null) {
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return clazz;
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}
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}
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}
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if (dexElementsSuppressedExceptions != null) {
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suppressed.addAll(Arrays.asList(dexElementsSuppressedExceptions));
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}
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return null;
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}
寻找大致过程PathClassLoader ->BaseDexClassLoader.findClass(String name)->PathList.findClass(name, suppressedExceptions).
大致相关的5个类：
PathClassLoader 用来加载应用程序的dex
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/**
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* Provides a simple {@link ClassLoader} implementation that operates on a list
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* of files and directories in the local file system, but does not attempt to
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* load classes from the network. Android uses this class for its system class
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* loader and for its application class loader(s).
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*/
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public class PathClassLoader extends BaseDexClassLoader {
DexClassLoader 这个类是可以用来从.jar文件和.apk文件中加载classed.dex。（必须要在应用程序的目录下面。最终是加载jar、apk文件里的dex文件）
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/**
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* A class loader that loads classes from {@code .jar} and {@code .apk} files
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* containing a {@code classes.dex} entry. This can be used to execute code not
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* installed as part of an application.
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*
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* <p>This class loader requires an application-private, writable directory to
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* cache optimized classes. Use {@code Context.getCodeCacheDir()} to create
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* such a directory: <pre> {@code
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* File dexOutputDir = context.getCodeCacheDir();
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* }</pre>
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*
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* <p><strong>Do not cache optimized classes on external storage.</strong>
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* External storage does not provide access controls necessary to protect your
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* application from code injection attacks.
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*/
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public class DexClassLoader extends BaseDexClassLoader {
BaseDexClassLoader PathClassLoader和DexClassLoader继承这个类。BaseDexClassLoader查找类的方法：
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/**
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* Base class for common functionality between various dex-based
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* {@link ClassLoader} implementations.
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*/
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public class BaseDexClassLoader extends ClassLoader {
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private final DexPathList pathList;
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//.... code
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@Override
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protected Class<?> findClass(String name) throws ClassNotFoundException {
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List<Throwable> suppressedExceptions = new ArrayList<Throwable>();
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Class c = pathList.findClass(name, suppressedExceptions);
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if (c == null) {
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ClassNotFoundException cnfe = new ClassNotFoundException("Didn't find class \"" + name + "\" on path: " + pathList);
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for (Throwable t : suppressedExceptions) {
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cnfe.addSuppressed(t);
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}
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throw cnfe;
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}
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return c;
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}
pathList.findClass(name),附DexPathList类代码
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final class DexPathList {
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/**
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* List of dex/resource (class path) elements.
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* Should be called pathElements, but the Facebook app uses reflection
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* to modify 'dexElements' (http://b/7726934).
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*/
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private Element[] dexElements;
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/**
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* Finds the named class in one of the dex files pointed at by
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* this instance. This will find the one in the earliest listed
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* path element. If the class is found but has not yet been
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* defined, then this method will define it in the defining
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* context that this instance was constructed with.
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*
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* @param name of class to find
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* @param suppressed exceptions encountered whilst finding the class
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* @return the named class or {@code null} if the class is not
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* found in any of the dex files
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*/
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public Class findClass(String name, List<Throwable> suppressed) {
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for (Element element : dexElements) {
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DexFile dex = element.dexFile;
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if (dex != null) {
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Class clazz = dex.loadClassBinaryName(name, definingContext, suppressed);
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if (clazz != null) {
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return clazz;
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}
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}
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}
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if (dexElementsSuppressedExceptions != null) {
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suppressed.addAll(Arrays.asList(dexElementsSuppressedExceptions));
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}
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return null;
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}
DexFile.loadClassBinaryName方法：
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public final class DexFile {
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/**
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* See {@link #loadClass(String, ClassLoader)}.
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*
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* This takes a "binary" class name to better match ClassLoader semantics.
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*
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* @hide
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*/
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public Class loadClassBinaryName(String name, ClassLoader loader, List<Throwable> suppressed) {
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return defineClass(name, loader, mCookie, this, suppressed);
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}
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private static Class defineClass(String name, ClassLoader loader, Object cookie,
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DexFile dexFile, List<Throwable> suppressed) {
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Class result = null;
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try {
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result = defineClassNative(name, loader, cookie, dexFile);
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} catch (NoClassDefFoundError e) {
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if (suppressed != null) {
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suppressed.add(e);
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}
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} catch (ClassNotFoundException e) {
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if (suppressed != null) {
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suppressed.add(e);
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}
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}
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return result;
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}
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private static native Class defineClassNative(String name, ClassLoader loader, Object cookie,
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DexFile dexFile)
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throws ClassNotFoundException, NoClassDefFoundError;
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}
类所在路径:
总结：一个ClassLoader可以包含多个dex文件，每个dex文件是一个Element，多个dex文件排列成一个有序的数组dexElements，当找类的时候，会按顺序遍历dex文件，然后从当前遍历的dex文件中找类，如果找类则返回，如果找不到从下一个dex文件继续查找。(出自安卓App热补丁动态修复技术介绍)
热修复原理：
ClassLoader加载类会遍历dexElments数组，从dex文件中不断寻找你要的class，找到后立即返回class，后面的dex文件不再读取。热修复就是把有问题class打包成fixDex文件，插入dexElements靠前的位置，让修复的class优先被找到。
手动实现热修复
1步：通过使用dx.bat将修复的class打包成fix.dex。
命令 : dx --dex --output=E:\patch\dex\fix.dex E:\patch\cls
--output=E:\patch\dex\fix.dex dex输出路径
E:\patch\cls 打包指定目录下面的class字节文件(注意要包括全路径的文件夹，也可以有多个class)
3歩：将fix.dex文件搬到应用目录下，如/data/data/pkgName/odex/
4歩：通过反射将fix.dex添加到dexElements比较靠前的位置（要在bug class所在的dex前面）。
这里借助工具类FixDexUtils.
5歩：自定义MyApplication，onCreate方法中调用FixDexUtils.loadFixedDex(this);
FixDexUtils代码：
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public class FixDexUtils {
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private static final String DEX_DIR = "odex";
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private static HashSet<File> loadedDex = new HashSet<File>();
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static {
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loadedDex.clear();
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}
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public static void loadFixedDex(Context context) {
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if (context == null) {
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return;
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}
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//遍历所有的修复的dex
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File fileDir = context.getDir(DEX_DIR, Context.MODE_PRIVATE);
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File[] listFiles = fileDir.listFiles();
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for (File file : listFiles) {
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if (file.getName().startsWith("classes") && file.getName().endsWith(".dex")) {
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loadedDex.add(file);
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}
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}
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//合并之前的dex
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doDexInject(context, fileDir, loadedDex);
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}
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private static void doDexInject(final Context appContext, File filesDir, HashSet<File> loadedDex) {
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String optimizeDir = filesDir.getAbsolutePath() + File.separator + "opt_dex";
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File fopt = new File(optimizeDir);
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if (!fopt.exists()) {
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fopt.mkdirs();
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}
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//1.加载应用程序的dex
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//2.加载指定的修复的dex文件。
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//3.合并
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try {
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PathClassLoader pathLoader = (PathClassLoader) appContext.getClassLoader();
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for (File dex : loadedDex) {
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//2.加载指定的修复的dex文件。
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DexClassLoader classLoader = new DexClassLoader(
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dex.getAbsolutePath(),
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fopt.getAbsolutePath(),
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null,
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pathLoader
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);
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///通过反射获得classLoader、pathLoader的pathList
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Object dexObj = getPathList(classLoader);
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Object pathObj = getPathList(pathLoader);
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//通过反射获得classLoader、pathLoader的DexElements
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Object mDexElementsList = getDexElements(dexObj);
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Object pathDexElementsList = getDexElements(pathObj);
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//将classLoader、pathLoader的DexElements合并
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Object dexElements = combineArray(mDexElementsList, pathDexElementsList);
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//重新给pathLoader的PathList里面的dexElements赋值
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setField(pathObj, pathObj.getClass(), "dexElements", dexElements);
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}
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} catch (Exception e) {
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e.printStackTrace();
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}
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}
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private static void setField(Object obj, Class<?> cl, String field, Object value) throws Exception {
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Field localField = cl.getDeclaredField(field);
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localField.setAccessible(true);
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localField.set(obj, value);
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}
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private static Object getField(Object obj, Class<?> cl, String field)
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throws Exception {
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Field localField = cl.getDeclaredField(field);
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localField.setAccessible(true);
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return localField.get(obj);
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}
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//反射获得BaseDexClassLoader的pathList
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private static Object getPathList(Object baseDexClassLoader) throws Exception {
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return getField(baseDexClassLoader, Class.forName("dalvik.system.BaseDexClassLoader"), "pathList");
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}
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//反射DexPathList的dexElements
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private static Object getDexElements(Object obj) throws Exception {
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return getField(obj, obj.getClass(), "dexElements");
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}
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/**
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* 两个数组合并
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*
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* @param arrayLhs
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* @param arrayRhs
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* @return
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*/
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private static Object combineArray(Object arrayLhs, Object arrayRhs) {
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Class<?> localClass = arrayLhs.getClass().getComponentType();
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int i = Array.getLength(arrayLhs);
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int j = i + Array.getLength(arrayRhs);
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Object result = Array.newInstance(localClass, j);
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for (int k = 0; k < j; ++k) {
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if (k < i) {
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Array.set(result, k, Array.get(arrayLhs, k));
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} else {
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Array.set(result, k, Array.get(arrayRhs, k - i));
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}
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}
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return result;
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}
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}
代码：热修复简单实例
参考
Android dex分包方案
安卓App热补丁动态修复技术介绍
Alibaba-AndFix
Bug热修复框架原理及源码解析
Android 热修复Nuwa的原理及Gradle插件源码解析
Android中热修复框架Robust原理解析+并将框架代码从"闭源"变成"开源"(下篇)