热修复这项技术，基本上已经成为Android项目比较重要的模块了。主要因为项目在上线之后，都难免会有各种问题，而依靠发版去修复问题，成本太高了。
现在热修复的技术基本上有阿里的AndFix、QZone的方案、美团提出的思想方案以及腾讯的Tinker等。
其中AndFix可能接入是最简单的一个（和Tinker命令行接入方式差不多），不过AndFix兼容性有一定的问题，QZone方案对性能会有一定的影响，且在Art模式下出现内存错乱的问题，美团提出的思想方案主要是基于Instant Run的原理，目前尚未开源，兼容性较好。
这么看来，如果选择开源方案，Tinker目前是最佳的选择，下面来看看Tinker的大致的原理分析。
1. 原理概述
Tinker将old.apk和new.apk做了diff，拿到patch.dex后将其与本机中apk的classes.dex做了合并，生成新的classes.dex，运行时通过反射将合并后的dex文件放置在加载的dexElements数组的前面。
运行时替代的原理，其实和Qzone的方案差不多，都是去反射修改dexElements。两者的差异是：Qzone是直接将patch.dex插到数组的前面；而Tinker是合并后的全量dex插在数组的前面。因为Qzone方案中提到的CLASS_ISPREVERIFIED的解决方案存在问题。
Android的ClassLoader体系中加载类一般使用的是PathClassLoader和DexClassLoader，大家只需要明白，Android使用PathClassLoader作为其类加载器，DexClassLoader可以从.jar和.apk类型的文件内部加载classes.dex文件就好了。对于加载类，无非是给个classname，然后去findClass，PathClassLoader和DexClassLoader都继承自BaseDexClassLoader。在BaseDexClassLoader中有如下源码：
#BaseDexClassLoader
@Override
protected Class<?> findClass(String name) throws ClassNotFoundException {
Class clazz = pathList.findClass(name);
if (clazz == null) {
throw new ClassNotFoundException(name);
}
return clazz;
}
#DexPathList
public Class findClass(String name) {
for (Element element : dexElements) {
DexFile dex = element.dexFile;
if (dex != null) {
Class clazz = dex.loadClassBinaryName(name, definingContext);
if (clazz != null) {
return clazz;
}
}
}
return null;
}
#DexFile
public Class loadClassBinaryName(String name, ClassLoader loader) {
return defineClass(name, loader, mCookie);
}
private native static Class defineClass(String name, ClassLoader loader, int cookie);可以看出BaseDexClassLoader中有个pathList对象，pathList中包含一个DexFile的集合dexElements，而对于类加载就是遍历这个集合，通过DexFile去寻找。
通俗点说，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。那么这样的话，我们可以在这个数组的第一个元素放置我们的patch.jar，里面包含修复过的类，这样的话，当遍历findClass的时候，我们修复的类就会被查找到，从而替代有bug的类。
本片文章Tinker源码分析的两条线：
（1）应用启动时，从默认目录加载合并后的classes.dex
（2）patch下发后，合成classes.dex至目标目录
2．源码浅析
2.1 加载patch
加载的代码实际上在生成的Application中调用的，其父类为TinkerApplication，在其attachBaseContext中辗转会调用到loadTinker()方法，在该方法内部，反射调用了TinkerLoader的tryLoad方法。继而调用了tryLoadPatchFilesInternal方法。
@Override
public Intent tryLoad(TinkerApplication app, int tinkerFlag, boolean tinkerLoadVerifyFlag) {
Intent resultIntent = new Intent();
long begin = SystemClock.elapsedRealtime();
tryLoadPatchFilesInternal(app, tinkerFlag, tinkerLoadVerifyFlag, resultIntent);
long cost = SystemClock.elapsedRealtime() - begin;
ShareIntentUtil.setIntentPatchCostTime(resultIntent, cost);
return resultIntent;
}private void tryLoadPatchFilesInternal(TinkerApplication app, int tinkerFlag, boolean tinkerLoadVerifyFlag, Intent resultIntent) {
// 省略大量安全性校验代码
if (isEnabledForDex) {
//tinker/patch.info/patch-641e634c/dex
boolean dexCheck = TinkerDexLoader.checkComplete(patchVersionDirectory, securityCheck, resultIntent);
if (!dexCheck) {
//file not found, do not load patch
Log.w(TAG, "tryLoadPatchFiles:dex check fail");
return;
}
}
//now we can load patch jar
if (isEnabledForDex) {
boolean loadTinkerJars = TinkerDexLoader.loadTinkerJars(app, tinkerLoadVerifyFlag, patchVersionDirectory, resultIntent, isSystemOTA);
if (!loadTinkerJars) {
Log.w(TAG, "tryLoadPatchFiles:onPatchLoadDexesFail");
return;
}
}
}其中TinkerDexLoader.checkComplete主要是用于检查下发的meta文件中记录的dex信息（meta文件，可以查看生成patch的产物，在assets/dex-meta.txt），检查meta文件中记录的dex文件信息对应的dex文件是否存在，并把值存在TinkerDexLoader的静态变量dexList中。
TinkerDexLoader.loadTinkerJars传入四个参数，分别为application，tinkerLoadVerifyFlag（注解上声明的值，传入为false），patchVersionDirectory当前version的patch文件夹，intent，当前patch是否仅适用于art。
@TargetApi(Build.VERSION_CODES.ICE_CREAM_SANDWICH)
public static boolean loadTinkerJars(Application application, boolean tinkerLoadVerifyFlag,
String directory, Intent intentResult, boolean isSystemOTA) {
PathClassLoader classLoader = (PathClassLoader) TinkerDexLoader.class.getClassLoader();
String dexPath = directory + "/" + DEX_PATH + "/";
File optimizeDir = new File(directory + "/" + DEX_OPTIMIZE_PATH);
ArrayList<File> legalFiles = new ArrayList<>();
final boolean isArtPlatForm = ShareTinkerInternals.isVmArt();
for (ShareDexDiffPatchInfo info : dexList) {
//for dalvik, ignore art support dex
if (isJustArtSupportDex(info)) {
continue;
}
String path = dexPath + info.realName;
File file = new File(path);
legalFiles.add(file);
}
// just for art
if (isSystemOTA) {
parallelOTAResult = true;
parallelOTAThrowable = null;
Log.w(TAG, "systemOTA, try parallel oat dexes!!!!!");
TinkerParallelDexOptimizer.optimizeAll(
legalFiles, optimizeDir,
new TinkerParallelDexOptimizer.ResultCallback() {
}
);
SystemClassLoaderAdder.installDexes(application, classLoader, optimizeDir, legalFiles);
return true;找出仅支持art的dex，且当前patch是否仅适用于art时，并行去loadDex。关键是最后的installDexes：
@SuppressLint("NewApi")
public static void installDexes(Application application, PathClassLoader loader, File dexOptDir, List<File> files)
throws Throwable {
if (!files.isEmpty()) {
ClassLoader classLoader = loader;
if (Build.VERSION.SDK_INT >= 24) {
classLoader = AndroidNClassLoader.inject(loader, application);
}
//because in dalvik, if inner class is not the same classloader with it wrapper class.
//it won't fail at dex2opt
if (Build.VERSION.SDK_INT >= 23) {
V23.install(classLoader, files, dexOptDir);
} else if (Build.VERSION.SDK_INT >= 19) {
V19.install(classLoader, files, dexOptDir);
} else if (Build.VERSION.SDK_INT >= 14) {
V14.install(classLoader, files, dexOptDir);
} else {
V4.install(classLoader, files, dexOptDir);
}
//install done
sPatchDexCount = files.size();
Log.i(TAG, "after loaded classloader: " + classLoader + ", dex size:" + sPatchDexCount);
if (!checkDexInstall(classLoader)) {
//reset patch dex
SystemClassLoaderAdder.uninstallPatchDex(classLoader);
throw new TinkerRuntimeException(ShareConstants.CHECK_DEX_INSTALL_FAIL);
}
}
}这里实际上就是根据不同的系统版本，去反射处理dexElements。我们看一下V19的实现：
private static final class V19 {
private static void install(ClassLoader loader, List<File> additionalClassPathEntries,
File optimizedDirectory)
throws IllegalArgumentException, IllegalAccessException,
NoSuchFieldException, InvocationTargetException, NoSuchMethodException, IOException {
Field pathListField = ShareReflectUtil.findField(loader, "pathList");
Object dexPathList = pathListField.get(loader);
ArrayList<IOException> suppressedExceptions = new ArrayList<IOException>();
ShareReflectUtil.expandFieldArray(dexPathList, "dexElements", makeDexElements(dexPathList,
new ArrayList<File>(additionalClassPathEntries), optimizedDirectory,
suppressedExceptions));
if (suppressedExceptions.size() > 0) {
for (IOException e : suppressedExceptions) {
Log.w(TAG, "Exception in makeDexElement", e);
throw e;
}
}
}
}（1）找到PathClassLoader（BaseDexClassLoader）对象中的pathList对象
（2）根据pathList对象找到其中的makeDexElements方法，传入patch相关的对应的实参，返回Element[]对象
（3）拿到pathList对象中原本的dexElements方法
（4）步骤2与步骤3中的Element[]数组进行合并，将patch相关的dex放在数组的前面
（5）最后将合并后的数组，设置给pathList
2.2 合成patch
入口为onReceiveUpgradePatch()方法：
TinkerInstaller.onReceiveUpgradePatch(getApplicationContext(),
Environment.getExternalStorageDirectory().getAbsolutePath() + "/patch_signed.apk");上述代码会调用DefaultPatchListener中的onPatchReceived方法：
# DefaultPatchListener
@Override
public int onPatchReceived(String path) {
int returnCode = patchCheck(path);
if (returnCode == ShareConstants.ERROR_PATCH_OK) {
TinkerPatchService.runPatchService(context, path);
} else {
Tinker.with(context).getLoadReporter().onLoadPatchListenerReceiveFail(new File(path), returnCode);
}
return returnCode;
}首先对Tinker的相关配置（isEnable）以及patch的合法性进行检测，如果合法，则调用TinkerPatchService.runPatchService(context, path)。
public static void runPatchService(Context context, String path) {
try {
Intent intent = new Intent(context, TinkerPatchService.class);
intent.putExtra(PATCH_PATH_EXTRA, path);
intent.putExtra(RESULT_CLASS_EXTRA, resultServiceClass.getName());
context.startService(intent);
} catch (Throwable throwable) {
TinkerLog.e(TAG, "start patch service fail, exception:" + throwable);
}
}TinkerPatchService是IntentService的子类，这里通过intent设置了两个参数，一个是patch的路径，一个是resultServiceClass，该值是调用Tinker.install的时候设置的，默认为DefaultTinkerResultService.class。由于是IntentService，直接看onHandleIntent即可。
@Override
protected void onHandleIntent(Intent intent) {
final Context context = getApplicationContext();
Tinker tinker = Tinker.with(context);
String path = getPatchPathExtra(intent);
File patchFile = new File(path);
boolean result;
increasingPriority();
PatchResult patchResult = new PatchResult();
result = upgradePatchProcessor.tryPatch(context, path, patchResult);
patchResult.isSuccess = result;
patchResult.rawPatchFilePath = path;
patchResult.costTime = cost;
patchResult.e = e;
AbstractResultService.runResultService(context, patchResult, getPatchResultExtra(intent));
}比较清晰，主要关注upgradePatchProcessor.tryPatch方法，调用的是UpgradePatch.tryPatch。这里有个有意思的地方increasingPriority()，其内部实现为：
private void increasingPriority() {
TinkerLog.i(TAG, "try to increase patch process priority");
try {
Notification notification = new Notification();
if (Build.VERSION.SDK_INT < 18) {
startForeground(notificationId, notification);
} else {
startForeground(notificationId, notification);
// start InnerService
startService(new Intent(this, InnerService.class));
}
} catch (Throwable e) {
TinkerLog.i(TAG, "try to increase patch process priority error:" + e);
}
}如果你对“保活”这个话题比较关注，那么对这段代码一定不陌生，主要是利用系统的一个漏洞来启动一个前台Service。下面继续回到tryPatch方法：
# UpgradePatch
@Override
public boolean tryPatch(Context context, String tempPatchPath, PatchResult patchResult) {
Tinker manager = Tinker.with(context);
final File patchFile = new File(tempPatchPath);
//it is a new patch, so we should not find a exist
SharePatchInfo oldInfo = manager.getTinkerLoadResultIfPresent().patchInfo;
String patchMd5 = SharePatchFileUtil.getMD5(patchFile);
//use md5 as version
patchResult.patchVersion = patchMd5;
SharePatchInfo newInfo;
//already have patch
if (oldInfo != null) {
newInfo = new SharePatchInfo(oldInfo.oldVersion, patchMd5, Build.FINGERPRINT);
} else {
newInfo = new SharePatchInfo("", patchMd5, Build.FINGERPRINT);
}
//check ok, we can real recover a new patch
final String patchDirectory = manager.getPatchDirectory().getAbsolutePath();
final String patchName = SharePatchFileUtil.getPatchVersionDirectory(patchMd5);
final String patchVersionDirectory = patchDirectory + "/" + patchName;
//copy file
File destPatchFile = new File(patchVersionDirectory + "/" + SharePatchFileUtil.getPatchVersionFile(patchMd5));
// check md5 first
if (!patchMd5.equals(SharePatchFileUtil.getMD5(destPatchFile))) {
SharePatchFileUtil.copyFileUsingStream(patchFile, destPatchFile);
}
//we use destPatchFile instead of patchFile, because patchFile may be deleted during the patch process
if (!DexDiffPatchInternal.tryRecoverDexFiles(manager, signatureCheck, context, patchVersionDirectory,
destPatchFile)) {
TinkerLog.e(TAG, "UpgradePatch tryPatch:new patch recover, try patch dex failed");
return false;
}
return true;
}拷贝patch文件拷贝至私有目录，然后调用DexDiffPatchInternal.tryRecoverDexFiles：
protected static boolean tryRecoverDexFiles(Tinker manager, ShareSecurityCheck checker, Context context,
String patchVersionDirectory, File patchFile) {
String dexMeta = checker.getMetaContentMap().get(DEX_META_FILE);
boolean result = patchDexExtractViaDexDiff(context, patchVersionDirectory, dexMeta, patchFile);
return result;
}直接看patchDexExtractViaDexDiff：
private static boolean patchDexExtractViaDexDiff(Context context, String patchVersionDirectory, String meta, final File patchFile) {
String dir = patchVersionDirectory + "/" + DEX_PATH + "/";
if (!extractDexDiffInternals(context, dir, meta, patchFile, TYPE_DEX)) {
TinkerLog.w(TAG, "patch recover, extractDiffInternals fail");
return false;
}
final Tinker manager = Tinker.with(context);
File dexFiles = new File(dir);
File[] files = dexFiles.listFiles();
...files遍历执行：DexFile.loadDex
return true;
}核心代码主要在extractDexDiffInternals中：
private static boolean extractDexDiffInternals(Context context, String dir, String meta, File patchFile, int type) {
//parse meta
ArrayList<ShareDexDiffPatchInfo> patchList = new ArrayList<>();
ShareDexDiffPatchInfo.parseDexDiffPatchInfo(meta, patchList);
File directory = new File(dir);
//I think it is better to extract the raw files from apk
Tinker manager = Tinker.with(context);
ZipFile apk = null;
ZipFile patch = null;
ApplicationInfo applicationInfo = context.getApplicationInfo();
String apkPath = applicationInfo.sourceDir; //base.apk
apk = new ZipFile(apkPath);
patch = new ZipFile(patchFile);
for (ShareDexDiffPatchInfo info : patchList) {
final String infoPath = info.path;
String patchRealPath;
if (infoPath.equals("")) {
patchRealPath = info.rawName;
} else {
patchRealPath = info.path + "/" + info.rawName;
}
File extractedFile = new File(dir + info.realName);
ZipEntry patchFileEntry = patch.getEntry(patchRealPath);
ZipEntry rawApkFileEntry = apk.getEntry(patchRealPath);
patchDexFile(apk, patch, rawApkFileEntry, patchFileEntry, info, extractedFile);
}
return true;
}这里的代码比较关键了，可以看出首先解析了meta里面的信息，meta中包含了patch中每个dex的相关数据。然后通过Application拿到sourceDir，其实就是本机apk的路径以及patch文件；根据mate中的信息开始遍历，其实就是取出对应的dex文件，最后通过patchDexFile对两个dex文件做合并。
private static void patchDexFile(
ZipFile baseApk, ZipFile patchPkg, ZipEntry oldDexEntry, ZipEntry patchFileEntry,
ShareDexDiffPatchInfo patchInfo, File patchedDexFile) throws IOException {
InputStream oldDexStream = null;
InputStream patchFileStream = null;
oldDexStream = new BufferedInputStream(baseApk.getInputStream(oldDexEntry));
patchFileStream = (patchFileEntry != null ? new BufferedInputStream(patchPkg.getInputStream(patchFileEntry)) : null);
new DexPatchApplier(oldDexStream, patchFileStream).executeAndSaveTo(patchedDexFile);
}通过ZipFile拿到其内部文件的InputStream，其实就是读取本地apk对应的dex文件，以及patch中对应dex文件，对二者的通过executeAndSaveTo方法进行合并至patchedDexFile，即patch的目标私有目录。至于合并算法，这里其实才是Tinker比较核心的地方，感兴趣可以参考这篇文章！
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