MultiDex加载流程分析

只介绍主要流程，multiDex源码已经上传到https://github.com/AlexSmille/google-android-support-source-analyze,包含了BaseDexClassloader等一些android源码。

1.判断当前是否需要动态加载类

在5.0以上，会自动加载一个apk中所有的classesX.dex，但是低于5.0时需要手动导入。

具体判断是否需要手动导入如下：

 static boolean isVMMultidexCapable(String versionString) {//You can verify which runtime is in use by calling System.getProperty("java.vm.version").// If ART is in use, the property's value is "2.0.0" or higher.boolean isMultidexCapable = false;//2.1.0的时候开始支持默认加载外部classes.dex,根据vm版本号判断if (versionString != null) {Matcher matcher = Pattern.compile("(\\d+)\\.(\\d+)(\\.\\d+)?").matcher(versionString);if (matcher.matches()) {try {int major = Integer.parseInt(matcher.group(1));int minor = Integer.parseInt(matcher.group(2));isMultidexCapable = (major > VM_WITH_MULTIDEX_VERSION_MAJOR)|| ((major == VM_WITH_MULTIDEX_VERSION_MAJOR)&& (minor >= VM_WITH_MULTIDEX_VERSION_MINOR));} catch (NumberFormatException e) {// let isMultidexCapable be false}}}return isMultidexCapable;}

如果已经支持自动导入，则结束流程，否则进入下一步。

2.构造必要参数，调用`doInstallation()`方法

调用doInstallation()传入四个参数

Context mainContext：上下文对象。
File sourceApk：安装额apk的存放路径。/data/app/….
File dataDir: apk的数据存放目录。/data/data/package/
String secondaryFolderName:存放缓存的目录文件名secondary-dexes
String prefsKeyPrefix:
boolean reinstallOnPatchRecoverableException：如果导入失败，是重试还是直接抛出异常。

3.`doInstallation()`方法判断一些必要条件

if (installedApk.contains(sourceApk)) {// 是否包含要导入的apk,防止重复导入return;}// 将apk添加到集合中installedApk.add(sourceApk);// 5.0以上不用导入  IS_VM_MULTIDEX_CAPABLEif (Build.VERSION.SDK_INT > MAX_SUPPORTED_SDK_VERSION) {Log.w(TAG, "MultiDex is not guaranteed to work in SDK version "+ Build.VERSION.SDK_INT + ": SDK version higher than "+ MAX_SUPPORTED_SDK_VERSION + " should be backed by "+ "runtime with built-in multidex capabilty but it's not the "+ "case here: java.vm.version=\""+ System.getProperty("java.vm.version") + "\"");}}

4.构造存放`classesX.dex`的目录`/data/data/package/code_cache/secondary-dexes`

File dexDir = getDexDir(mainContext, dataDir, secondaryFolderName);

创建dexDir，即存放dex的目录。getDexDir()代码如下：

private static File getDexDir(Context context, File dataDir, String secondaryFolderName)throws IOException {// data/data/package/code_cacheFile cache = new File(dataDir, CODE_CACHE_NAME);try {mkdirChecked(cache);} catch (IOException e) {/* If we can't emulate code_cache, then store to filesDir. This means abandoning useless* files on disk if the device ever updates to android 5+. But since this seems to* happen only on some devices running android 2, this should cause no pollution.*/cache = new File(context.getFilesDir(), CODE_CACHE_NAME);mkdirChecked(cache);}// data/data/package/code_cache/secondary-dexesFile dexDir = new File(cache, secondaryFolderName);mkdirChecked(dexDir);return dexDir;}

5.构造`MultiDexExtractor`对象，准备提取`classesX.dex`

// MultiDexExtractor is taking the file lock and keeping it until it is closed.// Keep it open during installSecondaryDexes and through forced extraction to ensure no// extraction or optimizing dexopt is running in parallel.MultiDexExtractor extractor = new MultiDexExtractor(sourceApk, dexDir);

6.通过`MultiDexExtractor`提取并存储到`data/data/package/code_cache/secondary-dexes/`目录下

在此步骤中，会将apk中的classesX.dex提取并存放为data/data/package/code_cache/secondary-dexes/package--1.apk.classN.zip，以便后续加载到内存中。

关键方法MultiDexExtractor.load()方法

List<? extends File> load(Context context, String prefsKeyPrefix, boolean forceReload)throws IOException {Log.i(TAG, "MultiDexExtractor.load(" + sourceApk.getPath() + ", " + forceReload + ", " +prefsKeyPrefix + ")");if (!cacheLock.isValid()) {throw new IllegalStateException("MultiDexExtractor was closed");}List<ExtractedDex> files;if (!forceReload && !isModified(context, sourceApk, sourceCrc, prefsKeyPrefix)) {// apk信息没有发生变化try {// 加载已存在的classX.zipfiles = loadExistingExtractions(context, prefsKeyPrefix);} catch (IOException ioe) {Log.w(TAG, "Failed to reload existing extracted secondary dex files,"+ " falling back to fresh extraction", ioe);files = performExtractions();putStoredApkInfo(context, prefsKeyPrefix, getTimeStamp(sourceApk), sourceCrc,files);}} else {if (forceReload) {Log.i(TAG, "Forced extraction must be performed.");} else {Log.i(TAG, "Detected that extraction must be performed.");}// 重新从apk中提取classesX.dexfiles = performExtractions();putStoredApkInfo(context, prefsKeyPrefix, getTimeStamp(sourceApk), sourceCrc,files);}Log.i(TAG, "load found " + files.size() + " secondary dex files");return files;}

根据apk是否有新变化，判断是直接读取之前获取的classX.zip文件，还是从apk中重新提取classes.dex。

看一下提取的方法

private List<ExtractedDex> performExtractions() throws IOException {final String extractedFilePrefix = sourceApk.getName() + EXTRACTED_NAME_EXT;// It is safe to fully clear the dex dir because we own the file lock so no other process is// extracting or running optimizing dexopt. It may cause crash of already running// applications if for whatever reason we end up extracting again over a valid extraction.// 清楚缓存目录clearDexDir();List<ExtractedDex> files = new ArrayList<ExtractedDex>();// 获取apk的zip对象final ZipFile apk = new ZipFile(sourceApk);int secondaryNumber = 2;// 从apk中获取对应classesX.dexZipEntry dexFile = apk.getEntry(DEX_PREFIX + secondaryNumber + DEX_SUFFIX);while (dexFile != null) {String fileName = extractedFilePrefix + secondaryNumber + EXTRACTED_SUFFIX;ExtractedDex extractedFile = new ExtractedDex(dexDir, fileName);files.add(extractedFile);Log.i(TAG, "Extraction is needed for file " + extractedFile);int numAttempts = 0;boolean isExtractionSuccessful = false;while (numAttempts < MAX_EXTRACT_ATTEMPTS && !isExtractionSuccessful) {numAttempts++;// Create a zip file (extractedFile) containing only the secondary dex file// (dexFile) from the apk.// 创建zip文件，并提取.dex文件extract(apk, dexFile, extractedFile, extractedFilePrefix);// .... 后面的都是一些判断}}return files;}

此段对应的log如下

// MultidexExtractor.load()
07-04 16:19:52.296 I/MultiDex( 4387): MultiDexExtractor.load(/data/app/com.spearbothy.android-2.apk, false, )
// 有新信息变化，删除老的缓存文件
07-04 16:19:52.306 I/MultiDex( 4387): Detected that extraction must be performed.
07-04 16:19:52.306 I/MultiDex( 4387): Trying to delete old file /data/data/com.spearbothy.android/code_cache/secondary-dexes/com.spearbothy.android-1.apk.classes2.zip of size 185848
07-04 16:19:52.306 I/MultiDex( 4387): Deleted old file /data/data/com.spearbothy.android/code_cache/secondary-dexes/com.spearbothy.android-1.apk.classes2.zip
07-04 16:19:52.306 I/MultiDex( 4387): Trying to delete old file /data/data/com.spearbothy.android/code_cache/secondary-dexes/com.spearbothy.android-1.apk.classes2.dex of size 675216
07-04 16:19:52.306 I/MultiDex( 4387): Deleted old file /data/data/com.spearbothy.android/code_cache/secondary-dexes/com.spearbothy.android-1.apk.classes2.dex
// 提取apk中的class文件
07-04 16:19:52.386 I/MultiDex( 4387): Extraction is needed for file /data/data/com.spearbothy.android/code_cache/secondary-dexes/com.spearbothy.android-2.apk.classes2.zip
07-04 16:19:52.386 I/MultiDex( 4387): Extracting /data/data/com.spearbothy.android/code_cache/secondary-dexes/tmp-com.spearbothy.android-2.apk.classes762107569.zip
07-04 16:19:56.016 I/MultiDex( 4387): Renaming to /data/data/com.spearbothy.android/code_cache/secondary-dexes/com.spearbothy.android-2.apk.classes2.zip
07-04 16:19:56.016 I/MultiDex( 4387): Extraction succeeded '/data/data/com.spearbothy.android/code_cache/secondary-dexes/com.spearbothy.android-2.apk.classes2.zip': length 3843460 - crc: 28802720

7.加载类到`BaseDexClassLoader`中

在第6步的时候，将.dex文件提取出来保存到了对应的目录，那么只需要加载就可以了。

// 获取提出来的`dex`文件目录
files = extractor.load(mainContext, prefsKeyPrefix, true);
// 导入
installSecondaryDexes(loader, dexDir, files);

private static void installSecondaryDexes(ClassLoader loader, File dexDir,List<? extends File> files)throws IllegalArgumentException, IllegalAccessException, NoSuchFieldException,InvocationTargetException, NoSuchMethodException, IOException, SecurityException,ClassNotFoundException, InstantiationException {if (!files.isEmpty()) {if (Build.VERSION.SDK_INT >= 19) {V19.install(loader, files, dexDir);} else if (Build.VERSION.SDK_INT >= 14) {V14.install(loader, files);} else {V4.install(loader, files);}}}

不同版本，加载的方式不同，所以根据sdk版本选择不同的加载方式。因为5.0以上不需要加载，所以只需要到19即可。

虽然加载的方式不同，但核心的就是将每一个.zip转化为DexPathList.Element，添加到DexPathList的elements字段中。