加载多个dex

下面是多dex加载的时序图： 

Android项目有两种方式支持多dex：

1. 项目中的Application类继承MultiDexApplication。
2. 在自己的Application类的attachBaseContext方法中调用MultiDex.install(this);。


我从MultiDexApplication这个类开始分析。

MultiDexApplication类继承了Application，并重载了attachBaseContext方法，在这个方法中调用了MultiDex.install(this);。

 public class MultiDexApplication extends Application {
   @Override
   protected void attachBaseContext(Context base) {
     super.attachBaseContext(base);
     MultiDex.install(this);
   }
 }

MultiDex.install 方法：

 /**
  * Patches the application context class loader by appending extra dex files
  * loaded from the application apk. This method should be called in the
  * attachBaseContext of your {@link Application}, see
  * {@link MultiDexApplication} for more explanation and an example.
  *
  * @param context application context.
  * @throws RuntimeException if an error occurred preventing the classloader
  *         extension.
  */
 public static void install(Context context) {
     Log.i(TAG, "install");

     ......

     try {
         ApplicationInfo applicationInfo = getApplicationInfo(context);
         if (applicationInfo == null) {
             // Looks like running on a test Context, so just return without patching.
             return;
         }

         synchronized (installedApk) {
             String apkPath = applicationInfo.sourceDir;
             // installedApk的类型是：Set。
             // 如果这个apk已经安装，则不重复安装。
             if (installedApk.contains(apkPath)) {
                 return;
             }
             installedApk.add(apkPath);

             ......

             // 类加载器应该直接或间接继承于BaseDexClassLoader。
             // 修改BaseDexClassLoader类中的DexPathList pathList字段，追加额外的DEX文件项。
             /* The patched class loader is expected to be a descendant of
              * dalvik.system.BaseDexClassLoader. We modify its
              * dalvik.system.DexPathList pathList field to append additional DEX
              * file entries.
              */
             ClassLoader loader;

             ......

             // dex将会输出到SECONDARY_FOLDER_NAME目录。
             File dexDir = new File(applicationInfo.dataDir, SECONDARY_FOLDER_NAME);
             List files = MultiDexExtractor.load(context, applicationInfo, dexDir, false);
             // 校验这些zip文件是否合法。
             if (checkValidZipFiles(files)) {
                 // 安装提取出来的zip文件。
                 installSecondaryDexes(loader, dexDir, files);
             } else {
                 Log.w(TAG, "Files were not valid zip files.  Forcing a reload.");
                 // 最后一个参数是true，代表强制加载。
                 // Try again, but this time force a reload of the zip file.
                 files = MultiDexExtractor.load(context, applicationInfo, dexDir, true);

                 // 校验这些zip文件是否合法。
                 if (checkValidZipFiles(files)) {
                     // 安装提取出来的zip文件。
                     installSecondaryDexes(loader, dexDir, files);
                 } else {
                     // Second time didn't work, give up
                     throw new RuntimeException("Zip files were not valid.");
                 }
             }
         }

     } catch (Exception e) {
         Log.e(TAG, "Multidex installation failure", e);
         throw new RuntimeException("Multi dex installation failed (" + e.getMessage() + ").");
     }
     Log.i(TAG, "install done");
 }

MultiDexExtractor.load 获得/导出apk中多出的dex，这些dex导出后会被打包成zip文件：

 /**
  * 提取/获得apk中多dex的提取zip文件。
  * 如果不是加载已经存在的文件的情况，则还要保存apk的信息：时间戳、crc值、apk中dex的总个数。
  *
  * Extracts application secondary dexes into files in the application data
  * directory.
  *
  * @return a list of files that were created. The list may be empty if there
  *         are no secondary dex files.
  * @throws IOException if encounters a problem while reading or writing
  *         secondary dex files
  */
 static List load(Context context, ApplicationInfo applicationInfo, File dexDir,
         boolean forceReload) throws IOException {
     Log.i(TAG, "MultiDexExtractor.load(" + applicationInfo.sourceDir + ", " + forceReload + ")");
     final File sourceApk = new File(applicationInfo.sourceDir);

     long currentCrc = getZipCrc(sourceApk);

     List files;
     // isModified方法判断apk是否被修改过。
     if (!forceReload && !isModified(context, sourceApk, currentCrc)) {
         try {
             // 加载已经存在的文件，如果有的文件不存在，或者不是zip文件，则会抛出异常。
             files = loadExistingExtractions(context, sourceApk, dexDir);
         } catch (IOException ioe) {
             Log.w(TAG, "Failed to reload existing extracted secondary dex files,"
                     + " falling back to fresh extraction", ioe);
             // 从apk中提取出多dex，然后将这些dex逐个打包为zip文件，最终返回提取出来的zip文件列表。
             files = performExtractions(sourceApk, dexDir);
             // getTimeStamp方法中调用的是sourceApk.lastModified()方法。
             // putStoredApkInfo方法存储apk的信息：时间戳、crc值、apk中dex的总个数。
             putStoredApkInfo(context, getTimeStamp(sourceApk), currentCrc, files.size() + 1);

         }
     } else {
         Log.i(TAG, "Detected that extraction must be performed.");
         // 这里的performExtractions和putStoredApkInfo同上。
         files = performExtractions(sourceApk, dexDir);
         putStoredApkInfo(context, getTimeStamp(sourceApk), currentCrc, files.size() + 1);
     }

     Log.i(TAG, "load found " + files.size() + " secondary dex files");
     return files;
 }

MultiDexExtractor.performExtractions 方法：

 /**
  * 从apk中提取出多dex，然后将这些dex逐个打包为zip文件。
  * @param sourceApk apk文件。
  * @param dexDir 输出目录。
  * @return 提取出来的zip文件列表。
  */
 private static List performExtractions(File sourceApk, File dexDir)
         throws IOException {

     final String extractedFilePrefix = sourceApk.getName() + EXTRACTED_NAME_EXT;

     // 如果文件没有正确的前缀，则删除。
     // Ensure that whatever deletions happen in prepareDexDir only happen if the zip that
     // contains a secondary dex file in there is not consistent with the latest apk.  Otherwise,
     // multi-process race conditions can cause a crash loop where one process deletes the zip
     // while another had created it.
     prepareDexDir(dexDir, extractedFilePrefix);

     List files = new ArrayList();

     final ZipFile apk = new ZipFile(sourceApk);
     try {

         int secondaryNumber = 2;

         ZipEntry dexFile = apk.getEntry(DEX_PREFIX + secondaryNumber + DEX_SUFFIX);
         while (dexFile != null) {
             // 输出的文件名。
             String fileName = extractedFilePrefix + secondaryNumber + EXTRACTED_SUFFIX;
             // 输出的文件。
             File extractedFile = new File(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++;

                 // 提取apk中的多dex文件，然后打包成一个zip文件。
                 // Create a zip file (extractedFile) containing only the secondary dex file
                 // (dexFile) from the apk.
                 extract(apk, dexFile, extractedFile, extractedFilePrefix);

                 // 验证提取的文件是否是一个zip文件。
                 // Verify that the extracted file is indeed a zip file.
                 isExtractionSuccessful = verifyZipFile(extractedFile);

                 // Log the sha1 of the extracted zip file
                 Log.i(TAG, "Extraction " + (isExtractionSuccessful ? "success" : "failed") +
                         " - length " + extractedFile.getAbsolutePath() + ": " +
                         extractedFile.length());
                 if (!isExtractionSuccessful) {
                     // Delete the extracted file
                     extractedFile.delete();
                     if (extractedFile.exists()) {
                         Log.w(TAG, "Failed to delete corrupted secondary dex '" +
                                 extractedFile.getPath() + "'");
                     }
                 }
             }
             if (!isExtractionSuccessful) {
                 throw new IOException("Could not create zip file " +
                         extractedFile.getAbsolutePath() + " for secondary dex (" +
                         secondaryNumber + ")");
             }
             secondaryNumber++;
             dexFile = apk.getEntry(DEX_PREFIX + secondaryNumber + DEX_SUFFIX);
         }
     } finally {
         try {
             apk.close();
         } catch (IOException e) {
             Log.w(TAG, "Failed to close resource", e);
         }
     }

     return files;
 }

MultiDexExtractor.putStoredApkInfo 方法：

 private static void putStoredApkInfo(Context context, long timeStamp, long crc,
         int totalDexNumber) {
     SharedPreferences prefs = getMultiDexPreferences(context);
     SharedPreferences.Editor edit = prefs.edit();
     edit.putLong(KEY_TIME_STAMP, timeStamp);    // 时间戳
     edit.putLong(KEY_CRC, crc); // crc值。
     /* SharedPreferences.Editor doc says that apply() and commit() "atomically performs the
      * requested modifications" it should be OK to rely on saving the dex files number (getting
      * old number value would go along with old crc and time stamp).
      */
     edit.putInt(KEY_DEX_NUMBER, totalDexNumber);    // dex总个数。
     apply(edit);
 }

MultiDex.installSecondaryDexes 方法，对dex进行安装：

 private static void installSecondaryDexes(ClassLoader loader, File dexDir, List files)
         throws IllegalArgumentException, IllegalAccessException, NoSuchFieldException,
         InvocationTargetException, NoSuchMethodException, IOException {
     // 安装。
     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, dexDir);
         } else {
             V4.install(loader, files);
         }
     }
 }

这个方法的代码非常简单，挑选 V19.install 分析：

 /**
  * 安装多dex。
  * @param loader
  * @param additionalClassPathEntries zip文件列表，这些zip文件中都只有一个文件classes.dex。
  * @param optimizedDirectory 优化的dex存放的目录。
  */
 private static void install(ClassLoader loader, List additionalClassPathEntries,
         File optimizedDirectory)
                 throws IllegalArgumentException, IllegalAccessException,
                 NoSuchFieldException, InvocationTargetException, NoSuchMethodException {
     // 被打补丁的类加载器应该直接或间接继承BaseDexClassLoader。
     // 我们修改DexPathList pathList字段，追加额外的DEX文件项。
     /* The patched class loader is expected to be a descendant of
      * dalvik.system.BaseDexClassLoader. We modify its
      * dalvik.system.DexPathList pathList field to append additional DEX
      * file entries.
      */
     // dexPathList = load.pathList;
     Field pathListField = findField(loader, "pathList");
     Object dexPathList = pathListField.get(loader);

     ArrayList suppressedExceptions = new ArrayList();
     // makeDexElements方法调用了DexPathList中的makeDexElements方法，这个方法可以加载并优化dex、zip、jar。
     // expandFieldArray方法将makeDexElements返回的数组patch到dexPathList.dexElements中。
     expandFieldArray(dexPathList, "dexElements", makeDexElements(dexPathList,
             new ArrayList(additionalClassPathEntries), optimizedDirectory,
             suppressedExceptions));

     ......
 }

V19.makeDexElements 方法：

 /**
  * A wrapper around
  * {@code private static final dalvik.system.DexPathList#makeDexElements}.
  */
 private static Object[] makeDexElements(
         Object dexPathList, ArrayList files, File optimizedDirectory,
         ArrayList suppressedExceptions)
                 throws IllegalAccessException, InvocationTargetException,
                 NoSuchMethodException {
     Method makeDexElements =
             findMethod(dexPathList, "makeDexElements", ArrayList.class, File.class,
                     ArrayList.class);

     // return (DexPathList.Element[]) dexPathList.makeDexElements(files, optimizedDirectory, suppressedExceptions)
     return (Object[]) makeDexElements.invoke(dexPathList, files, optimizedDirectory,
             suppressedExceptions);
 }

MultiDex.expandFieldArray 方法：

 /**
  * 原字段内容加上扩展的数组元素替换相应字段的内容，这个字段是一个数组。
  *
  * Replace the value of a field containing a non null array, by a new array containing the
  * elements of the original array plus the elements of extraElements.
  * @param instance the instance whose field is to be modified.
  * @param fieldName the field to modify.
  * @param extraElements elements to append at the end of the array.
  */
 private static void expandFieldArray(Object instance, String fieldName,
         Object[] extraElements) throws NoSuchFieldException, IllegalArgumentException,
         IllegalAccessException {
     // combined = new [original.length + extraElements.length];
     Field jlrField = findField(instance, fieldName);
     Object[] original = (Object[]) jlrField.get(instance);
     Object[] combined = (Object[]) Array.newInstance(
             original.getClass().getComponentType(), original.length + extraElements.length);

     System.arraycopy(original, 0, combined, 0, original.length);
     System.arraycopy(extraElements, 0, combined, original.length, extraElements.length);
     // 替换对象中的数组字段。
     jlrField.set(instance, combined);
 }