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Android源码版本：5.0.2_r1
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);
}
}
/**
* 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<String>。
// 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<File> 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");
}
/**
* 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<File> 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<File> 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;
}
/**
* 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<File> 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<File> files = new ArrayList<File>();
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;
}
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);
}
private static void installSecondaryDexes(ClassLoader loader, File dexDir, List<File> 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);
}
}
}
/**
* 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<File> 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<IOException> suppressedExceptions = new ArrayList<IOException>();
// 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<File>(additionalClassPathEntries), optimizedDirectory,
suppressedExceptions));
......
}
/**
* A wrapper around
* {@code private static final dalvik.system.DexPathList#makeDexElements}.
*/
private static Object[] makeDexElements(
Object dexPathList, ArrayList<File> files, File optimizedDirectory,
ArrayList<IOException> 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);
}
/**
* 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 <Type>[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);
}原文：http://blog.csdn.net/zylc369/article/details/44660007