本文主要从源码角度出发，分析MultiDex的实现原理。
出处： Allen's Zone
作者： Allen Feng
分析
调用MultiDex的方式有多种，不论是直接使用官方提供的MultiDexApplication，还是继承MultiDexApplication，或者是重写自定义Application的attachBaseContext方法，最后都会调用到MultiDex.install(this);：
@Override
protected void attachBaseContext(Context base) {
super.attachBaseContext(base);
MultiDex.install(this);
}
MultiDex.install是整个MultiDex的入口点，我们以此为切入点开始分析：
public static void install(Context context) {
Log.i(TAG, "install");
// 检查当前系统是否支持multidex
if (IS_VM_MULTIDEX_CAPABLE) {
Log.i(TAG, "VM has multidex support, MultiDex support library is disabled.");
try {
clearOldDexDir(context);
} catch (Throwable t) {
Log.w(TAG, "Something went wrong when trying to clear old MultiDex extraction, "
+ "continuing without cleaning.", t);
}
return;
}
// MultiDex最低只支持到1.6
if (Build.VERSION.SDK_INT < MIN_SDK_VERSION) {
throw new RuntimeException("Multi dex installation failed. SDK " + Build.VERSION.SDK_INT
+ " is unsupported. Min SDK version is " + MIN_SDK_VERSION + ".");
}
try {
ApplicationInfo applicationInfo = getApplicationInfo(context);
if (applicationInfo == null) {
// Looks like running on a test Context, so just return without patching.
return;
}
synchronized (installedApk) {
// sourceDir对应于/data/app/<package-name>.apk
String apkPath = applicationInfo.sourceDir;
// 若给定apk已经install过，直接退出
if (installedApk.contains(apkPath)) {
return;
}
installedApk.add(apkPath);
// MultiDex 最高只支持到20(Android 4.4W)，更高的版本不能保证正常工作
if (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") + "\"");
}
/*
* 待Patch的class loader应该是BaseDexClassLoaderd的子类，
* MultiDex主要通过修改pathList字段来添加更多的dex
*/
ClassLoader loader;
try {
loader = context.getClassLoader();
} catch (RuntimeException e) {
/* Ignore those exceptions so that we don't break tests relying on Context like
* a android.test.mock.MockContext or a android.content.ContextWrapper with a
* null base Context.
*/
Log.w(TAG, "Failure while trying to obtain Context class loader. " +
"Must be running in test mode. Skip patching.", e);
return;
}
if (loader == null) {
// Note, the context class loader is null when running Robolectric tests.
Log.e(TAG,
"Context class loader is null. Must be running in test mode. "
+ "Skip patching.");
return;
}
// MultiDex的二级dex文件将存放在 /data/data/<package-name>/secondary-dexes 下
File dexDir = new File(context.getFilesDir(), SECONDARY_FOLDER_NAME);
// 从apk中查找并解压二级dex文件到/data/data/<package-name>/secondary-dexes
List<File> files = MultiDexExtractor.load(context, applicationInfo, dexDir, false);
// 检查dex压缩文件的完整性
if (checkValidZipFiles(files)) {
// 开始安装dex
installSecondaryDexes(loader, dexDir, files);
} else {
Log.w(TAG, "Files were not valid zip files. Forcing a reload.");
// 第一次检查失败，MultiDex会尽责的再检查一次
files = MultiDexExtractor.load(context, applicationInfo, dexDir, true);
if (checkValidZipFiles(files)) {
// 开始安装dex
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");
}
这个方法涵盖了MultiDex安装的整个流程：
1. 检查虚拟机版本判断是否需要MultiDex;
在ART虚拟机中（部分4.4机器及5.0以上的机器），采用了Ahead-of-time（AOT）compilation技术，系统在apk的安装过程中，会使用自带的dex2oat工具对apk中可用的dex文件进行编译，并生成一个可在本地机器上运行的odex(optimized dex)文件，这样做会提高应用的启动速度。（但是安装速度降低了）
若不需要使用MultiDex，将使用clearOldDexDir清除/data/data/pkgName/code-cache/secondary-dexes目录下下所有文件
2. 根据applicationInfo.sourceDir的值获取安装的apk路径
安装完成的apk路径为/data/app/<package-name>.apk
3. 检查apk是否执行过MultiDex.install，若已经安装直接退出
4. 使用MultiDexExtractor.load获取apk中可用的二级dex列表
static List<File> load(Context context, ApplicationInfo applicationInfo, File dexDir, boolean forceReload) throws IOException {
Log.i("MultiDex", "MultiDexExtractor.load(" + applicationInfo.sourceDir + ", " + forceReload + ")");
File sourceApk = new File(applicationInfo.sourceDir);
long currentCrc = getZipCrc(sourceApk);
List files;
if(!forceReload && !isModified(context, sourceApk, currentCrc)) {
try {
files = loadExistingExtractions(context, sourceApk, dexDir);
} catch (IOException var9) {
Log.w("MultiDex", "Failed to reload existing extracted secondary dex files, falling back to fresh extraction", var9);
files = performExtractions(sourceApk, dexDir);
putStoredApkInfo(context, getTimeStamp(sourceApk), currentCrc, files.size() + 1);
}
} else {
Log.i("MultiDex", "Detected that extraction must be performed.");
files = performExtractions(sourceApk, dexDir);
putStoredApkInfo(context, getTimeStamp(sourceApk), currentCrc, files.size() + 1);
}
Log.i("MultiDex", "load found " + files.size() + " secondary dex files");
return files;
}
MultiDexExtractor.load会先判断是否需要从apk中解压dex文件，主要判断依据是：上次保存的apk（zip文件）的CRC校验码和last modify日期与dex的总数量是否与当前apk相同。此外，forceReload也会决定是否需要重新解压，这个参数后文会提到。
如果需要解压dex文件，将会使用performExtractions将.dex从apk中解压出来，解压路径为
/data/data/<package-name>/code_cache/secondary-dexes/<package-name>.apk.classes2.zip
/data/data/<package-name>/code_cache/secondary-dexes/<package-name>.apk.classes3.zip
...
private static List<File> performExtractions(File sourceApk, File dexDir) throws IOException {
// extractedFilePrefix值为<package-name>.apk.classes
String extractedFilePrefix = sourceApk.getName() + ".classes";
prepareDexDir(dexDir, extractedFilePrefix);
ArrayList files = new ArrayList();
ZipFile apk = new ZipFile(sourceApk);
try {
int e = 2;
// 扫描apk内所有classes2.dex、classes3.dex...文件
for(ZipEntry dexFile = apk.getEntry("classes" + e + ".dex"); dexFile != null; dexFile = apk.getEntry("classes" + e + ".dex")) {
// 解压路径为 /data/data/<package-name>/secondary-dexes/<package-name>.classes2.dex.zip 、/data/data/<package-name>/secondary-dexes/<package-name>.classes3.dex.zip ...
String fileName = extractedFilePrefix + e + ".zip";
File extractedFile = new File(dexDir, fileName);
files.add(extractedFile);
Log.i("MultiDex", "Extraction is needed for file " + extractedFile);
int numAttempts = 0;
boolean isExtractionSuccessful = false;
// 每个dex文件都会尝试3次解压
while(numAttempts < 3 && !isExtractionSuccessful) {
++numAttempts;
extract(apk, dexFile, extractedFile, extractedFilePrefix);
isExtractionSuccessful = verifyZipFile(extractedFile);
Log.i("MultiDex", "Extraction " + (isExtractionSuccessful?"success":"failed") + " - length " + extractedFile.getAbsolutePath() + ": " + extractedFile.length());
if(!isExtractionSuccessful) {
extractedFile.delete();
if(extractedFile.exists()) {
Log.w("MultiDex", "Failed to delete corrupted secondary dex \'" + extractedFile.getPath() + "\'");
}
}
}
if(!isExtractionSuccessful) {
throw new IOException("Could not create zip file " + extractedFile.getAbsolutePath() + " for secondary dex (" + e + ")");
}
++e;
}
} finally {
try {
apk.close();
} catch (IOException var16) {
Log.w("MultiDex", "Failed to close resource", var16);
}
}
return files;
}
解压成功后，会保存本次解压所使用的apk信息，用于下次调用MultiDexExtractor.load时判断是否需要重新解压：
private static void putStoredApkInfo(Context context, long timeStamp, long crc, int totalDexNumber) {
SharedPreferences prefs = getMultiDexPreferences(context);
Editor edit = prefs.edit();
// apk最后修改时间戳
edit.putLong("timestamp", timeStamp);
// apk的CRC校验码
edit.putLong("crc", crc);
// dex的总数量
edit.putInt("dex.number", totalDexNumber);
apply(edit);
}
如果apk未被修改，将会调用loadExistingExtractions方法，直接加载上一次解压出来的文件：
private static List<File> loadExistingExtractions(Context context, File sourceApk, File dexDir) throws IOException {
Log.i("MultiDex", "loading existing secondary dex files");
String extractedFilePrefix = sourceApk.getName() + ".classes";
int totalDexNumber = getMultiDexPreferences(context).getInt("dex.number", 1);
ArrayList files = new ArrayList(totalDexNumber);
for(int secondaryNumber = 2; secondaryNumber <= totalDexNumber; ++secondaryNumber) {
String fileName = extractedFilePrefix + secondaryNumber + ".zip";
File extractedFile = new File(dexDir, fileName);
if(!extractedFile.isFile()) {
throw new IOException("Missing extracted secondary dex file \'" + extractedFile.getPath() + "\'");
}
files.add(extractedFile);
if(!verifyZipFile(extractedFile)) {
Log.i("MultiDex", "Invalid zip file: " + extractedFile);
throw new IOException("Invalid ZIP file.");
}
}
return files;
}
不管是调用了loadExistingExtractions还是performExtractions，都会返回一个解压后的<package-name>.apk.classes2.zip、<package-name>.apk.classes3.zip...File列表，供下一步使用。
5. 两次校验dex压缩包的完整性
通过上一步得到解压后的dex File列表后，在MultiDex中会两次检查zip文件的完整性：
public static void install(Context context) {
...
try {
...
synchronized (installedApk) {
...
// 从apk中查找并解压二级dex文件到/data/data/<package-name>/secondary-dexes
List<File> files = MultiDexExtractor.load(context, applicationInfo, dexDir, false);
// 检查dex压缩文件的完整性
if (checkValidZipFiles(files)) {
// 开始安装dex
installSecondaryDexes(loader, dexDir, files);
} else {
Log.w(TAG, "Files were not valid zip files. Forcing a reload.");
// 第一次检查失败，MultiDex会尽责的再检查一次
files = MultiDexExtractor.load(context, applicationInfo, dexDir, true);
if (checkValidZipFiles(files)) {
// 开始安装dex
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");
}
若第一次校验失败（dex文件损坏等），MultiDex会重新调用MultiDexExtractor.load方法重查找加载二级dex文件列表，值得注意的是此时forceReload的值为true，会强制重新从apk中解压dex文件。
6. 开始dex的安装
经过上面的重重检验和解压，终于到了最关键的一步：将二级dex添加到我们classLoader中
private static void installSecondaryDexes(ClassLoader loader, File dexDir, List<File> files) throws IllegalArgumentException, IllegalAccessException, NoSuchFieldException, InvocationTargetException, NoSuchMethodException, IOException {
if(!files.isEmpty()) {
if(VERSION.SDK_INT >= 19) {
MultiDex.V19.install(loader, files, dexDir);
} else if(VERSION.SDK_INT >= 14) {
MultiDex.V14.install(loader, files, dexDir);
} else {
MultiDex.V4.install(loader, files);
}
}
}
由于SDK版本不同，ClassLoader中的实现存在差异，所以使用了三个分支去执行dex的安装。这里我们选择MultiDex.V14.install进行分析，其他两个大同小异：
先明确入参：
| 入参 | 含义 |
|---|---|
| ClassLoader loader | 通过context.getClassLoader获取到的默认类加载器 |
| List<File> additionalClassPathEntries | 二级dex文件解压后的路径(通过步骤4获得) |
| optimizedDirectory | 对应/data/data/<package-name>/code_cache/secondary-dexes/目录 |
private static Field findField(Object instance, String name) throws NoSuchFieldException {
Class clazz = instance.getClass();
while(clazz != null) {
try {
Field e = clazz.getDeclaredField(name);
if(!e.isAccessible()) {
e.setAccessible(true);
}
return e;
} catch (NoSuchFieldException var4) {
clazz = clazz.getSuperclass();
}
}
throw new NoSuchFieldException("Field " + name + " not found in " + instance.getClass());
}
private static Method findMethod(Object instance, String name, Class... parameterTypes) throws NoSuchMethodException {
Class clazz = instance.getClass();
while(clazz != null) {
try {
Method e = clazz.getDeclaredMethod(name, parameterTypes);
if(!e.isAccessible()) {
e.setAccessible(true);
}
return e;
} catch (NoSuchMethodException var5) {
clazz = clazz.getSuperclass();
}
}
throw new NoSuchMethodException("Method " + name + " with parameters " + Arrays.asList(parameterTypes) + " not found in " + instance.getClass());
}
private static final class V14 {
private V14() {
}
private static void install(ClassLoader loader, List<File> additionalClassPathEntries, File optimizedDirectory) throws IllegalArgumentException, IllegalAccessException, NoSuchFieldException, InvocationTargetException, NoSuchMethodException {
// 通过反射获取 ClassLoader中的pathList
Field pathListField = MultiDex.findField(loader, "pathList");
Object dexPathList = pathListField.get(loader);
// 先调用pathList的makeDexElements，然后将生成的Element[]传入expandFieldArray中
MultiDex.expandFieldArray(dexPathList, "dexElements", makeDexElements(dexPathList, new ArrayList(additionalClassPathEntries), optimizedDirectory));
}
private static Object[] makeDexElements(Object dexPathList, ArrayList<File> files, File optimizedDirectory) throws IllegalAccessException, InvocationTargetException, NoSuchMethodException {
Method makeDexElements = MultiDex.findMethod(dexPathList, "makeDexElements", new Class[]{ArrayList.class, File.class});
return (Object[])((Object[])makeDexElements.invoke(dexPathList, new Object[]{files, optimizedDirectory}));
}
}
/libcore/dalvik/src/main/java/dalvik/system/BaseDexClassLoader.java
public class BaseDexClassLoader extends ClassLoader {
...
/** structured lists of path elements */
private final DexPathList pathList;
...
public BaseDexClassLoader(String dexPath, File optimizedDirectory, String libraryPath, ClassLoader parent) {
super(parent);
this.originalPath = dexPath;
this.originalLibraryPath = libraryPath;
this.pathList =
new DexPathList(this, dexPath, libraryPath, optimizedDirectory);
}
}
/libcore/dalvik/src/main/java/dalvik/system/DexPathList.java
/*package*/ final class DexPathList {
....
/**
* Makes an array of dex/resource path elements, one per element of
* the given array.
*/
private static Element[] makeDexElements(ArrayList<File> files,
File optimizedDirectory) {
ArrayList<Element> elements = new ArrayList<Element>();
/*
* Open all files and load the (direct or contained) dex files
* up front.
*/
for (File file : files) {
File zip = null;
DexFile dex = null;
String name = file.getName();
if (name.endsWith(DEX_SUFFIX)) {
// Raw dex file (not inside a zip/jar).
try {
dex = loadDexFile(file, optimizedDirectory);
} catch (IOException ex) {
System.logE("Unable to load dex file: " + file, ex);
}
} else if (name.endsWith(APK_SUFFIX) || name.endsWith(JAR_SUFFIX)
|| name.endsWith(ZIP_SUFFIX)) {
zip = file;
try {
dex = loadDexFile(file, optimizedDirectory);
} catch (IOException ignored) {
/*
* IOException might get thrown "legitimately" by
* the DexFile constructor if the zip file turns
* out to be resource-only (that is, no
* classes.dex file in it). Safe to just ignore
* the exception here, and let dex == null.
*/
}
} else {
System.logW("Unknown file type for: " + file);
}
if ((zip != null) || (dex != null)) {
elements.add(new Element(file, zip, dex));
}
}
return elements.toArray(new Element[elements.size()]);
}
...
}
所以MultiDex在安装开始时，会先通过反射调用BaseDexClassLoader里
DexPathList类型的pathList字段，接着通过pathList调用DexPathList的makeDexElements方法，将上面解压得到的additionalClassPathEntries（二级dex文件列表）封装成Element数组。
需要注意的是，makeDexElements最终会去进行dex2opt操作，这是一个比较耗时的过程，如果全部放在main线程去处理的话，比较影响用户体验，甚至可能引起ANR。
dex2opt后，/data/data/<package-name>/code_cache/secondary-dexes/下的会出现优化后的文件：<package-name>.apk.classes2.dex等
最后调用MultiDex.expandFieldArray：
private static void expandFieldArray(Object instance, String fieldName, Object[] extraElements) throws NoSuchFieldException, IllegalArgumentException, IllegalAccessException {
Field jlrField = findField(instance, fieldName);
Object[] original = (Object[])((Object[])jlrField.get(instance));
Object[] combined = (Object[])((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);
jlrField.set(instance, combined);
}
expandFieldArray同样是通过反射调用，找到pathList中的dexElements字段，并将上一步生成的封装了二级dex的Element数组添加到dexElements之后，完成整个安装流程
总结
通过上面的分析，我们可以总结出来MultiDex的原理如下：
- apk在Applicaion实例化之后，会检查系统版本是否支持MultiDex，判断二级dex是否需要安装；
- 如果需要安装则会从apk中解压出classes2.dex并将其拷贝到应用的/data/data/<package-name>/code_cache/secondary-dexes/目录下；
- 通过反射将classes2.dex等注入到当前的ClassLoader的pathList中，完成整体安装流程。
