目的是为了解决65535问题，支持的SDK是4以上，低了会抛异常，Android5.0以上的虚拟机本来就可以支持Dex分包加载
主要原理：为应用的DexClassLoader动态地添加dex文件
流程分析
基本流程
1、校验（Vm是否已经支持分包如21+，最低SDK版本是4，是否已经分包过了）
2、清理旧的的dex分包的目录下文件，data/data/packageName/file/secondary-dexes
3、Dex包读取，存放目录data/data/packageName/code_cache/secondary-dexes
- 3.1 主要是读取apk压缩包下的的classes2.dex、classesN.dex依次写入
/data/data/pkgName/code_cache/secondary-dexes/base.apk.classesN.zip下
4、校验分包的dex压缩包是否有效，无效再进行一次分包
5、Dex压缩包文件安装加载，通过DexPathList#makeDexElements的方法进行dex的加载，用返回的Element数组扩充原来ClassLoader下的Elements实现加载
public static void install(Context context) {
if (IS_VM_MULTIDEX_CAPABLE) {
Log.i(TAG, "VM has multidex support, MultiDex support library is disabled.");
return;
}
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) {
String apkPath = applicationInfo.sourceDir;
if (installedApk.contains(apkPath)) { //是否已经安装了
return;
}
installedApk.add(apkPath);
if (Build.VERSION.SDK_INT > MAX_SUPPORTED_SDK_VERSION) {
//警告:高于20的可以使用内建的dex分包能力
}
/*
*/
ClassLoader loader;
try {
loader = context.getClassLoader();
} catch (RuntimeException e) {
// 测试MockContext
return;
}
if (loader == null) {
// Robolectric tests
return;
}
try {
clearOldDexDir(context); //清理应用内部文件存储目录（一般data/data/pkg-name/）下的secondary-dexes目录
} catch (Throwable t) {
}
// data/data/packageName/code_cache/secondary-dexes
File dexDir = new File(applicationInfo.dataDir, SECONDARY_FOLDER_NAME);
List files = MultiDexExtractor.load(context, applicationInfo, dexDir, false); //返回分包后的zip文件列表
if (checkValidZipFiles(files)) { //检验zip文件是否有效
installSecondaryDexes(loader, dexDir, files);
} else {
//如果第一失败了，再进行一次相同的加载操作
}
}
} catch (Exception e) {
}
}
如何安装
DexClassLoader在构造的时候就会读取指定目录下的zip、dex、jar等文件，加载成DexFile，并构造成Element数组，记录在成员pathList下，以后类的加载都会尝试在这些DexFile中寻找，而在dex分包后，就需要自己把"新的dex的文件路径" 告诉DexClassLoader，这里以SDK19+为例子来说(对14,15,16,17and18来说区别在于DexPathList#makeDexElements方法签名的改变，4到13的改变稍微有点大，但现在也不会开发14以下的了就不细看了)
private static void installSecondaryDexes(ClassLoader loader, File dexDir, List files) {
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);
}
}
}
主要用DexPathList#makeDexElements的方法进行dex的加载，用返回的Element数组扩充原来ClassLoader下的Elements
private static final class V19 {
private static void install(ClassLoader loader, List additionalClassPathEntries, File optimizedDirectory) {
Field pathListField = findField(loader, "pathList"); //loader#pathList字段，DexPathList类型
Object dexPathList = pathListField.get(loader);
ArrayList suppressedExceptions = new ArrayList();
expandFieldArray(dexPathList, "dexElements", makeDexElements(dexPathList, new ArrayList(additionalClassPathEntries), optimizedDirectory, suppressedExceptions));
if (suppressedExceptions.size() > 0) {
for (IOException e : suppressedExceptions) {
Log.w(TAG, "Exception in makeDexElement", e);
}
//.....
}
}
/**
* {@code private static final dalvik.system.DexPathList#makeDexElements}.
* 这个方法用来执行DexPathList#makeDexElements的方法输入需要加载的dex目录，返回`Element`数组
*/
private static Object[] makeDexElements(
Object dexPathList, ArrayList files, File optimizedDirectory, ArrayList suppressedExceptions) {
Method makeDexElements = findMethod(dexPathList, "makeDexElements", ArrayList.class, File.class, ArrayList.class);
return (Object[]) makeDexElements.invoke(dexPathList, files, optimizedDirectory,suppressedExceptions);
}
}
Dex读取
Dex的读取在MultiDexExtractor#load方法进行
MultiDexExtractor.java
static List load(Context context, ApplicationInfo applicationInfo, File dexDir, boolean forceReload) throws IOException {
final File sourceApk = new File(applicationInfo.sourceDir); //data/app/packageName/base.apk
long currentCrc = getZipCrc(sourceApk); //返回一个crc32值，类似MD5?反正应该是获取一个文件的标志
List files;
//检验安装文件是否发生了改变，如果是重新加载
if (!forceReload && !isModified(context, sourceApk, currentCrc)) {
//...
files = loadExistingExtractions(context, sourceApk, dexDir);
//...
} else {
files = performExtractions(sourceApk, dexDir);
putStoredApkInfo(context, getTimeStamp(sourceApk), currentCrc, files.size() + 1); //dex分包情况记录在sp,以便下次可以根据别配加载
}
return files;
}
private static boolean isModified(Context context, File archive, long currentCrc) {
SharedPreferences prefs = getMultiDexPreferences(context);//multidex.version
return (prefs.getLong(KEY_TIME_STAMP, NO_VALUE) != getTimeStamp(archive)) || (prefs.getLong(KEY_CRC, NO_VALUE) != currentCrc);
}
主要来看怎么读取DEX，获取apk文件的名字classesNdex的ZipEntry，写入到文件data/data/packageName/code_cache/secondary-dexes/base.apk.classesN.zip，N为dex的数量，2开始。因为Android系统在启动app时只加载了第一个Classes.dex，其他的DEX需要我们人工进行安装
private static List performExtractions(File sourceApk, File dexDir) throws IOException {
final String extractedFilePrefix = sourceApk.getName() + "classes"; //base.apk.classes
// 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); //删除非base.apk.classes为前缀的文件
List files = new ArrayList();
final ZipFile apk = new ZipFile(sourceApk); //data/app/packageName/base.apk
try {
int secondaryNumber = 2;
ZipEntry dexFile = apk.getEntry("classes" + secondaryNumber + "dex"); //获取ZipEntry
while (dexFile != null) {
String fileName = extractedFilePrefix + secondaryNumber + "zip"; //base.classes2.zip，往后便是base.classes3.dex、base.classes4.dex、base.classesN.dex
File extractedFile = new File(dexDir, fileName); //data/data/packageName/code_cache/secondary-dexes/base.classes2.zip
files.add(extractedFile);
int numAttempts = 0;
boolean isExtractionSuccessful = false;
while (numAttempts < 3 && !isExtractionSuccessful) { //最多3次尝试
numAttempts++;
// Create a zip file (extractedFile) containing only the secondary dex file (dexFile) from the apk.
extract(apk, dexFile, extractedFile, extractedFilePrefix); //ZipEntry写入到指定文件
isExtractionSuccessful = verifyZipFile(extractedFile); //是否是有效的zip文件
// Log the sha1 of the extracted zip file
if (!isExtractionSuccessful) {
// Delete the extracted file
extractedFile.delete();
//...
}
}
if (!isExtractionSuccessful) {
//...
}
secondaryNumber++;
dexFile = apk.getEntry(DEX_PREFIX + secondaryNumber + DEX_SUFFIX);
} //end while
} finally {
//..
}
return files;
}
删除data/data/packageName/code_cache/secondary-dexes/目录下所有非base.apk.classes开头的文件
/**
* This removes any files that do not have the correct prefix.
*/
private static void prepareDexDir(File dexDir, final String extractedFilePrefix) throws IOException {
/* mkdirs() has some bugs, especially before jb-mr1 and we have only a maximum of one parent
* to create, lets stick to mkdir().
*/
File cache = dexDir.getParentFile();
mkdirChecked(cache); //`data/data/packageName/code_cache/`
mkdirChecked(dexDir); //`data/data/packageName/code_cache/secondary-dexes/`
// Clean possible old files
FileFilter filter = new FileFilter() {
@Override
public boolean accept(File pathname) {
return !pathname.getName().startsWith(extractedFilePrefix); //过滤base.apk.classes前缀的文件
}
};
File[] files = dexDir.listFiles(filter);
if (files == null) {
return;
}
for (File oldFile : files) {
if (!oldFile.delete()) {
Log.w(TAG, "Failed to delete old file " + oldFile.getPath());
} else {
Log.i(TAG, "Deleted old file " + oldFile.getPath());
}
}
}
把ZipEntry写入文件，具体文件data/data/packageName/code_cache/secondary-dexes/base.apk.classesN.zip
/**
* apk ： apk的压缩包文件
* dexFile : Apk文件zip解压后得到的从dex文件，classes2.dex…classesN.dex
* extractTo : data/data/packageName/code_cache/secondary-dexes/base.apk.classesN.zip
* extractedFilePrefix : base.apk.classes
*/
private static void extract(ZipFile apk, ZipEntry dexFile, File extractTo, String extractedFilePrefix) {
InputStream in = apk.getInputStream(dexFile);
ZipOutputStream out = null;
File tmp = File.createTempFile(extractedFilePrefix, "zip", extractTo.getParentFile());
try {
out = new ZipOutputStream(new BufferedOutputStream(new FileOutputStream(tmp)));
try {
ZipEntry classesDex = new ZipEntry("classes.dex");
// keep zip entry time since it is the criteria used by Dalvik
classesDex.setTime(dexFile.getTime());
out.putNextEntry(classesDex);
byte[] buffer = new byte[BUFFER_SIZE];
int length = in.read(buffer);
while (length != -1) {
out.write(buffer, 0, length);
length = in.read(buffer);
}
out.closeEntry();
} finally {
out.close();
}
if (!tmp.renameTo(extractTo)) {
//...
}
} finally {
closeQuietly(in);
tmp.delete(); // return status ignored
}
}
参考
- MultiDex安装过程源码分析
- Android编译及Dex过程源码分析
- Android分包MultiDex原理详解
- 美团Android DEX自动拆包及动态加载简介