目的是為了解決65535問題，支持的SDK是4以上，低了會拋異常，Android5.0以上的虛擬機本來就可以支持Dex分包加載
主要原理：為應用的DexClassLoader動態(tài)地添加dex文件
流程分析
基本流程
1、校驗（Vm是否已經支持分包如21+，最低SDK版本是4，是否已經分包過了）
2、清理舊的的dex分包的目錄下文件，data/data/packageName/file/secondary-dexes
3、Dex包讀取，存放目錄data/data/packageName/code_cache/secondary-dexes
- 3.1 主要是讀取apk壓縮包下的的classes2.dex、classesN.dex依次寫入
/data/data/pkgName/code_cache/secondary-dexes/base.apk.classesN.zip下
4、校驗分包的dex壓縮包是否有效，無效再進行一次分包
5、Dex壓縮包文件安裝加載，通過DexPathList#makeDexElements的方法進行dex的加載，用返回的Element數(shù)組擴充原來ClassLoader下的Elements實現(xiàn)加載
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)) { //是否已經安裝了
return;
}
installedApk.add(apkPath);
if (Build.VERSION.SDK_INT > MAX_SUPPORTED_SDK_VERSION) {
//警告:高于20的可以使用內建的dex分包能力
}
/*
*/
ClassLoader loader;
try {
loader = context.getClassLoader();
} catch (RuntimeException e) {
// 測試MockContext
return;
}
if (loader == null) {
// Robolectric tests
return;
}
try {
clearOldDexDir(context); //清理應用內部文件存儲目錄（一般data/data/pkg-name/）下的secondary-dexes目錄
} catch (Throwable t) {
}
// data/data/packageName/code_cache/secondary-dexes
File dexDir = new File(applicationInfo.dataDir, SECONDARY_FOLDER_NAME);
List<File> files = MultiDexExtractor.load(context, applicationInfo, dexDir, false); //返回分包后的zip文件列表
if (checkValidZipFiles(files)) { //檢驗zip文件是否有效
installSecondaryDexes(loader, dexDir, files);
} else {
//如果第一失敗了，再進行一次相同的加載操作
}
}
} catch (Exception e) {
}
}
如何安裝
DexClassLoader在構造的時候就會讀取指定目錄下的zip、dex、jar等文件，加載成DexFile，并構造成Element數(shù)組，記錄在成員pathList下，以后類的加載都會嘗試在這些DexFile中尋找，而在dex分包后，就需要自己把"新的dex的文件路徑" 告訴DexClassLoader，這里以SDK19+為例子來說(對14,15,16,17and18來說區(qū)別在于DexPathList#makeDexElements方法簽名的改變，4到13的改變稍微有點大，但現(xiàn)在也不會開發(fā)14以下的了就不細看了)
private static void installSecondaryDexes(ClassLoader loader, File dexDir, List<File> 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的方法進行dex的加載，用返回的Element數(shù)組擴充原來ClassLoader下的Elements
private static final class V19 {
private static void install(ClassLoader loader, List<File> additionalClassPathEntries, File optimizedDirectory) {
Field pathListField = findField(loader, "pathList"); //loader#pathList字段，DexPathList類型
Object dexPathList = pathListField.get(loader);
ArrayList<IOException> suppressedExceptions = new ArrayList<IOException>();
expandFieldArray(dexPathList, "dexElements", makeDexElements(dexPathList, new ArrayList<File>(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}.
* 這個方法用來執(zhí)行DexPathList#makeDexElements的方法輸入需要加載的dex目錄，返回`Element`數(shù)組
*/
private static Object[] makeDexElements(
Object dexPathList, ArrayList<File> files, File optimizedDirectory, ArrayList<IOException> suppressedExceptions) {
Method makeDexElements = findMethod(dexPathList, "makeDexElements", ArrayList.class, File.class, ArrayList.class);
return (Object[]) makeDexElements.invoke(dexPathList, files, optimizedDirectory,suppressedExceptions);
}
}
Dex讀取
Dex的讀取在MultiDexExtractor#load方法進行
MultiDexExtractor.java
static List<File> 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); //返回一個crc32值，類似MD5?反正應該是獲取一個文件的標志
List<File> files;
//檢驗安裝文件是否發(fā)生了改變，如果是重新加載
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分包情況記錄在sp,以便下次可以根據(jù)別配加載
}
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);
}
主要來看怎么讀取DEX，獲取apk文件的名字classesNdex的ZipEntry，寫入到文件data/data/packageName/code_cache/secondary-dexes/base.apk.classesN.zip，N為dex的數(shù)量，2開始。因為Android系統(tǒng)在啟動app時只加載了第一個Classes.dex，其他的DEX需要我們人工進行安裝
private static List<File> 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); //刪除非base.apk.classes為前綴的文件
List<File> files = new ArrayList<File>();
final ZipFile apk = new ZipFile(sourceApk); //data/app/packageName/base.apk
try {
int secondaryNumber = 2;
ZipEntry dexFile = apk.getEntry("classes" + secondaryNumber + "dex"); //獲取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次嘗試
numAttempts++;
// Create a zip file (extractedFile) containing only the secondary dex file (dexFile) from the apk.
extract(apk, dexFile, extractedFile, extractedFilePrefix); //ZipEntry寫入到指定文件
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;
}
刪除data/data/packageName/code_cache/secondary-dexes/目錄下所有非base.apk.classes開頭的文件
/**
* 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); //過濾base.apk.classes前綴的文件
}
};
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寫入文件，具體文件data/data/packageName/code_cache/secondary-dexes/base.apk.classesN.zip
/**
* apk ： apk的壓縮包文件
* dexFile : Apk文件zip解壓后得到的從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
}
}
