Multidex記錄三：原始碼解析

MultiDex · 發表 2018-10-23 02:24:36

摘要：為什麼要用記錄呢，因為我從開始接觸Android時我們的專案就在65535的邊緣。不久Google就出了multidex的解決方案。我們也已經接入multidex好多年，但我自己還沒有接入，知其然而不知其所以然。出了問題只能自己找原始碼來分析。前兩篇文章 Multidex記錄一：介紹和使用 ...

為什麼要用記錄呢，因為我從開始接觸Android時我們的專案就在65535的邊緣。不久Google就出了multidex的解決方案。我們也已經接入multidex好多年，但我自己還沒有接入，知其然而不知其所以然。出了問題只能自己找原始碼來分析。前兩篇文章 ofollow,noindex">Multidex記錄一：介紹和使用和 Multidex記錄二：缺陷&解決分別講述了怎麼接入和接入時遇到的問題，本博文只是對multidex原始碼學習過程中的分析和理解的記錄。

關於 Multidex 的相關知識點前兩章已經講的差不多了，這篇文章只分析 Multidex 的安裝。

流程圖

原始碼分析

我們先來看看 MultiDex 的安裝日誌：

I/MultiDex: VM with version 1.6.0 does not have multidex support
Installing application
MultiDexExtractor.load(/data/app/com.xxx.xxx-1.apk, false, )
I/MultiDex: Blocking on lock /data/data/com.xxx.xxx/code_cache/secondary-dexes/MultiDex.lock
/data/data/com.xxx.xxx/code_cache/secondary-dexes/MultiDex.lock locked
Detected that extraction must be performed.
I/MultiDex: Extraction is needed for file /data/data/com.xxx.xxx/code_cache/secondary-dexes/com.xxx.xxx-1.apk.classes2.zip
Extracting /data/data/com.xxx.xxx/code_cache/secondary-dexes/tmp-com.xxx.xxx-1.apk.classes1415547735.zip
I/MultiDex: Renaming to /data/data/com.xxx.xxx/code_cache/secondary-dexes/com.xxx.xxx-1.apk.classes2.zip
Extraction succeeded - length /data/data/com.xxx.xxx/code_cache/secondary-dexes/com.xxx.xxx-1.apk.classes2.zip: 4238720 - crc: 2971858359
Extraction is needed for file /data/data/com.xxx.xxx/code_cache/secondary-dexes/com.xxx.xxx-1.apk.classes3.zip
Extracting /data/data/com.xxx.xxx/code_cache/secondary-dexes/tmp-com.xxx.xxx-1.apk.classes-1615165740.zip
I/MultiDex: Renaming to /data/data/com.xxx.xxx/code_cache/secondary-dexes/com.xxx.xxx-1.apk.classes3.zip
Extraction succeeded - length /data/data/com.xxx.xxx/code_cache/secondary-dexes/com.xxx.xxx-1.apk.classes3.zip: 3106018 - crc: 3138243730
Extraction is needed for file /data/data/com.xxx.xxx/code_cache/secondary-dexes/com.xxx.xxx-1.apk.classes4.zip
Extracting /data/data/com.xxx.xxx/code_cache/secondary-dexes/tmp-com.xxx.xxx-1.apk.classes-469912688.zip
I/MultiDex: Renaming to /data/data/com.xxx.xxx/code_cache/secondary-dexes/com.xxx.xxx-1.apk.classes4.zip
Extraction succeeded - length /data/data/com.xxx.xxx/code_cache/secondary-dexes/com.xxx.xxx-1.apk.classes4.zip: 2163715 - crc: 1148318293
load found 3 secondary dex files
I/MultiDex: install done

第二次啟動時的日誌：

I/MultiDex: VM with version 1.6.0 does not have multidex support
Installing application
MultiDexExtractor.load(/data/app/com.xxx.xxx-1.apk, false, )
Blocking on lock /data/data/com.xxx.xxx/code_cache/secondary-dexes/MultiDex.lock
/data/data/com.xxx.xxx/code_cache/secondary-dexes/MultiDex.lock locked
I/MultiDex: loading existing secondary dex files
load found 3 secondary dex files
install done

初始化資訊

public final class MultiDex {
static final String TAG = "MultiDex";
//老版本dex檔案存放路徑
private static final String OLD_SECONDARY_FOLDER_NAME = "secondary-dexes";
//dex檔案存放路徑 code_cache/secondary-dexes
private static final String SECONDARY_FOLDER_NAME;
//Multidex最高支援的版本，大於20Android系統已支援
private static final int MAX_SUPPORTED_SDK_VERSION = 20;
//Multidex最低支援的版本
private static final int MIN_SDK_VERSION = 4;
//vm的版本資訊
private static final int VM_WITH_MULTIDEX_VERSION_MAJOR = 2;
private static final int VM_WITH_MULTIDEX_VERSION_MINOR = 1;
//apk路徑
private static final Set<String> installedApk;
//是否支援Multidex
private static final boolean IS_VM_MULTIDEX_CAPABLE;

static {
//SECONDARY_FOLDER_NAME=code_cache/secondary-dexes
SECONDARY_FOLDER_NAME = "code_cache" + File.separator + "secondary-dexes";
installedApk = new HashSet();
//VM是否已經支援自動Multidex
IS_VM_MULTIDEX_CAPABLE = isVMMultidexCapable(System.getProperty("java.vm.version"));
}

Multidex安裝

public static void install(Context context) {
//VM是否已經支援自動Multidex
if (IS_VM_MULTIDEX_CAPABLE) {
Log.i("MultiDex", "VM has multidex support, MultiDex support library is disabled.");
} else if (VERSION.SDK_INT < 4) {
//Multidex最低支援的版本
throw new RuntimeException("Multi dex installation failed. SDK " + VERSION.SDK_INT + " is unsupported. Min SDK version is " + 4 + ".");
} else {
try {
/***部分程式碼省略***/
//多執行緒鎖
synchronized(installedApk) {
//apkPath = data/data/com.xxx.xxx/
String apkPath = applicationInfo.sourceDir;
if (installedApk.contains(apkPath)) {
return;
}
installedApk.add(apkPath);
/***部分程式碼省略***/
//清除 /data/data/com.xxx.xxx/files/secondary-dexes 目錄下的檔案
try {
clearOldDexDir(context);
} catch (Throwable var8) {
Log.w("MultiDex", "Something went wrong when trying to clear old MultiDex extraction, continuing without cleaning.", var8);
}
//data/data/com.xxx.xxx/code_cache/secondary-dexes
File dexDir = new File(applicationInfo.dataDir, SECONDARY_FOLDER_NAME);
//解壓apk，獲得dex的zip檔案列表
List<File> files = MultiDexExtractor.load(context, applicationInfo, dexDir, false);
//校驗zip檔案
if (checkValidZipFiles(files)) {
//安裝dex檔案
installSecondaryDexes(loader, dexDir, files);
} else {
//校驗失敗，重新執行解壓（解壓失敗直接丟擲異常）和安裝
Log.w("MultiDex", "Files were not valid zip files.Forcing a reload.");
files = MultiDexExtractor.load(context, applicationInfo, dexDir, true);
if (!checkValidZipFiles(files)) {
throw new RuntimeException("Zip files were not valid.");
}

installSecondaryDexes(loader, dexDir, files);
}
}
}
/***部分程式碼省略***/
}
}

Multidex獲取dex檔案

載入dex檔案

/**
 * @param context
 * @param applicationInfo
 * @param dexDir /data/data/com.xxx.xxx/code_cache/secondary-dexes/
 * @param forceReload 是否強制重新載入
 * @return 包含dex的zip檔案列表
 * @throws IOException
 *if an error occurs when writing the file.
 */
static List<File> load(Context context, ApplicationInfo applicationInfo, File dexDir, boolean forceReload) throws IOException {
//data/data/com.xxx.xxx/
File sourceApk = new File(applicationInfo.sourceDir);
//apk的迴圈冗餘校驗碼
long currentCrc = getZipCrc(sourceApk);
List files;
//是否強制執行reload和是否已經解壓過apk
if (!forceReload && !isModified(context, sourceApk, currentCrc)) {
try {
//是否已存在zip檔案
files = loadExistingExtractions(context, sourceApk, dexDir);
} catch (IOException var9) {
files = performExtractions(sourceApk, dexDir);
putStoredApkInfo(context, getTimeStamp(sourceApk), currentCrc, files.size() + 1);
}
} else {
//獲取apk中的classes2.dex並且壓縮為zip檔案
files = performExtractions(sourceApk, dexDir);
//儲存當前apk的資訊，作為下次有效快取的明證
putStoredApkInfo(context, getTimeStamp(sourceApk), currentCrc, files.size() + 1);
}
return files;
}
/**
 * @param context
 * @param timeStamp apk的最後一次修改時間
 * @param crc apk的迴圈冗餘校驗碼
 * @param totalDexNumber apk中一共有幾個dex
 */
private static void putStoredApkInfo(Context context, long timeStamp, long crc, int totalDexNumber) {
SharedPreferences prefs = getMultiDexPreferences(context);
Editor edit = prefs.edit();
edit.putLong("timestamp", timeStamp);
edit.putLong("crc", crc);
edit.putInt("dex.number", totalDexNumber);
apply(edit);
}

獲取已經存在的dex的壓縮包

/**
 * @param context
 * @param dexDir /data/data/com.xxx.xxx/code_cache/secondary-dexes/
 * @param sourceApk data/app/com.xxx.xxx-1.apk
 * @return 包含dex的zip檔案列表
 * @throws IOException
 *if an error occurs when writing the file.
 */
private static List<File> loadExistingExtractions(Context context, File sourceApk, File dexDir) throws IOException {
//extractedFilePrefix = com.xxx.xxx.apk.classes
String extractedFilePrefix = sourceApk.getName() + ".classes";
//totalDexNumber=apk中dex的數量
int totalDexNumber = getMultiDexPreferences(context).getInt("dex.number", 1);
List<File> files = new ArrayList(totalDexNumber);
//主dex已經載入過了，載入class2.dex,class3.dex......
for(int secondaryNumber = 2; secondaryNumber <= totalDexNumber; ++secondaryNumber) {
//fileName = com.xxx.xxx.apk.classes2.zip
String fileName = extractedFilePrefix + secondaryNumber + ".zip";
//extractedFile = data/data/com.xxx.xxx/code_cache/secondary-dexes/com.wuba.bangjob-1.apk.classes2.zip
File extractedFile = new File(dexDir, fileName);
if (!extractedFile.isFile()) {
throw new IOException("Missing extracted secondary dex file '" + extractedFile.getPath() + "'");
}

files.add(extractedFile);
//校驗zip檔案是否完整
if (!verifyZipFile(extractedFile)) {
Log.i("MultiDex", "Invalid zip file: " + extractedFile);
throw new IOException("Invalid ZIP file.");
}
}

return files;
}

生成dex的壓縮zip檔案

/**
 * @param sourceApk data/app/com.xxx.xxx-1.apk
 * @param dexDir /data/data/com.xxx.xxx/code_cache/secondary-dexes/
 * @return 包含dex的zip檔案列表
 * @throws IOException
 *if an error occurs when writing the file.
 */
private static List<File> performExtractions(File sourceApk, File dexDir) throws IOException {
//extractedFilePrefix = com.xxx.xxx.apk.classes
String extractedFilePrefix = sourceApk.getName() + ".classes";
//刪除data/data/com.xxx.xxx/code_cache/secondary-dexes/目錄下的檔案
prepareDexDir(dexDir, extractedFilePrefix);
List<File> files = new ArrayList();
ZipFile apk = new ZipFile(sourceApk);
try {
//從class2.dex開始
int secondaryNumber = 2;
for(ZipEntry dexFile = apk.getEntry("classes" + secondaryNumber + ".dex"); dexFile != null; dexFile = apk.getEntry("classes" + secondaryNumber + ".dex")) {
//fileName = com.xxx.xxx.apk.classes2.zip
String fileName = extractedFilePrefix + secondaryNumber + ".zip";
File extractedFile = new File(dexDir, fileName);
files.add(extractedFile);
//最多三次嘗試生成dex的zip檔案
int numAttempts = 0;
//標識是否生成dex的zip檔案
boolean isExtractionSuccessful = false;
while(numAttempts < 3 && !isExtractionSuccessful) {
++numAttempts;
extract(apk, dexFile, extractedFile, extractedFilePrefix);
//校驗壓縮檔案是否完整，否則刪除重來
isExtractionSuccessful = verifyZipFile(extractedFile);
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 (" + secondaryNumber + ")");
}

++secondaryNumber;
}
}/***部分程式碼省略***/
return files;
}

將classes2.dex放入zip檔案中

/**
 * @param apk apk的壓縮包
 * @param dexFile apk中的classes2.dex檔案
 * @param extractTo /data/data/com.xxx.xxx/code_cache/secondary-dexes/com.xxx.xxx.apk.classes2.zip
 * @param extractedFilePrefixcom.wuba.bangjob-1.apk.classes
 * @throws IOException
 *if an error occurs when writing the file.
 */
private static void extract(ZipFile apk, ZipEntry dexFile, File extractTo, String extractedFilePrefix) throws IOException, FileNotFoundException {
InputStream in = apk.getInputStream(dexFile);
ZipOutputStream out = null;
///data/data/com.wuba.bangjob/code_cache/secondary-dexes/com.xxx.xxx.apk.classes1415547735.zip
File tmp = File.createTempFile(extractedFilePrefix, ".zip", extractTo.getParentFile());
try {
out = new ZipOutputStream(new BufferedOutputStream(new FileOutputStream(tmp)));
try {
//將classes2.dex進行壓縮內容名稱為classes.dex
ZipEntry classesDex = new ZipEntry("classes.dex");
classesDex.setTime(dexFile.getTime());
out.putNextEntry(classesDex);
byte[] buffer = new byte[16384];
for(int length = in.read(buffer); length != -1; length = in.read(buffer)) {
out.write(buffer, 0, length);
}

out.closeEntry();
} finally {
out.close();
}
//重新命名檔案為com.xxx.xxx.apk.classes2.zip
if (!tmp.renameTo(extractTo)) {
throw new IOException("Failed to rename \"" + tmp.getAbsolutePath() + "\" to \"" + extractTo.getAbsolutePath() + "\"");
}
}/***部分程式碼省略***/
}

dex檔案的裝載

將含有載入含有dex的壓縮包進行夾雜，相關知識點參考： Android類載入之PathClassLoader和DexClassLoader 。

為什麼需要做版本的區分，就是因為版本見類載入的實現是有些差異的。

/**
 * @param loader
 * @param dexDir /data/data/xxx.xxx.xxx/code_cache/secondary-dexes/
 * @param files dex的壓縮zip檔案
 */
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) {//KitKat(19)
MultiDex.V19.install(loader, files, dexDir);
} else if (VERSION.SDK_INT >= 14) {//IceCreamSandwich(14,15),JellyBean(16,17,18)
MultiDex.V14.install(loader, files, dexDir);
} else {
MultiDex.V4.install(loader, files);
}
}
}

Android類載入之PathClassLoader和DexClassLoader 這篇文章將的是 VERSION.SDK_INT >= 19 那麼我們先看看第一個case：

private static final class V19 {
private V19() {
}
/**
* @param loader PathClassLoader
* @param additionalClassPathEntries dex壓縮包路徑
* @param optimizedDirectory opt之後的dex檔案目錄
*/
private static void install(ClassLoader loader, List<File> additionalClassPathEntries, File optimizedDirectory) throws IllegalArgumentException, IllegalAccessException, NoSuchFieldException, InvocationTargetException, NoSuchMethodException {
//PathClassLoader的DexPathList型別變數pathList
Field pathListField = MultiDex.findField(loader, "pathList");
Object dexPathList = pathListField.get(loader);
ArrayList<IOException> suppressedExceptions = new ArrayList();
//進行dex的opt併合並dex的Element
MultiDex.expandFieldArray(dexPathList, "dexElements", makeDexElements(dexPathList, new ArrayList(additionalClassPathEntries), optimizedDirectory, suppressedExceptions));
//處理異常
if (suppressedExceptions.size() > 0) {
Iterator i$ = suppressedExceptions.iterator();
while(i$.hasNext()) {
IOException e = (IOException)i$.next();
Log.w("MultiDex", "Exception in makeDexElement", e);
}
//一直感覺Google原始碼這塊有些問題，loader應該改為dexPathList。因為dexElementsSuppressedExceptions變數是屬於DexPathList的成員
Field suppressedExceptionsField = MultiDex.findField(loader, "dexElementsSuppressedExceptions");
IOException[] dexElementsSuppressedExceptions = (IOException[])((IOException[])suppressedExceptionsField.get(loader));
if (dexElementsSuppressedExceptions == null) {
dexElementsSuppressedExceptions = (IOException[])suppressedExceptions.toArray(new IOException[suppressedExceptions.size()]);
} else {
IOException[] combined = new IOException[suppressedExceptions.size() + dexElementsSuppressedExceptions.length];
suppressedExceptions.toArray(combined);
System.arraycopy(dexElementsSuppressedExceptions, 0, combined, suppressedExceptions.size(), dexElementsSuppressedExceptions.length);
dexElementsSuppressedExceptions = combined;
}
suppressedExceptionsField.set(loader, dexElementsSuppressedExceptions);
}

}
//呼叫DexPathList的makeDexElements方法進行dex的opt
private static Object[] makeDexElements(Object dexPathList, ArrayList<File> files, File optimizedDirectory, ArrayList<IOException> suppressedExceptions) throws IllegalAccessException, InvocationTargetException, NoSuchMethodException {
Method makeDexElements = MultiDex.findMethod(dexPathList, "makeDexElements", ArrayList.class, File.class, ArrayList.class);
return (Object[])((Object[])makeDexElements.invoke(dexPathList, files, optimizedDirectory, suppressedExceptions));
}
}

19 > VERSION.SDK_INT >= 14 的處理支援比 VERSION.SDK_INT >= 19 少了異常處理。是因為DexPathList的 makeDexElements 方法有了修改。

/**
 * VERSION.SDK_INT >= 19
 */
private static Element[] makeDexElements(ArrayList<File> files, File optimizedDirectory,
ArrayList<IOException> suppressedExceptions);

/**
 * 19 > VERSION.SDK_INT >= 14
 */
private static Element[] makeDexElements(ArrayList<File> files,File optimizedDirectory);

VERSION_SDK_INT < 14 的情況和前面的都不相同是因為 PathClassLoader 的是實現就不相同。。。。

/**
 * VERSION.SDK_INT == 13
 */
public class PathClassLoader extends ClassLoader {

private final String path;
private final String libPath;

/*
* Parallel arrays for jar/apk files.
*
* (could stuff these into an object and have a single array;
* improves clarity but adds overhead)
*/
private final String[] mPaths;
private final File[] mFiles;
private final ZipFile[] mZips;
private final DexFile[] mDexs;
/***部分程式碼省略***/
}

apk解壓結果預覽

文章到這裡就全部講述完啦，若有其他需要交流的可以留言哦~！~！

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