MultiDex的具體實(shí)現(xiàn)過(guò)程

一、代碼示例

1氏仗、Gradle修改

修改Gradle的配置吉捶，支持multidex:

android {
    compileSdkVersion 21
    buildToolsVersion "21.1.0"
    defaultConfig {
        ...
        minSdkVersion 14
        targetSdkVersion 21
        ...
        // Enabling multidex support.
        multiDexEnabled true
    }
    ...
}
dependencies {
  compile 'com.android.support:multidex:1.0.0'
}

2、manifest修改

在manifest文件中廓鞠，在application標(biāo)簽下添加MultidexApplication Class的引用

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.android.multidex.myapplication">
    <application
        ...
        android:name="android.support.multidex.MultiDexApplication">
        ...
    </application>
</manifest>

二帚稠、實(shí)現(xiàn)源碼

1、MultiDexApplication

public class MultiDexApplication extends Application {
    public MultiDexApplication() {
    }

    protected void attachBaseContext(Context base) {
        super.attachBaseContext(base);
        MultiDex.install(this);
    }
}

使用 MultiDexApplication的原因就是它重寫(xiě)了Application床佳，主要是可以將其他dex文件注入到系統(tǒng)的ClassLoader

2、MultiDex

MultiDex.install榄审，這個(gè)方法涵蓋了MultiDex安裝的整個(gè)流程：

publicstaticvoidinstall(Context context){
     Log.i(TAG, "install");

     // 檢查當(dāng)前系統(tǒng)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對(duì)應(yīng)于/data/app/<package-name>.apk
             String apkPath = applicationInfo.sourceDir;

             // 若給定apk已經(jīng)install過(guò)砌们，直接退出
             if (installedApk.contains(apkPath)) {
                 return;
             }
             installedApk.add(apkPath);


             // MultiDex 最高只支持到20(Android 4.4W)，更高的版本不能保證正常工作

             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") + "\"");
             }

            // 得到classLoader搁进，它就是PathClassLoader是BaseDexClassLoaderd的子類浪感，
            // MultiDex主要通過(guò)修改pathList字段來(lái)添加更多的dex
            // 后面就可以從這個(gè)PathClassLoader中拿到DexPathList中的Element數(shù)組
            // 這個(gè)數(shù)組里面就包括由系統(tǒng)加載第一個(gè)dex包
             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的二級(jí)dex文件將存放在 /data/data/<package-name>/secondary-dexes 下

             File dexDir = new File(context.getFilesDir(), SECONDARY_FOLDER_NAME);

             // 從apk中查找并解壓二級(jí)dex文件到/data/data/<package-name>/secondary-dexes
             List<File> files = MultiDexExtractor.load(context, applicationInfo, dexDir, false);

             // 檢查dex壓縮文件的完整性
             if (checkValidZipFiles(files)) {

                 // 開(kāi)始安裝dex
                 installSecondaryDexes(loader, dexDir, files);
             } else {
                 Log.w(TAG, "Files were not valid zip files. Forcing a reload.");

               /** 
               第一次檢查失敗，MultiDex會(huì)盡責(zé)的再檢查一次
               若第一次校驗(yàn)失敱省（dex文件損壞等）影兽，
               MultiDex會(huì)重新調(diào)用MultiDexExtractor.load方法重查找加載二級(jí)dex文件列表，
               值得注意的是此時(shí)forceReload的值為true莱革，會(huì)強(qiáng)制重新從apk中解壓dex文件
               */
                 files = MultiDexExtractor.load(context, applicationInfo, dexDir, true);

                 if (checkValidZipFiles(files)) {

                     // 開(kāi)始安裝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");
 }

1峻堰、檢查虛擬機(jī)版本

跟虛擬機(jī)版本判斷是否需要MultiDex;

在ART虛擬機(jī)中（部分4.4機(jī)器及5.0以上的機(jī)器）讹开，采用了Ahead-of-time（AOT）compilation技術(shù)，系統(tǒng)在apk的安裝過(guò)程中捐名，會(huì)使用自帶的dex2oat工具對(duì)apk中可用的dex文件進(jìn)行編譯旦万，并生成一個(gè)可在本地機(jī)器上運(yùn)行的odex(optimized dex)文件，這樣做會(huì)提高應(yīng)用的啟動(dòng)速度镶蹋。（但是安裝速度降低了）

若不需要使用MultiDex成艘，將使用clearOldDexDir清除/data/data/pkgName/code-cache/secondary-dexes目錄下下所有文件

2、獲取安裝路徑

根據(jù)applicationInfo.sourceDir的值獲取安裝的apk路徑贺归；安裝完成的apk路徑為 /data/app/.apk

3淆两、安裝檢測(cè)

檢查apk是否執(zhí)行過(guò)MultiDex.install，若已經(jīng)安裝直接退出

4拂酣、獲取二級(jí)dex列表

MultiDexExtractor.load獲取apk中二級(jí)dex列表

5琼腔、校驗(yàn)dex壓縮包的完整性

6、開(kāi)始dex的安裝

installSecondaryDexes 安裝dex

// loader對(duì)應(yīng)的就是PathClassLoader
// dexDir是dex的存放目錄
// files對(duì)應(yīng)的就是dex文件
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版本不同踱葛，ClassLoader中的實(shí)現(xiàn)存在差異丹莲，所以使用了三個(gè)分支去執(zhí)行dex的安裝。這里我們選擇MultiDex.V14.install進(jìn)行分析尸诽，其他兩個(gè)大同小異：

V14.install

private static void install(ClassLoader loader, List<File> additionalClassPathEntries, File optimizedDirectory) throws IllegalArgumentException, IllegalAccessException, NoSuchFieldException, InvocationTargetException, NoSuchMethodException {
    // 這個(gè)方法就是使用反射來(lái)得到loader的pathList字段
    Field pathListField = MultiDex.findField(loader, "pathList");
    // 得到loader的pathList字段后甥材，我們就可以得到這個(gè)字段的值，也就是DexPathList對(duì)象
    Object dexPathList = pathListField.get(loader);
    // 這個(gè)方法就是將其他的dex文件Element數(shù)組和第一個(gè)dex的Element數(shù)組合并
    // makeDexElements方法就是用來(lái)得到其他dex的Elements數(shù)組
    MultiDex.expandFieldArray(dexPathList, "dexElements", makeDexElements(dexPathList, new ArrayList(additionalClassPathEntries), optimizedDirectory));
}

MultiDex在安裝開(kāi)始時(shí)性含，會(huì)先通過(guò)反射調(diào)用 BaseDexClassLoader 里DexPathList類型的pathList字段洲赵，接著通過(guò)pathList調(diào)用 DexPathList 的makeDexElements方法，將上面解壓得到的additionalClassPathEntries（二級(jí)dex文件列表）封裝成Element數(shù)組商蕴。

需要注意的是叠萍，makeDexElements最終會(huì)去進(jìn)行dex2opt操作，這是一個(gè)比較耗時(shí)的過(guò)程绪商，如果全部放在main線程去處理的話苛谷，比較影響用戶體驗(yàn)，甚至可能引起ANR格郁。

7腹殿、合并數(shù)組

調(diào)用 MultiDex.expandFieldArray 合并dex 數(shù)組

// instance對(duì)應(yīng)的就是pathList對(duì)象
// fieldName 對(duì)應(yīng)的就是字段名，我們要得到的就是pathList對(duì)象里面的dexElements數(shù)組
// extraElements對(duì)應(yīng)的就是其他dex對(duì)應(yīng)的Element數(shù)組
private static void expandFieldArray(Object instance, String fieldName, Object[] extraElements) throws NoSuchFieldException, IllegalArgumentException, IllegalAccessException {
    // 得到Element數(shù)組字段
    Field jlrField = findField(instance, fieldName);
    // 得到pathList對(duì)象里面的dexElements數(shù)組
    Object[] original = (Object[])((Object[])jlrField.get(instance));
    // 創(chuàng)建一個(gè)新的數(shù)組用來(lái)存放合并之后的結(jié)果
    Object[] combined = (Object[])((Object[])Array.newInstance(original.getClass().getComponentType(), original.length + extraElements.length));
    // 將第一個(gè)dex的Elements數(shù)組復(fù)制到創(chuàng)建的數(shù)組中去
    System.arraycopy(original, 0, combined, 0, original.length);
    // 將其他dex的Elements數(shù)組復(fù)制到創(chuàng)建的數(shù)組中去
    System.arraycopy(extraElements, 0, combined, original.length, extraElements.length);
    // 將得到的這個(gè)合并的新數(shù)組的值設(shè)置到pathList對(duì)象的Element數(shù)組字段上
    jlrField.set(instance, combined);
}

expandFieldArray同樣是通過(guò)反射調(diào)用例书，找到pathList中的dexElements字段锣尉，并將上一步生成的封裝了二級(jí)dex的Element數(shù)組添加到dexElements之后，完成整個(gè)安裝流程

8决采、其他涉及的方法

MultiDexExtractor.load(……獲取apk中可用的二級(jí)dex列表

/**
MultiDexExtractor.load會(huì)先判斷是否需要從apk中解壓dex文件自沧，主要判斷依據(jù)是：上次保存的apk（zip文件）的CRC校驗(yàn)碼和last modify日期與dex的總數(shù)量是否與當(dāng)前apk相同。此外树瞭，forceReload也會(huì)決定是否需要重新解壓
*/
private static List<File> load(Context context, ApplicationInfo applicationInfo, File dexDir,booleanforceReload)throwsIOException{

    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;
}

解壓apk

private static List<File> performExtractions(File sourceApk, File dexDir)throwsIOException{

        // extractedFilePrefix值為<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;

            // 掃描apk內(nèi)所有classes2.dex拇厢、classes3.dex...文件
            for(ZipEntry dexFile = apk.getEntry("classes" + e + ".dex"); dexFile != null; dexFile = apk.getEntry("classes" + e + ".dex")) {

                // 解壓路徑為 /data/data/<package-name>/secondary-dexes/<package-name>.classes2.dex.zip 爱谁、/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;

                // 每個(gè)dex文件都會(huì)嘗試3次解壓
                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;
    }

解壓成功后，會(huì)保存本次解壓所使用的apk信息旺嬉，用于下次調(diào)用MultiDexExtractor.load時(shí)判斷是否需要重新解壓：

private static void putStoredApkInfo(Context context,longtimeStamp,longcrc,inttotalDexNumber){
    SharedPreferences prefs = getMultiDexPreferences(context);
    Editor edit = prefs.edit();

    // apk最后修改時(shí)間戳
    edit.putLong("timestamp", timeStamp);

    // apk的CRC校驗(yàn)碼
    edit.putLong("crc", crc);

    // dex的總數(shù)量
    edit.putInt("dex.number", totalDexNumber);
    apply(edit);
}

如果apk未被修改管行，將會(huì)調(diào)用loadExistingExtractions方法，直接加載上一次解壓出來(lái)的文件

privatestaticList<File>loadExistingExtractions(Context context, File sourceApk, File dexDir)throwsIOException{
        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;
}