参考
https://juejin.im/post/5d95f4a4f265da5b8f10714b
MultiDex 原理
在进行源码分析之前，需要会使用才行，可以先看multidex的文档。
一般使用是调用MultiDex.install(Context context)，具体下面看下MultiDex的install 方法做了什么事
public static void install(Context context) { Log.i("MultiDex", "Installing application"); if (IS_VM_MULTIDEX_CAPABLE) { //5.0 以上VM基本支持多dex，啥事都不用干 Log.i("MultiDex", "VM has multidex support, MultiDex support library is disabled."); } else if (VERSION.SDK_INT < 4) { // throw new RuntimeException("MultiDex installation failed. SDK " + VERSION.SDK_INT + " is unsupported. Min SDK version is " + 4 + "."); } else { ... doInstallation(context, new File(applicationInfo.sourceDir), new File(applicationInfo.dataDir), "secondary-dexes", "", true); ... Log.i("MultiDex", "install done"); } }
从入口的判断来看，
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如果虚拟机本身就支持加载多个dex文件，那就啥都不用做；
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如果是不支持加载多个dex（5.0以下是不支持的），则走到 doInstallation 方法。
继续看doInstallation 方法
private static void doInstallation(Context mainContext, File sourceApk, File dataDir, String secondaryFolderName, String prefsKeyPrefix, boolean reinstallOnPatchRecoverableException) throws 各种Exception ... { synchronized(installedApk) { if (!installedApk.contains(sourceApk)) { installedApk.add(sourceApk); if (VERSION.SDK_INT > 20) { Log.w(...); } ClassLoader loader; try { loader = mainContext.getClassLoader(); } catch (RuntimeException var25) { Log.w("MultiDex", "Failure while trying to obtain Context class loader. Must be running in test mode. Skip patching.", var25); return; } if (loader == null) { Log.e("MultiDex", "Context class loader is null. Must be running in test mode. Skip patching."); } else { try { clearOldDexDir(mainContext); } catch (Throwable var24) { Log.w("MultiDex", "Something went wrong when trying to clear old MultiDex extraction, continuing without cleaning.", var24); } // 获取非主dex文件 File dexDir = getDexDir(mainContext, dataDir, secondaryFolderName); MultiDexExtractor extractor = new MultiDexExtractor(sourceApk, dexDir); IOException closeException = null; try { // 1. 这个load方法，第一次没有缓存，会非常耗时 List files = extractor.load(mainContext, prefsKeyPrefix, false); try { // 2. 安装dex installSecondaryDexes(loader, dexDir, files); } catch (IOException var26) { ... } } finally { try { extractor.close(); } catch (IOException var23) { closeException = var23; } } if (closeException != null) { throw closeException; } } } } }
先看注释1，MultiDexExtractor#load
List<? extends File> load(Context context, String prefsKeyPrefix, boolean forceReload) throws IOException { Log.i("MultiDex", "MultiDexExtractor.load(" + this.sourceApk.getPath() + ", " + forceReload + ", " + prefsKeyPrefix + ")"); if (!this.cacheLock.isValid()) { throw new IllegalStateException("MultiDexExtractor was closed"); } else { List files; if (!forceReload && !isModified(context, this.sourceApk, this.sourceCrc, prefsKeyPrefix)) { try { //读缓存的dex files = this.loadExistingExtractions(context, prefsKeyPrefix); } catch (IOException var6) { Log.w("MultiDex", "Failed to reload existing extracted secondary dex files, falling back to fresh extraction", var6); //读取缓存的dex失败，可能是损坏了，那就重新去解压apk读取，跟else代码块一样 files = this.performExtractions(); //保存标志位到sp，下次进来就走if了，不走else putStoredApkInfo(context, prefsKeyPrefix, getTimeStamp(this.sourceApk), this.sourceCrc, files); } } else { if (forceReload) { Log.i("MultiDex", "Forced extraction must be performed."); } else { Log.i("MultiDex", "Detected that extraction must be performed."); } //没有缓存，解压apk读取 files = this.performExtractions(); //保存dex信息到sp，下次进来就走if了，不走else putStoredApkInfo(context, prefsKeyPrefix, getTimeStamp(this.sourceApk), this.sourceCrc, files); } Log.i("MultiDex", "load found " + files.size() + " secondary dex files"); return files; } }
查找dex文件，有两个逻辑，有缓存就调用loadExistingExtractions方法，没有缓存或者缓存读取失败就调用performExtractions方法，然后再缓存起来。
既然用到缓存，那么performExtractions 方法想必应该是很耗时的，分析一下代码：
private List<MultiDexExtractor.ExtractedDex> performExtractions() throws IOException { //先确定命名格式 String extractedFilePrefix = this.sourceApk.getName() + ".classes"; this.clearDexDir(); List<MultiDexExtractor.ExtractedDex> files = new ArrayList(); ZipFile apk = new ZipFile(this.sourceApk); try { int secondaryNumber = 2; //apk已经是改为zip格式了，解压遍历zip文件，里面是dex文件， //名字有规律，如classes1.dex,class2.dex for(ZipEntry dexFile = apk.getEntry("classes" + secondaryNumber + ".dex"); dexFile != null; dexFile = apk.getEntry("classes" + secondaryNumber + ".dex")) { //文件名：xxx.classes1.zip String fileName = extractedFilePrefix + secondaryNumber + ".zip"; //创建这个classes1.zip文件 MultiDexExtractor.ExtractedDex extractedFile = new MultiDexExtractor.ExtractedDex(this.dexDir, fileName); //classes1.zip文件添加到list files.add(extractedFile); Log.i("MultiDex", "Extraction is needed for file " + extractedFile); int numAttempts = 0; boolean isExtractionSuccessful = false; while(numAttempts < 3 && !isExtractionSuccessful) { ++numAttempts; //这个方法是将classes1.dex文件写到压缩文件classes1.zip里去，最多重试三次 extract(apk, dexFile, extractedFile, extractedFilePrefix); ... } if (!isExtractionSuccessful) { throw new IOException("Could not create zip file " + extractedFile.getAbsolutePath() + " for secondary dex (" + secondaryNumber + ")"); } ++secondaryNumber; } } finally { try { apk.close(); } catch (IOException var17) { Log.w("MultiDex", "Failed to close resource", var17); } } return files; }
这里的逻辑就是解压apk，遍历出里面的dex文件，例如class1.dex，class2.dex，然后又压缩成class1.zip，class2.zip...，然后返回zip文件列表。
思考为什么这里要压缩呢？ 后面涉及到ClassLoader加载类原理的时候会分析ClassLoader支持的文件格式。
第一次加载才会执行解压和压缩过程，第二次进来读取sp中保存的dex信息，直接返回file list，所以第一次启动的时候比较耗时。
dex文件列表找到了，回到上面MultiDex#doInstallation方法的注释2，找到的dex文件列表，然后调用installSecondaryDexes方法进行安装，怎么安装呢？
private static void installSecondaryDexes(ClassLoader loader, File dexDir, List<? extends File> files) throws 各种Exception { 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); } else { MultiDex.V4.install(loader, files); } } }
看SDK 19 以上的实现
private static final class V19 { private V19() { } static void install(ClassLoader loader, List<? extends File> additionalClassPathEntries, File optimizedDirectory) throws 各种Exception { //1 反射ClassLoader 的 pathList 字段 Field pathListField = MultiDex.findField(loader, "pathList"); Object dexPathList = pathListField.get(loader); ArrayList<IOException> suppressedExceptions = new ArrayList(); // 2 扩展数组 MultiDex.expandFieldArray(dexPathList, "dexElements", makeDexElements(dexPathList, new ArrayList(additionalClassPathEntries), optimizedDirectory, suppressedExceptions)); ... } 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)); } }
上边方法的步骤：
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反射ClassLoader 的 pathList 字段
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找到pathList 字段对应的类的makeDexElements 方法
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通过MultiDex.expandFieldArray 这个方法扩展 dexElements 数组，怎么扩展？看下代码：
private static void expandFieldArray(Object instance, String fieldName, Object[] extraElements) throws NoSuchFieldException, IllegalArgumentException, IllegalAccessException { Field jlrField = findField(instance, fieldName); //取出原来的dexElements 数组 Object[] original = (Object[])((Object[])jlrField.get(instance)); //新的数组 Object[] combined = (Object[])((Object[])Array.newInstance(original.getClass().getComponentType(), original.length + extraElements.length)); //原来数组内容拷贝到新的数组 System.arraycopy(original, 0, combined, 0, original.length); //dex2、dex3...拷贝到新的数组 System.arraycopy(extraElements, 0, combined, original.length, extraElements.length); //将dexElements 重新赋值为新的数组 jlrField.set(instance, combined); }
就是创建一个新的数组，把原来数组内容（主dex）和要增加的内容（dex2、dex3...）拷贝进去，反射替换原来的dexElements为新的数组，如下图
看起来有点眼熟，Tinker热修复的原理也是通过反射将修复后的dex添加到这个dex数组去，不同的是热修复是添加到数组最前面，而MultiDex是添加到数组后面。