基于MuiltDex可以实现Hot Patch在不重新发布app的情况下，采用补丁的方式升级应用，用来快速解决一些线上的问题
google官方提供了MuiltDex的支持 compile 'com.android.support:multidex:1.0.0'
github地址：https://github.com/casidiablo/multidex
MuiltDex的基本原理是在打包的时候，将dex文件分拆成多个，并且在程序启动的时候动态加载，具体的使用方法可以参考 http://developer.android.com/intl/zh-cn/tools/building/multidex.html
下面是多dex加载的时序图：
1 public class MultiDexApplication extends Application {
2 @Override
3 protected void attachBaseContext(Context base) {
4 super.attachBaseContext(base);
5 MultiDex.install(this);
6 }
7 }
在使用MuiltDex的时候，需要继承MultiDexApplication，可以看到在attachBaseContext回调中，调用了MultiDex.install，在android中Application、Activity、Service都是继承自Context的，在attachBaseContext之后，Context才创建完成，才能只用Context。
MultiDex.install主要做了以下几个事情 * 检查虚拟机是否原生支持MultiDex，如果支持，支持返回：具体的判断是System.getProperty("java.vm.version").通过虚拟机的版本号，大于2.1表示原生支持。 * SDK版本小于4，不支持，直接返回。 * SDK版本大于20，抛个警告 * 核心处理逻辑如下：通过MultiDexExtractor.load获取FIles列表，通过installSecondaryDexes处理获取的ClassLoader和Files
1 File dexDir = new File(applicationInfo.dataDir, SECONDARY_FOLDER_NAME);
2 List<File> files = MultiDexExtractor.load(context, applicationInfo, dexDir, false);
3 if (checkValidZipFiles(files)) {
4 installSecondaryDexes(loader, dexDir, files);
5 } else {
6 Log.w(TAG, "Files were not valid zip files. Forcing a reload.");
7 // Try again, but this time force a reload of the zip file.
8 files = MultiDexExtractor.load(context, applicationInfo, dexDir, true);
9
10 if (checkValidZipFiles(files)) {
11 installSecondaryDexes(loader, dexDir, files);
12 } else {
13 // Second time didn't work, give up
14 throw new RuntimeException("Zip files were not valid.");
15 }
16 }MultiDexExtractor.load的逻辑如下：
1)loadExistingExtractions：读取已经解压出来的文件
2)performExtractions：解压文件
1 static List<File> load(Context context, ApplicationInfo applicationInfo, File dexDir, boolean forceReload) throws IOException {
2 Log.i(TAG, "MultiDexExtractor.load(" + applicationInfo.sourceDir + ", " + forceReload + ")");
3 final File sourceApk = new File(applicationInfo.sourceDir);
4
5 long currentCrc = getZipCrc(sourceApk);
6
7 List<File> files;
8 if (!forceReload && !isModified(context, sourceApk, currentCrc)) {
9 try {
10 files = loadExistingExtractions(context, sourceApk, dexDir);
11 } catch (IOException ioe) {
12 Log.w(TAG, "Failed to reload existing extracted secondary dex files,"
13 + " falling back to fresh extraction", ioe);
14 files = performExtractions(sourceApk, dexDir);
15 putStoredApkInfo(context, getTimeStamp(sourceApk), currentCrc, files.size() + 1);
16
17 }
18 } else {
19 Log.i(TAG, "Detected that extraction must be performed.");
20 files = performExtractions(sourceApk, dexDir);
21 putStoredApkInfo(context, getTimeStamp(sourceApk), currentCrc, files.size() + 1);
22 }
23
24 Log.i(TAG, "load found " + files.size() + " secondary dex files");
25 return files;
26 }installSecondaryDexes的主要逻辑如下：主要是根据不同的sdk版本来处理，主要是通过反射来替换ClassLoader的path或者pathList
1 private static void installSecondaryDexes(ClassLoader loader, File dexDir, List<File> files) throws IllegalArgumentException, IllegalAccessException, NoSuchFieldException, InvocationTargetException, NoSuchMethodException,IOException {
2 if (!files.isEmpty()) {
3 if (Build.VERSION.SDK_INT >= 19) {
4 V19.install(loader, files, dexDir);
5 } else if (Build.VERSION.SDK_INT >= 14) {
6 V14.install(loader, files, dexDir);
7 } else {
8 V4.install(loader, files);
9 }
10 }
11 }
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