虽然Android 5.0及其以上系统允许加载多dex了，但是对于低于5.0版本的系统来说，由于Android系统最开始只允许加载一个dex，而一个dex的方法数又不能65535，所以程序在启动时会出现超出方法数错误。
为了解决这个问题，Google提出了MultiDex方案。但是Android的小伙伴们引用了这个方案后会发现，虽然MultiDex方案解决了低版本多dex问题，但是启动速度变慢了。坑爹了。
那么，我们如何优化它，解决启动速度慢的问题呢？
我在《MultiDex源码解析》这篇文章中讲解了MultiDex原理，我们会发现之所以引入MultiDex方案启动速度会变慢，是因为Dalvik虚拟机在加载非主dex（classes2.dex, classes3.dex....）文件时，需要解压apk，并压缩dex文件，转成odex文件。这些都是耗时操作。那么，如果我们将启动的所有类都加载到主dex（classes.dex）中，是不是就能加快启动速度了呢？
那么，就需要我们先找出哪些类是启动时需要，但是却不在主dex中的。我们可以使用以下代码找出这些类。
public class MultiDexUtils {
private static final String EXTRACTED_NAME_EXT = ".classes";
private static final String EXTRACTED_SUFFIX = ".zip";
private static final String SECONDARY_FOLDER_NAME = "code_cache" + File.separator + "secondary-dexes";
private static final String PREFS_FILE = "multidex.version";
private static final String KEY_DEX_NUMBER = "dex.number";
private static SharedPreferences getMultiDexPreferences(Context context) {
return context.getSharedPreferences(PREFS_FILE,
Build.VERSION.SDK_INT < Build.VERSION_CODES.HONEYCOMB
? Context.MODE_PRIVATE
: Context.MODE_PRIVATE | Context.MODE_MULTI_PROCESS);
}
/**
* get all the dex path
*
* @param context the application context
* @return all the dex path
* @throws PackageManager.NameNotFoundException
* @throws IOException
*/
private static List<String> getSourcePaths(Context context) throws
PackageManager.NameNotFoundException, IOException {
final ApplicationInfo applicationInfo = context.getPackageManager().getApplicationInfo(context.getPackageName(), 0);
final File sourceApk = new File(applicationInfo.sourceDir);
final File dexDir = new File(applicationInfo.dataDir, SECONDARY_FOLDER_NAME);
final List<String> sourcePaths = new ArrayList<>();
sourcePaths.add(applicationInfo.sourceDir); //add the default apk path
//the prefix of extracted file, ie: test.classes
final String extractedFilePrefix = sourceApk.getName() + EXTRACTED_NAME_EXT;
//the total dex numbers
final int totalDexNumber = getMultiDexPreferences(context).getInt(KEY_DEX_NUMBER, 1);
for (int secondaryNumber = 2; secondaryNumber <= totalDexNumber; secondaryNumber++) {
//for each dex file, ie: test.classes2.zip, test.classes3.zip...
final String fileName = extractedFilePrefix + secondaryNumber + EXTRACTED_SUFFIX;
final File extractedFile = new File(dexDir, fileName);
if (extractedFile.isFile()) {
sourcePaths.add(extractedFile.getAbsolutePath());
//we ignore the verify zip part
} else {
throw new IOException("Missing extracted secondary dex file '" +
extractedFile.getPath() + "'");
}
}
return sourcePaths;
}
/**
- get all the external classes name in "classes2.dex", "classes3.dex" ....
*
- @param context the application context
- @return all the classes name in the external dex
- @throws PackageManager.NameNotFoundException
- @throws IOException
*/
private static List<String> getExternalDexClasses(Context context) throws
PackageManager.NameNotFoundException, IOException {
final List<String> paths = getSourcePaths(context);
if(paths.size() <= 1) {
// no external dex
return null;
}
// the first element is the main dex, remove it.
paths.remove(0);
final List<String> classNames = new ArrayList<>();
for (String path : paths) {
try {
DexFile dexfile = null;
if (path.endsWith(EXTRACTED_SUFFIX)) {
//NOT use new DexFile(path), because it will throw "permission error in /data/dalvik-cache"
dexfile = DexFile.loadDex(path, path + ".tmp", 0);
} else {
dexfile = new DexFile(path);
}
final Enumeration<String> dexEntries = dexfile.entries();
while (dexEntries.hasMoreElements()) {
classNames.add(dexEntries.nextElement());
}
} catch (IOException e) {
throw new IOException("Error at loading dex file '" +
path + "'");
}
}
return classNames;
}
/**
- Get all loaded external classes name in "classes2.dex", "classes3.dex" ....
- @param context
- @return get all loaded external classes
*/
public static List<String> getLoadedExternalDexClasses(Context context) {
try {
final List<String> externalDexClasses = getExternalDexClasses(context);
if (externalDexClasses != null
&& !externalDexClasses.isEmpty()) {
final ArrayList<String> classList = new
ArrayList<>();
final java.lang.reflect.Method m = ClassLoader.class.getDeclaredMethod("findLoadedClass", new
Class[]{String.class});
m.setAccessible(true);
final ClassLoader cl = context.getClassLoader();
for (String clazz : externalDexClasses) {
if (m.invoke(cl, clazz) != null) {
classList.add(clazz.replaceAll("\\.", "/").replaceAll("#34;, ".class"));
}
}
return classList;
}
} catch (Exception e) {
e.printStackTrace();
}
return null;
}
}调用MultiDexUtils的静态方法getLoadedExternalDexClasses可以找出这些类。
然后，我们将这些类放到multidex-config.txt文件中。
并将这个文件配置到app目录下的build.gradle文件中。
multiDexKeepFile file('multidex-config.txt')这样，这些类就会打包到主dex中了。