在app项目下新建multidex.keep文件，将指定类保存到此文件下

build.gradle中添加：（这个脚本将在编译项目的时候把multidex.keep 和 由Gradle生成的maindexlist.txt 结合在一起。）

android.applicationVariants.all { variant ->    task "fix${variant.name.capitalize()}MainDexClassList" << {        logger.info "Fixing main dex keep file for $variant.name"        File keepFile = new File("$buildDir/intermediates/multi-dex/$variant.buildType.name/maindexlist.txt")        keepFile.withWriterAppend { w ->            // Get a reader for the input file            w.append('\n')            new File("${projectDir}/multidex.keep").withReader { r ->                // And write data from the input into the output                w << r << '\n'            }            logger.info "Updated main dex keep file for ${keepFile.getAbsolutePath()}\n$keepFile.text"        }    }}tasks.whenTaskAdded { task ->    android.applicationVariants.all { variant ->        if (task.name == "create${variant.name.capitalize()}MainDexClassList") {            task.finalizedBy "fix${variant.name.capitalize()}MainDexClassList"        }    }}

附：

下面的方法是将App启动时查找加载除class.dex外的class文件：

• 在你认为app启动结束的地方运行下面util类中的getLoadedExternalDexClasses 

• 把上面这个方法返回的列表添加到你的 multidex.keep 文件然后重新编译。

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 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     */    public List
    
      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
     
       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     */    public List
      
        getExternalDexClasses(Context context) throws PackageManager.NameNotFoundException, IOException {        final List
       
         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
        
          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
         
           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 List
          
            getLoadedExternalDexClasses(Context context) { try { final List
           
             externalDexClasses = getExternalDexClasses(context); if (externalDexClasses != null && !externalDexClasses.isEmpty()) { final ArrayList
            
              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("$", ".class")); } } return classList; } } catch (Exception e) { e.printStackTrace(); } return null; }}