Android插件化（一）：使用改进的MultiDex动态加载assets中的apk
简介
为了解决65535方法数超标的问题，Google推荐使用MultiDex来加载classes2.dex,classes3.dex等等，其基本思想就是在运行时动态修改ClassLoader，以达到动态加载类的目的。为了更好的理解MultiDex的工作原理，可以先看一下ClassLoader的工作原理[1].然后参见PathClassLoader的源码，当然，它继承自BaseDexClassLoader，主要源码都在BaseDexClassLoader中。MultiDex加载离线apk的过程如下： 
我们可以在Application的onCreate方法或者Activity的attachBaseContext方法中开发加载。
动态加载assets中的apk
根据MultiDex的源码，我们可以修改其install方法，然后从assets资源中解压出所需要加载的apk文件，然后调用installSecondaryDexes方法，将其加载到当前Application的ClassLoader当中，这样，在运行的时候，就可以通过当前的ClassLoader查找到离线apk中的类了。
[AssetsMultiDexLoader.class]
package net.mobctrl.hostapk;
import java.io.File;
import java.io.FilenameFilter;
import java.io.IOException;
import java.lang.reflect.Array;
import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.HashSet;
import java.util.List;
import java.util.ListIterator;
import java.util.Set;
import java.util.zip.ZipFile;
import android.content.Context;
import android.content.pm.ApplicationInfo;
import android.content.pm.PackageManager;
import android.content.pm.PackageManager.NameNotFoundException;
import android.os.Build;
import android.util.Log;
import dalvik.system.DexFile;
/**
* @Author Zheng Haibo
* @Mail mochuan.zhb@alibaba-inc.com
* @Company Alibaba Group
* @PersonalWebsite http://www.mobctrl.net
* @version $Id: AssetsDex.java, v 0.1 2015年12月10日 下午5:36:23 mochuan.zhb Exp $
* @Description ClassLoader
*/
public class AssetsMultiDexLoader {
private static final String TAG = "AssetsApkLoader";
private static boolean installed = false;
private static final int MAX_SUPPORTED_SDK_VERSION = 20;
private static final int MIN_SDK_VERSION = 4;
private static final Set<String> installedApk = new HashSet<String>();
private AssetsMultiDexLoader() {
}
/**
* 安装Assets中的apk文件
*
* @param context
*/
public static void install(Context context) {
Log.i(TAG, "install...");
if (installed) {
return;
}
try {
clearOldDexDir(context);
} catch (Throwable t) {
Log.w(TAG,
"Something went wrong when trying to clear old MultiDex extraction, "
+ "continuing without cleaning.", t);
}
AssetsManager.copyAllAssetsApk(context);
Log.i(TAG, "install");
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) {
String apkPath = applicationInfo.sourceDir;
if (installedApk.contains(apkPath)) {
return;
}
installedApk.add(apkPath);
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")
+ "\"");
}
/*
* The patched class loader is expected to be a descendant of
* dalvik.system.BaseDexClassLoader. We modify its
* dalvik.system.DexPathList pathList field to append additional
* DEX file entries.
*/
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;
}
// 获取dex文件列表
File dexDir = context.getDir(AssetsManager.APK_DIR, Context.MODE_PRIVATE);
File[] szFiles = dexDir.listFiles(new FilenameFilter() {
@Override
public boolean accept(File dir, String filename) {
return filename.endsWith(AssetsManager.FILE_FILTER);
}
});
List<File> files = new ArrayList<File>();
for (File f : szFiles) {
Log.i(TAG, "load file:" + f.getName());
files.add(f);
}
Log.i(TAG, "loader before:" + context.getClassLoader());
installSecondaryDexes(loader, dexDir, files);
Log.i(TAG, "loader end:" + context.getClassLoader());
}
} catch (Exception e) {
Log.e(TAG, "Multidex installation failure", e);
throw new RuntimeException("Multi dex installation failed ("
+ e.getMessage() + ").");
}
installed = true;
Log.i(TAG, "install done");
}
private static ApplicationInfo getApplicationInfo(Context context)
throws NameNotFoundException {
PackageManager pm;
String packageName;
try {
pm = context.getPackageManager();
packageName = context.getPackageName();
} 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 ApplicationInfo from Context. "
+ "Must be running in test mode. Skip patching.", e);
return null;
}
if (pm == null || packageName == null) {
// This is most likely a mock context, so just return without
// patching.
return null;
}
ApplicationInfo applicationInfo = pm.getApplicationInfo(packageName,
PackageManager.GET_META_DATA);
return applicationInfo;
}
private static void installSecondaryDexes(ClassLoader loader, File dexDir,
List<File> files) throws IllegalArgumentException,
IllegalAccessException, NoSuchFieldException,
InvocationTargetException, NoSuchMethodException, IOException {
if (!files.isEmpty()) {
if (Build.VERSION.SDK_INT >= 19) {
V19.install(loader, files, dexDir);
} else if (Build.VERSION.SDK_INT >= 14) {
V14.install(loader, files, dexDir);
} else {
V4.install(loader, files);
}
}
}
/**
* Locates a given field anywhere in the class inheritance hierarchy.
*
* @param instance
* an object to search the field into.
* @param name
* field name
* @return a field object
* @throws NoSuchFieldException
* if the field cannot be located
*/
private static Field findField(Object instance, String name)
throws NoSuchFieldException {
for (Class<?> clazz = instance.getClass(); clazz != null; clazz = clazz
.getSuperclass()) {
try {
Field field = clazz.getDeclaredField(name);
if (!field.isAccessible()) {
field.setAccessible(true);
}
return field;
} catch (NoSuchFieldException e) {
// ignore and search next
}
}
throw new NoSuchFieldException("Field " + name + " not found in "
+ instance.getClass());
}
/**
* Locates a given method anywhere in the class inheritance hierarchy.
*
* @param instance
* an object to search the method into.
* @param name
* method name
* @param parameterTypes
* method parameter types
* @return a method object
* @throws NoSuchMethodException
* if the method cannot be located
*/
private static Method findMethod(Object instance, String name,
Class<?>... parameterTypes) throws NoSuchMethodException {
for (Class<?> clazz = instance.getClass(); clazz != null; clazz = clazz
.getSuperclass()) {
try {
Method method = clazz.getDeclaredMethod(name, parameterTypes);
if (!method.isAccessible()) {
method.setAccessible(true);
}
return method;
} catch (NoSuchMethodException e) {
// ignore and search next
}
}
throw new NoSuchMethodException("Method " + name + " with parameters "
+ Arrays.asList(parameterTypes) + " not found in "
+ instance.getClass());
}
/**
* Replace the value of a field containing a non null array, by a new array
* containing the elements of the original array plus the elements of
* extraElements.
*
* @param instance
* the instance whose field is to be modified.
* @param fieldName
* the field to modify.
* @param extraElements
* elements to append at the end of the array.
*/
private static void expandFieldArray(Object instance, String fieldName,
Object[] extraElements) throws NoSuchFieldException,
IllegalArgumentException, IllegalAccessException {
Field jlrField = findField(instance, fieldName);
Object[] original = (Object[]) jlrField.get(instance);
Object[] combined = (Object[]) Array.newInstance(original.getClass()
.getComponentType(), original.length + extraElements.length);
System.arraycopy(original, 0, combined, 0, original.length);
System.arraycopy(extraElements, 0, combined, original.length,
extraElements.length);
jlrField.set(instance, combined);
}
private static void clearOldDexDir(Context context) throws Exception {
File dexDir = context.getDir(AssetsManager.APK_DIR, Context.MODE_PRIVATE);
if (dexDir.isDirectory()) {
Log.i(TAG, "Clearing old secondary dex dir (" + dexDir.getPath()
+ ").");
File[] files = dexDir.listFiles();
if (files == null) {
Log.w(TAG, "Failed to list secondary dex dir content ("
+ dexDir.getPath() + ").");
return;
}
for (File oldFile : files) {
Log.i(TAG, "Trying to delete old file " + oldFile.getPath()
+ " of size " + oldFile.length());
if (!oldFile.delete()) {
Log.w(TAG, "Failed to delete old file " + oldFile.getPath());
} else {
Log.i(TAG, "Deleted old file " + oldFile.getPath());
}
}
if (!dexDir.delete()) {
Log.w(TAG,
"Failed to delete secondary dex dir "
+ dexDir.getPath());
} else {
Log.i(TAG, "Deleted old secondary dex dir " + dexDir.getPath());
}
}
}
/**
* Installer for platform versions 19.
*/
private static final class V19 {
private static void install(ClassLoader loader,
List<File> additionalClassPathEntries, File optimizedDirectory)
throws IllegalArgumentException, IllegalAccessException,
NoSuchFieldException, InvocationTargetException,
NoSuchMethodException {
/*
* The patched class loader is expected to be a descendant of
* dalvik.system.BaseDexClassLoader. We modify its
* dalvik.system.DexPathList pathList field to append additional DEX
* file entries.
*/
Field pathListField = findField(loader, "pathList");
Object dexPathList = pathListField.get(loader);
ArrayList<IOException> suppressedExceptions = new ArrayList<IOException>();
expandFieldArray(
dexPathList,
"dexElements",
makeDexElements(dexPathList, new ArrayList<File>(
additionalClassPathEntries), optimizedDirectory,
suppressedExceptions));
if (suppressedExceptions.size() > 0) {
for (IOException e : suppressedExceptions) {
Log.w(TAG, "Exception in makeDexElement", e);
}
Field suppressedExceptionsField = findField(loader,
"dexElementsSuppressedExceptions");
IOException[] dexElementsSuppressedExceptions = (IOException[]) suppressedExceptionsField
.get(loader);
if (dexElementsSuppressedExceptions == null) {
dexElementsSuppressedExceptions = suppressedExceptions
.toArray(new IOException[suppressedExceptions
.size()]);
} else {
IOException[] combined = new IOException[suppressedExceptions
.size() + dexElementsSuppressedExceptions.length];
suppressedExceptions.toArray(combined);
System.arraycopy(dexElementsSuppressedExceptions, 0,
combined, suppressedExceptions.size(),
dexElementsSuppressedExceptions.length);
dexElementsSuppressedExceptions = combined;
}
suppressedExceptionsField.set(loader,
dexElementsSuppressedExceptions);
}
}
/**
* A wrapper around
* {@code private static final dalvik.system.DexPathList#makeDexElements}
* .
*/
private static Object[] makeDexElements(Object dexPathList,
ArrayList<File> files, File optimizedDirectory,
ArrayList<IOException> suppressedExceptions)
throws IllegalAccessException, InvocationTargetException,
NoSuchMethodException {
Method makeDexElements = findMethod(dexPathList, "makeDexElements",
ArrayList.class, File.class, ArrayList.class);
return (Object[]) makeDexElements.invoke(dexPathList, files,
optimizedDirectory, suppressedExceptions);
}
}
/**
* Installer for platform versions 14, 15, 16, 17 and 18.
*/
private static final class V14 {
private static void install(ClassLoader loader,
List<File> additionalClassPathEntries, File optimizedDirectory)
throws IllegalArgumentException, IllegalAccessException,
NoSuchFieldException, InvocationTargetException,
NoSuchMethodException {
/*
* The patched class loader is expected to be a descendant of
* dalvik.system.BaseDexClassLoader. We modify its
* dalvik.system.DexPathList pathList field to append additional DEX
* file entries.
*/
Field pathListField = findField(loader, "pathList");
Object dexPathList = pathListField.get(loader);
expandFieldArray(
dexPathList,
"dexElements",
makeDexElements(dexPathList, new ArrayList<File>(
additionalClassPathEntries), optimizedDirectory));
}
/**
* A wrapper around
* {@code private static final dalvik.system.DexPathList#makeDexElements}
* .
*/
private static Object[] makeDexElements(Object dexPathList,
ArrayList<File> files, File optimizedDirectory)
throws IllegalAccessException, InvocationTargetException,
NoSuchMethodException {
Method makeDexElements = findMethod(dexPathList, "makeDexElements",
ArrayList.class, File.class);
return (Object[]) makeDexElements.invoke(dexPathList, files,
optimizedDirectory);
}
}
/**
* Installer for platform versions 4 to 13.
*/
private static final class V4 {
private static void install(ClassLoader loader,
List<File> additionalClassPathEntries)
throws IllegalArgumentException, IllegalAccessException,
NoSuchFieldException, IOException {
/*
* The patched class loader is expected to be a descendant of
* dalvik.system.DexClassLoader. We modify its fields mPaths,
* mFiles, mZips and mDexs to append additional DEX file entries.
*/
int extraSize = additionalClassPathEntries.size();
Field pathField = findField(loader, "path");
StringBuilder path = new StringBuilder(
(String) pathField.get(loader));
String[] extraPaths = new String[extraSize];
File[] extraFiles = new File[extraSize];
ZipFile[] extraZips = new ZipFile[extraSize];
DexFile[] extraDexs = new DexFile[extraSize];
for (ListIterator<File> iterator = additionalClassPathEntries
.listIterator(); iterator.hasNext();) {
File additionalEntry = iterator.next();
String entryPath = additionalEntry.getAbsolutePath();
path.append(':').append(entryPath);
int index = iterator.previousIndex();
extraPaths[index] = entryPath;
extraFiles[index] = additionalEntry;
extraZips[index] = new ZipFile(additionalEntry);
extraDexs[index] = DexFile.loadDex(entryPath, entryPath
+ ".dex", 0);
}
pathField.set(loader, path.toString());
expandFieldArray(loader, "mPaths", extraPaths);
expandFieldArray(loader, "mFiles", extraFiles);
expandFieldArray(loader, "mZips", extraZips);
expandFieldArray(loader, "mDexs", extraDexs);
}
}
}
[AssetsManager.java]
package net.mobctrl.hostapk;
import java.io.File;
import java.io.FileOutputStream;
import java.io.InputStream;
import java.io.OutputStream;
import android.content.Context;
import android.content.res.AssetManager;
import android.util.Log;
/**
* @Author Zheng Haibo
* @Mail mochuan.zhb@alibaba-inc.com
* @Company Alibaba Group
* @PersonalWebsite http://www.mobctrl.net
* @version $Id: AssetsManager.java, v 0.1 2015年12月11日 下午4:41:10 mochuan.zhb Exp $
* @Description
*/
public class AssetsManager {
public static final String TAG = "AssetsApkLoader";
//从assets复制出去的apk的目标目录
public static final String APK_DIR = "third_apk";
//文件结尾过滤
public static final String FILE_FILTER = ".apk";
/**
* 将资源文件中的apk文件拷贝到私有目录中
*
* @param context
*/
public static void copyAllAssetsApk(Context context) {
AssetManager assetManager = context.getAssets();
long startTime = System.currentTimeMillis();
try {
File dex = context.getDir(APK_DIR, Context.MODE_PRIVATE);
dex.mkdir();
String []fileNames = assetManager.list("");
for(String fileName:fileNames){
if(!fileName.endsWith(FILE_FILTER)){
return;
}
InputStream in = null;
OutputStream out = null;
in = assetManager.open(fileName);
File f = new File(dex, fileName);
if (f.exists() && f.length() == in.available()) {
Log.i(TAG, fileName+"no change");
return;
}
Log.i(TAG, fileName+" chaneged");
out = new FileOutputStream(f);
byte[] buffer = new byte[2048];
int read;
while ((read = in.read(buffer)) != -1) {
out.write(buffer, 0, read);
}
in.close();
in = null;
out.flush();
out.close();
out = null;
Log.i(TAG, fileName+" copy over");
}
Log.i(TAG,"###copyAssets time = "+(System.currentTimeMillis() - startTime));
} catch (Exception e) {
e.printStackTrace();
}
}
}
以上就是从assets中加载apk的核心代码。
DEMO运行
打开BundleApk项目，编译成apk。然后将BundleApk.apk文件拷贝到HostApk项目的assets目录，在HostApk的MainActivity方法的onCreate当中，调用AssetsMultiDexLoader.install(getApplicationContext());加载BundleApk.apk。然后我们就可以通过如下两种方式调用BundleApk中的类：
- Class.forName
由于我们的HostApk没有BundleApk类的引用，所以我们需要用反射的方式调用。
private void loadClass(){
try{
Class<?> clazz = Class.forName("net.mobctrl.normal.apk.FileUtils");
Constructor<?> constructor = clazz.getConstructor();
Object bundleUtils = constructor.newInstance();
Method printSumMethod = clazz.getMethod("print", Context.class,String.class);
printSumMethod.setAccessible(true);
printSumMethod.invoke(bundleUtils,
getApplicationContext(), "Hello");
}catch(Exception e){
e.printStackTrace();
}
}- loadClass
我们也可以直接获取当前的ClassLoader对象，然后调用其loadClass方法
@TargetApi(Build.VERSION_CODES.ICE_CREAM_SANDWICH)
private void loadApk() {
try {
Class<?> clazz = getClassLoader()
.loadClass("net.mobctrl.normal.apk.Utils");
Constructor<?> constructor = clazz.getConstructor();
Object bundleUtils = constructor.newInstance();
Method printSumMethod = clazz.getMethod("printSum", Context.class,
int.class, int.class, String.class);
printSumMethod.setAccessible(true);
Integer sum = (Integer)printSumMethod.invoke(bundleUtils,
getApplicationContext(), 10, 20, "计算结果");
System.out.println("debug:sum = " + sum);
} catch (ClassNotFoundException e) {
e.printStackTrace();
} catch (SecurityException e) {
e.printStackTrace();
} catch (NoSuchMethodException e) {
e.printStackTrace();
} catch (IllegalArgumentException e) {
e.printStackTrace();
} catch (InstantiationException e) {
e.printStackTrace();
} catch (IllegalAccessException e) {
e.printStackTrace();
} catch (InvocationTargetException e) {
e.printStackTrace();
}
}参考文章
[1].ClassLoader的工作原理
[2].PathClassLoader源码
[3].MultiDex源码