1. 问题的产生
当Android系统安装一个应用的时候，有一步是对Dex进行优化，这个过程有一个专门的工具来处理，叫DexOpt。DexOpt的执行过程是在第一次加载Dex文件的时候执行的。这个过程会生成一个ODEX文件，即Optimised Dex。执行ODex的效率会比直接执行Dex文件的效率要高很多。
但是在早期的Android系统中，DexOpt有一个问题，DexOpt会把每一个类的方法id检索起来，存在一个链表结构里面。但是这个链表的长度是用一个short类型来保存的，导致了方法id的数目不能够超过65536个。当一个项目足够大的时候，显然这个方法数的上限是不够的。尽管在新版本的Android系统中，DexOpt修复了这个问题，但是我们仍然需要对低版本的Android系统做兼容。
2.Multidex的出现
在android5.0之前，每一个android应用中只会含有一个dex文件，但是这个dex的方法数量被限制在65535之内，这就是著名的64K(64*1024)事件。为了解决这个问题，Google官方推出了这个类似于补丁一样的support-library。关于这个库的使用，对于新手来说不免会遇到一些坑的，本篇文章重点在于MultiDex的使用原理的分析,关于它的使用爬坑以及优化方案,请看我的其他两篇文章。其实你不知道MultiDex到底有多坑和Android MultiDex初次启动APP优化方案优雅的实现。使用这个库后，我们的APP不再只会仅有一个dex文件，可能会产生多个dex文件，这样就避免了64K问题。
比如:微信app经过分包后,我们解压其apk文件:
解压之后的微信apk
3.Multidex的基本使用
- 1.配置build.gradle
android {
compileSdkVersion 21
buildToolsVersion "21.1.0"
defaultConfig {
...
// Enabling multidex support.
multiDexEnabled true
}
...
}
dependencies {
compile 'com.android.support:multidex:1.0.0'
}
- 2.配置AndroidManifest.xml
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.android.multidex.myapplication">
<application
...
android:name="android.support.multidex.MultiDexApplication">
...
</application>
</manifest>
- 3.若是有自定义的Application则可以直接继承MultiDexApplication
public class MyApplication extends MultiDexApplication{
// ...........
}
- 4.若是不想继承MultiDexApplication, 也可以复写attachBaseContext方法
@Override
protected void attachBaseContext(Context base) {
super.attachBaseContext(base);
MultiDex.install(this) ;
}
- 5.使用productFlavors(初次引入MultiDex 需要很长的时间，为减少开发阶段，运行时间，使用productFlavors)
android {
//定义不同的flavors
productFlavors {
//开发阶段
dev {
//运行在Android 5.0以上版本的设备中，使用ART-supported格式生成multidex的速度要快得多
minSdkVersion 21
}
prod {
//实际支持的最低版本
minSdkVersion 14
}
}
}
4.原理分析之android中的类加载机制
在分析MultiDex.install(Context)之前，先了解一下android中的类是如何加载的。在android中，类的加载可以分为DexClassLoader和PathClassLoader，这里先看看他们各自的实现：
DexClassLoader
/**
* A class loader that loads classes from {@code .jar} and {@code .apk} files
* containing a {@code classes.dex} entry. This can be used to execute code not
* installed as part of an application.
* 从包含dex文件的jar或是apk中加载classes。该ClassLoader可以用来加载外部的classes，
* 也就是可以加载没有预先安装的含有dex文件的jar或是apk。
*
* <p>This class loader requires an application-private, writable directory to
* cache optimized classes. Use {@code Context.getDir(String, int)} to create
* such a directory: <pre> {@code
* File dexOutputDir = context.getDir("dex", 0);
* }</pre>
*
* <p><strong>Do not cache optimized classes on external storage.</strong>
* External storage does not provide access controls necessary to protect your
* application from code injection attacks.
*/
public class DexClassLoader extends BaseDexClassLoader {
/**
* Creates a {@code DexClassLoader} that finds interpreted and native
* code. Interpreted classes are found in a set of DEX files contained
* in Jar or APK files.
*
* <p>The path lists are separated using the character specified by the
* {@code path.separator} system property, which defaults to {@code :}.
*
* @param dexPath the list of jar/apk files containing classes and
* resources, delimited by {@code File.pathSeparator}, which
* defaults to {@code ":"} on Android
* 含有classes和resources的文件(jar/apk)路径，多个文件以 File.pathSeparator(Linux为":"Windows为";")分割开来,比如:xx/xx/aa.apk:yy/yy/bb.apk
* @param optimizedDirectory directory where optimized dex files
* should be written; must not be {@code null}
* dex文件所在的根路径，不能为null
* @param libraryPath the list of directories containing native
* libraries, delimited by {@code File.pathSeparator}; may be
* {@code null}
* SO文件所在的路径
* @param parent the parent class loader
*/
public DexClassLoader(String dexPath, String optimizedDirectory,
String libraryPath, ClassLoader parent) {
super(dexPath, new File(optimizedDirectory), libraryPath, parent);
}
}
PathClassLoader
/**
* Provides a simple {@link ClassLoader} implementation that operates on a list
* of files and directories in the local file system, but does not attempt to
* load classes from the network. Android uses this class for its system class
* loader and for its application class loader(s).
* 不能加载从网络上获取的classes，Android用这个类来加载系统classes和已经安装的应用的classes
*/
public class PathClassLoader extends BaseDexClassLoader {
/**
* Creates a {@code PathClassLoader} that operates on a given list of files
* and directories. This method is equivalent to calling
* {@link #PathClassLoader(String, String, ClassLoader)} with a
* {@code null} value for the second argument (see description there).
*
* @param dexPath the list of jar/apk files containing classes and
* resources, delimited by {@code File.pathSeparator}, which
* defaults to {@code ":"} on Android
* @param parent the parent class loader
*/
public PathClassLoader(String dexPath, ClassLoader parent) {
super(dexPath, null, null, parent);
}
/**
* Creates a {@code PathClassLoader} that operates on two given
* lists of files and directories. The entries of the first list
* should be one of the following:
*
* <ul>
* <li>JAR/ZIP/APK files, possibly containing a "classes.dex" file as
* well as arbitrary resources.
* <li>Raw ".dex" files (not inside a zip file).
* </ul>
* dexPath只支持jar/zip/apk/dex四种文件
*
* The entries of the second list should be directories containing
* native library files.
*
* @param dexPath the list of jar/apk files containing classes and
* resources, delimited by {@code File.pathSeparator}, which
* defaults to {@code ":"} on Android
* @param libraryPath the list of directories containing native
* libraries, delimited by {@code File.pathSeparator}; may be
* {@code null}
* @param parent the parent class loader
*/
public PathClassLoader(String dexPath, String libraryPath,
ClassLoader parent) {
super(dexPath, null, libraryPath, parent);
}
}
从类的说明以及构造函数可以明显的知道DexClassLoader和PathClassLoader区别：DexClassLoader可以加载未安装的含有dex文件的jar或是apk，而PathClassLoader只能加载已安装好的含有dex文件的jar/apk/zip/dex。
对于它们的父类PathClassLoader:
public class BaseDexClassLoader extends ClassLoader {
/** structured lists of path elements */
private final DexPathList pathList;
public BaseDexClassLoader(String dexPath, File optimizedDirectory,
String libraryPath, ClassLoader parent) {
super(parent);
this.originalPath = dexPath;
this.pathList =
new DexPathList(this, dexPath, libraryPath, optimizedDirectory);
}
}
其中用到了一个类DexPathList:
/**
* A pair of lists of entries, associated with a {@code ClassLoader}.
* One of the lists is a dex/resource path — typically referred
* to as a "class path" — list, and the other names directories
* containing native code libraries. Class path entries may be any of:
* a {@code .jar} or {@code .zip} file containing an optional
* top-level {@code classes.dex} file as well as arbitrary resources,
* or a plain {@code .dex} file (with no possibility of associated
* resources).
*
* <p>This class also contains methods to use these lists to look up
* classes and resources.</p>
*/
/*package*/ final class DexPathList {
private static final String DEX_SUFFIX = ".dex";
private static final String JAR_SUFFIX = ".jar";
private static final String ZIP_SUFFIX = ".zip";
private static final String APK_SUFFIX = ".apk";
/** class definition context */
private final ClassLoader definingContext;
/** list of dex/resource (class path) elements */
private final Element[] dexElements;
/** list of native library directory elements */
private final File[] nativeLibraryDirectories;
public DexPathList(ClassLoader definingContext, String dexPath,
String libraryPath, File optimizedDirectory) {
if (definingContext == null) {
throw new NullPointerException("definingContext == null");
}
if (dexPath == null) {
throw new NullPointerException("dexPath == null");
}
if (optimizedDirectory != null) {
// 对于PathClassloader来说optimizedDirectory是恒等于null的
if (!optimizedDirectory.exists()) {
throw new IllegalArgumentException(
"optimizedDirectory doesn't exist: "
+ optimizedDirectory);
}
if (!(optimizedDirectory.canRead()
&& optimizedDirectory.canWrite())) {
throw new IllegalArgumentException(
"optimizedDirectory not readable/writable: "
+ optimizedDirectory);
}
}
// classLoader
this.definingContext = definingContext;
// 把每个dex文件的相关信息存储到Element这个对象中
this.dexElements =
makeDexElements(splitDexPath(dexPath), optimizedDirectory);
// 本地so文件路径
this.nativeLibraryDirectories = splitLibraryPath(libraryPath);
}
//......
}
首先是通过splitDexPath()得到含有dex文件的jar/zip/dex/apk集合：
/**
* Splits the given dex path string into elements using the path
* separator, pruning out any elements that do not refer to existing
* and readable files. (That is, directories are not included in the
* result.)
*/
private static ArrayList<File> splitDexPath(String path) {
return splitPaths(path, null, false);
}
/**
* Splits the given path strings into file elements using the path
* separator, combining the results and filtering out elements
* that don't exist, aren't readable, or aren't either a regular
* file or a directory (as specified). Either string may be empty
* or {@code null}, in which case it is ignored. If both strings
* are empty or {@code null}, or all elements get pruned out, then
* this returns a zero-element list.
*/
private static ArrayList<File> splitPaths(String path1, String path2,
boolean wantDirectories) {
ArrayList<File> result = new ArrayList<File>();
splitAndAdd(path1, wantDirectories, result);
splitAndAdd(path2, wantDirectories, result);
return result;
}
/**
* Helper for {@link #splitPaths}, which does the actual splitting
* and filtering and adding to a result.
*/
private static void splitAndAdd(String path, boolean wantDirectories,
ArrayList<File> resultList) {
if (path == null) {
return;
}
// 路径分离
String[] strings = path.split(Pattern.quote(File.pathSeparator));
for (String s : strings) {
File file = new File(s);
if (! (file.exists() && file.canRead())) {
continue;
}
/*
* Note: There are other entities in filesystems than
* regular files and directories.
*/
if (wantDirectories) {
if (!file.isDirectory()) {
continue;
}
} else {
if (!file.isFile()) {
continue;
}
}
resultList.add(file);
}
}
然后通过makeDexElements()来把dex信息保存到Element对象中：
/**
* Makes an array of dex/resource path elements, one per element of
* the given array.
*/
private static Element[] makeDexElements(ArrayList<File> files,
File optimizedDirectory) {
ArrayList<Element> elements = new ArrayList<Element>();
/*
* Open all files and load the (direct or contained) dex files
* up front.
*/
for (File file : files) {
ZipFile zip = null;
DexFile dex = null;
String name = file.getName();
if (name.endsWith(DEX_SUFFIX)) {// 直接是.dex文件
// Raw dex file (not inside a zip/jar).
try {
dex = loadDexFile(file, optimizedDirectory);
} catch (IOException ex) {
System.logE("Unable to load dex file: " + file, ex);
}
} else if (name.endsWith(APK_SUFFIX) || name.endsWith(JAR_SUFFIX)
|| name.endsWith(ZIP_SUFFIX)) {// 其他三种格式文件:都是压缩包啊
try {
zip = new ZipFile(file);
} catch (IOException ex) {
/*
* Note: ZipException (a subclass of IOException)
* might get thrown by the ZipFile constructor
* (e.g. if the file isn't actually a zip/jar
* file).
*/
System.logE("Unable to open zip file: " + file, ex);
}
try {
dex = loadDexFile(file, optimizedDirectory);
} catch (IOException ignored) {
/*
* IOException might get thrown "legitimately" by
* the DexFile constructor if the zip file turns
* out to be resource-only (that is, no
* classes.dex file in it). Safe to just ignore
* the exception here, and let dex == null.
*/
}
} else {
System.logW("Unknown file type for: " + file);
}
// 格式符合要求，放到数组中
if ((zip != null) || (dex != null)) {
// 依靠DexFile创建Element对象
elements.add(new Element(file, zip, dex));
}
}
return elements.toArray(new Element[elements.size()]);
}
/**
* Constructs a {@code DexFile} instance, as appropriate depending
* on whether {@code optimizedDirectory} is {@code null}.
* .dex文件转成DexFile对象
*/
private static DexFile loadDexFile(File file, File optimizedDirectory)
throws IOException {
if (optimizedDirectory == null) {
return new DexFile(file);
} else {
String optimizedPath = optimizedPathFor(file, optimizedDirectory);
return DexFile.loadDex(file.getPath(), optimizedPath, 0);
}
}
/**
* Converts a dex/jar file path and an output directory to an
* output file path for an associated optimized dex file.
* 主要是对非.dex结尾的文件加上.dex后缀，统一为.dex后缀
* 没有使用.odex为后缀，是因为构建系统会把.odex文件认为是只含有资源的文件
*/
private static String optimizedPathFor(File path,
File optimizedDirectory) {
/*
* Get the filename component of the path, and replace the
* suffix with ".dex" if that's not already the suffix.
*
* We don't want to use ".odex", because the build system uses
* that for files that are paired with resource-only jar
* files. If the VM can assume that there's no classes.dex in
* the matching jar, it doesn't need to open the jar to check
* for updated dependencies, providing a slight performance
* boost at startup. The use of ".dex" here matches the use on
* files in /data/dalvik-cache.
*/
String fileName = path.getName();
if (!fileName.endsWith(DEX_SUFFIX)) {
int lastDot = fileName.lastIndexOf(".");
if (lastDot < 0) {
fileName += DEX_SUFFIX;
} else {
StringBuilder sb = new StringBuilder(lastDot + 4);
sb.append(fileName, 0, lastDot);
sb.append(DEX_SUFFIX);
fileName = sb.toString();
}
}
File result = new File(optimizedDirectory, fileName);
return result.getPath();
}
以上，是android中类加载过程，从中可以发现，android中的classloader能加载的文件格式只能为zip/apk/dex/jar，加载目标是把dex文件转化成一个Element对象，以供后续查找使用。
5.原理分析之MultiDex.install(Context)
我们先来看一下源码:
/**
* Patches the application context class loader by appending extra dex files
* loaded from the application apk. This method should be called in the
* attachBaseContext of your {@link Application}, see
* {@link MultiDexApplication} for more explanation and an example.
*
* @param context application context.
* @throws RuntimeException if an error occurred preventing the classloader
* extension.
*/
public static void install(Context context) {
Log.i(TAG, "install");
if (IS_VM_MULTIDEX_CAPABLE) {
Log.i(TAG, "VM has multidex support, MultiDex support library is disabled.");
return;
}
if (Build.VERSION.SDK_INT < MIN_SDK_VERSION) {// 最低版本是4
throw new RuntimeException("Multi dex installation failed. SDK " + Build.VERSION.SDK_INT
+ " is unsupported. Min SDK version is " + MIN_SDK_VERSION + ".");
}
try {
// 获取到应用的相关信息
ApplicationInfo applicationInfo = getApplicationInfo(context);
if (applicationInfo == null) {
// Looks like running on a test Context, so just return without patching.
return;
}
synchronized (installedApk) {
// dataDir = /data/data/com.example.android.animationsdemo
// apkPath = sourceDir = /data/app/com.example.android.animationsdemo-1.apk
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 {
//dalvik.system.PathClassLoader
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;
}
try {
// 清空放置dex文件的文件夹
clearOldDexDir(context);
} catch (Throwable t) {
Log.w(TAG, "Something went wrong when trying to clear old MultiDex extraction, "
+ "continuing without cleaning.", t);
}
// 创建放置dex文件的文件夹
// dexDir = /data/data/com.example.android.animationsdemo/code_cache/secondary-dexes
File dexDir = getDexDir(context, applicationInfo);
// 得到APK中所有dex对应的zip文件
List<File> files = MultiDexExtractor.load(context, applicationInfo, dexDir, false);
//
if (checkValidZipFiles(files)) {
// 加载dex到
installSecondaryDexes(loader, dexDir, files);
} else {
// 强制再转换一次
Log.w(TAG, "Files were not valid zip files. Forcing a reload.");
// Try again, but this time force a reload of the zip file.
files = MultiDexExtractor.load(context, applicationInfo, dexDir, true);
if (checkValidZipFiles(files)) {
installSecondaryDexes(loader, dexDir, files);
} else {
// Second time didn't work, give up
throw new RuntimeException("Zip files were not valid.");
}
}
}
} catch (Exception e) {
Log.e(TAG, "Multidex installation failure", e);
throw new RuntimeException("Multi dex installation failed (" + e.getMessage() + ").");
}
Log.i(TAG, "install done");
}
整个加载过程主要分为3步骤：
- clearOldDexDir()：清理过期数据
- MultiDexExtractor.load：把dex转换成对应的zip文件
- installSecondaryDexes：dex转换成对应的Element对象
- 清理过期文件:删除/data/data/(pkg)/files/secondary-dexes路径下的所有文件和该文件夹。源码如下:
private static void clearOldDexDir(Context context) throws Exception {
// dexDir = /data/data/com.example.android.animationsdemo/files/secondary-dexes
File dexDir = new File(context.getFilesDir(), OLD_SECONDARY_FOLDER_NAME);
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());
}
}
}
- 把dex文件转换成对应的zip文件：classesN.dex-->/data/data/(包名)/code_cache/secondary-dexes/(包名)-1.apk.classesN.zip。源码如下:
static List<File> load(Context context, ApplicationInfo applicationInfo, File dexDir,
boolean forceReload) throws IOException {
Log.i(TAG, "MultiDexExtractor.load(" + applicationInfo.sourceDir + ", " + forceReload + ")");
// sourceApk = /data/app/com.example.android.animationsdemo-1.apk
final File sourceApk = new File(applicationInfo.sourceDir);
long currentCrc = getZipCrc(sourceApk);
// Validity check and extraction must be done only while the lock file has been taken.
File lockFile = new File(dexDir, LOCK_FILENAME);
RandomAccessFile lockRaf = new RandomAccessFile(lockFile, "rw");
FileChannel lockChannel = null;
FileLock cacheLock = null;
List<File> files;
IOException releaseLockException = null;
try {
lockChannel = lockRaf.getChannel();
Log.i(TAG, "Blocking on lock " + lockFile.getPath());
cacheLock = lockChannel.lock();
Log.i(TAG, lockFile.getPath() + " locked");
if (!forceReload && !isModified(context, sourceApk, currentCrc)) {
// APK信息没有发生变化
try {
// dex对应的zip文件已经存在
files = loadExistingExtractions(context, sourceApk, dexDir);
} catch (IOException ioe) {
Log.w(TAG, "Failed to reload existing extracted secondary dex files,"
+ " falling back to fresh extraction", ioe);
files = performExtractions(sourceApk, dexDir);
putStoredApkInfo(context,
getTimeStamp(sourceApk), currentCrc, files.size() + 1);
}
} else {
// second dex 发生了改变
Log.i(TAG, "Detected that extraction must be performed.");
files = performExtractions(sourceApk, dexDir);
// 保存APK相关信息:时间戳、总dex数等
putStoredApkInfo(context, getTimeStamp(sourceApk), currentCrc, files.size() + 1);
}
} finally {
if (cacheLock != null) {
try {
cacheLock.release();
} catch (IOException e) {
Log.e(TAG, "Failed to release lock on " + lockFile.getPath());
// Exception while releasing the lock is bad, we want to report it, but not at
// the price of overriding any already pending exception.
releaseLockException = e;
}
}
if (lockChannel != null) {
closeQuietly(lockChannel);
}
closeQuietly(lockRaf);
}
if (releaseLockException != null) {
throw releaseLockException;
}
Log.i(TAG, "load found " + files.size() + " secondary dex files");
return files;
}
/**
* 把APK文件中的classesN.dex(N >= 2)转换成对应的zip文件，保存到集合中
* zip文件:/data/data/(包名)/code_cache/secondary-dexes/(包名)-1.apk.classesN.zip
*/
private static List<File> performExtractions(File sourceApk, File dexDir)
throws IOException {
// extractedFilePrefix = com.example.android.animationsdemo-1.apk.classes
final String extractedFilePrefix = sourceApk.getName() + EXTRACTED_NAME_EXT;
// Ensure that whatever deletions happen in prepareDexDir only happen if the zip that
// contains a secondary dex file in there is not consistent with the latest apk. Otherwise,
// multi-process race conditions can cause a crash loop where one process deletes the zip
// while another had created it.
prepareDexDir(dexDir, extractedFilePrefix);
List<File> files = new ArrayList<File>();
final ZipFile apk = new ZipFile(sourceApk);
try {
int secondaryNumber = 2;
// dexFile = classes2.dex
ZipEntry dexFile = apk.getEntry(DEX_PREFIX + secondaryNumber + DEX_SUFFIX);
while (dexFile != null) {
// fileName = com.example.android.animationsdemo-1.apk.classes2.zip
String fileName = extractedFilePrefix + secondaryNumber + EXTRACTED_SUFFIX;
// extractedFile = /data/data/com.example.android.animationsdemo/code_cache/secondary-dexes/com.example.android.animationsdemo-1.apk.classes2.zip
File extractedFile = new File(dexDir, fileName);
files.add(extractedFile);
Log.i(TAG, "Extraction is needed for file " + extractedFile);
int numAttempts = 0;
boolean isExtractionSuccessful = false;
while (numAttempts < MAX_EXTRACT_ATTEMPTS && !isExtractionSuccessful) {
// 最多尝试3次来把classesN.dex转成对应的zip文件
numAttempts++;
// Create a zip file (extractedFile) containing only the secondary dex file
// (dexFile) from the apk.
// 把classesN.dex文件转化为zip文件
extract(apk, dexFile, extractedFile, extractedFilePrefix);
// Verify that the extracted file is indeed a zip file.
isExtractionSuccessful = verifyZipFile(extractedFile);
// Log the sha1 of the extracted zip file
Log.i(TAG, "Extraction " + (isExtractionSuccessful ? "success" : "failed") +
" - length " + extractedFile.getAbsolutePath() + ": " +
extractedFile.length());
if (!isExtractionSuccessful) {
// Delete the extracted file
extractedFile.delete();
if (extractedFile.exists()) {
Log.w(TAG, "Failed to delete corrupted secondary dex '" +
extractedFile.getPath() + "'");
}
}
}
if (!isExtractionSuccessful) {
throw new IOException("Could not create zip file " +
extractedFile.getAbsolutePath() + " for secondary dex (" +
secondaryNumber + ")");
}
secondaryNumber++;
dexFile = apk.getEntry(DEX_PREFIX + secondaryNumber + DEX_SUFFIX);
}
} finally {
try {
apk.close();
} catch (IOException e) {
Log.w(TAG, "Failed to close resource", e);
}
}
return files;
}
/**
* apk = /data/app/com.example.android.animationsdemo-1.apk
* dexFile = classesN.dex
* extractTo = /data/data/com.example.android.animationsdemo/code_cache/secondary-dexes/com.example.android.animationsdemo-1.apk.classesN.zip
* extractedFilePrefix = com.example.android.animationsdemo-1.apk.classes
* 把classesN.dex文件转化为zip文件
*/
private static void extract(ZipFile apk, ZipEntry dexFile, File extractTo,
String extractedFilePrefix) throws IOException, FileNotFoundException {
InputStream in = apk.getInputStream(dexFile);
ZipOutputStream out = null;
// tmp = /data/data/com.example.android.animationsdemo/code_cache/secondary-dexes/com.example.android.animationsdemo-1.apk.classes.zip
File tmp = File.createTempFile(extractedFilePrefix, EXTRACTED_SUFFIX,
extractTo.getParentFile());
Log.i(TAG, "Extracting " + tmp.getPath());
try {
// 把classesN.dex写到tmp这个zip文件中，然后指定这个zip文件的下一个ZipEntry为classes.dex
// 最后把zip文件重命名为extractTo
out = new ZipOutputStream(new BufferedOutputStream(new FileOutputStream(tmp)));
try {
ZipEntry classesDex = new ZipEntry("classes.dex");
// keep zip entry time since it is the criteria used by Dalvik
classesDex.setTime(dexFile.getTime());
out.putNextEntry(classesDex);
byte[] buffer = new byte[BUFFER_SIZE];
int length = in.read(buffer);
while (length != -1) {
out.write(buffer, 0, length);
length = in.read(buffer);
}
out.closeEntry();
} finally {
out.close();
}
Log.i(TAG, "Renaming to " + extractTo.getPath());
if (!tmp.renameTo(extractTo)) {
throw new IOException("Failed to rename \"" + tmp.getAbsolutePath() +
"\" to \"" + extractTo.getAbsolutePath() + "\"");
}
} finally {
closeQuietly(in);
tmp.delete(); // return status ignored
}
}
- 加载dex信息到内存中
源码:
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);
}
}
}
v19的源码:
/**
* 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.
*/
// 通过反射的方式获取PathClassLoader中的pathList属性
Field pathListField = findField(loader, "pathList");
Object dexPathList = pathListField.get(loader);
// 通过反射的方式重新给DexPathList中的dexElements重新赋值
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(dexPathList, "dexElementsSuppressedExceptions");
IOException[] dexElementsSuppressedExceptions =
(IOException[]) suppressedExceptionsField.get(dexPathList);
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(dexPathList, 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 {
// 通过反射的方式调用dalvik.system.DexPathList.makeDexElements()，得到classesN.dex对应的Element
Method makeDexElements =
findMethod(dexPathList, "makeDexElements", ArrayList.class, File.class,
ArrayList.class);
return (Object[]) makeDexElements.invoke(dexPathList, files, optimizedDirectory,
suppressedExceptions);
}
}
private static void expandFieldArray(Object instance, String fieldName,
Object[] extraElements) throws NoSuchFieldException, IllegalArgumentException,
IllegalAccessException {
// 找到DexPathList类中的dexElements属性，它是Element[]
Field jlrField = findField(instance, fieldName);
// 目前内存具有的Elements,应该只有classes.dex对应的Element
Object[] original = (Object[]) jlrField.get(instance);
// classesN.dex对应的Element
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);
// 给instance对象中的属性jlrField重新赋值为combined
jlrField.set(instance, combined);
}
总结：本篇文章重点在于让读者对Multidex的使用及其原理有大体上的认识。关于Multidex在日常开发使用过程中遇到的一些坑点以及如何优雅的避免这些坑,请关注我的另外两篇文章。其实你不知道MultiDex到底有多坑和Android MultiDex初次启动APP优化方案优雅的实现。
