multidex的作用就不在此介绍了，相信每一个看到这篇文章的程序猿们都不会容忍过多的废话。直接步入正题，以下分析都是基于Android7.1.2。
multidex的入口方法是：MultiDex.install()
public static void install(Context context) {
Log.i(TAG, "install");
//判断虚拟机的版本是否大于2.1，如果是，则直接返回。dalvik虚拟机版本为1.x，而art虚拟机的版本为2.x。
if (IS_VM_MULTIDEX_CAPABLE) {
Log.i(TAG, "VM has multidex support, MultiDex support library is disabled.");
return;
}
//如果sdk版本小于4，则不支持multidex
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;
}
//同步
synchronized (installedApk) {
String apkPath = applicationInfo.sourceDir;
//已经执行过multidex，则直接返回
if (installedApk.contains(apkPath)) {
return;
}
//加入已执行列表
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 {
//获取classloader，后面加载dex文件的时候会用到
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 {
//清除旧的分包文件
clearOldDexDir(context);
} catch (Throwable t) {
Log.w(TAG, "Something went wrong when trying to clear old MultiDex extraction, "
+ "continuing without cleaning.", t);
}
//获取dex文件目录
File dexDir = getDexDir(context, applicationInfo);
//提取除主dex之外的dex文件
List files = MultiDexExtractor.load(context, applicationInfo, dexDir, false);
if (checkValidZipFiles(files)) {
//安装
installSecondaryDexes(loader, dexDir, files);
} else {
Log.w(TAG, "Files were not valid zip files. Forcing a reload.");
// Try again, but this time force a reload of the zip file.
//再尝试一次
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");
}
如上有两个方法需要重点分析：
MultiDexExtractor.load()
installSecondaryDexes()
MultiDexExtractor.load()方法用于提取dex文件，如下：
static List load(Context context, ApplicationInfo applicationInfo, File dexDir,
boolean forceReload) throws IOException {
Log.i(TAG, "MultiDexExtractor.load(" + applicationInfo.sourceDir + ", " + forceReload + ")");
final File sourceApk = new File(applicationInfo.sourceDir);
//计算crc校验结果，用于验证文件的完整性
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 files;
IOException releaseLockException = null;
try {
lockChannel = lockRaf.getChannel();
Log.i(TAG, "Blocking on lock " + lockFile.getPath());
//启用文件锁
cacheLock = lockChannel.lock();
Log.i(TAG, lockFile.getPath() + " locked");
//校验apk文件的完整性
if (!forceReload && !isModified(context, sourceApk, currentCrc)) {
try {
//获取dex文件列表
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);
//强制提取dex文件列表，并存储时间戳，crc和文件size信息
files = performExtractions(sourceApk, dexDir);
putStoredApkInfo(context,
getTimeStamp(sourceApk), currentCrc, files.size() + 1);
}
} else {
Log.i(TAG, "Detected that extraction must be performed.");
//强制提取dex文件列表，并存储时间戳，crc和文件size信息
files = performExtractions(sourceApk, dexDir);
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;
}
loadExistingExtractions的源码如下：
private static List loadExistingExtractions(Context context, File sourceApk, File dexDir)
throws IOException {
Log.i(TAG, "loading existing secondary dex files");
//dex文件前缀
final String extractedFilePrefix = sourceApk.getName() + EXTRACTED_NAME_EXT;
//dex文件的总数
int totalDexNumber = getMultiDexPreferences(context).getInt(KEY_DEX_NUMBER, 1);
final List files = new ArrayList(totalDexNumber);
//遍历dex文件，主dex除外
for (int secondaryNumber = 2; secondaryNumber <= totalDexNumber; secondaryNumber++) {
String fileName = extractedFilePrefix + secondaryNumber + EXTRACTED_SUFFIX;
//文件后缀为zip
File extractedFile = new File(dexDir, fileName);
if (extractedFile.isFile()) {
//将dex文件添加的列表中
files.add(extractedFile);
if (!verifyZipFile(extractedFile)) {
Log.i(TAG, "Invalid zip file: " + extractedFile);
throw new IOException("Invalid ZIP file.");
}
} else {
throw new IOException("Missing extracted secondary dex file '" +
extractedFile.getPath() + "'");
}
}
return files;
}
performExtractions执行了强制提取dex文件的功能，源码如下：
private static List performExtractions(File sourceApk, File dexDir)
throws IOException {
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.
//删除旧的dex文件
prepareDexDir(dexDir, extractedFilePrefix);
List files = new ArrayList();
//apk文件
final ZipFile apk = new ZipFile(sourceApk);
try {
int secondaryNumber = 2;
//获取dexapk文件中的dex文件
ZipEntry dexFile = apk.getEntry(DEX_PREFIX + secondaryNumber + DEX_SUFFIX);
while (dexFile != null) {
String fileName = extractedFilePrefix + secondaryNumber + EXTRACTED_SUFFIX;
//创建zip文件
File extractedFile = new File(dexDir, fileName);
files.add(extractedFile);
Log.i(TAG, "Extraction is needed for file " + extractedFile);
int numAttempts = 0;
boolean isExtractionSuccessful = false;
//每个dex文件的提取最多尝试3次
while (numAttempts < MAX_EXTRACT_ATTEMPTS && !isExtractionSuccessful) {
numAttempts++;
// Create a zip file (extractedFile) containing only the secondary dex file
// (dexFile) from the apk.
将dex文件写入到zip文件中
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 + ")");
}
//继续查找下一个dex文件
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;
}
在performExtractions()方法中，有一个非常重要的方法extract()，将dex文件写入到zip文件中去，源码如下：
private static void extract(ZipFile apk, ZipEntry dexFile, File extractTo,
String extractedFilePrefix) throws IOException, FileNotFoundException {
InputStream in = apk.getInputStream(dexFile);
ZipOutputStream out = null;
File tmp = File.createTempFile(extractedFilePrefix, EXTRACTED_SUFFIX,
extractTo.getParentFile());
Log.i(TAG, "Extracting " + tmp.getPath());
try {
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
}
}
到此dex文件提取相关流程就介绍完毕了，接下来看一下installSecondaryDexes()这个方法。
private static void installSecondaryDexes(ClassLoader loader, File dexDir, List files)
throws IllegalArgumentException, IllegalAccessException, NoSuchFieldException,
InvocationTargetException, NoSuchMethodException, IOException {
if (!files.isEmpty()) {
//sdk 19 android 4.4
//sdk14 android 4.0~4.0.2
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);
}
}
}
以V19为例说明，
private static void install(ClassLoader loader, List 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.
*/
//获取classloader的pathList字段
Field pathListField = findField(loader, "pathList");
Object dexPathList = pathListField.get(loader);
ArrayList suppressedExceptions = new ArrayList();
//这里分为两步，1 将dex文件封装成Element对象，2 将封装好的Element对象加入到classloader的pathList数组中去
expandFieldArray(dexPathList, "dexElements", makeDexElements(dexPathList,
new ArrayList(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);
}
}
makeDexElements源码如下：
//通过反射获取makeDexElements方法，并将dex文件封装到Element中
private static Object[] makeDexElements(
Object dexPathList, ArrayList files, File optimizedDirectory,
ArrayList suppressedExceptions)
throws IllegalAccessException, InvocationTargetException,
NoSuchMethodException {
Method makeDexElements =
findMethod(dexPathList, "makeDexElements", ArrayList.class, File.class,
ArrayList.class);
return (Object[]) makeDexElements.invoke(dexPathList, files, optimizedDirectory,
suppressedExceptions);
}
expandFieldArray源码如下：
private static void expandFieldArray(Object instance, String fieldName,
Object[] extraElements) throws NoSuchFieldException, IllegalArgumentException,
IllegalAccessException {
Field jlrField = findField(instance, fieldName);
//获取原始数组
Object[] original = (Object[]) jlrField.get(instance);
//新建扩展后的数组
Object[] combined = (Object[]) Array.newInstance(
original.getClass().getComponentType(), original.length + extraElements.length);
//将原始数组的内容拷贝到扩展后的数组中
System.arraycopy(original, 0, combined, 0, original.length);
//将需要插入的数组拷贝到扩展后的数组中
System.arraycopy(extraElements, 0, combined, original.length, extraElements.length);
//替换实例中的数组
jlrField.set(instance, combined);
}
到此multidex的源码就介绍完毕了。如果有使用过multidex的同学，一定对multidex可能会引起anr这个问题深恶痛绝，而且随着分包数量的增加，app启动速度会急剧变慢，极大的影响了app的体验。
针对这个问题，业内已经有了很好的解决方案。思路就是将非主dex的加载放到非UI线程，更准确的说是另开启一个线程，专门处理dex的加载，这样就能避免ANR问题的产生。