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MultiDex安装过程源码分析

2016-06-11 | Comments

1.解决65535问题的通用方法

当项目比较大时,通常我们会遇到65535问题,相信大家都知道需要用到MultiDex分包。

通常做法是,在Application中添加如下代码

@Override
protected void attachBaseContext(Context base) {
    super.attachBaseContext(base);
    MultiDex.install(this);
}

然后在build.gradle中添加

multiDexEnabled true

以上做法确实可以解决许多问题,但是在实际项目中,还是会存在一些问题。

关于MultiDex和热修复这块,计划是分3-4篇文章来写。因为我一下也吸收不了那么多,这一节主要是分析MultiDex源码。

关于为啥dex方法数限定65535?官方文档说的MultiDex的一些坑?以及更深层次的东西,打算自己研究相关dalvik源码后再写。

2.MultiDex源码分析

0x00 入口方法MultiDex.install(Context)

public static void install(Context context) {
    Log.i("MultiDex", "install");
    if(IS_VM_MULTIDEX_CAPABLE) {
        Log.i("MultiDex", "VM has multidex support, MultiDex support library is disabled.");
    } else if(VERSION.SDK_INT < 4) {
        throw new RuntimeException("Multi dex installation failed. SDK " + VERSION.SDK_INT + " is unsupported. Min SDK version is " + 4 + ".");
    } else {
        try {
            ApplicationInfo e = getApplicationInfo(context);
            if(e == null) {
                return;
            }

            Set var2 = installedApk;
            synchronized(installedApk) {
                String apkPath = e.sourceDir;
                if(installedApk.contains(apkPath)) {
                    return;
                }

                installedApk.add(apkPath);
                if(VERSION.SDK_INT > 20) {
                    Log.w("MultiDex", "MultiDex is not guaranteed to work in SDK version " + VERSION.SDK_INT + ": SDK version higher than " + 20 + " 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") + "\"");
                }

                ClassLoader loader;
                try {
                    loader = context.getClassLoader();
                } catch (RuntimeException var9) {
                    Log.w("MultiDex", "Failure while trying to obtain Context class loader. Must be running in test mode. Skip patching.", var9);
                    return;
                }

                if(loader == null) {
                    Log.e("MultiDex", "Context class loader is null. Must be running in test mode. Skip patching.");
                    return;
                }

                try {
                    clearOldDexDir(context);
                } catch (Throwable var8) {
                    Log.w("MultiDex", "Something went wrong when trying to clear old MultiDex extraction, continuing without cleaning.", var8);
                }

                File dexDir = new File(e.dataDir, SECONDARY_FOLDER_NAME);
                List files = MultiDexExtractor.load(context, e, dexDir, false);
                if(checkValidZipFiles(files)) {
                    installSecondaryDexes(loader, dexDir, files);
                } else {
                    Log.w("MultiDex", "Files were not valid zip files.  Forcing a reload.");
                    files = MultiDexExtractor.load(context, e, dexDir, true);
                    if(!checkValidZipFiles(files)) {
                        throw new RuntimeException("Zip files were not valid.");
                    }

                    installSecondaryDexes(loader, dexDir, files);
                }
            }
        } catch (Exception var11) {
            Log.e("MultiDex", "Multidex installation failure", var11);
            throw new RuntimeException("Multi dex installation failed (" + var11.getMessage() + ").");
        }

        Log.i("MultiDex", "install done");
    }
}

依次做了如下事情:

1.先看JVM是否支持MultiDex,若JVM本身就支持则MultiDex库工程将被禁用

2.检查SDK版本号,要求最低版本为4

3.执行具体MultiDex操作:

(1)从ApplicationInfo.sourcecDir中获取APK路径apkPath,其值为/data/app/apkName.apk

(2)检查APK是否已安装,若APK已安装,则不进行后续操作。检查SDK版本号,版本号大于20不能保证MultiDex可正常Work

(3)清空旧的Dex目录。定位到旧Dex路径dexDir,即/data/data/pkgName/files/secondary-dexes,删除dexDir目录下所有文件。具体实现在clearOldDexDir(context)方法中。

(4)multi dex文件列表提取。从dex文件提取过程可阅读MultiDexExtractor.load(context, e, dexDir, false)源码

static List<File> load(Context context, ApplicationInfo applicationInfo, File dexDir, boolean forceReload) throws IOException {
    Log.i("MultiDex", "MultiDexExtractor.load(" + applicationInfo.sourceDir + ", " + forceReload + ")");
    File sourceApk = new File(applicationInfo.sourceDir);
    long currentCrc = getZipCrc(sourceApk);
    List files;
    if(!forceReload && !isModified(context, sourceApk, currentCrc)) {
        try {
            files = loadExistingExtractions(context, sourceApk, dexDir);
        } catch (IOException var9) {
            Log.w("MultiDex", "Failed to reload existing extracted secondary dex files, falling back to fresh extraction", var9);
            files = performExtractions(sourceApk, dexDir);
            putStoredApkInfo(context, getTimeStamp(sourceApk), currentCrc, files.size() + 1);
        }
    } else {
        Log.i("MultiDex", "Detected that extraction must be performed.");
        files = performExtractions(sourceApk, dexDir);
        putStoredApkInfo(context, getTimeStamp(sourceApk), currentCrc, files.size() + 1);
    }

    Log.i("MultiDex", "load found " + files.size() + " secondary dex files");
    return files;
}

以上dexDir路径为/data/data/pkgName/code_cache/secondary-dexes。

先检查forceReload和sourceApk isModified。(forceReload由提取过程是否出错决定,isModified通过比较sourceApk文件的CRC校验码和时间戳是否变化决定)

若不强制重新提取且sourceApk未经修改。则调用loadExistingExtractions(),直接加载已经存在的从dex文件。

private static List<File> loadExistingExtractions(Context context, File sourceApk, File dexDir) throws IOException {
    Log.i("MultiDex", "loading existing secondary dex files");
    String extractedFilePrefix = sourceApk.getName() + ".classes";
    int totalDexNumber = getMultiDexPreferences(context).getInt("dex.number", 1);
    ArrayList files = new ArrayList(totalDexNumber);

    for(int secondaryNumber = 2; secondaryNumber <= totalDexNumber; ++secondaryNumber) {
        String fileName = extractedFilePrefix + secondaryNumber + ".zip";
        File extractedFile = new File(dexDir, fileName);
        if(!extractedFile.isFile()) {
            throw new IOException("Missing extracted secondary dex file \'" + extractedFile.getPath() + "\'");
        }

        files.add(extractedFile);
        if(!verifyZipFile(extractedFile)) {
            Log.i("MultiDex", "Invalid zip file: " + extractedFile);
            throw new IOException("Invalid ZIP file.");
        }
    }

    return files;
}

以上代码段具体实现是,从dexDir路径下提取如下zip文件列表:

/data/data/pkgName/code_cache/secondary-dexes/apkName.apk.classes2.zip

/data/data/pkgName/code_cache/secondary-dexes/apkName.apk.classes3.zip

……

/data/data/pkgName/code_cache/secondary-dexes/apkName.apk.classesN.zip

若以上loadExistingExtractions过程中出现异常(缺少某个从dex,或zip文件格式不对),或者强制重新提取或sourceApk被修改,则调用performExtractions(File sourceApk, File dexDir),重新抽取从dex列表

private static List<File> performExtractions(File sourceApk, File dexDir) throws IOException {
    String extractedFilePrefix = sourceApk.getName() + ".classes";
    prepareDexDir(dexDir, extractedFilePrefix);
    ArrayList files = new ArrayList();
    ZipFile apk = new ZipFile(sourceApk);

    try {
        int e = 2;

        for(ZipEntry dexFile = apk.getEntry("classes" + e + ".dex"); dexFile != null; dexFile = apk.getEntry("classes" + e + ".dex")) {
            String fileName = extractedFilePrefix + e + ".zip";
            File extractedFile = new File(dexDir, fileName);
            files.add(extractedFile);
            Log.i("MultiDex", "Extraction is needed for file " + extractedFile);
            int numAttempts = 0;
            boolean isExtractionSuccessful = false;

            while(numAttempts < 3 && !isExtractionSuccessful) {
                ++numAttempts;
                extract(apk, dexFile, extractedFile, extractedFilePrefix);
                isExtractionSuccessful = verifyZipFile(extractedFile);
                Log.i("MultiDex", "Extraction " + (isExtractionSuccessful?"success":"failed") + " - length " + extractedFile.getAbsolutePath() + ": " + extractedFile.length());
                if(!isExtractionSuccessful) {
                    extractedFile.delete();
                    if(extractedFile.exists()) {
                        Log.w("MultiDex", "Failed to delete corrupted secondary dex \'" + extractedFile.getPath() + "\'");
                    }
                }
            }

            if(!isExtractionSuccessful) {
                throw new IOException("Could not create zip file " + extractedFile.getAbsolutePath() + " for secondary dex (" + e + ")");
            }

            ++e;
        }
    } finally {
        try {
            apk.close();
        } catch (IOException var16) {
            Log.w("MultiDex", "Failed to close resource", var16);
        }

    }

    return files;
}

以上代码段具体实现是:

先准备好dexDir,prepareDexDir(File dexDir, final String extractedFilePrefix)。从dexDir路径下过滤掉所有不是以apkName.apk.classes开头的文件,并删除这些文件。

从dex文件列表提取。从sourceApk的文件中找到classes2.dex…classesN.dex的ZipEntry入口,依次调用extract(apk, dexFile, extractedFile, extractedFilePrefix):

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, ".zip", extractTo.getParentFile());
    Log.i("MultiDex", "Extracting " + tmp.getPath());

    try {
        out = new ZipOutputStream(new BufferedOutputStream(new FileOutputStream(tmp)));

        try {
            ZipEntry classesDex = new ZipEntry("classes.dex");
            classesDex.setTime(dexFile.getTime());
            out.putNextEntry(classesDex);
            byte[] buffer = new byte[16384];

            for(int length = in.read(buffer); length != -1; length = in.read(buffer)) {
                out.write(buffer, 0, length);
            }

            out.closeEntry();
        } finally {
            out.close();
        }

        Log.i("MultiDex", "Renaming to " + extractTo.getPath());
        if(!tmp.renameTo(extractTo)) {
            throw new IOException("Failed to rename \"" + tmp.getAbsolutePath() + "\" to \"" + extractTo.getAbsolutePath() + "\"");
        }
    } finally {
        closeQuietly(in);
        tmp.delete();
    }

}

分析下extract(apk, dexFile, extractedFile, extractedFilePrefix):

参数如下:

  • apk: Apk文件/data/app/apkName.apk

  • dexFile: Apk文件zip解压后得到的从dex文件,classes2.dex…classesN.dex

  • extractedFile: dexFile写入的目标文件/data/data/pkgName/code_cache/secondary-dexes/apkName.apk.classes2.zip等

  • extractedFilePrefix: 前缀apkName.apk.classes

简言之,这个方法做的事情就是,将Apk文件解压后得到的classes2.dex, …, classesN.dex文件的内容依次写入到/data/data/pkgName/code_cache/secondary-dexes/apkName.apk.classes2.zip, …, /data/data/pkgName/code_cache/secondary-dexes/apkName.apk.classesN.zip压缩文件的classes.dex文件中。

若该方法执行异常,则最多可以重试3次。

最终performExtractions做的事情就是,将/data/app.apkName.apk文件解压得到的classes2.dex, …, classesN.dex文件的内容依次写入/data/data/pkgName/code_cache/secondary-dexes/apkName.apk.classes2.zip, …, /data/data/pkgName/code_cache/secondary-dexes/apkName.apk.classesN.zip压缩文件的classes.dex文件中,并返回这个zip列表。

得到从dex的zip列表后,SP中存入timestamp, crc, dex.number等Apk相关信息

得到从dex的zip列表后,调用installSecondaryDexes(ClassLoader loader, File dexDir, List files)进行安装

installSecondaryDexes(ClassLoader loader, File dexDir, List files)具体实现为:

根据SDK >= 19,SDK < 14, 14 <= SDK < 19三种情况V19, V14, V4分别调用不同实现。

以V19为例,安装过程实际做的事情是:

通过反射方法,找到BaseDexClassLoader实例中的DexPathList类型的pathList实例变量。

调用pathList实例的makeDexElements(ArrayList files,File optimizedDirectory)方法,将上面得到的zip文件列表封装成Element数组

修改pathList实例的dexElements数组变量的值,在原有基础上扩充上面的Element数组。

异常处理

makeDexElements的C实现中,会执行里相关dex2opt优化操作,这个是个耗时操作,更详细的可以看dalvik源码部分。

小结

本文主要讲述MultiDex的安装过程:

将/data/app/apkName.apk路径下解压得到的classes2.dex, …, classesN.dex,依次写入到/data/data/pkgName/code_cache/secondary-dexes/apkName.apk.classes2.zip等zip文件的classes.dex中,并返回这个zip列表。然后针对这个zip列表执行安装过程,具体过程是,将这个要安装的zip列表加入BaseDexClassLoader的pathList实例的dexElements数组中,其中会针对各dex文件进行dex2opt优化。一旦加入到了dexElements数组中,程序启动的时候,ClassLoader会加载dexElements数组中的元素,从而实现multi dex的安装。

这么看来,本文并没有讲述如果进行multi dex拆分。

未完待续。

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