Android 8.0 PKMS新变化：如何提取ABI

关于ABI的知识可以网上搜一下

在8.0之前，因为每次开机都要扫描APK，所以每次都要判断并设置ABI，这个过程比较花费时间，如果手机安装大量APK，势必会造成开机时间过慢
所以之前项目会对这一块做优化，将ABI信息写入一个xml文件中，首次开机或者安装删除APK就会更新这个文件，非首次开机就会直接读取里面的值
这次Android 8.0内部优化这一过程，而且比之前的方案更简单，下面来看看是怎么实现的：
之前在分析PKMS开机初始化的时候，知道最终会进入scanPackageDirtyLI方法内：

if ((scanFlags & SCAN_FIRST_BOOT_OR_UPGRADE) == 0) {
    PackageSetting foundPs = mSettings.getPackageLPr(pkg.packageName);
    if (foundPs != null) {
        primaryCpuAbiFromSettings = foundPs.primaryCpuAbiString;
        secondaryCpuAbiFromSettings = foundPs.secondaryCpuAbiString;
    }
}

可以看到，这部分逻辑是，如果不是第一次开机或者升级，就从mSettings中读取PackageSetting类型的值，再将其中存储的关于ABI的值保存到临时变量中
再往下看

if ((scanFlags & SCAN_NEW_INSTALL) == 0) {
    if ((scanFlags & SCAN_FIRST_BOOT_OR_UPGRADE) != 0) {
        Trace.traceBegin(TRACE_TAG_PACKAGE_MANAGER, "derivePackageAbi");
        final boolean extractNativeLibs = !pkg.isLibrary();
        derivePackageAbi(pkg, scanFile, cpuAbiOverride, extractNativeLibs,
                mAppLib32InstallDir);
        Trace.traceEnd(TRACE_TAG_PACKAGE_MANAGER);

        // Some system apps still use directory structure for native libraries
        // in which case we might end up not detecting abi solely based on apk
        // structure. Try to detect abi based on directory structure.
        if (isSystemApp(pkg) && !pkg.isUpdatedSystemApp() &&
                pkg.applicationInfo.primaryCpuAbi == null) {
            setBundledAppAbisAndRoots(pkg, pkgSetting);
            setNativeLibraryPaths(pkg, mAppLib32InstallDir);
        }
    } else {
        // This is not a first boot or an upgrade, don't bother deriving the
        // ABI during the scan. Instead, trust the value that was stored in the
        // package setting.
        pkg.applicationInfo.primaryCpuAbi = primaryCpuAbiFromSettings;
        pkg.applicationInfo.secondaryCpuAbi = secondaryCpuAbiFromSettings;

        setNativeLibraryPaths(pkg, mAppLib32InstallDir);

        if (DEBUG_ABI_SELECTION) {
            Slog.i(TAG, "Using ABIS and native lib paths from settings : " +
                pkg.packageName + " " + pkg.applicationInfo.primaryCpuAbi + ", " +
                pkg.applicationInfo.secondaryCpuAbi);
        }
    }
} 

这里会再次判断是否第一次开机或者升级
如果是的话，调用derivePackageAbi方法解析abi路径等相关信息存到pkg中

pkg.applicationInfo.primaryCpuAbi
pkg.applicationInfo.secondaryCpuAbi

如果不是，将上面第一次判断得到的临时变量的值复制到pkg中

获取到ABI后肯定要保存起来：
在commitPackageSettings方法中会调用：mSettings.insertPackageSettingLPw(pkgSetting, pkg)

[Settings.java]
void insertPackageSettingLPw(PackageSetting p, PackageParser.Package pkg) {

    addPackageSettingLPw(p, p.sharedUser);
}

private void addPackageSettingLPw(PackageSetting p, SharedUserSetting sharedUser) {
    //这里会加入到mPackages中
    mPackages.put(p.name, p);
}

之前分析PKMS的构造函数时候说到，在构造函数最后会执行mSettings.writeLPr()
writeLPr函数会把mPackages相关信息写入到package.xml等文件中
而在package.xml中就记录了ABI的信息：

第一次判断的时候如果不是首次开机或者升级就从mSettings的mPackages中读取PackageSetting类型的值
下面看看packages.xml中的值是怎么读取到mPackages中的
在PKMS构造函数的开始，会调用mSettings.readLPw

[Settings.java]
boolean readLPw(@NonNull List users) {
    //mBackupSettingsFilename = new File(mSystemDir, "packages-backup.xml");
    str = new FileInputStream(mBackupSettingsFilename);
    XmlPullParser parser = Xml.newPullParser();
    parser.setInput(str, StandardCharsets.UTF_8.name());
    if (tagName.equals("package")) {
        readPackageLPw(parser);
    }
}

readLPw的逻辑就是删除packages.xml，从packages-backup.xml中读取值
再按照不同的节点来来调用不同的方法解析，这里主要看readPackageLPw方法

primaryCpuAbiString = parser.getAttributeValue(null, "primaryCpuAbi");
secondaryCpuAbiString = parser.getAttributeValue(null, "secondaryCpuAbi");
//读取一些值后调用addPackageLPw方法
packageSetting = addPackageLPw(......, primaryCpuAbiString,secondaryCpuAbiString,......);

PackageSettinga ddPackageLPw(......) {
    p = new PackageSetting(......, primaryCpuAbiString, secondaryCpuAbiString,......);
    mPackages.put(name, p);
}

这样就加入到mPackages中了