Android 8.0 PKMS新变化之提取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：

<package name="com.google.andro
9d44
id.youtube" codePath="/system/app/YouTube" nativeLibraryPath="/system/app/YouTube/lib" primaryCpuAbi="arm64-v8a" ......>
<package name="com.android.chrome" codePath="/system/app/Chrome" nativeLibraryPath="/system/app/Chrome/lib" primaryCpuAbi="armeabi-v7a" secondaryCpuAbi="arm64-v8a" .......>

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<UserInfo> 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