[Android][Framework]裁剪SystemServer服务以及关闭SystemFeature

本文链接 http://wossoneri.github.io/2018/08/30/[Android][Framework]crop-SystemServer-and-SystemFeature/

SystemServer服务裁剪

有些系统，因为应用场景的不同，需要的服务也不一样。比如Android Things，为了应对IOT的应用场景，它就裁剪掉了很多服务。下面介绍一下裁剪服务的方法。

关于服务，要提一下SystemServer，具体介绍见另一篇文章：http://wossoneri.github.io。SystemServer启动了系统的核心服务，除此之外，SystemServer还启动了很多其他服务，具体是在startOtherServices()方法中。我们要裁剪不需要的服务就可以从这里入手。

比如要关闭打印机服务：

可以直接把相关启动服务的代码注释掉：

//mSystemServiceManager.startService(PRINT_MANAGER_SERVICE_CLASS);

当然这样修改后，以后如果要再打开，还需要再次修改SystemServer，然后编译jar包，push到设备使其生效。

所以建议使用下面的改法：

首先定义boolean变量，从全局属性读取配置，

boolean disablePrinter = SystemProperties.getBoolean("config.disable_printer", false);

然后在启动服务的代码段添加对这个属性的判断：

if (!disablePrinter && mPackageManager.hasSystemFeature(PackageManager.FEATURE_PRINTING)) {
    mSystemServiceManager.startService(PRINT_MANAGER_SERVICE_CLASS);
}

之后在MakeFile增加自定义的系统权限：

PRODUCT_PROPERTY_OVERRIDES += \
    config.disable_printer=true

以后如果要打开这个服务，就把true变成false就可以了。

如果要调试，从修改设备的 /system/build.prop 然后重启即可。非常方便有木有！

如果要修改，删掉out目录下的build.prop，重新编译system（或者直接修改build.prop然后make snod），烧录启动系统之后，运行如下命令进行验证：

service check printer

这样就不会再启动不需要的服务了。

裁剪服务引发的问题

服务不是你不让它Start就完事儿了，系统那么大，总有一些地方会获取服务对象做一些调用处理。比如我们刚裁减掉了打印机服务，然后打开Settings就遇到了crash：

E AndroidRuntime: FATAL EXCEPTION: main
E AndroidRuntime: Process: com.android.settings, PID: 3496
E AndroidRuntime: java.lang.RuntimeException: Unable to start activity ComponentInfo{com.android.settings/com.android.settings.Settings}: java.lang.NullPointerException: Attempt to invoke interface method 'java.util.List android.print.IPrintManager.getPrintServices(int, int)' on a null object reference
E AndroidRuntime: 	at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2665)
E AndroidRuntime: 	at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726)
E AndroidRuntime: 	at android.app.ActivityThread.-wrap12(ActivityThread.java)
E AndroidRuntime: 	at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477)
E AndroidRuntime: 	at android.os.Handler.dispatchMessage(Handler.java:102)
E AndroidRuntime: 	at android.os.Looper.loop(Looper.java:154)
E AndroidRuntime: 	at android.app.ActivityThread.main(ActivityThread.java:6119)
E AndroidRuntime: 	at java.lang.reflect.Method.invoke(Native Method)
E AndroidRuntime: 	at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:900)
E AndroidRuntime: 	at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:790)
E AndroidRuntime: Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'java.util.List android.print.IPrintManager.getPrintServices(int, int)' on a null object reference
E AndroidRuntime: 	at android.print.PrintManager.getPrintServices(PrintManager.java:635)
E AndroidRuntime: 	at android.print.PrintServicesLoader.onStartLoading(PrintServicesLoader.java:88)
E AndroidRuntime: 	at android.content.Loader.startLoading(Loader.java:290)
E AndroidRuntime: 	at android.app.LoaderManagerImpl$LoaderInfo.start(LoaderManager.java:283)
E AndroidRuntime: 	at android.app.LoaderManagerImpl.installLoader(LoaderManager.java:579)
E AndroidRuntime: 	at android.app.LoaderManagerImpl.createAndInstallLoader(LoaderManager.java:566)
E AndroidRuntime: 	at android.app.LoaderManagerImpl.initLoader(LoaderManager.java:619)
E AndroidRuntime: 	at com.android.settings.search.DynamicIndexableContentMonitor.register(DynamicIndexableContentMonitor.java:136)
E AndroidRuntime: 	at com.android.settings.SettingsActivity.onStart(SettingsActivity.java:868)
E AndroidRuntime: 	at android.app.Instrumentation.callActivityOnStart(Instrumentation.java:1249)
E AndroidRuntime: 	at android.app.Activity.performStart(Activity.java:6737)
E AndroidRuntime: 	at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2628)
E AndroidRuntime: 	... 9 more

通过堆栈信息，可以知道PrintManager.getPrintServices出现了空指针。这里也不用看代码就能猜到，因为我们开机没有启动打印服务，所以肯定get不到这个服务的。

然后考虑修改方案，增加非空保护是不是就可以了？Naive！我们的目的是裁剪打印服务，所以我们的修改点并不在这个服务本身，而是删除所以调用这个服务的地方。

所以堆栈显示的PrintManager，PrintServicesLoader什么的统统不要改，我们要看代码结构，看看是怎么调用进来的。通过阅读代码，了解到系统里有很多Loader类型的对象，其中一个子类就是PrintServicesLoader。然后这些Loader是由LoaderManager管理启动的。而LoaderManager在DynamicIndexableContentMonitor.java出现过一次初始化操作。

看下DynamicIndexableContentMonitor.java代码：

public void register(Activity activity, int loaderId) {
    ...
	boolean hasFeaturePrinting = mContext.getPackageManager().hasSystemFeature(PackageManager.FEATURE_PRINTING);
    ...
	if (hasFeaturePrinting) {
		activity.getLoaderManager().initLoader(loaderId, null, this);
	}
    ...

有木有发现一个熟悉的代码？

对，代码里再次出现了一个有关SystemFeature的判断！上一次出现时SystemServer启动服务前也做了相同的判断。

所以要裁剪掉打印机服务，我们只需要将FEATURE_PRINTING关闭即可。

通过修改SystemFeature判断后，在SystemServer里面的裁剪代码就可以不再添加了。但是有些服务的裁剪Android并没有添加系统特性的处理，所以还是建议使用我的方法进行裁剪。

SystemFeature加载流程

先看一看FEATURE_PRINTING

PackageManager.java

/**
 * Feature for {@link #getSystemAvailableFeatures} and {@link #hasSystemFeature}:
 * The device supports printing.
 */
@SdkConstant(SdkConstantType.FEATURE)
public static final String FEATURE_PRINTING = "android.software.print";
/**
 * Get a list of features that are available on the
 * system.
 *
 * @return An array of FeatureInfo classes describing the features
 * that are available on the system, or null if there are none(!!).
 */
public abstract FeatureInfo[] getSystemAvailableFeatures();
/**
 * Check whether the given feature name is one of the available features as
 * returned by {@link #getSystemAvailableFeatures()}. This tests for the
 * presence of <em>any</em> version of the given feature name; use
 * {@link #hasSystemFeature(String, int)} to check for a minimum version.
 *
 * @return Returns true if the devices supports the feature, else false.
 */
public abstract boolean hasSystemFeature(String name);
/**
 * Check whether the given feature name and version is one of the available
 * features as returned by {@link #getSystemAvailableFeatures()}. Since
 * features are defined to always be backwards compatible, this returns true
 * if the available feature version is greater than or equal to the
 * requested version.
 *
 * @return Returns true if the devices supports the feature, else false.
 */
public abstract boolean hasSystemFeature(String name, int version);

都是抽象方法，我们去PMS查找对应的实现

PackageManagerService.java

public @NonNull ParceledListSlice<FeatureInfo> getSystemAvailableFeatures() {
    synchronized (mPackages) {
        final ArrayList<FeatureInfo> res = new ArrayList<>(mAvailableFeatures.values());
        final FeatureInfo fi = new FeatureInfo();
        fi.reqGlEsVersion = SystemProperties.getInt("ro.opengles.version",
                                                    FeatureInfo.GL_ES_VERSION_UNDEFINED);
        res.add(fi);
        return new ParceledListSlice<>(res);
    }
}
@Override
public boolean hasSystemFeature(String name, int version) {
    synchronized (mPackages) {
        final FeatureInfo feat = mAvailableFeatures.get(name);
        if (feat == null) {
            return false;
        } else {
            return feat.version >= version;
        }
    }
}

这里的逻辑都是通过mAvailableFeatures得到所有的feature，查找该成员变量的相关代码

final ArrayMap<String, FeatureInfo> mAvailableFeatures;
SystemConfig systemConfig = SystemConfig.getInstance();
mGlobalGids = systemConfig.getGlobalGids();
mSystemPermissions = systemConfig.getSystemPermissions();
mAvailableFeatures = systemConfig.getAvailableFeatures();

了解到，首先获取一个SystemConfig的单例，然后通过getAvailableFeatures方法获取可用的feature。

SystemConfig.java

// These are the features this devices supports that were read from the
// system configuration files.
final ArrayMap<String, FeatureInfo> mAvailableFeatures = new ArrayMap<>();
public ArrayMap<String, FeatureInfo> getAvailableFeatures() {
    return mAvailableFeatures;
}
private void addFeature(String name, int version) {
    FeatureInfo fi = mAvailableFeatures.get(name);
    if (fi == null) {
        fi = new FeatureInfo();
        fi.name = name;
        fi.version = version;
        mAvailableFeatures.put(name, fi);
    } else {
        fi.version = Math.max(fi.version, version);
    }
}
private void removeFeature(String name) {
    if (mAvailableFeatures.remove(name) != null) {
        Slog.d(TAG, "Removed unavailable feature " + name);
    }
}

根据mAvailableFeatures的注释，设备支持的feature是从配置文件里读取出来的。调用读取配置文件的地方是：

SystemConfig() {
    // Read configuration from system
    readPermissions(Environment.buildPath(
        Environment.getRootDirectory(), "etc", "sysconfig"), ALLOW_ALL);
    // Read configuration from the old permissions dir
    readPermissions(Environment.buildPath(
        Environment.getRootDirectory(), "etc", "permissions"), ALLOW_ALL);
    // Allow ODM to customize system configs around libs, features and apps
    int odmPermissionFlag = ALLOW_LIBS | ALLOW_FEATURES | ALLOW_APP_CONFIGS;
    readPermissions(Environment.buildPath(
        Environment.getOdmDirectory(), "etc", "sysconfig"), odmPermissionFlag);
    readPermissions(Environment.buildPath(
        Environment.getOdmDirectory(), "etc", "permissions"), odmPermissionFlag);
    // Only allow OEM to customize features
    readPermissions(Environment.buildPath(
        Environment.getOemDirectory(), "etc", "sysconfig"), ALLOW_FEATURES);
    readPermissions(Environment.buildPath(
        Environment.getOemDirectory(), "etc", "permissions"), ALLOW_FEATURES);
}

到此就很明白了，它是读取了几个目录：

• /system/etc/permission
• /system/etc/sysconfig
• /oem/etc/permission
• /oem/etc/sysconfig
• /odm/etc/permission
• /odm/etc/sysconfig

然后遍历xml文件，进行解析处理。SystemFeature就是解析的Feature标签。

最后再总结一下加载流程：

屏蔽SystemFeature

知道原理就好做了，在系统扫描的几个目录中使用grep命令查找控制打印机的字串，找到:

/system/etc/permission/handheld_core_hardware.xml

    <!-- basic system services -->
    <feature name="android.software.app_widgets" />
    <feature name="android.software.connectionservice" />
    <feature name="android.software.voice_recognizers" notLowRam="true" />
    <feature name="android.software.backup" />
    <feature name="android.software.home_screen" />
    <feature name="android.software.input_methods" />
    <feature name="android.software.print" />   <------这个就是打印特性

将其注释掉就可以在手机进行测试了。

但是，我们还需要修改源码，保证以后编译系统这个值都是被屏蔽的。

查找MakeFile，找到如下:

PRODUCT_COPY_FILES := \ frameworks/native/data/etc/handheld_core_hardware.xml:system/etc/permissions/handheld_core_hardware.xml

这个文件在源码中的位置是frameworks/native/data/etc/。找到该源码文件，将不要的Feature注释掉，然后重新编译源码，启动系统，一切正常！打印机相关的服务彻底被屏蔽掉了，系统启动速度，资源消耗又变小了一点点。嗯，是很小的一点点，我们还可以把VR，红外线等等很多服务裁剪掉，以适应不同应用场景下的精简系统。