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[Android][Framework]裁剪SystemServer服务以及关闭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
/**
* 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
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。
// 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。
具体的feature说明可以看官方文档
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