App层使用pms服务流程，先通过Context的getPackageManager取得PackageManger对象后，再调用PackageManger相关操作方法完成相应处理。
而这个Context实质是ContextImpl，在ActivityThread中创建应用时生成的。这个类提供了getPackageManager()，内部通过ActivityThread.getPackageManager()获得IPackageManager对象，再生成ApplicationPackageManager对象，返回给APP层。
PackageManger类是一个抽象类，ApplicationPackageManager继承自PackageManger，使用PMS服务对象进行封装，进而间接操作PMS服务，完成相应操作。
ActivityThread.getPackageManger()，是通过ServiceManager.getService()获取了PMS服务对象，再生成PMS服务代理对象，最后返回给ContextImpl。
PackageManagerService.java//实现PMS服务端接口，对客户端提供各种操作。
public class PackageManagerService extends IPackageManager.Stub {
@Override
public boolean onTransact(int code, Parcel data, Parcel reply, int flags)
throws RemoteException {
try {
return super.onTransact(code, data, reply, flags);
} catch (RuntimeException e) {
if (!(e instanceof SecurityException) && !(e instanceof IllegalArgumentException)) {
Slog.wtf(TAG, "Package Manager Crash", e);
}
throw e;
}
}
//main方法由SystemServer调用。
public static final IPackageManager main(Context context, Installer installer,
boolean factoryTest, boolean onlyCore) {
PackageManagerService m = new PackageManagerService(context, installer,
factoryTest, onlyCore);
ServiceManager.addService("package", m); //注册服务
return m;
}
}
PackageManager.java //抽象类，作为Context的getPackageManager返回类型。
public abstract class PackageManager {
}
ApplicationPackageManager.java//继承自PackageManager，使用PMS对象封装各类操作。
final class ApplicationPackageManager extends PackageManager {
private IPackageManager mPM; //PMS服务对象
ApplicationPackageManager(ContextImpl context,
IPackageManager pm) {
mContext = context;
mPM = pm;
}
}
ContextImpl.java//ActivityThread中创建该对象，并通过getPackageManager返回ApplicationPackageManager对象给应用层。
class ContextImpl extends Context {
@Override
public PackageManager getPackageManager() {
if (mPackageManager != null) {
return mPackageManager;
}
IPackageManager pm = ActivityThread.getPackageManager(); //获取PMS服务
if (pm != null) {
// Doesn't matter if we make more than one instance.
return (mPackageManager = new ApplicationPackageManager(this, pm));//使用PMS服务对像初始化ApplicationPackageManager
}
return null;
}
}
ActivityThread.java //生成ContextImpl对象，及通过ServiceManager获取PMS服务对象。
public final class ActivityThread {
public static IPackageManager getPackageManager() {
if (sPackageManager != null) {
//Slog.v("PackageManager", "returning cur default = " + sPackageManager);
return sPackageManager;
}
IBinder b = ServiceManager.getService("package");//通过ServiceManager获取PMS服务对象。
//Slog.v("PackageManager", "default service binder = " + b);
sPackageManager = IPackageManager.Stub.asInterface(b); //返回服务调用代理
//Slog.v("PackageManager", "default service = " + sPackageManager);
return sPackageManager;
}
private Context createBaseContextForActivity(ActivityClientRecord r,
final Activity activity) {
ContextImpl appContext = new ContextImpl(); //创建appContext， 生成ContextImpl对象。
appContext.init(r.packageInfo, r.token, this);
appContext.setOuterContext(activity);
}
}