简单分析 mPM.getPackageInfo() 调用流程

上文分析了 mContext.getPackageManager() 调用流程，本文再简单说下 mPM.getPacakgeInfo()调用流程。

这里就涉及到了 binder 知识。

mPM 是 ApplicationPackageManager.java 类型，此类封装了 IPackageManager 的实现。

    @Override    public PackageInfo getPackageInfo(String packageName, int flags)            throws NameNotFoundException {        return getPackageInfoAsUser(packageName, flags, mContext.getUserId());    }    @Override    public PackageInfo getPackageInfoAsUser(String packageName, int flags, int userId)            throws NameNotFoundException {        try {            PackageInfo pi = mPM.getPackageInfo(packageName, flags, userId);            if (pi != null) {                return pi;            }        } catch (RemoteException e) {            throw e.rethrowFromSystemServer();        }        throw new NameNotFoundException(packageName);    }

这里我们就要分析 IPacakgeManager.java，这个java文件是编译时转好 AIDL 文件生成的，这个AIDL文件位置：

base/core/java/android/content/pm/IPackageManager.aidl

/**
 *  See {@link PackageManager} for documentation on most of the APIs
 *  here.
 *
 *  {@hide}
 */
interface IPackageManager {
    void checkPackageStartable(String packageName, int userId);
    boolean isPackageAvailable(String packageName, int userId);
    PackageInfo getPackageInfo(String packageName, int flags, int userId);

.........................

说白了，aidl 文件顾名思义就是定义接口描述：接口名称，接口参数，返回值类型。跨进程通讯双方遵循这个格式传递数据。

Android在编译代码的时候转化未 IPackageManager.java文件：

@Override public android.content.pm.PackageInfo getPackageInfo(java.lang.String packageName, int flags, int userId) throws android.os.RemoteException{android.os.Parcel _data = android.os.Parcel.obtain();android.os.Parcel _reply = android.os.Parcel.obtain();android.content.pm.PackageInfo _result;try {_data.writeInterfaceToken(DESCRIPTOR);_data.writeString(packageName);_data.writeInt(flags);_data.writeInt(userId);mRemote.transact(Stub.TRANSACTION_getPackageInfo, _data, _reply, 0);_reply.readException();if ((0!=_reply.readInt())) {_result = android.content.pm.PackageInfo.CREATOR.createFromParcel(_reply);}else {_result = null;}}finally {_reply.recycle();_data.recycle();}return _result;}

从上面自动生成的代码可以看出，getPackageInfo 函数讲传入参数打包成 Parcel请求，作为一个业务调用 mRemote.transact 函数发送到系统进程。

这个 mRemote 是个代理类，在 IPacakgeManager.java 实例化时就被创建：

/** * Cast an IBinder object into an android.content.pm.IPackageManager interface, * generating a proxy if needed. */public static android.content.pm.IPackageManager asInterface(android.os.IBinder obj){if ((obj==null)) {return null;}android.os.IInterface iin = obj.queryLocalInterface(DESCRIPTOR);if (((iin!=null)&&(iin instanceof android.content.pm.IPackageManager))) {return ((android.content.pm.IPackageManager)iin);}return new android.content.pm.IPackageManager.Stub.Proxy(obj);}

..........

private static class Proxy implements android.content.pm.IPackageManager{private android.os.IBinder mRemote;Proxy(android.os.IBinder remote){mRemote = remote;}

.............

这个 transact 函数是在 frameworks/base/core/java/android/os/Binder.java 中由 BinderProxy 类实现的：

final class BinderProxy implements IBinder {    public native boolean pingBinder();    public native boolean isBinderAlive();    public IInterface queryLocalInterface(String descriptor) {        return null;    }    public boolean transact(int code, Parcel data, Parcel reply, int flags) throws RemoteException {        Binder.checkParcel(this, code, data, "Unreasonably large binder buffer");        if (Binder.isTracingEnabled()) { Binder.getTransactionTracker().addTrace(); }        return transactNative(code, data, reply, flags);    }

从前面 IPackageManager 从 ActivityThread.java 中获取到的，这个从 ServiceManager 获取的 IBinder 接口来自于 PackageManagerService.java

frameworks/base/services/core/java/com/android/server/pm/PackageManagerService.java

public class PackageManagerService extends IPackageManager.Stub {

....

     public static PackageManagerService main(Context context, Installer installer,            boolean factoryTest, boolean onlyCore) {        // Self-check for initial settings.        PackageManagerServiceCompilerMapping.checkProperties();        PackageManagerService m = new PackageManagerService(context, installer,                factoryTest, onlyCore);        m.enableSystemUserPackages();        ServiceManager.addService("package", m);        return m;    }

从IPackageManager.ｊａｖａ我们可以看到 IPackageManager.Stub继承自　android.os.Binder。

所以那个　transact 函数最终掉一个到　Binder.java 里的实现，最终走到　ｃ　语言　ｓｏ　库里执行。