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。