int pid = Binder.getCallingPid();
int uid = Binder.getCallingUid();
String callingApp = mContext.getPackageManager().getNameForUid(uid);

//程序A：
public class MainActivityA extends Activity {
    TextView textView;
    
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        textView = (TextView)findViewById(R.id.textView1);
        WriteSettings(this, "123");
    }

    public void WriteSettings(Context context, String data) {
        FileOutputStream fOut = null;
        OutputStreamWriter osw = null;
        try {
            //默认建立在data/data/xxx/file/ 
            fOut = this.openFileOutput("settings.dat", MODE_PRIVATE);            
            osw = new OutputStreamWriter(fOut);
            osw.write(data);
            osw.flush();
            Toast.makeText(context, "Settings saved", Toast.LENGTH_SHORT)
                    .show();
        } catch (Exception e) {
            e.printStackTrace();
            Toast.makeText(context, "Settings not saved", Toast.LENGTH_SHORT)
                    .show();
        } finally {
            try {
                osw.close();
                fOut.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}

//程序B：
public class MainActivityB extends Activity {
    TextView textView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        textView = (TextView) this.findViewById(R.id.textView1);
        
        try {
            //获取程序A的context
            Context ctx = this.createPackageContext(
                    "com.example.shareusertesta", Context.CONTEXT_IGNORE_SECURITY);
            String msg = ReadSettings(ctxDealFile);
            Toast.makeText(this, "DealFile2 Settings read" + msg,
                    Toast.LENGTH_SHORT).show();
            WriteSettings(ctx, "deal file2 write");
        } catch (NameNotFoundException e) {
            // TODO Auto-generated catch block
            e.printStackTrace();
        }
    }

    public String ReadSettings(Context context) {
        FileInputStream fIn = null;
        InputStreamReader isr = null;
        char[] inputBuffer = new char[255];
        String data = null;
        try {
            //此处调用并没有区别，但context此时是从程序A里面获取的
            fIn = context.openFileInput("settings.dat");
            isr = new InputStreamReader(fIn);
            isr.read(inputBuffer);
            data = new String(inputBuffer);
            textView.setText(data);
            Toast.makeText(context, "Settings read", Toast.LENGTH_SHORT).show();
        } catch (Exception e) {
            e.printStackTrace();
            Toast.makeText(context, "Settings not read", Toast.LENGTH_SHORT)
                    .show();
        } finally {
            try {
                isr.close();
                fIn.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
        return data;
    }

    public void WriteSettings(Context context, String data) {
        FileOutputStream fOut = null;
        OutputStreamWriter osw = null;
        try {
            fOut = context.openFileOutput("settings.dat", MODE_PRIVATE);
            //此处调用并没有区别，但context此时是从程序A里面获取的
            osw = new OutputStreamWriter(fOut);
            osw.write(data);
            osw.flush();
            Toast.makeText(context, "Settings saved", Toast.LENGTH_SHORT)
                    .show();

        } catch (Exception e) {
            e.printStackTrace();
            Toast.makeText(context, "Settings not saved", Toast.LENGTH_SHORT)
                    .show();

        } finally {
            try {
                osw.close();
                fOut.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }
    }
}