在Java / Android中，为什么我可以从ParcelFileDescriptor获取整数文件描述符？

Question

I have solved a problem I had while coding on the NDK, but I'm not sure the solution is canonical, or if there is a canonical way to do this. First a description of what I did:

It appears that I cannot access an integer file descriptor value using Java's File or FileDescriptor objects: http://developer.android.com/reference/java/io/File.html http://developer.android.com/reference/java/io/FileDescriptor.html

But I can using Android's ParcelFileDescriptor object: http://developer.android.com/reference/android/os/ParcelFileDescriptor.html

So I can get the integer fd to my native code in this way:

pfd = ParcelFileDescriptor.open(new File("blah"), MODE_WRITE_ONLY);
myNativeFunction(pfd.getFd());

Why does this work? Wouldn't the file descriptor integer field be private in the File object, since I can't access it even when I own the object? So how does the ParcelFileDescriptor get to access this presumably private field just by being passed the object in one of its public methods? Do I even want to know?

Answer 1

The Java.io.File and Java.io.FileDescriptor are part of the base Java APIs defined by Sun, which, by design, never expose an integer file descriptor to the. This is likely done for the sake of abstraction, ie to make Java programs portable to custom operating systems that don't use simple integers to identify opened file descriptors.

On the other hand, android.os.ParcelFileDescriptor is an Android-specific API, and on Android, which is based on Linux, integers are always used to model file descriptors, so it's ok to expose it.

You probably don't need to know exactly how ParcelFileDescriptor performs its magic though. Just my 2 cents.

Answer 2

how does the ParcelFileDescriptor get to access this presumably private field

The integer file descriptor is not part of java.io.File class at all. This class wraps a file name and can be used for names that don't exist or even cannot exist on the file system. When you work with Java File object, the Linux file is not opened. So, to open the file, we use one of the many classes like FileInputStream , FileReader , etc.

FileDescriptor class, defined in the core Java API, can be used for some manipulations with open files; it does "know" the actual int file descriptor (you can check its source code ), but Android SDK isolates this number, as @danske correctly explained.

android.os.ParcelFileDescriptor uses system library libnativehelper.so to find the value, you can see the relevant source code , too. You can actually look up the sources of the nativehelper , too.