Possible to Override system .so library in App

Question

I have to modify the Http Live Streaming implementation of Android Media Player. The implementation is under the stagefright library http://androidxref.com/4.0.4/xref/frameworks/base/media/libstagefright/httplive/LiveDataSource.cpp

I think these library will compile to a libstagefright.so which should be part of the Android system.

My question is if I make some changes to this library and compile a new libstagefright.so. If I load this new libstagefright.so in my new application and call up the media player, will it use the code in my new libstagefright.so?

Answer 1

You will not be able to replace the original library, since when you try to loadLibrary it will load the library from within /system/lib. So unless you replace that (which is not possible on unrooted devices), you won't be able to load your custom code.

https://github.com/android/platform_system_core/blob/66ed50af6870210ce013a5588a688434a5d48ee9/rootdir/init.environ.rc.in sets the LD_LIBRARY_PATH by default. And loads it from these paths if available. If not, then your application's lib directory will be searched; but not the other way around.

I tried this with libwebkit.so in the past on various mainstream devices and haven't had any luck getting it to load instead of the one in /system/lib.

You can learn more by looking at:

• doLoad from here https://android.googlesource.com/platform/libcore/+/41d00b744b7772f9302fdb94dddadb165b951220/luni/src/main/java/java/lang/Runtime.java
• findLibrary here http://developer.android.com/reference/dalvik/system/BaseDexClassLoader.html#findLibrary(java.lang.String)

I'm pretty sure you can't replace the default class loader either for security reasons.

What you can do, though, is a straightforward fork the Media Player and have it load your modified libstagefright-modified.so. There could be other solutions, haven't looked at Media Player's code.

Answer 2

Knowing that all you want to do is parse the data before it gets to the MediaPlayer, I suggest not trying to alter the Android libraries. As soulseekah mentioned, it's not going to work without a rooted device. There are other options, although they both have drawbacks.

1) If you are only targeting recent versions (4.2 or later, I believe), you can take a look at new classes added to the android.media package, like MediaExtractor and MediaCodec. I'm not greatly familiar with those because they aren't available on the hardware with which I work, but they could be useful in getting to the raw data. Here is a decent sample of using them to play video. The drawback is those classes aren't available in earlier versions.

2) The other option is to put a local proxy on the device. Connect the MediaPlayer to the proxy and make the request to the media server yourself. See my answer here for a little more info on that. With a proxy, you will see all the data that comes through, giving you a chance to parse the ID3 tags. There is the drawback that you will have to parse the TS packets to put together an elementary stream (essentially doing the demuxer's job), but it will work with any version of Android. TS streams aren't difficult to disassemble, and ID3 tags aren't time consuming to parse, so I think this is a reasonable approach.