Question

I plan on using Necessitas to port Qt code to the Android platform. On a first sight I noticed despite being native code, everything still passes through the Dalvik VM.

My question is does this introduce overhead? Java is less efficient than native c++ to begin with and Dalvik is rather immature compared to vanilla Java, which is the cause of my concerns.

Answer 1

In the Android documentation you can find the following tip:

Native code isn’t necessarily more efficient than Java. For one thing,
there’s a cost associated with the Java-native transition, and the JIT
can’t optimize across these boundaries. If you’re allocating native
resources (memory on the native heap, file descriptors, or whatever),
it can be significantly more difficult to arrange timely collection of
these resources. You also need to compile your code for each
architecture you wish to run on (rather than rely on it having a JIT).
You may even have to compile multiple versions for what you consider
the same architecture: native code compiled for the ARM processor in
the G1 can’t take full advantage of the ARM in the Nexus One, and code
compiled for the ARM in the Nexus One won’t run on the ARM in the G1.

Of coarse, Dalvik code is slower than pure C/C++ optimized for the platform. But communication between native code and Java code happens through JNI which is the main source of the overhead.

So the answer for your question is yes, JNI introduces additional overhead. But if you want to port existing C/C++ code, ndk is the best choice in your case.