Question

I am using a thirdparty jar file in my android project. When I add the jar to the build paths, via eclipse project -> properties -> java build path -> Libraries -> add external jar.

The project compiles fine until that line of code runs which uses any class from that jar and I get class not found exception at runtime and the program crashes.

I solved it by copying the third party jar file to the libs folder of my project. The class not found exception goes away.

Question:
Can someone please shed some light why this is required and why does it not just work with a reference from the project properties despite the fact it compiles fine.. Why do I have to explicitly copy it to libs folder ?

Answer 1

When I add the jar to the build paths, via eclipse project -> properties -> java build path -> Libraries -> add external jar.

That is not the proper procedure for use with the Android build tools.

I solved it by copying the third party jar file to the libs folder of my project. The class not found exception goes away.

Correct.

Can someone please shed some light why this is required and why does it not just work with a reference from the project properties despite the fact it compiles fine.. Why do I have to explicitly copy it to libs folder ?

Because only the JARs in libs/ are added to the APK file. The Java build path is pure a compile-time thing, not tied into Android’s packaging of APK files.