unzip -x dex2jar-version.zip -d /home/panxiaobo
use dex2jar to generate .jar file. dex2jar will generate a file named someApk-dex2jar.jar in the working folder.
linux sh /home/panxiaobo/dex2jar-version/d2j-dex2jar.sh /home/panxiaobo/someApk.apk
windows C:\dex2jar-version\d2j-dex2jar.bat someApk.apk
use a decompiler to view the source.
that's it