After we were teased with a video of the Xperia Z3 running Android 5.0 AOSP, Sony is now releasing the source codes for not only the Z3, but the Xperia Z2 and even Z1, too. The AOSP source codes mean that developers can now start coining away custom ROMs for all flagship Xperias released this and last year, and the results shouldn't be far behind.
The beauty of AOSP is that devs can mod and add features as they see fit, without the need for having to overcome the manufacturer's UI overlay boundaries. If you are eager to check the AOSP codes out, you can take a lead, and dive into the Xperia section of Sony's Developer World site from the source link below.
As per Sony, you can "download the needed software binaries for Lollipop, and follow the AOSP build instructions to download, configure, build and finally flash the source code onto your device." Of course, these binaries are with limited functionality and not suitable for daily drivers as-is, but an enterprising developer with some skills can quickly make a swan out of that ugly duckling.
Since all recent Xperia flagships have an unlocked bootloader, Android 5.0 Lollipop for your Z3, Z2 or Z1 isn't a pipe dream anymore, and might happen much sooner than the official "early 2015" timeframe that Sony revealed earlier. Actually, Sony was recently looking for beta testers of Android 5.0 Lollipop on the Z3 and Z3 Compact, so things are starting to align nicely for a swift update.