[j-g-faustus]

Because Java was originally designed for set-top TV boxes and appliances, where it's kind of a big deal that you don't need to know or care what OS or processor each appliance is using internally.

https://web.archive.org/web/20050420081440/http://java.sun.c...

When the appliance market didn't pan out, they went for web browsers and Java applets. Bytecodes were a feature because browsers didn't exectute native code, and because it allowed for sandboxing to limit the attack surface.

Even when Java became more popular on the server than in the browser, the "write once, run everywhere" was considered a major feature: The same bytecode could be distributed everywhere; no need to maintain a heap of different build environments for different CPU architecture and OS combinations.

[mike_hearn]

I'd say the appliance market did pan out actually. BluRay players all contain an embedded JVM, as do many other kinds of set top box, as do of course all Android smart TVs.

Abstracting the CPU has worked out pretty well for the Java platform. Look at how easy the 64 bit transition was for the Java world vs the C++ world. Visual Studio is still not a 64 bit app and yet Java IDEs hardly even noticed the change. The transition on Linux was just a disaster zone, every distro came up with their own way of handling the incompatible flavours of each binary.

In addition, a simple JIT compiled instruction set makes on the fly code generation a lot easier in many cases and it's a common feature of Java frameworks. For instance the java.lang.reflect.Proxy feature is one I was using just the other day and it works by generating and loading bytecode at runtime. On the fly code generation is considered a black art for native apps and certainly extremely non portable, but is relatively commonplace and approachable in Java.

[pjmlp]

Plenty of other platforms do support bytecodes, JIT and AOT on the same toolchain.

So they could keep the WORA story and still offer AOT as an option, which actually most commercial JDKs do.

Just Sun was against providing it at all on Java SE, but they actually supported it on Java Embedded.

[j-g-faustus]

I was commenting on "why did they design Java that way in the first place", as opposed to (say) Go.

I agree that once the primary use of Java moved outside the browser, there was no particular reason to not give the option of AOT too. I'm not sure why Sun was so adamantly opposed to the idea.

If I recall correctly, Sun really wanted to stick with JIT on Java Embedded too, they just couldn't get it to run fast enough on embedded hardware. For desktop and servers, they considered bytecode interpretation and JIT "fast enough".

[pjmlp]

Sure and actually that is where mobile OSes are moving.

We now have bitcode on iDevices, DEX on Android and MSIL/MDIL on WinRT.

Still, both iDevices and Windows Store take, what I consider the best approach, to do AOT on the store for each supported target.

As Google found out, using AOT on the device doesn't scale. I just don't get why they went back to an overly complicated architecture of Interpreter/JIT/PGO → AOT, instead of following the same path as the competition and serve freshly baked AOT binaries.