This post originated from an RSS feed registered with Java Buzz by Norman Richards. Original Post: Java Virtual Machine != Java Language Feed Title: Orb [norman richards] Feed URL: http://members.capmac.org/~orb/blog.cgi/tech/java?flav=rss Feed Description: Monkey number 312,978,199 Latest Java Buzz Posts Latest Java Buzz Posts by Norman Richards Latest Posts From Orb [norman richards]

Henri Yandell dug into a Java class file and discovered how .class is implemented. If you think that is a hack, just wait till you see how inner classes access variables in the outer class. It sure isn't pretty.

If you stick to the assumption that the Java virtual machine is equivalent to the Java language, I can certainly see why it .class might seem like a hack. Initially, that was true, but with every post 1.1 Java release, the JVM and the Java language drift further apart. The JVM isn't moving. A few optional class/field/method attributes are added here or there, but the bytecode and operation of the JVM aren't changing. That leaves it up to the compiler writers to find ways to implement new Java features on the existing JVM structure.

There's no Java bytecode to load a Class object directly, so the compiler has to find a way to implement it. There's no concept of inner classes, dynamic proxies, or assertions, leaving it up to the compiler to fend for itself. (the JVM does play some minor role in configuring assertion enable/disable, but that's really outside of the JVM specification)

Looking forward to 1.5, metadata, generics and autoboxing and enums are all a combination of the compiler and the Java libraries. There's really nothing new for the JVM. I don't

One person's hack is another person's implmentation detail. Since there is very good reason to not keep the JVM view of the world in sync with the Java language view of the world, Java language implementation in the JVM is going to get more and more interesting implementation details.

I think a strong understanding how the Java language maps onto the Java virtual machine is critical for senior level programmer types. You want to live in the Java abstraction, but to truly understand it you really have to get to the level below it. That is really the motivation for the Java Internals talk I'm giving at the Dallas Lonestar Software Symposium next week. I'm more than a bit biased, but if you are going to conference then take a look at that session. If you have never looked beneath the hood, I promise you'll find the session invaluable.