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Why is Java faster when using a JIT vs. compiling to machine code?

I have heard that Java must use a JIT to be fast. This makes perfect sense when comparing to interpretation, but why can’t someone make an ahead-of-time compiler that generates fast Java code? I know about gcj, but I don’t think its output is typically faster than Hotspot for example.

Are there things about the language that make this difficult? I think it comes down to just these things:

  • Reflection
  • Classloading

What am I missing? If I avoid these features, would it be possible to compile Java code once to native machine code and be done?

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Answer

The real killer for any AOT compiler is:

Class.forName(...)

This means that you cannot write a AOT compiler which covers ALL Java programs as there is information available only at runtime about the characteristics of the program. You can, however, do it on a subset of Java which is what I believe that gcj does.

Another typical example is the ability of a JIT to inline methods like getX() directly in the calling methods if it is found that it is safe to do so, and undoing it if appropriate, even if not explicitly helped by the programmer by telling that a method is final. The JIT can see that in the running program a given method is not overriden and is therefore in this instance can be treated as final. This might be different in the next invocation.


Edit 2019: Oracle has introduced GraalVM which allows AOT compilation on a subset of Java (a quite large one, but still a subset) with the primary requirement that all code is available at compile time. This allows for millisecond startup time of web containers.

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