'Smart JVM and JIT Micro-Optimizations

Over time, Sun's JVM and JIT have gotten pretty smart. Things that used to be common knowledge as being a necessary micro-optimization are no longer needed, because it gets taken care of for you.

For example, it used to be the case that you should mark all possible classes as final, so the JVM inlines as much code as possible. However now, the JIT knows whether your class is final based on what classes get loaded in at runtime, and if you load a class to make the original one non-final-able, it un-inlines the methods and un-marks it as final.

What other smart micro-optimizations does the JVM or JIT do for you?

EDIT: I made this a community wiki; I'd like to collect these over time.

javajvmjitmicro-optimization


Solution 1:[1]

It's beyond impressive. All of these are things you can't do in C++ (certainly to the same extent Java does). Keep in mind that early versions of Java started the "slow" reputation by not having these things, and we keep improving significantly over time. This is still a big research area.

Solution 2:[2]

Oracle has a wiki on Performance techniques used in the Hotspot JVM.

Solution 3:[3]

Java is smarter at inlining as it can

  • inline code only available at runtime or even dynamically generated.
  • inline virtual methods (up to two at once)
  • perform escape analysis on inlined methods and the methods they were inlined to. (Much harder to do in C++)

Sources

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Source: Stack Overflow

Solution Source
Solution 1 Glorfindel
Solution 2
Solution 3 Peter Lawrey

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