Is it faster to access final local variables than class variables in Java?

by

Accessing local variable or parameter is a single step operation: take a variable located at offset N on the stack. If you function has 2 arguments (simplified):

  • N = 0 – this
  • N = 1 – first argument
  • N = 2 – second argument
  • N = 3 – first local variable
  • N = 4 – second local variable
  • …

So when you access local variable, you have one memory access at fixed offset (N is known at compilation time). This is the bytecode for accessing first method argument (int):

iload 1  //N = 1

However when you access field, you are actually performing an extra step. First you are reading “local variable” this just to determine the current object address. Then you are loading a field (getfield) which has a fixed offset from this. So you perform two memory operations instead of one (or one extra). Bytecode:

aload 0  //N = 0: this reference
getfield total I  //int total

So technically accessing local variables and parameters is faster than object fields. In practice, many other factors may affect performance (including various levels of CPU cache and JVM optimizations).

final is a different story. It is basically a hint for the compiler/JIT that this reference won’t change so it can make some heavier optimizations. But this is much harder to track down, as a rule of thumb use final whenever possible.

More Related Contents:

  1. Fastest way to strip all non-printable characters from a Java String
  2. Do java finals help the compiler create more efficient bytecode? [duplicate]
  3. Optimizing a while loop in Java
  4. In ArrayBlockingQueue, why copy final member field into local final variable?
  5. Java Reflection Performance
  6. Integers caching in Java [duplicate]
  7. Java Integer compareTo() – why use comparison vs. subtraction?
  8. Java NIO FileChannel versus FileOutputstream performance / usefulness
  9. Fastest way to determine if an integer’s square root is an integer
  10. How to efficiently remove duplicates from an array without using Set
  11. Where do I find a standard Trie based map implementation in Java? [closed]
  12. for loop optimization
  13. Optimization by Java Compiler
  14. Should I use Java’s String.format() if performance is important?
  15. Why to use StringBuffer in Java instead of the string concatenation operator
  16. How does memory reordering help processors and compilers?
  17. How to quickly determine if a method is overridden in Java
  18. Java HashMap performance optimization / alternative
  19. Is the ternary operator faster than an “if” condition in Java [duplicate]
  20. Would Java inline method(s) during optimization?
  21. Which of these pieces of code is faster in Java?
  22. Most efficient way of converting String to Integer in java
  23. What’s wrong with using associativity by compilers?
  24. Most efficient way to see if an ArrayList contains an object in Java
  25. Why defining class as final improves JVM performance?
  26. How to remove large if-else-if chain [duplicate]
  27. ‘MergeSort Algorithm’ – What’s the better implementation in JAVA? [closed]
  28. ‘ … != null’ or ‘null != ….’ best performance?
  29. Java for-loop optimization
  30. What’s the fastest way to read from System.in in Java?

Leave a Comment