Java Performance Tuning

Variables

Local (temporary) variables and method-argument variables are the fastest variables to access and update. Local variables remain on the stack, so they can be manipulated directly; the manipulation of local variables depends on both the VM and underlying machine implementation. Heap variables (static and instance variables) are manipulated in heap memory through the Java VM-assigned bytecodes that apply to these variables. There are special bytecodes for accessing the first four local variables and parameters on a method stack. Arguments are counted first; then, if there are less than four passed arguments, local variables are counted. For nonstatic methods, this always takes the first slot. longs and doubles each take two slots. Theoretically, this means that methods with no more than three parameters and local variables combined (four for static methods) should be slightly faster than equivalent methods with a larger number of parameters and local variables. This also means that any variables allocated the special bytecodes should be slightly faster to manipulate. In practice, I have found any effect is small or negligible, and it is not worth the effort involved to limit the number of arguments and variables.

Instance and static variables can be up to an order of magnitude slower to operate on when compared to method arguments and local variables. You can see this clearly with a simple test comparing local and static loop counters:

package tuning.exception; public ...