5.5 Subroutines

Another kind of special branch is the jsr, for jump subroutine. It's like a goto that remembers where it came from. Or you can think of it as a method invocation that doesn't create a new stack frame. For example,

 state1: jsr get_next_character   ; Go to the subroutine that leaves                          ; the next char on top of the stack lookupswitch             ; Jump to the next state    65: state1            ; Go to state 1 if the value is 65    66: state2            ; Go to state 2 if the value is 66    default: state3       ; Otherwise, go to state 3 get_next_character: ; At this point, there's a returnAddress on top of the stack astore_3                 ; Store it in variable 3 ;; Do code to get the next character, ;; and leave it on top of the stack ret 3                    ; Return to the location in variable 3 

The jsr is used to take code that would otherwise have to be replicated in several places and put it in a single place. It's not quite as powerful as putting it into a separate method, since there's no explicit facilities for passing arguments. It is less expensive to use, however, since the machine doesn't have to create a whole new stack frame.

When jsr is executed, it branches to the location specified by the label, and it leaves a special kind of value on the stack called a returnAddress to represent the return address. It's your responsibility to remember this value by storing it in a local variable. The value is a kind of reference, so you use the aload and astore instructions to store its value. However, you can't call methods on it, store it in a field, or use it as an argument to a method. As with the other branches, the label must be inside the body of the same method as the jsr instruction.

At the end of a subroutine, you can return to the instruction after the jsr with a ret. A ret is different from a return, which is used to return from an invocation. The argument to ret is a number that represents a local variable. That variable should contain the return address that was on the stack at the beginning of the subroutine.



Programming for the Java Virtual Machine
Programming for the Javaв„ў Virtual Machine
ISBN: 0201309726
EAN: 2147483647
Year: 1998
Pages: 158
Authors: Joshua Engel

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