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Editorial Team
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Asked: 2026-06-14T04:07:13+00:00

Consider two lines from code: … rs.next(); //rs is ResultSet interface. rs.getString(name); … Byte

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Consider two lines from code:

...
rs.next();  //rs is ResultSet interface.
rs.getString("name");
...

Byte code generated by javap is :

...
35:  invokeinterface #9,  1; //InterfaceMethod java/sql/ResultSet.next:()Z
   40:  pop
   41:  aload   4
   43:  ldc     #10; //String name
   45:  invokeinterface #11,  2; //InterfaceMethod java/sql/ResultSet.getString:
(Ljava/lang/String;)Ljava/lang/String;
   50:  pop
   51:  aload_2
   52:  invokeinterface #12,  1; //InterfaceMethod java/sql/Connection.close:()V

 ...

I am trying to interpret this bytecode.

For line rs.getString("name"); bytecode starts from line 43 to 51.

1)At line 45 method is INVOKED and a string object is returned as represented by Ljava/lang/String.Is this right OR here, only method is being loaded from constant pool with index #11 TO STACK and executed at line 50 ???

2) where does the string object returned by rs.getString("name"); lies? on stack or on heap as i think aload_2 is pushing some value on stack.

Actually here, i am confused in the following:

If suppose i have :

 ...
rs.next();
rs.getString("name");
rs.getString("name");
rs.getString("name");
rs.getString("name");
...10 more times...
...

and all return a same name. Then there will be 10 different string objects with same value.So it will be memory wasting and is a case for using intern(). But if these will be on stack then will it still be considered as memory wastage and need intern()???

3) I think string object returned by rs.getString("name"); is not an interned string so definitely it will not be on Permanent Generation memory area??

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1 Answer

  1. Editorial Team

    The relevant part of the bytecode for the call

    rs.getString("name")

    is

    41:  aload   4
43:  ldc     #10; //String name
45:  invokeinterface #11,  2; //InterfaceMethod java/sql/ResultSet.getString:(Ljava/lang/String;)Ljava/lang/String;
50:  pop

    Taking this line one opcode at a time:

    41:  aload   4

    pushes the object stored in local variable #4 (rs) onto the stack

    43:  ldc     #10; //String name

    pushes the string constant "name" onto the stack

    45:  invokeinterface #11,  2; //InterfaceMethod java/sql/ResultSet.getString:(Ljava/lang/String;)Ljava/lang/String;

    pops the top 2 items off the stack to call the interface method java/sql/ResultSet.getString:(Ljava/lang/String;)Ljava/lang/String; and pushes the result (a Ljava/lang/String;) onto the stack.

    50:  pop

    then pops the result from the previous method off of the stack.

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