Why do indexes to constantpool take up different amount of bytes in classfile format

Question

I have been learning about the java class format and i was just wondering why sometimes an opcode with a constant pool entry following takes up two bytes in the class file such as with InvokeStatic, but then op-codes such as ldc with an integer index take up only one byte in the class file. Is there any pattern behind this, i am asking this because i am writing a simple byte-code manipulation library and i would like to be able to know weather to write a constant pool index as a byte or a short without hard-coding every single command into the library.

For example `ldc_w` takes up two bytes while `ldc` takes up one. — johnchen902, Jul 02 '13 at 11:41

score 1 · Accepted Answer · answered Jul 02 '13 at 14:42

1

Yes there is a pattern: every instruction except ldc takes a two byte index.

Presumably the designers of the bytecode format deciding that loading constants was such a common task that they should provide a shorter instruction for it.

answered Jul 02 '13 at 14:42

Antimony

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still why not do that for all instructions – Popgalop Jul 08 '13 at 21:58
@Popgalop You'd have to ask the designers of Java for that. Probably they just didn't think it'd be worth it. – Antimony Jul 09 '13 at 03:02

Why do indexes to constantpool take up different amount of bytes in classfile format

1 Answers1