Why is the Java class file format missing constant pool tag 2?

Question

The JVM specification for Java 1.0.2 lists the following constant pool entry types:

+-----------------------------+-------+
|        Constant Type        | Value |
+-----------------------------+-------+
| CONSTANT_Class              |     7 |
| CONSTANT_Fieldref           |     9 |
| CONSTANT_Methodref          |    10 |
| CONSTANT_InterfaceMethodref |    11 |
| CONSTANT_String             |     8 |
| CONSTANT_Integer            |     3 |
| CONSTANT_Float              |     4 |
| CONSTANT_Long               |     5 |
| CONSTANT_Double             |     6 |
| CONSTANT_NameAndType        |    12 |
| CONSTANT_Utf8               |     1 |
+-----------------------------+-------+

Subsequent JVM specs have added more constant pool entry types but haven't ever filled the "2" spot. Why is there a gap there?

Probably for the same reason [there's no 17, either](https://docs.oracle.com/javase/specs/jvms/se8/html/jvms-4.html#jvms-4.4). — azurefrog, Oct 09 '17 at 19:38
@jordiburgos, nope, I'm just curious. There must be a reason, and I'd like to know what it is! — 0xbe5077ed, Oct 09 '17 at 21:35
@azurefrog, 17 is added for CONSTANT_Dynamic, but 2, 13, 14 are still missing, also want to know why — Jakim, Aug 08 '20 at 09:43

Jakim · Accepted Answer · 2020-08-08T12:19:49.963

I did some research and found some clue, for the constant pool tag 2, it seems to be held open under the Constant_Unicode but has never been used, because UTF-8 is already there, and UTF-8 is widely adopted, so if there is constant written in unicode, UTF-8 can handle it, and UTF-8 has a number of advantages than other encoding scheme, so I guess this historical fact might explain why 2 is missing, I guess it can be reused for other purposes if necessary.

Some statements from here: https://bugs.openjdk.java.net/browse/JDK-8161256

For 13, 14, it should have different specific reasons why it was opened but never got used.

13 and 14 were used by early prototypes of the invokedynamic/methodhandles stuff. — Holger, Dec 07 '21 at 11:38

Why is the Java class file format missing constant pool tag 2?

1 Answers1