CODEWITHSUNDEEP
javajvm.class-file
Curious Learner
Curious Learner

Reputation: 463

Why is the constant pool (in Java classfile) indexed from 1 (and not 0)? What is the constant_pool[0] entry reserved for?

From the JVM specs (Chapter 4.1 "The ClassFile structure"), it is stated that "The constant_pool table is indexed from 1 to constant_pool_count - 1."

I'm curious why they're skipping [0] and what is this entry reserved for.

Upvotes: 6

Views: 1565

Answers (2)

Antimony
Antimony

Reputation: 39451

They skipped index 0 so that it can be used for cases where you would normally reference a constant pool entry, but instead want to indicate "nothing". It is the constant pool equivalent of a null pointer.

The most notable use for index 0 is for "catch all" exception handlers. An exception handler can either point to the constant pool entry for the class of exceptions it wants to handle, or just use index 0 to catch everything (this is equivalent to catching java/lang/Throwable). In practice, the compiler will generate catch all exception handlers to implement finally, synchronized blocks, and the cleanup portions of try with resources, among other things.

Other uses for index 0 include:

  • The superclass of java/lang/Object
  • The name of a parameter with no name
  • The outer class for classes which are not the member of another class (i.e. top level classes, local classes, and anonymous classes)
  • The inner name of anonymous classes
  • The enclosing method for classes which are not immediately enclosed in a method
  • Version info for a module with no version info
  • Dependencies for a module with no dependency info

Upvotes: 12

John Bollinger
John Bollinger

Reputation: 180286

There is no reserved entry. "[I]ndexed from 1" means that the first entry in the constant pool corresponds to index 1, so there isn't any constant_pool[0]. Rather, there is a reserved index, 0, that is known with certainty to not be a valid index into the constant pool.

Documented uses for that reserved index seem to be rare, but at least section 4.7.24 specifies a use: method parameters' names are represented by indexes into the constant pool, and for unnamed parameters, that index is given as 0.

Note also that the name constant_pool_count is misleading. The specification explicitly says that there are only constant_pool_count - 1 entries in the constant pool (see the structure declaration at the top of section 4.1, and compare to some of the other members).

Upvotes: 1

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