What is the purpose of the Java Constant Pool?

Question

I am currently trying to dig deeper into the specification of the Java Virtual Machine. I have been reading Inside the JVM book online and there is one confusing abstraction I can't seem to grasp: Constant Pool. here is the excerpt from the book:

For each type it loads, a Java virtual machine must store a constant pool. A constant pool is an ordered set of constants used by the type, including literals (string, integer, and floating point constants) and symbolic references to types, fields, and methods. Entries in the constant pool are referenced by index, much like the elements of an array. Because it holds symbolic references to all types, fields, and methods used by a type, the constant pool plays a central role in the dynamic linking of Java programs

I have several questions about the above and CP in general:

1. Is CP located in .class file for each type?
2. What does the author mean by "symbolic reference"?
3. What is the Constant Pool's purpose, in simple English?

Answer 1

I think understanding how the frame is constructed using a diagram would help.

The frame is where the operands (operation instructions) reside and that is where the dynamic linking occurs. It is a shorthand way, so to speak, using the constant pool to keep track of the class and it's members.

Each frame contains a reference to the runtime constant pool. The reference points to the constant pool for the class of the method being executed for that frame. This reference helps to support dynamic linking.

C/C++ code is typically compiled to an object file then multiple object files are linked together to product a usable artifact such as an executable or dll. During the linking phase symbolic references in each object file are replaced with an actual memory address relative to the final executable. In Java this linking phase is done dynamically at runtime.

When a Java file is compiled, all references to variables and methods are stored in the class's constant pool as a symbolic reference. A symbolic reference is a logical reference not a reference that actually points to a physical memory location.

Here is a link to James Blooms JVM Internals for more details.