Where does Java's String constant pool live, the heap or the stack?

Question

I know the concept of a constants pool and the String constant pool used by JVMs to handle String literals. But I don't know which type of memory is used by the JVM to store String constant literals. The stack or the heap? Since its a literal which is not associated with any instance I would assume that it will be stored in stack. But if it's not referred by any instance the literal has to be collected by GC run (correct me if I am wrong), so how is that handled if it is stored in the stack?

Answer 1

The answer is technically neither. According to the Java Virtual Machine Specification, the area for storing string literals is in the runtime constant pool. The runtime constant pool memory area is allocated on a per-class or per-interface basis, so it's not tied to any object instances at all. The runtime constant pool is a subset of the method area which "stores per-class structures such as the runtime constant pool, field and method data, and the code for methods and constructors, including the special methods used in class and instance initialization and interface type initialization". The VM spec says that although the method area is logically part of the heap, it doesn't dictate that memory allocated in the method area be subject to garbage collection or other behaviors that would be associated with normal data structures allocated to the heap.

Answer 2

As explained by this answer, the exact location of the string pool is not specified and can vary from one JVM implementation to another.

It is interesting to note that until Java 7, the pool was in the permgen space of the heap on hotspot JVM but it has been moved to the main part of the heap since Java 7:

Area: HotSpot
Synopsis: In JDK 7, interned strings are no longer allocated in the permanent generation of the Java heap, but are instead allocated in the main part of the Java heap (known as the young and old generations), along with the other objects created by the application. This change will result in more data residing in the main Java heap, and less data in the permanent generation, and thus may require heap sizes to be adjusted. Most applications will see only relatively small differences in heap usage due to this change, but larger applications that load many classes or make heavy use of the String.intern() method will see more significant differences. RFE: 6962931

And in Java 8 Hotspot, Permanent Generation has been completely removed.