Difference in String pool between Java 6 and 7? Answer

In Java, the string pool is a special memory area that stores string literals, allowing for efficient string sharing. The behavior of the string pool has evolved between Java 6 and Java 7. Here are the key differences:

String Pool Location:

Java 6: In Java 6 and earlier versions, the string pool was stored in the PermGen (Permanent Generation) area of the Java Virtual Machine (JVM) heap. The PermGen space was primarily used for storing metadata about loaded classes, interned strings, and other JVM-specific data.

Java 7: In Java 7 and later versions, the string pool was moved out of the PermGen space and placed in the main heap area called the "heap" or the "Young Generation." This change was introduced as part of the PermGen removal in Java 8.

String Interning:

Java 6: In Java 6, string literals were automatically interned and stored in the string pool. Additionally, you could manually intern strings using the intern() method, which would return a canonical representation of the string if it already existed in the pool.

Java 7: In Java 7, string literals continued to be automatically interned and stored in the string pool. However, the behavior of the intern() method was changed. Instead of storing a copy of the string in the pool, the intern() method returned a reference to the original string stored in the heap if it already existed, or a reference to the new string if it didn't exist.

String Pool Memory Management:

Java 6: In Java 6, the string pool in the PermGen space had a fixed maximum size. If the string pool became full, subsequent string literals could not be added to the pool, potentially leading to OutOfMemoryError in extreme cases.

Java 7: In Java 7, with the string pool residing in the main heap, the memory management of the string pool was aligned with the overall heap management. This allowed for more flexible memory allocation and reduced the likelihood of OutOfMemoryError due to string pool limitations.

Overall, the changes in Java 7 regarding the string pool primarily focused on the relocation of the pool from the PermGen space to the main heap area, resulting in improved memory management and more consistent behavior of the intern() method.

It's worth noting that the string pool behavior continued to evolve in subsequent Java versions. For example, in Java 8, the PermGen space was removed, and in Java 9, the string pool was further optimized with the introduction of the Compact Strings feature.