When String instances in Constant Pool will be GC'ed?

Question

Hi all,
I'm involved in an application development; where we are storing numerous string objects into multiple HashMaps. Since the string objects created/placed in HashMap may have duplicate values; we're calling String::intern() method to make use of String Constant Pool feature in Java. But, my concern is - When the Constant String instance will be removed from Pool? Is it follow the same behaviour that if no references are presentm it is eligible for GC; i.e., removal of an instance from all HashMaps?
Please help.
Thanks & regards,
R Kaja Mohideen

Answer 1

EJP wrote:
How do you figure?Doesn't say so in the Javadoc. It talks about another pool ...You're taking "A pool of strings, initially empty, is maintained privately by the class String." as meaning it's not "the constant pool"? I wouldn't interpret it that way. And based on the JVM spec, I don't see anything saying the intern()ed Strings go to a different pool than "the constant pool". And if it is a separate pool, then what does go into "the constant pool"?
http://java.sun.com/docs/books/jvms/second_edition/html/Overview.doc.html#22972
"A runtime constant pool is a per-class or per-interface runtime representation of the constant_pool table in a class file (§4.4). It contains several kinds of constants, ranging from numeric literals known at compile time to method and field references that must be resolved at run time. "
"Each runtime constant pool is allocated from the Java virtual machine's method area (§3.5.4)."
http://java.sun.com/docs/books/jvms/second_edition/html/Overview.doc.html#6656
"The Java virtual machine has a method area that is shared among all Java virtual machine threads. [...] It stores per-class structures such as the runtime constant pool, field and method data, and the code for methods and constructors [...]"
http://java.sun.com/docs/books/jvms/second_edition/html/ConstantPool.doc.html#73272
"The Java virtual machine maintains a per-type constant pool (§3.5.5),"
"A string literal (§2.3) is derived from a CONSTANT_String_info structure (§4.4.3) in the binary representation of a class or interface. The CONSTANT_String_info structure gives the sequence of Unicode characters constituting the string literal.
The Java programming language requires that identical string literals (that is, literals that contain the same sequence of characters) must refer to the same instance of class String. In addition, if the method String.intern is called on any string, the result is a reference to the same class instance that would be returned if that string appeared as a literal. Thus,
("a" + "b" + "c").intern() == "abc"
must have the value true.
To derive a string literal, the Java virtual machine examines the sequence of characters given by the CONSTANT_String_info structure.
If the method String.intern has previously been called on an instance of class String containing a sequence of Unicode characters identical to that given by the CONSTANT_String_info structure, then the result of string literal derivation is a reference to that same instance of class String.
Otherwise, a new instance of class String is created containing the sequence of Unicode characters given by the CONSTANT_String_info structure; that class instance is the result of string literal derivation. Finally, the intern method of the new String instance is invoked."
Edited by: jverd on Jan 23, 2012 8:09 AM