Lifetime and behavior of the String Constant Pool

If you 'inline' a String constant in Java, I'm aware in the VM spec that it says this String is put in the constant pool.
What is the lifetime of the constant pool and what is it tied to? Is there one constant pool per Class, per Class instance, or per VM?
If an instance is GC'd, do it's constants previously moved to the constant pool get destroyed, or do they live on until I create another instance of that Class?
Thanks, Kevin

Is the constant pool created at compile time and written into the Class file?Yes.
Each class has it's own constant pool which contains constant values and references to other classes and fields.
It is created a compile time.
A class (definition) is loaded by a classloader at runtime.
In theory you could have the same class loaded twice by different classloaders and each would have a copy of the class definition.
As far as I know, a class definition can only be removed from memory by nulling/garbage collecting the classloader that loaded it.
regards,
Owen

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I was reading an article about weak references and came across a peice of code
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I never heard of String constant pool could any one enlighten me. I didnt get the point of what above paragraph says also.
Jubs

No it won't: it will only be removed when the garbage collector rears
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kind regards,
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What is the little square box in the string constant input?

Hello LabVIEW World -
I am attempting to understand a LabVIEW code that I did not write. At one point, there is a string (Equal?) comparison that is comparing a string from a VISA Read to a string constant input. The string constant has a little square box inside of it.
Does anyone know what this little square box is? (See attached screenshot)
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Attachments:
LittleSquare.JPG ‏69 KB

When I place string constants like this I will display then either as hex or in the codes mode. Either of these will make the code more readable. Using the default ASCII string display is useless other than letting you know it is an unprintable character.
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thanks very much for your help!
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SQL> hope this helps.

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Hello all,
I'm trying to generate a PDF file (from scratch, using a C++ library - but that's not relevant to my question) that uses links to specify the location of linked files relative to the root of a drive - NOT relative to the PDF file itself (i.e. I don't want to use ../../).
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e.g. To link to the file in c:\images\test.jpg from a PDF file you know is also on the c: drive - the links written form is: //images/test.jpg (as per table 43)
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I can see what's happening - and want to know if there is something I can do (or should be doing) to fix it.
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Adam
(I've attached a example.pdf file that contains a link to \images\test.jpg - which works on the desktop, but not in the browser)

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

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

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

What checks the String pool

Hi there.
With regard to the string pool,
my book states that whenever the compiler
encounters a string literal that it will check the pool to see if
there is a matching string...
This doesn't seem right to me.
I would have imagined that the JVM would perform this operation at runtime.
Maybe I'm wrong, but I googled it and I came across some sites that indicate that
the JVM checks the string pool.
Im just wondering which is it, the JVM or the compiler.
Thanks & regards.

jverd wrote:
Both.
String s1 = "abc";
String s2 = "abc";Compiler sees the first line, looks for "abc" in the class' constant pool, doesn't find it, adds it, places reference to its position for the assignment to s1.
Compiler sees the second line, looks for "abc", finds it, places reference to its position for the assignment to s2.
At runtime, when class is loaded, VM loads class' String constant pool. When time comes to execute the above lines, the bytecode tells it which constant pool entry to reference.Great answer (No, I'm not sarcastic)

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Hi,
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Syntax
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Regards,
Nadim
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National Instruments

Lifetime and behavior of the String Constant Pool

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