Java string literal pool

[Jun Zhuang]

Hi Kris,

Thanks for the response. Regarding the 2nd question, I want to make sure I understand your answer.  The handling of new String("Cat") can be broken into two steps:

Step #1: Handling the “Cat” expression
If ( Does not already exist )
Create an instance on the heap
Add a reference to the StringTable for the above instance and return the reference

else
Return the reference from the StringTable

                Question: I assume the reference returned is consumed by the String () constructor, is it using the existing object to create a new one?

 Step #2: Handling the new String()
           Action: Create an instance and return reference

Either way, TWO instance of String object representing Cat will end up on the heap, right? If I have another statement String str2 = new String("Cat"); a third such instance will be created?

Thanks much,
Jun

--------------------------------------------------------------------------------------------------------------------------------------------
#2: Is the following statement correct?
        String str = new String("Cat");
In above statement, either 1 or 2 string will be created. If there is already a string literal “Cat” in the pool, then only one string “str” will be created in the pool. If there is no string literal “Cat” in the pool, then it will be first created in the pool and then in the heap space, so total 2 string objects will be created.
>From http://www.journaldev.com/797/what-is-java-string-pool#comment-36152
 For this statement, yes, either 1 or 2 java.lang.String instances are going to be created. But I wouldn't frame it that way, since it's potentially confusing.
The correct way to frame this is to make a clear distinction between one-time actions and normal runtime actions.

The "Cat" expression is a reference to a compile-time constant of type java.lang.String. At runtime, there will be a one-time resolution for such a reference. Indeed, such resolution will probe the StringTable to see if a matching String instance is already referenced by the StringTable, if so return that reference; otherwise create a java.lang.String instance from the Symbol representing "Cat", intern the reference into the StringTable, and return that reference.

The "new String(...)" expression, on the other hand, is a "new" expression. Semantically it should always create a new String instance every time.

- Kris

[1]: http://hg.openjdk.java.net/jdk8u/jdk8u/hotspot/file/312e113bc3ed/src/share/vm/classfile/symbolTable.hpp#l255


Apprciate your answers,
Jun

Jun Zhuang
Sr. Performance QA Engineer | Hobsons<https://www.hobsons.com/?utm_source=outlook&utm_medium=email&utm_campaign=banner_02.12.16_general>
T: +1 513 746 2288<tel:%2B1%20513%20746%202288> | jun.zhuang at hobsons.com<mailto:jun.zhuang at hobsons.com>
50 E-Business Way, Suite 300 | Cincinnati, OH 45241 | USA



[Upgraded by Hobsons - Subscribe Today]<https://geo.itunes.apple.com/us/podcast/upgraded/id1086217543?mt=2>





Upgraded by Hobsons - Subscribe Today
_______________________________________________
hotspot-gc-use mailing list
hotspot-gc-use at openjdk.java.net<mailto:hotspot-gc-use at openjdk.java.net>
http://mail.openjdk.java.net/mailman/listinfo/hotspot-gc-use

-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mail.openjdk.java.net/pipermail/hotspot-gc-use/attachments/20160830/ef8a0cd3/attachment-0001.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: image001.png
Type: image/png
Size: 13602 bytes
Desc: image001.png
URL: <http://mail.openjdk.java.net/pipermail/hotspot-gc-use/attachments/20160830/ef8a0cd3/image001-0001.png>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: image871000.png
Type: image/png
Size: 13602 bytes
Desc: image871000.png
URL: <http://mail.openjdk.java.net/pipermail/hotspot-gc-use/attachments/20160830/ef8a0cd3/image871000-0001.png>