Hi Jun, Comments below inline:
On Mon, Aug 29, 2016 at 6:29 AM, Jun Zhuang <jun.zhu...@hobsons.com> wrote: > Hi, > > > > I was reading about the Java8 Metaspace the other day, then got into the > topic of string pool. I got conflicting information regarding a couple of > things, I wonder if I can get a definitive answer from the experts? > > > > #1: Is the string literal/constant pool a heap area that holds actual > string literal objects or a place that holds references to string literal > objects? > > Reference: http://stackoverflow.com/questions/11700320/is-string- > literal-pool-a-collection-of-references-to-the-string-object-or-a-col > > > What you're referring to is called the "StringTable" in HotSpot JVM [1]. You can think of it as being semantically equivalent to a HashMap<String> in Java, which holds references to the java.lang.String objects being interned, instead of storing the actual string contents. > #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 | jun.zhu...@hobsons.com > > 50 E-Business Way, Suite 300 | Cincinnati, OH 45241 | USA > > > > [image: Upgraded by Hobsons - Subscribe Today] > <https://geo.itunes.apple.com/us/podcast/upgraded/id1086217543?mt=2> > > _______________________________________________ > hotspot-gc-use mailing list > hotspot-gc-use@openjdk.java.net > http://mail.openjdk.java.net/mailman/listinfo/hotspot-gc-use > >
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