string literals and permanent generation memory area

Question

When we say that interned strings are stored in permanent generation area then does the same applies for string literals also? Or it is only for strings interned by inter()?

Actually blog posts usually say that string pool contains reference to string object while actual string object is somewhere in heap. also there is very much confusion that whether permanent generation is IN heap or outside of it. (i used jcosole it is showing permanent gen different from heap.many posts say it as a part of heap and many say it is different)

Edit: Also when I ran:

public class stringtest2{
  public static void main(String args[]){
    int i=0;
    List<String> list=new ArrayList<String>();
    while(true){
      String s="hello"+i;
      String s1=i+"hello";
      String s2=i+"hello"+i;
      System.out.println(s);
      s.intern();

      s1.intern();
      s2.intern();
      list.add(s);
      list.add(s1);
      list.add(s2);
      i++;
    }
  }
}

I was expecting Java.lang.OutOfMemoryError: PermGen space But i got :

Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
        at java.util.Arrays.copyOf(Arrays.java:2760)
        at java.util.Arrays.copyOf(Arrays.java:2734)
        at java.util.ArrayList.ensureCapacity(ArrayList.java:167)
        at java.util.ArrayList.add(ArrayList.java:351)
        at stringtest2.main(stringtest2.java:20)

Shouldn't it be Java.lang.OutOfMemoryError: PermGen space

Answer 1

When we say that interned strings are stored in permanent generation area then does the same applies for string literals also?

Literal strings are interned. So yes, in Java 6-.

From Java 7, interned strings are not stored in permanent generation any longer. They are stored in the main part of the heap like any other objects you would create.

Shouldn't it be Java.lang.OutOfMemoryError: PermGen space

The exception you get is caused by the creation of an array which lives on the heap. To try to get an "out of permgen memory" error, you could try to remove the list.add() lines. Note however that interned strings can be garbage collected so even doing that will still not cause the exception you expect.

Cf RFE 6962931:

In JDK 7, interned strings are no longer allocated in the permanent generation of the Java heap, but are instead allocated in the main part of the Java heap (known as the young and old generations), along with the other objects created by the application. This change will result in more data residing in the main Java heap, and less data in the permanent generation, and thus may require heap sizes to be adjusted. Most applications will see only relatively small differences in heap usage due to this change, but larger applications that load many classes or make heavy use of the String.intern() method will see more significant differences.

Answer 2

All String literals are interned automatically, as described in the String JavaDoc:

All literal strings and string-valued constant expressions are interned.

I would expect the behaviour to be consistent between strings you manually intern and any string literals.