How String matters to Garbage Collection and Memory Leak in Java?

➟ What is Memory Leak?

      The application is unintentionally holding references to objects, and this prevents the objects from being garbage collected. This is the Java language equivalent of a memory leak.

➟ What is String Pool?

      String pool (String intern pool) is a special storage area in Java heap. When a string is created and if the string already exists in the pool, the reference of the existing string will be returned, instead of creating a new object and returning its reference.

➤ Some Examples

• Both below are same. It will create new instance only if it is not available in pool, and then add in pool.

String s1 = "Vishal"; 
String s2 = new String("Vishal").intern();

• Below line will create new instance each time.

String s3 = new String("Vishal");

➟ String.substring Method

➤ Before JDK 7 update 6

• String was using counter, offset variable for managing the operations like Substring.
• String was sharing same char[] of original String while calling substring method, thus original String was not eligible for GC.
• Substring method was fast compare to new version.

➤ After JDK 7 update 6

• Counter and offset variables has been removed from String class.
• String is not sharing char[] of original String while calling substring method.
• Substring method is slow compare to old version, because it will create new char[] for each call of substring.

➟ Changes done for String.substring method update in JDK 7u6

➟ Memory Leak Bug Example in before JDK 7u6 (Source)

public class TestGC {
    private String largeString = new String(new byte[100000]);
    
    String getSmallString() {
// if caller stores this substring, this object will not be gc'ed
        return this.largeString.substring(0,2);
//      return new String(this.largeString.substring(0,2)); // no error here!
    }
    
    public static void main(String[] args) {
        java.util.ArrayList list = new java.util.ArrayList();
        for (int i = 0; i < 1000000; i++) {
            TestGC gc = new TestGC();
            list.add(gc.getSmallString());
        }
    }
}

➟ Java 8 Update

➤ String Deduplication :

• A new optimization has been made that enables the G1 collector to identify strings which are duplicated more than once across your heap.
• It will handle it by pointing into the same internal char[] array.
• It will avoid multiple copies of the same string from residing inefficiently within the heap.