Java J2EE Technologies: Mechanism of String ( java.lang.String ) till Java 1.6

String

String is one of the famous and frequently used java Class from java.lang package.

If an instances of String is created in heap memory then its state can be never changed, means the value of the string can not be changed because String is an immutable class.

What is immutable?

The synonyms of immutable are – unchangeable, fixed, permanent and etc...

In Java world also its refers the same meaning, once an object is created then its state cannot be changed.

Let’s see with simple example:

String initialization is one of the unique features of String, the following are the few way of String initialization.

String s1 = “abc”;

String s2 = new String (s1);

String s3 = s2;

Since String is Class like other Java Classes we can create object with new operator. String is also called literal so similar to primitive type initialization String literal can be assigned value without new operator.

How JVM allocates memory for String?

We know JVM is responsible for memory allocation for all JAVA Classes but do you know JVM will always perform some additional work w.r.t memory management. String memory management is one of the additional feature of String Class. Lets see how JVM works for String memory management.

Whenever Class Loader encounter String literal ( String str = "value" ), first it will check in the String Literal Pool/String Constant Pool (You can see what is String pool and its mechanism below). If the value exists in the pool then JVM will take the reference of the existing value and assigned to String variable. If the value is not exists in the pool then JVM will create new String instance in the heap memory and immediately the reference of the new object will be register in String Pool for future reference.

What is String Literal Pool / String Constant Pool ?

Like other Java classes the String objects is also created and maintained in heap memory, here the additional memory management of String class is String Literal Pool /String Constant pool, during run time the JVM will collect all the String literal reference and register it in the String Pool which is similar to stack table.

Usually we calling this String Pool as Perm Gen Space, it is neither heap memory nor stack memory.

Let’s see the String mechanism with more details.

String s1 =”abc”;

String s2 = new String (“abc”);

What you think about above two statements? Will it point same object? How JVM will work for these statements?

Statement 1: As we have seen before “Whenever Class Loader encounter String literal (String s1 =”abc”), first it will check whether the value is exist in the String Literal Pool/String Constant Pool or not. If the value is exists in the pool then JVM will take the reference of the existing value and assigned in the String variable if not exists only the JVM will create new Object”.

Statement 2: Since new operator is used JVM will directly create new Object in heap memory even though if there is a String Object with same value in String Pool. As per Java rule JVM will always give high priority for new operator.

Once the above two statements executed then the reference and object of s1 and s2 will be available in memory as follows

So what do think about the output of the following statements?

System.out.println(s1.equalIgnoreCase(s1)); // ans: true

System.out.println(s1==s1); // ans: false

Do you think is there any possible way to optimize the string objects which are created by new operator?

Yes, String’s intern () native method can be used to optimize the String object’s memory. Let’s see from the above statement.

s2 = s2.intern();

Now check the output of following statement

System.out.println(s1==s2); // ans : true

What intern () will do?

Whenever the intern native method is invoked, it will check from String Literal/String Constant pool. If the pool already exists the same value of String Object then reference of the string will be returned if not exist then the reference will be added in the pool.

After intern () native method executed the reference and object of s1 and s2 will be as follows in the memory.