What is transient keyword in Java?

Background

In one of my earlier post, I had covered what serialization in Java is and how transient keyword can be used in that context. You can take a look at that post -

• Serialization in Java

We saw that in serialization context, instance variables marked as transient will not be serialized and on deserialization, they get the default values. In this post, we will see some more details about transient keyword with an example.

Transient use cases

• The transient keyword should be used when you do not want to serialize your instance variables. Eg.
• An instance of Logger class. There is no state associated with a logger instance, so we do not need to serialize it.
• Similarly, any secure data like passwords, which you may not want to serialize.
• You cannot use any classes in the JDK that does not implement Serializable as references in your class that is Serializable. It needs to be marked transient. Else it will throw “java.io.NotSerializableException” exception.

The transient and final keyword

Transient treats final keyword a bit differently. So let's take an example to understand this,

//final field 1
public final transient String myName = "Aniket";
//final field 2
public final transient Logger myLogger = LoggerFactory.getLogger(MyClass.class.getName());

If you consider above fields in a class, by our above logic they should not be serialized since they are marked as transient. However, if any final variable is evaluated as a "constant expression" like in case of myName above, it will be serialized. So in above case myName is serialized and myLogger is not.

On the side note, recall serialVersionUID which is static and final. It is the only static variable that gets serialized. static instances do not form the state of the object, so by very definition of serialization, it is not serialized.

Use of transient keyword in HashMap.

If you see HashMap implementation you can see the array that backs it is marked as transient.

      /**
      * The table, resized as necessary. Length MUST Always be a power of two.
  */
      transient Entry[] table;

If this array is not serialized, how does it get deserialized back to restore instance state? Class does implement serializable -

public class HashMap<K,V> extends AbstractMap<K,V>
    implements Map<K,V>, Cloneable, Serializable {


    private static final long serialVersionUID = 362498820763181265L;

Reason for this is that two instances of same class do not generate the same hashcode (unless of course, you override the hashcode method to do so). The native implementation uses the objects memory location which will be different for the object before serialization and after deserialization. So it is not guaranteed that the objects will be in the same bucket and in the same location as that of hashmap that was serialized. This is why the array itself is not serialized by marking it as transient. So how is the instance state restored? 

For this in HashMap implementation, they override writeObject and readObject method. In the writeObject method, all entries from the entry array are read in a sequence and serialized. Similarly, in readObject for deserialization, it is read in the same order and then stored in its own internal entry table array. So the buckets and position are dynamically calculated during deserialization with the same data.

PS: This is a good interview question :)

Related Links

• Serialization in Java(OSFG)