Does object state set in object constructor visible from all threads?

Question

For example -

public class Request {
    public String id; //is it visible to other threads after construction?
    public Request(String id){
        this.id= id;
    }   
}

Answer 1

As it is your class is not thread safe and a thread could observe a null value for id even after the constructor has finished.

To make sure id is visible to all threads after construction, you have several possibilities:

• make the field final
• make the field volatile
• safely publish the Request object.

Safe publication idioms include:

• initialising the instance from a static initialiser
• marking the reference to the instance as volatile
• marking the reference to the instance as final
• synchronizing all accesses

See also this other post which explains the importance of marking fields final to guarantee the thread safety of immutable objects.