Java: Final field freeze on object reachable from final fields

concurrency, java

Solution

Does this mean that if I have a final field in class A of type Class B, which in turn have a final field of type Integer, then final field freeze for an instance of class A completes only after the final field freeze for b.c have already happened?

I think I would carefully say that final field freeze in this case means that when you create an instance of A and safely publish it, other objects will never see an uninitialized value for b or c.

I would also say that when you are creating the instance of B inside A, other initialization code inside A will never see an uninitialized value for c.

One case where I have encountered real questions around final field freeze is for example a class that contains a (mutable) HashMap, intended only for read, initialized during construction:

public class DaysOfWeek {
    private final Map daysOfWeek = new HashMap();
    public DaysOfWeek() { 
      // prepopulate my map
      daysOfWeek.put(0, "Sunday");
      daysOfWeek.put(1, "Monday");
      // etc
    }

    public String getDayName(int dayOfWeek) {
      return daysOfWeek(dayOfWeek);
    }
}

The question arises here: assuming this object is safely published, and given that there is no synchronization here, is it safe for other threads to call getDayName()? The answer is yes, because final field freeze guarantees that the HashMap and everything reachable from it (here it's just strings, but could be arbitrarily complex objects) is frozen at the end of construction. [If you want to actually modify this map after construction, then you'll need explicit synchronization around reads and writes.] Here's a lengthier blog exploring the topic and check the comments for some interesting responses by people like Brian Goetz.

btw I'm the author of the refcard

Problem

DZone refcard titled "Core Java Concurrency" states: Once set, final field values cannot be changed. Marking an object reference field as final does not prevent objects referenced from that field from changing later. For example, a final ArrayList field cannot be changed to a different ArrayList, but objects may be added or removed on the list instance. and Final field freeze includes not just the final fields in the object but also all objects reachable from those final fields. I am not entirely clear about the second statement. Does this mean that if I have a final field in class A of type Class B, which in turn have a final field of type Integer, then final field freeze for an instance of class A completes only after the final field freeze for `b.c` have already happened? ``` public class A{ public final B b = new B(); } public class B{ public final Integer c = 10; } ```

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