Must all properties of an immutable object be final?

The main difference between an immutable object (all properties final) and an effectively immutable object (properties aren’t final but can’t be changed) is safe publication.

You can safely publish an immutable object in a multi threaded context without having to worry about adding synchronization, thanks to the guarantees provided by the Java Memory Model for final fields:

final fields also allow programmers to implement thread-safe immutable objects without synchronization. A thread-safe immutable object is seen as immutable by all threads, even if a data race is used to pass references to the immutable object between threads. This can provide safety guarantees against misuse of an immutable class by incorrect or malicious code. final fields must be used correctly to provide a guarantee of immutability.

As a side note, it also enables to enforce immutability (if you try to mutate those fields in a future version of your class because you have forgotten it should be immutable, it won’t compile).

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Clarifications

• Making all the fields of an object final does not make it immutable – you also need to make sure that (i) its state can’t change (for example, if the object contains a final List, no mutating operations (add, remove…) must be done after construction) and (ii) you don’t let this escape during construction
• An effectively immutable object is thread safe once it has been safely published

• Example of unsafe publication:

  class EffectivelyImmutable {
      static EffectivelyImmutable unsafe;
      private int i;
      public EffectivelyImmutable (int i) { this.i = i; }
      public int get() { return i; }
  }
  
  // in some thread
  EffectivelyImmutable.unsafe = new EffectivelyImmutable(1);
  
  //in some other thread
  if (EffectivelyImmutable.unsafe != null
      && EffectivelyImmutable.unsafe.get() != 1)
      System.out.println("What???");
  

  This program could in theory print What???. If i were final, that would not be a legal outcome.