Is String get/set threadsafe?

Question

Let's say I have the following,

public class Foo{
    private String bar;

    public String getBar(){
        return bar;
    }

    public void setBar(String bar){
        this.bar = bar;
    }
}

Are these methods automatically threadsafe due to the immutable nature of the String class, or is some locking mechanism required?

Answer 1

No, this is not threadsafe. Foo is mutable, so if you want to ensure that different threads see the same value of bar – that is, consistency – either:

• Make bar volatile, or
• Make the methods synchronized, or
• Use an AtomicReference<String>.

The reads and writes of bar are themselves atomic, but atomicity is not thread safety.

http://docs.oracle.com/javase/tutorial/essential/concurrency/atomic.html

---

For in-depth coverage of Java concurrency, grab a copy of Java Concurrency in Practice (aka JCIP).

Answer 2

You're setting references, and as such String's immutability doesn't come into play. You're not affecting the contents of String.