CODEWITHSUNDEEP
javathread-safetyimmutability
mre
mre

Reputation: 44240

Is String get/set threadsafe?

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?

Upvotes: 9

Views: 15434

Answers (4)

Matt Ball
Matt Ball

Reputation: 359786

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).

Upvotes: 20

duffymo
duffymo

Reputation: 308763

No, not safe.

This is Foo mutable behavior; String's immutability does not accrue to Foo.

public class Foo{
    private String bar;

    public synchronized String getBar(){
        return bar;
    }

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

Upvotes: 5

Theodoros Chatzigiannakis
Theodoros Chatzigiannakis

Reputation: 29213

No, it's not thread safe.

While String is immutable, the issue comes from the field of Foo. To make this more apparent, consider for example a method whose job would be to append (rather than replace) the value of bar. When it's called from multiple threads, some writes could be lost. The same (lost writes) can happen with your simple setter too, even if it's not obvious initially in this case.

Upvotes: 3

Brian Agnew
Brian Agnew

Reputation: 272257

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

Upvotes: 7

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