Page History

...

This noncompliant code example consists of the immutable Helper class:

// Immutable Helper
public final class Helper {
  private final int n;

  public Helper(int n) {
    this.n = n;
  }
  // ...
}

and a mutable Foo class:

final class Foo {
  private Helper helper;

  public Helper getHelper() {
    return helper;
  }

  public void setHelper(int num) {
    helper = new Helper(num);
  }
}

The getHelper() method publishes the mutable helper field. Because the Helper class is immutable, it cannot be changed after it is initialized

. Furthermore, because Helper is immutable, it is always constructed properly before its reference is made visible, in compliance with rule TSM03-J. Do not publish partially initialized objects. Unfortunately, a separate thread could observe a stale reference in the helper field of the Foo class.

...

This compliant solution synchronizes the methods of the Foo class to ensure that no thread sees a stale Helper reference.

final class Foo {
  private Helper helper;

  public synchronized Helper getHelper() {
    return helper;
  }

  public synchronized void setHelper(int num) {
    helper = new Helper(num);
  }
}

...

References to immutable member objects can be made visible by declaring them volatile.

final class Foo {
  private volatile Helper helper;

  public Helper getHelper() {
    return helper;
  }

  public void setHelper(int num) {
    helper = new Helper(num);
  }
}

...

This compliant solution wraps the mutable reference to the immutable Helper object within an AtomicReference wrapper that can be updated atomically.

final class Foo {
  private final AtomicReference<Helper> helperRef =
      new AtomicReference<Helper>();

  public Helper getHelper() {
    return helperRef.get();
  }

  public void setHelper(int num) {
    helperRef.set(new Helper(num));
  }
}

...

Automated Detection

Some static analysis tools are capable of detecting violations of this rule.

Bibliography

...

||Completed||Priority||Locked||CreatedDate||CompletedDate||Assignee||Name||
|F|M|F|1270826173609|          |dmohindr|"Unfortunately, a separate thread -could- *can* observe a stale reference in the helper field of the Foo class."|
|T|M|F|1270826698362|1271441478121|svoboda|"This compliant solution synchronizes the methods of *class* Foo -class- " (it sounds strange with class occuring after Foo)|

      07. Visibility and Atomicity (VNA)