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A common misconception is that shared references to immutable objects are immediately visible across multiple threads as soon as they are updated. For example, a developer can mistakenly believe that a class containing fields that refer only to immutable objects is itself immutable and consequently thread-safe.

Section 14.10.2, "Final Fields and Security," of Java Programming Language, Fourth Edition [JPL 2006] states:

Compliant Solution (immutable member accessed using synchronization)

If the Helper class is immutable, it cannot be changed after it is initialized and will always be properly constructed. However, this does not prevent a thread from accessing the helper field of class Foo before the Helper instance is assigned.

According to Arnold et al. \[[JPL 06|AA. Java References#JPL 06]\], 14.10.2. Final Fields and Security:

... indeed you can use final fields to define immutable objects. There is a common misconception that shared access to immutable objects does not require any synchronization because the state of the object never changes. This is a misconception in general because it relies on the assumption that a thread will be guaranteed to see the initialized state of the immutable object, and that need not be the case. The problem is that, while the shared object is immutable, the reference used to access the shared object is itself shared and often mutable. Consequently, a correctly synchronized program must synchronize access to that shared reference, but often programs do not do this, because programmers do not recognize the need to do it. For example, suppose one thread creates a String object and stores a reference to it in a static field. A second thread then uses that reference to access the string. There is no guarantee, based on what we've discussed so far, that the values written by the first thread when constructing the string will be seen by the second thread when it accesses the string.

References to both immutable and mutable objects must be made visible to all the threads. Immutable objects can be shared safely among multiple threads. However, references to mutable objects can be made visible before the objects are fully constructed. TSM03-J. Do not publish partially initialized objects describes object construction and visibility issues specific to mutable objects.

Noncompliant Code Example

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 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.

Compliant Solution (Synchronization)

This compliant solution synchronizes the methods of the Foo class to ensure that no thread sees a stale Helper reference:In this compliant solution, the Helper class is made immutable. However, the reference to the immutable Helper object instance is shared and as a result, mutable. Consequently, there is an additional requirement of synchronizing the methods of class Foo to ensure that no thread sees a partially initialized helper.

final class Foo {
  private Helper helper;

  public synchronized Helper getHelper() {
    return helper;
  }

  public synchronized void initializesetHelper(int num) {
    helper = new Helper(42num);
  }
}

// Immutable Helper
public class Helper

The immutable Helper class remains unchanged.

Compliant Solution (volatile)

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);
  }
}

The immutable Helper class remains unchanged.

Compliant Solution (java.util.concurrent Utilities)

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 int nAtomicReference<Helper> helperRef =
      new AtomicReference<Helper>();

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

  public void setHelper(int nnum) {
    thishelperRef.n = nset(new Helper(num));
  }
}

The immutable Helper class remains unchanged.

Risk Assessment

The incorrect assumption that classes that contain only references to immutable objects are themselves immutable can cause serious thread-safety issues.

Automated Detection

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

Bibliography

Issue Tracking

||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)|

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