CON09-J. Ensure visibility of shared references to immutable objects

A common misconception is to assume that shared references to immutable objects are made visible across multiple threads as soon as they are updated. For instance, a developer may misconceive that a class containing fields referring to only immutable objects, is immutable. However, this class is not even thread-safe.

The Java™ Programming Language, Fourth Edition [[JPL 06]], "Section 14.10.2. Final Fields and Security" states:

... 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.

Immutable objects can be safely shared amongst multiple threads. However, mutable objects may not be fully constructed when their references are made visible. References to either kinds of objects must be made visible to all the threads. The guideline [CON26-J. Do not publish partially initialized objects] focuses on object construction and visibility issues specific to mutable objects.

Noncompliant Code Example

This noncompliant code example consists of an immutable class Helper:

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

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

and a mutable class Foo:

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.

As the Helper class is [immutable], it cannot be changed after it is initialized and is always properly constructed before its reference is made visible, in compliance with [CON26-J. Do not publish partially initialized objects]. However, some thread can observe the helper field of class Foo to contain a stale reference.

Compliant Solution (synchronization)

This compliant solution synchronizes the methods of class Foo 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);
  }
}

The immutable Helper class is 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 uses an AtomicReference wrapper around the immutable Helper object.

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));
  }
}

The immutable Helper class remains unchanged.

Risk Assessment

The assumption that classes containing immutable objects are immutable is misleading and can cause serious thread-safety issues.

Automated Detection

TODO

Related Vulnerabilities

Search for vulnerabilities resulting from the violation of this rule on the CERT website.

References

[[API 06]]
[[JPL 06]], 14.10.2. Final Fields and Security:

[!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_left.png!]      [!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_up.png!]      [!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_right.png!]