CON11-J. Do not assume that declaring an object volatile guarantees visibility of its members

According to the Java Language Specification [[JLS 05]], section, 8.3.1.4 volatile Fields:

A field may be declared volatile, in which case the Java memory model (§17) ensures that all threads see a consistent value for the variable.

Notably, this applies only to primitive fields and immutable member objects. The visibility guarantee does not extend to mutable, thread-unsafe objects even if their references are declared volatile. A thread may not observe a recent write to a member object's field from another thread.

Declaring an object volatile to ensure visibility of the most up-to-date object state does not work without the use of synchronization, unless the object is [immutable].

In the absence of synchronization, the effect of declaring a field volatile is that when one thread sets the field to a new value, other threads can see the new object reference immediately. If the referenced object is immutable, this has the effect that other threads also see a consistent view of the state of the object. However, if the object is mutable and thread-unsafe, other threads may see a partially-constructed object, or an object in a (temporarily) inconsistent state [[Goetz 07]]. Declaring the object volatile does not prevent this issue.

Technically the object does not have to be strictly immutable. If it can be proved that the member object is thread-safe by design, the field that will hold its reference may be declared as volatile. However, this approach burdens maintainability and should be avoided as far as possible.

Noncompliant Code Example (Arrays)

This noncompliant code example shows an array object (arrays are objects in Java) that is declared volatile.

class Foo {
  volatile private int[] arr = new int[20];

  public int getFirst() {
    return arr[0];
  }

  public void setFirst(int n) {
    arr[0] = n;
  }

  // ...
}

It appears that when a value is written by a thread to one of the array elements, it becomes instantly visible to other threads. This is misleading because the volatile keyword just makes the array reference visible to all threads and does not affect the actual data contained within the array. For example, when a thread assigns a new value to arr[1] another thread that is attempting to read the value of arr[1], may observe an inconsistent value.

This happens because there is no [happens-before] relation between the thread that calls setFirst() and the thread that calls getFirst(). Normally a happens-before relation exists between a thread that writes to a volatile variable and a thread that subsequently reads it. But this code is neither writing to nor reading from a volatile variable. The array's 'volatility' applies only to the array reference, not to the array elements.

Compliant Solution (AtomicIntegerArray)

This compliant solution suggests using the java.util.concurrent.atomic.AtomicIntegerArray concurrency utility. Using its set(index, value) method ensures that the write is atomic and the resulting value is made visible to other threads. The other threads can retrieve a value from a specific index by using the get(index) method.

class Foo {
  AtomicIntegerArray aia = new AtomicIntegerArray(5);

  public int getFirst() {
    return aia.get(0);
  }

  public void setFirst(int n) {
    aia.set(0, 10);
  }

  // ...
}

In this compliant solution, the AtomicIntegerArray guarantees a happens-before relation between a thread that calls aia.set() and a thread that subsequently calls aia.get(). However, if a thread calls getFirst() first, it sees the default value of the atomic integer (0).

Compliant Solution (synchronization)

To ensure visibility, accessor methods may synchronize access while performing operations on nonvolatile elements of an array which is declared volatile. Note that the array need not be volatile for the code to be thread-safe.

class Foo {
  private int[] arr = new int[20];

  public synchronized int getFirst() {
    return arr[1];
  }

  public synchronized void setFirst(int n) {
    arr[1] = n;
  }

Synchronization establishes a happens-before relation between the thread that calls setFirst() and the thread that subsequently calls getFirst(). Consequently, the array's element set by setFirst() is guaranteed to be visible to getFirst().

Noncompliant Code Example (Mutable object)

This noncompliant code example declares an instance field of type Properties as volatile. The referenced object can be mutated using the put() method. This makes objects of class Foo mutable.

class Foo {
  private volatile Properties properties;

  public Foo() {
    properties = new Properties();
    // Load some useful values into properties
  }

  public String get(String s) {
    return properties.getProperty(s);
  }

  public void put(String key, String value) {
    // Perform validation of value before inserting
    properties.setProperty(key, value);
  }
}

If one thread calls get() while another calls put(), the first thread may receive a stale, or an internally inconsistent value from the Properties object because the operations within put() modify the state of the Properties object instance. Declaring the object volatile does not prevent this data race.

Compliant Solution (immutable)

This compliant solution renders the Foo class immutable. Consequently, once it is properly constructed, no thread can modify the state of the Properties object instance and cause a data race.

class Foo {
  private final Properties properties;

  public Foo() {
    properties = new Properties();
    // Load some useful values into properties
  }

  public String get(String s) {
    return properties.getProperty(s);
  }
}

The shortcoming of making Foo immutable is that the put() method can no longer be accommodated. The Foo class is [immutable] because all its fields are final and the properties field is being safely published.

Noncompliant Code Example (cheap read-write lock)

This noncompliant code example attempts to use the cheap read-write lock trick [[Goetz 07]]. The properties field is declared volatile in order to synchronize reads and writes. The non-atomic put() method is synchronized as well.

class Foo {
  private volatile Properties properties;

  public Foo() {
    properties = new Properties();
    // Load some useful values into properties
  }

  public String get(String s) {
    return properties.getProperty(s);
  }

  public synchronized void put(String key, String value) {
    // Perform validation of value
    properties.setProperty(key, value);
  }
}

The cheap read-write lock trick is often used with primitive types that require nonatomic operations to be performed on them, such as increment. However, the trick does not work with objects, because the visibility of volatile does not extend to their members. Consequently, if one thread adds a property using put, it may not be visible to other threads. There is no [happens-before relation] between the write and a subsequent read of the property.

Compliant Solution (synchronized)

This compliant solution uses method synchronization to ensure thread safety.

class Foo {
  private final Properties properties;

  public Foo() {
    properties = new Properties();
    // Load some useful values into properties
  }

  public synchronized String get(String s) {
    return properties.getProperty(s);
  }

  public synchronized void put(String key, String value) {
    // Perform validation of value
    properties.setProperty(key, value);
  }
}

Note that the properties field is not declared volatile because this solution achieves thread-safety using syncronization. The field is declared final so that its reference is not published when it is in a partially initialized state (see [CON26-J. Do not publish partially initialized objects]).

Noncompliant Code Example (mutable sub-object)

This noncompliant code example declares the field FORMAT as volatile. However, the field stores a reference to a mutable object, DateFormat.

class DateHandler {
  private static volatile DateFormat FORMAT =
    DateFormat.getDateInstance(DateFormat.MEDIUM);

  public static Date parse(String str) throws ParseException {
    return FORMAT.parse(str);
  }
}

In the presence of multiple threads, this results in subtle thread safety issues because DateFormat is not thread-safe [[API 06]]. For instance, a thread may observe correctly formatted output for a completely different date when it invokes parse() with a known date.

Compliant Solution (instance per call/defensive copying)

This compliant solution creates and returns a new DateFormat instance for every invocation of the parse() method. [[API 06]]

class DateHandler {
  public static Date parse(String str) throws ParseException {
    DateFormat format = DateFormat.getDateInstance(DateFormat.MEDIUM);
    return format.parse(str);
  }
}	

This does not violate [OBJ11-J. Defensively copy private mutable class members before returning their references] because the class no longer contains internal mutable state, but a local field, format.

Compliant Solution (synchronization)

This compliant solution synchronizes the parse() method and consequently, the class DateHandler is thread-safe [[API 06]]. There is no requirement for declaring the FORMAT field as volatile.

class DateHandler {
  private static DateFormat FORMAT =
    DateFormat.getDateInstance(DateFormat.MEDIUM);

  public static Date parse(String str) throws ParseException {
    synchronized (FORMAT) {
      return FORMAT.parse(str);
    }
  }
}

Compliant Solution (ThreadLocal storage)

This compliant solution uses a ThreadLocal object to store one DateFormat instance per thread.

class DateHandler {
  private static final ThreadLocal<DateFormat> df = new ThreadLocal<DateFormat>() {
    protected DateFormat initialValue() {
      return DateFormat.getDateInstance(DateFormat.MEDIUM);
    }
  };
  // ...
}

Risk Assessment

Assuming that declaring a field volatile guarantees visibility of the members of the referenced object may lead threads to observe stale values of the members.

Automated Detection

TODO

Related Vulnerabilities

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

References

[[Goetz 07]] Pattern #2: "one-time safe publication"
[[Miller 09]] Mutable Statics
[[API 06]] Class java.text.DateFormat
[[JLS 05]]

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