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

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

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

  // ...
}

final class Foo {
  private final AtomicIntegerArray atomicArray = 
    new AtomicIntegerArray(20);

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

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

  // ...
}

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

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

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

 of the Java Language Specification \[[JLS 05|AA. Java References#JLS 05]\] states the following:
{quote}
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.
{quote}

Notably, this applies only to primitive fields and immutable member objects. The visibility guarantee does not extend to mutable objects that are not thread-safe, even if their references are declared {{volatile}}. A thread may not observe a recent write from another thread to a member field of such an object. Declaring an object {{volatile}} to ensure visibility of its state does not work without the use of synchronization, unless the object is [immutable|BB. Definitions#immutable]. If the object is mutable and not thread-safe, other threads might see a partially-constructed object or an object in a (temporarily) inconsistent state \[[Goetz 07|AA. Java References#Goetz 07]\]. 

Technically, the object does not have to be strictly immutable to be used safely. 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 to using volatile burdens maintainability and should be avoided.


h2. Noncompliant Code Example (Arrays)

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

{code:bgColor=#FFcccc}
final class Foo {
  volatile private int[] arr = new int[20];

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

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

  // ...
}
{code}

The assumption that a value written to an array element by one thread is visible to other threads is incorrect, because the {{volatile}} keyword only makes the array reference visible 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\[1\]}} might observe a stale value.

This happens because there is no [happens-before|BB. Definitions#happens-before order] relation between the thread that calls {{setFirst()}} and the thread that calls {{getFirst()}}.  A happens-before relation exists between a thread that writes to a volatile variable and a thread that subsequently reads it. This code is neither writing to nor reading from a volatile variable. 


h2. Compliant Solution ({{AtomicIntegerArray}})

This compliant solution uses the {{java.util.concurrent.atomic.AtomicIntegerArray}} class. Its {{set(index, value)}} method ensures that the write is atomic and the resulting value is visible to other threads. The other threads can retrieve a value from a specific index using the {{get(index)}} method.

{code:bgColor=#ccccff}
final class Foo {
  private finalvolatile AtomicIntegerArrayMap<String, atomicArray = new AtomicIntegerArray(20)String> map;
 
  public int getFirstFoo() {
    return atomicArray.get(0);map = new HashMap<String, String>();
    // Load some useful values into map
  }
 
  public voidString setFirstget(intString ns) {
    atomicArrayreturn map.set(0, 10get(s);
  }
 
  // ...
}
{code}

{{AtomicIntegerArray}} guarantees a [happens-before|BB. Definitions#happens-before order] relation between a thread that calls {{atomicArray.set()}} and a thread that subsequently calls {{atomicArray.get()}}. However, if a thread calls {{getFirst()}} first, it sees the default value of the atomic integer (0).


h2. Compliant Solution (Synchronization)

To ensure visibility, accessor methods may synchronize access, while performing operations on non-volatile elements of an array that is declared {{volatile}}. Note that the code is thread-safe, even though the array reference is not volatile.

{code:bgColor=#ccccff}
public void put(String key, String value) {
    // Validate the values before inserting
    if (!value.matches("[\\w]*")) {
      throw new IllegalArgumentException();
    }
    map.put(key, value);
  }
}

final class Foo {
  private int[] arr = new int[20];
volatile Map<String, String> map;
  public synchronized int getFirst() {
    return arr[1];Foo() {
    map = new HashMap<String, String>();
    // Load some useful values into map
  }

  public synchronizedString void setFirstget(intString ns) {
    arr[1] = n;return map.get(s);
  }
}
{code}

Synchronization establishes a [happens-before|BB. Definitions#happens-before order] relation between the thread  public synchronized void put(String key, String value) {
    // Validate the values before inserting
    if (!value.matches("[\\w]*")) {
      throw new IllegalArgumentException();
    }
    map.put(key, value);
  }
}

 that calls {{setFirst()}} and the thread that subsequently calls {{getFirst()}}. Consequently, the array element set by {{setFirst()}} is guaranteed to be visible to {{getFirst()}}.


h2. Noncompliant Code Example (Mutable Object)

This noncompliant code example declares an instance field of the {{Properties}} type as {{volatile}}. The instance of the {{Properties}} object can be mutated using the {{put()}} method, and that makes the {{properties}} field mutable.

{code:bgColor=#FFcccc}
final class Foo {
  private volatile Properties properties;
final Map<String, String> map;
  public Foo() {
    propertiesmap = new HashMap<String, PropertiesString>();
    // Load some useful values into propertiesmap
  }

  public synchronized String get(String s) {
    return propertiesmap.getPropertyget(s);
  }

  public synchronized void put(String key, String value) {
    // Validate the values before inserting
    properties.setProperty(key, value);
  }
}
{code}

If one thread calls {{get()}} while another calls {{put()}}, the first thread may receive a stale or 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. 

There is no time of check, time of use (TOCTOU) vulnerability in {{put()}}, despite the presence of the validation logic, because the validation is performed on the immutable {{value}} argument and not the shared {{Properties}} instance.


h2. Compliant Solution (Immutable)

This compliant solution renders the {{properties}} field immutable by not providing a {{put()}} method.  Consequently, once it is properly constructed, no thread can modify the state of the {{Properties}} object instance and cause a data race.

{code:bgColor=#ccccff}
final 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);
  }
}
{code}

The {{Foo}} class is also [immutable|BB. Definitions#immutable], because all its fields are {{final}} and the {{properties}} field is being published safely. The disadvantage of making {{Foo}} immutable is that the {{put()}} method can no longer be accommodated. When that is unacceptable, the {{get()}} and {{put()}} methods must use some form of synchronization to prevent data races.


h2. Noncompliant Code Example (Volatile-Read, Synchronized-Write)

This noncompliant code example attempts to use the volatile-read, synchronized-write technique described by Goetz \[[Goetz 07|AA. Java References#Goetz 07]\]. The {{properties}} field is declared {{volatile}} to synchronize reads and writes of the field. The {{put()}} method is also synchronized to ensure that its statements are executed atomically.

{code:bgColor=#ffcccc}
final 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) {
    // Validate the values before inserting
    // ...
    properties.setProperty(key, value);
  }
}
{code}

The volatile-read, synchronized-write technique uses synchronization to preserve atomicity of compound operations such as increment and provides faster access times for atomic reads. However, it does not work with mutable objects, because the visibility of {{volatile}} does not extend to their members. Consequently, if one thread adds a property using {{put}}, other threads may not observe this change. There is no [happens-before relation|BB. Definitions#happens-before order] between the write and a subsequent read of the property.

This technique is also discussed in [CON01-J. Ensure that compound operations on shared variables are atomic].


h2. Compliant Solution (Synchronized)

This compliant solution uses method synchronization to ensure thread-safety. 

{code:bgColor=#ccccff}
final 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) {
    // Validate the values before inserting
    properties.setProperty(key, value);
  }
}
{code}

Note that the {{properties}} field is not declared {{volatile}}, because this solution achieves thread-safety using synchronization. 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] for more information). 


h2. Noncompliant Code Example (Mutable Sub-Object)

This noncompliant code example declares the {{FORMAT}} field {{volatile}}. However, the field stores a reference to a mutable object, {{java.text.DateFormat}}. 

{code:bgColor=#FFcccc}
final class DateHandler {
  private static volatile DateFormat FORMAT =
    DateFormat.getDateInstance(DateFormat.MEDIUM);

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

In the presence of multiple threads, this results in subtle thread-safety issues, because {{DateFormat}} is not thread-safe \[[API 06|AA. Java References#API 06]\]. For example, a thread might observe correctly formatted output for an arbitrary date when it invokes {{parse()}} with a known date. 

{mc}
// Calls DateHandler, demo code
public class DateCaller implements Runnable {
  public void run(){
    try {
      System.out.println(DateHandler.parse("Jan 1, 2010"));	
    } catch (ParseException e) {
  }	

  public static void main(String[] args) {
    for(int i=0;i<10;i++)
      new Thread(new DateCaller()).start();
  }
}
{mc} 


h2. Compliant Solution (Instance Per Call/Defensive Copying)

This compliant solution creates and returns a new {{DateFormat}} instance for every invocation of the {{parse()}} method. \[[API 06|AA. Java References#API 06]\]

{code:bgColor=#ccccff}
final class DateHandler {
  public static Date parse(String str) throws ParseException {
    DateFormat format = DateFormat.getDateInstance(DateFormat.MEDIUM);
    return format.parse(str);
  }
}	
{code}

This solution 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 rather only a local field, {{format}}. 


h2. Compliant Solution (Synchronization)

This compliant solution synchronizes statements within the {{parse()}} method, and consequently, the {{DateHandler}} class is thread-safe \[[API 06|AA. Java References#API 06]\]. There is no requirement for declaring the {{FORMAT}} field {{volatile}}. 

{code:bgColor=#ccccff}
final class DateHandler {
  private static DateFormat FORMAT =
    DateFormat.getDateInstance(DateFormat.MEDIUM);

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


h2. Compliant Solution ({{ThreadLocal}} Storage)

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

{code:bgColor=#ccccff}
final class DateHandler {
  private static final ThreadLocal<DateFormat> df = new ThreadLocal<DateFormat>() {
    @Override protected DateFormat initialValue() {
      return DateFormat.getDateInstance(DateFormat.MEDIUM);
    }
  };
  // ...
}
{code}


h2. Risk Assessment

Assuming that declaring a field {{volatile}} guarantees visibility of the members of the referenced object can cause threads to observe stale values of the members.

|| Rule || Severity || Likelihood || Remediation Cost || Priority || Level ||
| CON11-J | medium | probable | medium | {color:#cc9900}{*}P8{*}{color} | {color:#cc9900}{*}L2{*}{color} |

h3. Automated Detection

TODO

h3. Related Vulnerabilities

Any vulnerabilities resulting from the violation of this rule are listed on the [CERT website|https://www.kb.cert.org/vulnotes/bymetric?searchview&query=FIELD+KEYWORDS+contains+CON11-J].

h2. References

\[[Goetz 07|AA. Java References#Goetz 07]\] Pattern #2: "one-time safe publication"
\[[Miller 09|AA. Java References#Miller 09]\] Mutable Statics
\[[API 06|AA. Java References#API 06]\] Class {{java.text.DateFormat}}
\[[JLS 05|AA. Java References#JLS 05]\]

----
[!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_left.png!|CON10-J. Do not override thread-safe methods with methods that are not thread-safe]&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;[!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_up.png!|11. Concurrency (CON)]&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;[!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_right.png!|CON12-J. Avoid deadlock by requesting and releasing locks in the same order]

if (!value.matches("[\\w]*")) {
      throw new IllegalArgumentException();
    }
    map.put(key, value);
  }
}

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

  public static java.util.Date parse(String str) 
      throws ParseException {
    return format.parse(str);
  }
}

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

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

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

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