According to the _Java Language Specification_ \[[JLS 2005|AA. Bibliography#JLS 05]\], Section 8.3.1.4, "{{volatile}} Fields"
{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 non-thread-safe mutable objects, 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 \[[Goetz 2007|AA. Bibliography#Goetz 07]\].
Technically, the object does not have to be strictly immutable to be used safely. If it can be determined that a member object is thread-safe by design, the field that holds its reference may be declared volatile. However, this approach to using volatile decreases maintainability and should be avoided.
h2. Noncompliant Code Example (Arrays)
This noncompliant code example shows an array object that is declared volatile.
{code:bgColor=#FFcccc}
final class Foo {
private volatile int[] arr = new int[20];
public int getFirst() {
return arr[0];
}
public void setFirst(int n) {
arr[0] = n;
}
// ...
}
{code}
Values assigned to an array element by one thread, for example, by calling {{setFirst()}}, might not be visible to another thread calling {{getFirst()}} because the {{volatile}} keyword only makes the array reference visible and does not affect the actual data contained within the array.
The problem occurs because there is no [happens-before|BB. Definitions#happens-before order] relationship between the thread that calls {{setFirst()}} and the thread that calls {{getFirst()}}. A happens-before relationship exists between a thread that writes to a volatile variable and a thread that subsequently reads it. However, this code is neither writing to nor reading from a volatile variable.
h2. Compliant Solution ({{AtomicIntegerArray}})
To ensure that the writes to array elements are atomic and the resulting values are visible to other threads, this compliant solution uses the {{AtomicIntegerArray}} class defined in {{java.util.concurrent.atomic}}.
{code:bgColor=#ccccff}
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);
}
// ...
}
{code}
{{AtomicIntegerArray}} guarantees a [happens-before|BB. Definitions#happens-before order] relationship between a thread that calls {{atomicArray.set()}} and a thread that subsequently calls {{atomicArray.get()}}.
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 non-volatile.
{code:bgColor=#ccccff}
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;
}
}
{code}
Synchronization establishes a [happens-before|BB. Definitions#happens-before order] relationship between the thread that calls {{setFirst()}} and the thread that subsequently calls {{getFirst()}}, guaranteeing visibility.
h2. Noncompliant Code Example (Mutable Object)
This noncompliant code example declares the {{Properties}} instance field 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;
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) {
// Validate the values before inserting
if (!value.matches("[\\w]*")) {
throw new IllegalArgumentException();
}
properties.setProperty(key, value);
}
}
{code}
Interleaved calls to {{get()}} and {{put()}} may result in internally inconsistent values being retrieved from the {{Properties}} object because the operations within {{put()}} modify its state. Declaring the object volatile does not eliminate 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. Noncompliant Code Example (Volatile-Read, Synchronized-Write)
This noncompliant code example attempts to use the volatile-read, synchronized-write technique described by Goetz \[[Goetz 2007|AA. Bibliography#Goetz 07]\]. The {{properties}} field is declared volatile to synchronize its reads and writes. 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
if (!value.matches("[\\w]*")) {
throw new IllegalArgumentException();
}
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}} object references does not extend to object members. Consequently, there is no [happens-before|BB. Definitions#happens-before order] relationship between the write and a subsequent read of the property.
This technique is also discussed in [VNA02-J. Ensure that compound operations on shared variables are atomic].
h2. Compliant Solution (Synchronized)
This compliant solution uses method synchronization to guarantee visibility.
{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
if (!value.matches("[\\w]*")) {
throw new IllegalArgumentException();
}
properties.setProperty(key, value);
}
}
{code}
The {{properties}} field does not need to be volatile because the methods are synchronized. The field is declared final so that its reference is not published when it is in a partially initialized state (see guideline [TSM03-J. Do not publish partially initialized objects] for more information).
h2. Noncompliant Code Example (Mutable Sub-Object)
In this noncompliant code example, the volatile {{format}} field is used to store 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 java.util.Date parse(String str) throws ParseException {
return format.parse(str);
}
}
{code}
Because {{DateFormat}} is not thread-safe \[[API 2006|AA. Bibliography#API 06]\], the {{parse()}} method might return a value for {{Date}} that does not correspond to the {{str}} argument.
{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. \[[API 2006|AA. Bibliography#API 06]\]
{code:bgColor=#ccccff}
final class DateHandler {
public static java.util.Date parse(String str) throws ParseException {
return DateFormat.getDateInstance(DateFormat.MEDIUM).parse(str);
}
}
{code}
This solution does not violate guideline [OBJ09-J. Defensively copy private mutable class members before returning their references] because the class no longer contains internal mutable state.
h2. Compliant Solution (Synchronization)
This compliant solution synchronizes statements within the {{parse()}} method, making {{DateHandler}} thread-safe \[[API 2006|AA. Bibliography#API 06]\].
{code:bgColor=#ccccff}
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);
}
}
}
{code}
h2. Compliant Solution ({{ThreadLocal}} Storage)
This compliant solution uses a {{ThreadLocal}} object to create a separate {{DateFormat}} instance per thread.
{code:bgColor=#ccccff}
final class DateHandler {
private static final ThreadLocal<DateFormat> format = new ThreadLocal<DateFormat>() {
@Override protected DateFormat initialValue() {
return DateFormat.getDateInstance(DateFormat.MEDIUM);
}
};
// ...
}
{code}
h2. Risk Assessment
Incorrectly assuming that declaring a field volatile guarantees that the visibility of a referenced object's members can cause threads to observe stale values.
|| Guideline || Severity || Likelihood || Remediation Cost || Priority || Level ||
| VNA06-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 guideline are listed on the [CERT website|https://www.kb.cert.org/vulnotes/bymetric?searchview&query=FIELD+KEYWORDS+contains+CON11-J].
h2. Bibliography
\[[Goetz 2007|AA. Bibliography#Goetz 07]\] Pattern #2: "one-time safe publication"
\[[Miller 2009|AA. Bibliography#Miller 09]\] Mutable Statics
\[[API 2006|AA. Bibliography#API 06]\] Class {{java.text.DateFormat}}
\[[JLS 2005|AA. Bibliography#JLS 05]\]
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[!The CERT Oracle Secure Coding Standard for Java^button_arrow_left.png!|VNA05-J. Ensure atomicity when reading and writing 64-bit values] [!The CERT Oracle Secure Coding Standard for Java^button_arrow_up.png!|07. Visibility and Atomicity (VNA)] [!The CERT Oracle Secure Coding Standard for Java^button_arrow_right.png!|08. Locking (LCK)]
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