Synchronizing on the return value of the {{Object.getClass()}} method, rather than a class literal can lead to unexpected behavior. Whenever the implementing class is subclassed, the subclass locks on the subclass's type, which is a completely different {{Class}} object.
Section 4.3.2 "The Class Object" of the Java Language specification \[[JLS 05|AA. Java References#JLS 05]\] describes how method synchronization works:
{quote}
A class method that is declared {{synchronized}} synchronizes on the lock associated with the {{Class}} object of the class.
{quote}
This does not mean that a subclass using {{getClass()}} can only synchronize on the {{Class}} object of the base class. In fact, it will lock on its own {{Class}} object, which may or may not be what the programmer had in mind. The intent should be clearly documented or annotated. Note that if a subclass does not override an accessible noncompliant superclass's method, it inherits the method which may lead to the false conclusion that the superclass's intrinsic lock is available in the subclass.
h2. Noncompliant Code Example ({{getClass()}} lock object)
In this noncompliant code example, the {{parse()}} method of class {{Base}} parses a date and synchronizes on the class object returned by {{getClass()}}. The class {{Derived}} also inherits the {{parse()}} method. However, this inherited method synchronizes on the class object of {{Derived}} because of the particular return value of {{getClass()}}.
{{Derived}} also adds a method {{doSomethingAndParse()}} that locks on the class object of the base class because the developer misconstrued that the {{parse()}} method in {{Base}} always obtains a lock on the {{Base}} class object, and {{doSomethingAndParse()}} must follow the same locking policy. Consequently, the class {{Derived}} has two different locking strategies and is not thread-safe.
{code:bgColor=#FFcccc}
class Base {
static DateFormat FORMAT =
DateFormat.getDateInstance(DateFormat.MEDIUM);
public Date parse(String str) throws ParseException {
synchronized (getClass()) {
return FORMAT.parse(str);
}
}
}
class Derived extends Base {
public Date doSomethingAndParse(String str) throws ParseException {
synchronized(Base.class) {
// ...
return FORMAT.parse(str);
}
}
}
{code}
{mc}
// Hidden main() method to be put in class Derived
// Prints arbitrary date values and throws exceptions at times
public static void main(String[] args) {
for(int i = 0; i < 100; i++) {
new Thread(new Runnable() {
@Override public void run() {
try {
System.out.println(new Derived().parse("Jan 1, 2010"));
} catch (ParseException e) {
// Forward to handler
}
}
}).start();
new Thread(new Runnable() {
@Override public void run() {
try {
System.out.println(new Derived().doSomethingAndParse("Jan 2, 2010"));
} catch (ParseException e) {
// Forward to handler
}
}
}).start();
}
}
{mc}
h2. Compliant Solution (class name qualification)
Explicitly define the name of the class through name qualification ({{Base}} in this compliant solution) in the synchronized block.
{code:bgColor=#ccccff}
class Base {
static DateFormat FORMAT =
DateFormat.getDateInstance(DateFormat.MEDIUM);
public Date parse(String str) throws ParseException {
synchronized (Base.class) {
return FORMAT.parse(str);
}
}
}
// ...
{code}
This code example always synchronizes on the {{Base.class}} object, even if it is called from a {{Derived}} object.
The class object that is being used for synchronization should not be accessible to untrusted code. If the class is package-private, callers from other packages may not access the class object, ensuring its trustworthiness as an intrinsic lock object. For more information, see [CON04-J. Synchronize using ana private final lock object].
h2. Compliant Solution ({{Class.forName()}})
This compliant solution uses the {{Class.forName()}} method to synchronize on the {{Base}} class's {{Class}} object.
{code:bgColor=#ccccff}
class Base {
static DateFormat FORMAT =
DateFormat.getDateInstance(DateFormat.MEDIUM);
public Date parse(String str) throws ParseException {
synchronized (Class.forName("Base")) {
return FORMAT.parse(str);
}
}
}
// ...
{code}
The class object that is being used for synchronization should also not be accessible to untrusted code, see [CON04-J. Synchronize using ana private final lock object] for more information. Furthermore, care must be taken to ensure that untrusted inputs are not accepted as arguments while loading classes using {{Class.forname()}}. (See [SEC05-J. Do not expose standard APIs that use the immediate caller's class loader instance to untrusted code] for more information.)
h2. Risk Assessment
Synchronizing on the class object returned by {{getClass()}} can present a false sense of thread safety and result in non-deterministic behavior.
|| Rule || Severity || Likelihood || Remediation Cost || Priority || Level ||
| CON02- J | medium | probable | medium | {color:#cc9900}{*}P8{*}{color} | {color:#cc9900}{*}L2{*}{color} |
h3. Automated Detection
The following table summarizes the examples flagged as violations by FindBugs:
||Noncompliant Code Example||Flagged||Checker||Message||
|{{getClass()}} lock object|No|WL_USING_GETCLASS_RATHER_THAN_CLASS_LITERAL|n/a|
The following table summarizes the examples flagged as violations by [SureLogic Flashlight|http://www.surelogic.com/]:
||Noncompliant Code Example||Flagged||Message||
|{{getClass()}} lock object|No|No data available about field accesses|
h3. Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the [CERT website|https://www.kb.cert.org/vulnotes/bymetric?searchview&query=FIELD+KEYWORDS+contains+CON36-J].
h2. References
\[[API 06|AA. Java References#API 06]\]
\[[Findbugs 08|AA. Java References#Findbugs 08]\].
\[[Pugh 08|AA. Java References#Pugh 08]\] "Synchronization"
\[[Miller 09|AA. Java References#Miller 09]\] Locking
----
[!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_left.png!|VOID CON00-J. Synchronize access to shared mutable variables] [!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_up.png!|11. Concurrency (CON)] [!The CERT Sun Microsystems Secure Coding Standard for Java^button_arrow_right.png!|CON03-J. Do not use background threads during class initialization]
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