Misuse of synchronization primitives is a common source of concurrency issues. A significant number of concurrency vulnerabilities arise from locking on the wrong kind of object. -An analysis of the JDK 1.6.0 source code discovered 31 bugs that fell into this category \[[Pugh 08|AA. Java References#Pugh 08]\]- {mc} probably incorrect after splitting{mc}. It is important to recognize the entities with whom synchronization is required rather than indiscreetly scavenging for objects to synchronize on. {color:red}we need a more precise statement about what specifically this guideline requires{color} {mc} any suggestions? {mc} {mc} not yet {mc}
h2. Noncompliant Code Example ({{Boolean}} lock object)
This noncompliant code example synchronizes on a {{Boolean}} lock object.
{code:bgColor=#FFcccc}
private final Boolean initialized = Boolean.FALSE;
public void doSomething() {
synchronized(initialized) {
// ...
}
}
{code}
The {{Boolean}} type is unsuitable for locking purposes because it allows only two values: {{TRUE}} and {{FALSE}}. Boolean literals containing the same value share unique instances of class {{Boolean}} in the JVM. In this example, {{initialized}} references the instance corresponding to the value {{FALSE}}. If any other code inadvertently synchronizes on a {{Boolean}} literal with the value {{FALSE}}, the lock instance is reused and the system may become unresponsiveness or deadlock.
h2. Noncompliant Code Example (boxed primitive)
This noncompliant code example locks on a boxed {{Integer}} object.
{code:bgColor=#FFcccc}
int lock = 0;
private final Integer Lock = lock; // Boxed primitive Lock is shared
public void doSomething() {
synchronized(Lock) {
// ...
}
}
{code}
Boxed types may use the same instance for a range of integer values and consequently suffer from the same problem as {{Boolean}} constants. If the value of the primitive can be represented as a byte, the wrapper object is reused. Note that the use of the boxed {{Integer}} wrapper object is insecure; instances of the {{Integer}} object constructed using the {{new}} operator ({{new Integer(value)}}) are unique and not reused. In general, holding a lock on any data type that contains a boxed value is insecure.
h2. Compliant Solution (Integer)
This compliant solution recommends locking on a non-boxed Integer. The {{doSomething()}} method synchronizes using the intrinsic lock of the {{Integer}} instance, {{Lock}}.
{code:bgColor=#ccccff}
int lock = 0;
private final Integer Lock = new Integer(lock);
public void doSomething() {
synchronized(Lock) {
// ...
}
}
{code}
When explicitly constructed, an {{Integer}} object has a unique reference and its own intrinsic lock that is not shared with other {{Integer}} objects or boxed integers having the same value. While this is an acceptable solution, it may cause maintenance problems because developers might incorrectly assume that boxed integers are appropriate lock objects. A more appropriate solution is to synchronize on an internal private final lock {{Object}} as described in the following compliant solution.
h2. Noncompliant Code Example (interned {{String}} object)
This noncompliant code example locks on an interned {{String}} object.
{code:bgColor=#FFcccc}
private final String _lock = new String("LOCK").intern();
public void doSomething() {
synchronized(_lock) {
// ...
}
}
{code}
According to the Java API \[[API 06|AA. Java References#API 06]\], class {{java.lang.String}} documentation:
{quote}
When the {{intern()}} method is invoked, if the pool already contains a string equal to this {{String}} object as determined by the {{equals(Object)}} method, then the string from the pool is returned. Otherwise, this {{String}} object is added to the pool and a reference to this {{String}} object is returned.
{quote}
Consequently, an interned {{String}} object behaves like a global variable in the Java Virtual Machine (JVM). As demonstrated in this noncompliant code example, even if every instance of an object maintains its own field {{lock}}, the field references a common {{String}} constant. Locking on {{String}} constants has the same problem as locking {{Boolean}} constants.
Additionally, hostile code from any other package can exploit this vulnerability if the class is accessible (see [CON04-J. Synchronize using an internal private final lock object]).
h2. Noncompliant Code Example ({{String}} literal)
This noncompliant code example locks on a final {{String}} literal.
{code:bgColor=#FFcccc}
// This bug was found in jetty-6.1.3 BoundedThreadPool
private final String _lock = "LOCK";
// ...
synchronized(_lock) {
// ...
}
// ...
{code}
A {{String}} literal is a constant and is interned. Consequently, it suffers from the same pitfalls as the preceding noncompliant code example.
h2. Compliant Solution ({{String}} instance)
This compliant solution locks on a {{String}} instance that is not interned.
{code:bgColor=#ccccff}
private final String _lock = new String("LOCK");
public void doSomething() {
synchronized(_lock) {
// ...
}
}
{code}
A {{String}} instance differs from a {{String}} literal. The instance has a unique reference and its own intrinsic lock that is not shared by other string object instances or literals. A better approach is to synchronize on an internal private final lock object as shown in the following compliant solution.
h2. Compliant Solution (internal private final lock {{Object}})
This compliant solution synchronizes on an internal private final lock object. This is one of the few cases where a {{java.lang.Object}} instance is useful.
{code:bgColor=#ccccff}
private final Object lock = new Object();
public void doSomething() {
synchronized(lock) {
// ...
}
}
{code}
For more information on using an {{Object}} as a lock, see [CON04-J. Synchronize using an internal private final lock object].
h2. Risk Assessment
Synchronizing on an inappropriate object can provide 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||
|{{Boolean}} lock object|Yes|DL_SYNCHRONIZATION_ON_BOOLEAN|Synchronization on Boolean could deadlock|
|Boxed primitive|Yes|DL_SYNCHRONIZATION_ON_BOXED_PRIMITIVE|Synchronization on Integer could deadlock|
|interned {{String}} object|No|DL_SYNCHRONIZATION_ON_SHARED_CONSTANT|n/a|
|String literal|Yes||Synchronization on interned String could deadlock|
|{{getClass()}} lock object|No|WL_USING_GETCLASS_RATHER_THAN_CLASS_LITERAL|n/a|
|{{ReentrantLock}} lock object|No||n/a|
|Collection view|No|n/a|
The following table summarizes the examples flagged as violations by [SureLogic Flashlight|http://www.surelogic.com/]:
||Noncompliant Code Example||Flagged||Message||
|{{Boolean}} lock object|No|No obvious issues|
|Boxed primitive|No|No obvious issues|
|interned {{String}} object|No|No obvious issues|
|String literal|No||No data available about field accesses|
|{{getClass()}} lock object|No|No data available about field accesses|
|{{ReentrantLock}} lock object|No|No obvious issues|
|Collection view|No|No obvious issues|
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
\[[API 06|AA. Java References#API 06]\] Class String, Collections
\[[Findbugs 08|AA. Java References#Findbugs 08]\].
\[[Pugh 08|AA. Java References#Pugh 08]\] "Synchronization"
\[[Miller 09|AA. Java References#Miller 09]\] Locking
\[[Tutorials 08|AA. Java References#Tutorials 08]\] [Wrapper Implementations|http://java.sun.com/docs/books/tutorial/collections/implementations/wrapper.html]
----
[!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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