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Wiki MarkupCode that uses synchronization can sometimes be enigmatic and tricky to debug. Misuse of synchronization primitives is a common source of implementation errors. An analysis of the JDK 1.6.0 source code unveiled at least 31 bugs that fell into this category. \[[Pugh 08|AA. Java References#Pugh 08]\]of concurrency issues. Synchronizing on objects that may be reused can result in deadlock and nondeterministic behavior. Consequently, programs must never synchronize on objects that may be reused.

Noncompliant Code Example (Boolean

...

Lock Object)

This noncompliant code example uses synchronizes on a Boolean field for synchronization lock object.

private final Boolean initialized = Boolean.FALSE;

synchronizedpublic void doSomething(initialized) { 
  ifsynchronized (!initialized) {
    // Perform initialization
    initialized = Boolean.TRUE;...
  }
}

Wiki MarkupThere can be two possible valid values ({{true}} and {{false}} of the variable {{initialized}}, discounting {{null}}) that a {{Boolean}} can assume. Consequently, any other code that synchronizes on a {{Boolean}} variable with the same value, may cause unresponsiveness and deadlocks \[[Findbugs 08|AA. Java References#Findbugs 08]\]. 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 the Boolean class in the Java Virtual Machine (JVM). In this example, initialized refers to the instance corresponding to the value Boolean.FALSE. If any other code were to inadvertently synchronize on a Boolean literal with this value, the lock instance would be reused and the system could become unresponsive or could deadlock.

Noncompliant Code Example (Boxed

...

Primitive)

This noncompliant code example locks on a boxed Integer object.

private int lockcount = 0;
private final Integer Lock = lockcount; // Boxed primitive Lock will be shared
synchronized is shared

public void doSomething() {
  synchronized (Lock) {
 /*   count++;
    // ...
 */ }
}

Boxed types are allowed to may use the same instance for a range of integer values and ; consequently, they suffer from the same problems reuse problem as Boolean constants. Note that The wrapper object are reused when the value can be represented as a byte; JVM implementations are also permitted to reuse wrapper objects for larger ranges of values. While use of the intrinsic lock associated with the boxed Integer primitive is shared and not wrapper object is insecure; instances of the Integer object constructed using the new operator (new Integer(value)) itselfare unique and not reused. In general, holding a lock locks on any data structure type that contains a boxed value is are insecure.

Compliant Solution (Integer)

This compliant solution locks on a non-boxed Integera nonboxed Integer, using a variant of the private lock object idiom. The doSomething() method synchronizes using the intrinsic lock of the Integer instance, Lock.

private int lockcount = 0;
private final Integer Lock = new Integer(lockcount);

public void doSomething() {
  synchronized (Lock) {
 /*   count++;
    // ...
 */ }
}

When explicitly constructed, an Integer object has a unique reference and its own intrinsic lock that is distinct not shared by only from other Integer objects or , but also from boxed integers having that have the same value. While this is an acceptable solution, it may can cause maintenance problems . It is always better because developers can incorrectly assume that boxed integers are also appropriate lock objects. A more appropriate solution is to synchronize on a private final raw Object as described next.

Compliant Solution (private internal raw Object)

This compliant solution uses an internal private lock object. This is one of the few cases where a raw Object is useful.

private final Object lock = new Object();

synchronized(lock) {
// ...
}

For more information on using an Object as a lock, see CON04-J. Synchronize using an internal private final lock object.

lock object as described in the final compliant solution for this rule.

Noncompliant Code Example (Interned String Object

...

)

This noncompliant code example locks on an interned String object.

private final String _lock = new String("LOCK").intern();

public void doSomething() {
  synchronized (_lock) {
    //* ...
 */ }

...

}

According to the Java API \[[API 06|AA. Java References#API 06]\], class {{String}} documentationAPI class java.lang.String documentation [API 2006]:

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.

Consequently, an interned String object behaves like a global variable in the JVM. As demonstrated in this noncompliant code example, even if when every instance of an object maintains its own lock field lock, the field points fields all refer to a common String constant in the JVM. Trusted code that locks on the same String constant renders all synchronization attempts inadequate. Likewise. Locking on String constants has the same reuse problem as locking on Boolean constants.

Additionally, hostile code from any other package can exploit this vulnerability, if the class is accessible. See rule LCK00-J. Use private final lock objects to synchronize classes that may interact with untrusted code for more information.

Noncompliant Code Example (String

...

Literal)

This noncompliant code example locks on a final String literal.

// This bug was found in jetty-6.1.3 BoundedThreadPool
private final String _lock = "LOCK";

public void doSomething() {
  synchronized (_lock) {
    /*/ ...
 */ }
}

A String literal is a literals are constant and is are automatically interned. Consequently, it this example suffers from the same pitfalls as the preceding noncompliant code example.

Compliant Solution (String

...

Instance)

This compliant solution locks on a noninterned String instance that is not interned.

private final String _lock = new String("LOCK");

public void doSomething() {
  synchronized (_lock) {
    /*/ ...
 */ }
}

A String instance differs from a String literal. The instance has a unique reference and its own intrinsic lock , not shared by other string objects that is distinct from other String object instances or literals. A more suitable Nevertheless, a better approach is to use the private final internal raw Object discussed earlier.

Noncompliant Code Example (getClass() lock object)

Synchronizing on return values of the Object.getClass() method, rather than a class literal can also be counterproductive. Whenever the implementing class is subclassed, the subclass locks on a completely different Class object (subclass's type).

synchronized(getClass()) { /* ... */ }

Section 4.3.2 "The Class Object" of the Java Language specification \[[JLS 05|AA. Java References#JLS 05]\] describes how method synchronization works:

A class method that is declared synchronized synchronizes on the lock associated with the Class object of the class.

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 want the programmer had in mind. The intent should be appropriately documented or annotated.

Compliant Solution (class name qualification)

Explicitly define the name of the class through name qualification (superclass in this example) in the synchronization block.

synchronized(SuperclassName.class) { 
  // ... 
}

The class object being synchronized must not be accessible to hostile code. If the class is package-private, then external packages may not access the Class object, ensuring its trustworthiness as an intrinsic lock object. For more information, see CON04-J. Synchronize using an internal private final lock object.

Compliant Solution (Class.forName())

This compliant solution uses the Class.forName() method to synchronize on the superclass's Class object.

synchronized(Class.forName("SuperclassName")) { 
  // ... 
}

Again, the class object being synchronized must not be accessible to hostile code, as discussed in the previous example.

Noncompliant Code Example (ReentrantLock lock object)

This noncompliant code example incorrectly uses a ReentrantLock as the lock object.

final Lock lock = new ReentrantLock();
synchronized(lock) { /* ... */ }

This problem usually comes up in practice when refactoring from intrinsic locking to the java.util.concurrent dynamic locking utilities.

Compliant Solution (lock() and unlock())

Instead of using the intrinsic locks of objects that implement the Lock interface, including ReentrantLock, use the lock() and unlock() methods provided by the Lock interface.

synchronize on a private final lock object, as shown in the following compliant solution.

Compliant Solution (Private Final Lock Object)

This compliant solution synchronizes on a private final lock object. This is one of the few cases in which a java.lang.Object instance is useful.

private final Object

final Lock lock = new ReentrantLockObject();
lock.lock();
try {
  // ...
} finally {
  lock.unlock();
}

Noncompliant Code Example (collection view)

This noncompliant code example synchronizes on the view of a synchronized map.

private final Map<Integer, String> map = Collections.synchronizedMap(new HashMap<Integer, String>());
private final Set<Integer> set = map.keySet();
synchronized(set) {  // Incorrectly synchronizes on set
  for(Integer k : set) { 
public void doSomething() {
  synchronized (lock) {
    // Do something ...
  }
}

When using synchronization wrappers, the synchronization object must be the {{Collection}} object. The synchronization is necessary to enforce atomicity ([CON07-J. Do not assume that a grouping of calls to independently atomic methods is atomic]). This noncompliant code example demonstrates inappropriate synchronization resulting from locking on a Collection view instead of the Collection object itself \[[Tutorials 08|AA. Java References#Tutorials 08]\]. 

The Collections class documentation \[[API 06|AA. Java References#API 06]\] states:

It is imperative that the user manually synchronize on the returned map when iterating over any of its collection views... Failure to follow this advice may result in non-deterministic behavior.

Compliant Solution (collection lock object)

This compliant solution correctly synchronizes on the Collection object instead of the Collection view.

// ...
Map<Integer, String> map = Collections.synchronizedMap(new HashMap<Integer, String>());
synchronized(map) {  // Synchronize on map, not set
  for(Integer k : map) { 
    // Do something  
  }
}

Finally, it is more important to recognize the entities with whom synchronization is required rather than indiscreetly scavenging for variables or objects to synchronize on.

Risk Assessment

Synchronizing on an incorrect variable can provide a false sense of thread safety and result in nondeterministic behavior.

For more information on using an Object as a lock, see rule LCK00-J. Use private final lock objects to synchronize classes that may interact with untrusted code.

Risk Assessment

A significant number of concurrency vulnerabilities arise from locking on the wrong kind of object. It is important to consider the properties of the lock object rather than simply scavenging for objects on which to synchronize.

Automated Detection

TODO

Related Vulnerabilities

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

References

\[[API 06|AA. Java References#API 06]\] Class String, Collections
\[[Pugh 08|AA. Java References#Pugh 08]\] "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]

Some static analysis tools can detect violations of this rule.

Bibliography

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Image Added Image Added Image AddedVOID CON00-J. Synchronize access to shared mutable variables      11. Concurrency (CON)      CON03-J. Do not use background threads during class initialization