CON02-J. Always synchronize on the appropriate object

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]]. It is important to recognize the entities with whom synchronization is required rather than indiscreetly scavenging for objects to synchronize on. we need a more precise statement about what specifically this guideline requires

Noncompliant Code Example (Boolean lock object)

This noncompliant code example synchronizes on the (initialized) boolean.

private final Boolean initialized = Boolean.FALSE;

public void doSomething() {
  synchronized(initialized) { 
    // ...
  }
}

Variables of type Boolean are unsuitable as locks because they can only assume the values true and false. Consequently, Java may only allocate two Boolean objects--one which represents FALSE and the other which represents TRUE. In this example, initialized references the FALSE object. If any other code inadvertently synchronizes on the same object (a Boolean variable with the same value), the system may become unresponsiveness or deadlock.

Noncompliant Code Example (Boxed primitive)

This noncompliant code example locks on a boxed Integer object.

int lock = 0;
private final Integer Lock = lock; // Boxed primitive Lock is shared

public void doSomething() {
  synchronized(Lock) { 
    // ...
  }
}

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.

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.

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

public void doSomething() {
  synchronized(Lock) { 
    // ...
  }
}

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.

Compliant Solution (internal private final lock Object)

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

private final Object lock = new Object();

public void doSomething() {
  synchronized(lock) {
    // ...
  }
}

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

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]], class java.lang.String documentation:

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 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 points to a common String constant in the JVM. Trusted code that locks on the same String constant renders all synchronization attempts inadequate. Similarly, hostile code from any other package can exploit this vulnerability if the class is accessible.

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";

// ...
  synchronized(_lock) { 
    // ...
  }
// ...

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

Compliant Solution (String instance)

This compliant solution locks on a 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 that is not shared by other string object instances or literals. A more suitable approach is to use an internal private lock as 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).

public void doSomething() {
  synchronized(getClass()) {
    // ... 
  }
}

Section 4.3.2 "The Class Object" of the Java Language specification [[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 what the programmer had in mind. The intent should be clearly documented or annotated.

Compliant Solution (class name qualification)

Explicitly define the name of the class through name qualification (superclass in this compliant solution) in the synchronized block.

public void doSomething() {
  synchronized(SuperclassName.class) { 
    // ... 
  }
}

The class object that is being used for synchronization should not be accessible to hostile 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 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.

public void doSomething() {
  synchronized(Class.forName("SuperclassName")) { 
    // ... 
  }
}

The class object that is being used for synchronization should not be accessible to hostile code, as discussed in the previous compliant solution. 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.)

Noncompliant Code Example (ReentrantLock lock object)

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

private final Lock lock = new ReentrantLock();

public void doSomething() {
  synchronized(lock) {
    // ... 
  }
}

Similarly, it is inappropriate to lock on an object of a class that implements either the Lock or Condition interface (or both) of package java.util.concurrent.locks. Using intrinsic locks of these classes is a questionable practice even though the code may appear to function correctly. 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, such as ReentrantLock, use the lock() and unlock() methods provided by the Lock interface.

private final Lock lock = new ReentrantLock();

public void doSomething() {
  lock.lock();
  try {
    // ...
  } finally {
    lock.unlock();
  }
}

If there is no requirement of using the advanced functionality of the dynamic locking utilities of package java.util.concurrent, prefer using the Executor framework or other concurrency primitives such as synchronization and atomic classes.

Noncompliant Code Example (collection view)

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

// map has package-private accessibility
final Map<Integer, String> map = Collections.synchronizedMap(new HashMap<Integer, String>());
private final Set<Integer> set = map.keySet();

public void doSomething() {
  synchronized(set) {  // Incorrectly synchronizes on set
    for(Integer k : set) { 
      // ...
    }
  }
}

When using synchronization wrappers, the synchronization object should 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]].

The java.util.Collections interface's documentation [[API 06]] warns about the consequences of following this practice:

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 has package-private accessibility
final Map<Integer, String> map = Collections.synchronizedMap(new HashMap<Integer, String>());
private final Set<Integer> set = map.keySet();

public void doSomething() {
  synchronized(map) {  // Synchronize on map, not set
    for(Integer k : set) { 
      // ...
    }
  }
}

Risk Assessment

Synchronizing on an inappropriate object can provide a false sense of thread safety and result in non-deterministic behavior.

Automated Detection

The following table summarizes the examples flagged as violations by FindBugs:

The following table summarizes the examples flagged as violations by SureLogic Flashlight:

Related Vulnerabilities

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

References

[[API 06]] Class String, Collections
[[Findbugs 08]].
[[Pugh 08]] "Synchronization"
[[Miller 09]] Locking
[[Tutorials 08]] Wrapper Implementations

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