CON02-J. Always synchronize on the appropriate object

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]]

Noncompliant Code Example (Boolean lock object)

This noncompliant code example uses a Boolean field for synchronization.

private final Boolean initialized = Boolean.FALSE;

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

There can only be two possible valid values (true and false, discounting null) that initialized can assume. Consequently, any other code that synchronizes on a Boolean variable with the same value, may induce unresponsiveness and deadlocks [[Findbugs 08]].

Noncompliant Code Example (Boxed primitive)

This noncompliant code example locks on a boxed Integer object.

int lock = 0;
final Integer Lock = lock; // Boxed primitive Lock will be shared

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

Boxed types are allowed to use the same instance for a range of integer values and consequently, suffer from the same problems as Boolean constants. If the primitive can be represented as a byte, the wrapper object is reused. Note that the boxed Integer primitive is shared and not the Integer object (new Integer(value)) itself. In general, holding a lock on any data structure that contains a boxed value is insecure.

Compliant Solution (Integer)

This compliant solution locks on a non-boxed Integer.

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 by other Integer objects or boxed integers having the same value. While this is an acceptable solution, it may cause maintenance problems. It is always better to synchronize on a private final raw Object as described next.

Compliant Solution (private final internal raw Object)

This compliant solution uses an internal private final lock object. This is one of the few cases where a raw Object 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 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 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. Likewise, hostile code from any other package can exploit this vulnerability.

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, not shared by other string objects or literals. A more suitable 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).

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 appropriately 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 synchronization block.

public void doSomething() {
  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.

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

Again, the class object being synchronized must not be accessible to hostile code, as discussed in the previous compliant solution. Furthermore, care must be taken so 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.

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. 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.

final Lock lock = new ReentrantLock();

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

It is recommended to not use the intrinsic locks of objects of classes that implement Lock or Condition interfaces. If there is no real need 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.

private 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 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]].

The java.util.Collections interfaces' documentation [[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>());

public void doSomething() {
  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 inappropriate field can provide a false sense of thread safety and result in non-deterministic behavior.

Automated Detection

Currently, SureLogic Flashlight does not detect all violations of this guideline. It detects:

It does not detect:

Related Vulnerabilities

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

References

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

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