CON36-J. Always synchronize on the appropriate object

Code that uses synchronization can sometimes be enigmatic and tricky to debug. Misuse of synchronization primitives is a common source of implementation errors. The analysis of the JDK 1.6.0 source code unveiled 31 bugs that fell into this category. [[Pugh 08]]

Noncompliant Code Example

This noncompliant code example locks on a non-final object that is declared public. It is possible that untrusted code can change the value of the lock object and foil any attempts to synchronize.

public Object publicLock = new Object();
synchronized(publicLock) { 
  // body
}

Compliant Solution

This compliant solution synchronized on a private final object and is safe from malicious manipulation.

private final Object privateLock = new Object();
synchronized(privateLock) { 
  // body
}

Noncompliant Code Example

A String constant is interned in Java. According to [[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, a String constant behaves like a global variable in the JVM. As demonstrated in this noncompliant code example, even if each instance of an object maintains its own field lock, it points to a common String constant in the JVM. Legitimate code that locks on the same String constant will render all synchronization attempts inadequate. Likewise, hostile code from any other package can deliberately exploit this vulnerability.

// this bug was found in jetty-6.1.3 BoundedThreadPool
private final String _lock = "one";
synchronized(_lock) { /* ... */ }

Noncompliant Code Example

This noncompliant code example synchronizes on a mutable field instead of an object and demonstrates no mutual exclusion properties. This is because the thread that holds a lock on the field can modify the referenced object's value which allows another thread that is blocked on the unmodified value to resume, at the same time, granting access to a third thread that is blocked on the modified value. When aiming to modify a field, it is incorrect to synchronize on the same (or another) field as this is equivalent to synchronizing on the field's contents.

private Integer semaphore = new Integer(0);
synchronized(semaphore) { /* ... */ }

This is a mutual exclusion problem as opposed to the sharing issue discussed in the previous noncompliant code example. Note that the boxed Integer primitive is shared as shown below and not the Integer object (new Integer(value)) itself.

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

In general, holding a lock on any data structure that contains a boxed value can be dangerous.

Noncompliant Code Example

This noncompliant code example uses a Boolean field to synchronize. However, there can only be two possible values (true and false) that a Boolean can assume. Consequently, any other code that synchronizes on the same value can cause unresponsiveness and deadlocks [[Findbugs 08]].

private Boolean initialized = Boolean.FALSE;
synchronized(initialized) { 
  if (!initialized) {
    // perform initialization
    initialized = Boolean.TRUE;
  }
}

Compliant Solution

In the absence of an existing object to lock on, using a raw object to synchronize suffices.

private final Object lock = new Object();
synchronized(lock) { /* ... */ }

Note that the instance of the raw object should not be changed from within the synchronized block. For example, creating and storing the reference of a new object into the lock field. To prevent such modifications, declare the lock field final.

Noncompliant Code Example

Synchronizing on getClass() rather than a class literal can also be counterproductive. Whenever the implementing class is subclassed, the subclass locks on a completely different Class object.

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

This idea is sometimes easy to miss, especially when the Java Language Specification is misunderstood. Section 4.3.2 "The Class Object" of the 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 it is required to synchronize on the Class object.

Compliant Solution

Explicitly define the name of the class (superclass in this example) in the synchronization block. This can be achieved in two ways. One way is to explicitly pass the superclass's instance.

synchronized(SuperclassName.class) { ... }

The second way is to use the Class.forName() method.

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

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.

Noncompliant Code Example

When using synchronization wrappers, the synchronization object must be the Collection object. The synchronization is necessary to enforce atomicity ([CON38-J. Ensure atomicity of thread-safe code]). This noncompliant code example demonstrates inappropriate synchronization resulting from locking on a Collection view instead of the Collection itself [[Tutorials 08]].

Map<Integer, String> m = Collections.synchronizedMap(new HashMap<Integer, String>());
Set<Integer> s = m.keySet();
synchronized(s) {  // Incorrectly synchronizes on s
  for(Integer k : s) { /* do something */ }
}

Compliant Solution

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

// ...
synchronized(m) {  // Synchronize on m, not s
  for(Integer k : s) { /* do something */ }
}

Noncompliant Code Example

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

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

Compliant Solution

The proper mechanism to lock in this case is to explicitly use the lock() and unlock() methods provided by the ReentrantLock class.

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

Risk Assessment

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

Automated Detection

TODO

Related Vulnerabilities

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

References

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

CON35-J. Do not try to force thread shutdown      10. Concurrency (CON)      CON37-J. Never apply a lock to methods making network calls