CON38-J. Synchronize on a class literal instead of getClass()

Synchronizing on the return value of the Object.getClass() method, rather than a class literal can lead to unexpected behavior. Whenever the implementing class is subclassed, the subclass locks on the subclass's type, which is a completely different Class object.

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. Note that if a subclass does not override an accessible noncompliant superclass's method, it inherits the method which may lead to the false conclusion that the superclass's intrinsic lock is available in the subclass.

Noncompliant Code Example (getClass() lock object)

This noncompliant code example synchronizes on the class object returned by getClass().

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

class Derived extends Base {
  public Base getBase(); {
    // ...
  }

  public void doSomethingElse() {
    Base obj = getBase();
    obj.doSomething(); 
  }
}

The actual Class object that gets locked when Base.doSomething() gets invoked depends on the return value of getBase(). If getBase() returns a Derived, then doSomething() locks on Derived.class, not Base.class.

Compliant Solution (class name qualification)

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

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

This code example always synchronizes on the Base.class object, even if it is called from a Derived object.

The class object that is being used for synchronization should not be accessible to untrusted 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 Base class's Class object.

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

The class object that is being used for synchronization should also not be accessible to untrusted code, see CON04-J. Synchronize using an internal private final lock object for more information. 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.)

Risk Assessment

Synchronizing on the class object returned by getClass() can present 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]]
[[Findbugs 08]].
[[Pugh 08]] "Synchronization"
[[Miller 09]] Locking

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