MET32-J. Ensure that constructors do not call overridable methods

According to the Java Language Specification [[JLS 05]]:

Unlike C++, the Java programming language does not specify altered rules for method dispatch during the creation of a new class instance. If methods are invoked that are overridden in subclasses in the object being initialized, then these overriding methods are used, even before the new object is completely initialized.

This means that a method may use uninitialized data and cause runtime exceptions or lead to unanticipated outcomes.

Noncompliant Code Example

This noncompliant example invokes the doLogic method from the constructor. The super class doLogic method gets invoked on the first call, however, the overriding method is invoked on the second one. In the second call, the issue is that the constructor for SubClass initiates the super class's constructor which undesirably ends up calling SubClass's doLogic() method. The value of color is left as null because the initialization of the class SubClass has not yet concluded.

class BaseClass {
  public BaseClass() {
    doLogic();
  }
	
  public void doLogic() {
    System.out.println("This is super-class!");
  }	
}

class SubClass extends BaseClass {
  private String color = null;
  public SubClass() {
    super();	
    color = "Red";
  }
	
  public void doLogic() {
    System.out.println("This is sub-class! The color is :" + color); //color becomes null
    //other operations
  }
}

public class Overridable {
  public static void main(String[] args) {
    BaseClass bc = new BaseClass(); //prints "This is super-class!"
    BaseClass sc = new SubClass();  //prints "This is sub-class! The color is :null"		
  }
}

Compliant Solution

This compliant solution declares the doLogic method as final so that it is no longer overridable.

class BaseClass {
  public BaseClass() {
    doLogic();
  }
	
  public final void doLogic() {
    System.out.println("This is super-class!");
  }	
}

In addition to constructors, do not call overridable methods from the clone, readObject and readObjectNoData methods as it would allow attackers to obtain partially initialized instances of classes. An equally dangerous idea is to disobey this advice by calling an overridden method from a finalize method. This can prolong the subclass' life and in fact render the finalization call useless (See the example in OBJ02-J. Avoid using finalizers). Additionally, if the subclass's finalizer has terminated key resources, invoking its methods from the superclass might lead one to observe the object in an inconsistent state and in the worst case result in the infamous NullPointerException.

Risk Assessment

Allowing a constructor to call overridable methods may give an attacker access to this before an object is fully initialized which, in turn, could lead to a vulnerability.

Automated Detection

TODO

Related Vulnerabilities

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

References

[[JLS 05]] Chapter 8, Classes, 12.5 "Creation of New Class Instances"
[[SCG 07]] Guideline 4-3 Prevent constructors from calling methods that can be overridden
[[ESA 05]] Rule 62: Do not call non-final methods from within a constructor.

MET31-J. Ensure that hashCode() is overridden when equals() is overridden      10. Methods (MET)      MET33-J. Do not subject overloaded methods to polymorphic invocations