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

Noncompliant Code Example

This noncompliant code example invokes the doLogic() method from the class constructor. The superclass's 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() {
    // Color becomes null
    System.out.println("This is sub-class! The color is :" + color); 
    // 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.

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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 the finalize() method. This can prolong the subclass' life and in fact render the finalization call useless (See the example in OBJ08-J. Avoid using finalizers). Additionally, if the subclass's finalizer has terminated key resources, invoking its methods from the superclass may 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 the this reference 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

\[[JLS 05|AA. Java References#JLS 05]\] [Chapter 8, Classes|http://java.sun.com/docs/books/jls/third_edition/html/classes.html], 12.5 "Creation of New Class Instances"
\[[SCG 07|AA. Java References#SCG 07]\] Guideline 4-3 Prevent constructors from calling methods that can be overridden
\[[ESA 05|AA. Java References#ESA 05]\] Rule 62: Do not call non-final methods from within a constructor.

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