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Wiki MarkupAccording to the The Java Language Specification \[[JLS 2005|AA. Bibliography#JLS 05]\], Section 12, §12.5, "Creation of New Class Instances"[JLS 2015]:

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 Invocation of an overridable method during object construction may result in the use of uninitialized data, leading to runtime exceptions or lead to unanticipated outcomes. Calling overridable methods from constructors can also result in leak the escaping of the this reference before construction has concluded. (See guideline object construction is complete, potentially exposing uninitialized or inconsistent data to other threads (see TSM01-J. Do not let the ( this ) reference escape during object constructionfor additional information). As a result, a class's constructor must invoke (directly or indirectly) only methods in that class that are static, final or private.

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. results in the use of uninitialized data by the doLogic() method:

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

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

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

The doLogic() method is invoked from the superclass's constructor. When the superclass is constructed directly, the doLogic() method in the superclass is invoked and executes successfully. However, when the subclass initiates the superclass's construction, the subclass's doLogic() method is invoked instead. In this case, the value of color is still null because the subclass's constructor has not yet concluded.

Compliant Solution

This compliant solution declares the doLogic() method as final so that it is no longer overridable.cannot be overridden:

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

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. (See guidelines MET07-J. Do not invoke overridable methods on the clone under construction and SER11-J. Do not invoke overridable methods from the readObject method.) It is also insecure to call 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 guideline 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 a NullPointerException.

Risk Assessment

Allowing a constructor to call overridable methods may give can provide an attacker with 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 guideline on the CERT website.

Bibliography

\[[JLS 2005|AA. Bibliography#JLS 05]\] [Chapter 8, Classes|http://java.sun.com/docs/books/jls/third_edition/html/classes.html], 12.5 "Creation of New Class Instances"
\[[SCG 2007|AA. Bibliography#SCG 07]\] Guideline 4-3 Prevent constructors from calling methods that can be overridden
\[[ESA 2005|AA. Bibliography#ESA 05]\] Rule 62: Do not call non-final methods from within a constructor.

Automated detection of constructors that contain invocations of overridable methods is straightforward.

Related Guidelines

Bibliography

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Image Added Image Added Image AddedImage Removed      Methods (MET)      MET05-J. Do not use overloaded methods to differentiate between runtime types