OBJ01-J. Limit accessibility of fields

Invariants cannot be enforced for public nonfinal fields, or final fields that reference a mutable object. A protected member of an exported class represents a public commitment to an implementation detail.  Attackers can manipulate such fields to violate class invariants or they may be corrupted by multiple threads accessing them concurrently [Bloch 2008]. As a result, fields must be declared private or package-private.

Noncompliant Code Example (Public Primitive Field)

In this noncompliant code example, the total field tracks the total number of elements as they are added and removed from a container using the methods add() and remove() respectively.

public class Widget {
  public int total; // Number of elements

  void add() {
    if (total < Integer.MAX_VALUE) {      
      total++;
      // ...
    } else {
      throw new ArithmeticException("Overflow");
    }
  }

  void remove() {  
    if (total > 0) {      
      total--;
      // ...
    } else {
      throw new ArithmeticException("Overflow");
    }
  }
}

As a public field, total can be altered by client code independently of the add() and remove() methods.

Compliant Solution (Private Primitive Field)

Accessor methods provide controlled access to fields outside of the package in which their class is declared.  This compliant solution declares total as private and provides a public accessor.  The add() and remove() methods modify its value while preserving class invariants. 

public class Widget {
  private int total; // Declared private

  public int getTotal () {
    return total;
  }

  // Definitions for add() and remove() remain the same
}

Accessor methods can perform additional functions, such as input validation and security manager checks, before manipulating the state. 

Noncompliant Code Example (Public Mutable Field)

Programmers often incorrectly assume that declaring a field or variable final makes the referenced object immutable. Declaring variables that have a primitive type final does prevent changes to their values after initialization (by normal Java processing). However, when the field has a reference type, declaring the field final only makes the reference itself immutable. The final clause has no effect on the referenced object. According to the Java Language Specification, §4.12.4, "final Variables" [JLS 2013],

If a final variable holds a reference to an object, then the state of the object may be changed by operations on the object, but the variable will always refer to the same object.

This noncompliant code example declares a static final mutable hash map with public accessibility:

public static final HashMap<Integer, String> hm = new HashMap<Integer, String>();

Compliant Solution (Private Mutable Fields)

Mutable fields must be declared private:

private static final HashMap<Integer, String> hm = new HashMap<Integer, String>();

public static String getElement(int key) { 
  return hm.get(key);
}

Depending on the required functionality, accessor methods may return a copy of the HashMap or a value contained by the HashMap. This compliant solution adds an accessor method that returns the value of an element given its key in the HashMap.  Make sure that you do not return references to private mutable objects from accessor methods (see OBJ05-J. Do not return references to private mutable class members for details).

Noncompliant Code Example (Public Final Array)

A nonzero-length array is always mutable. Declaring a public final array is a potential security risk as the array elements may be modified by a client. 

public static final String[] items = {/* ... */};

Declaring the array reference final prevents modification of the reference but does not prevent clients from modifying the contents of the array.

Compliant Solution (Index Getter)

This compliant solution declares a public array and provides public methods to get individual items and array size. 

private static final String[] items = {/* ... */};

public static final String getItem(int index) {
  return items[index];
}

public static final int getItemCount() {
  return items.length;
}

Providing direct access to the array objects themselves is safe because String is immutable.

Compliant Solution (Clone the Array)

This compliant solution defines a private array and a public method that returns a copy of the array:

private static final String[] items = {/* ... */};

public static final String[] getItems() {
  return items.clone();
}

Because a copy of the array is returned, the original array values cannot be modified by a client. Note that a manual deep copy could be required when dealing with arrays of objects. This generally happens when the objects do not export a clone() method. Refer to OBJ06-J. Defensively copy mutable inputs and mutable internal components for more information.

As before, this method provides direct access to the array objects themselves, but this is safe because String is immutable. If the array contained mutable objects, the getItems() method could return an array of cloned objects instead.

Compliant Solution (Unmodifiable Wrappers)

This compliant solution constructs a public immutable list from the private array.  It is safe to share immutable objects without risk that the recipient can modify them [Mettler 2010].

private static final String[] items = { ... };

public static final List<String> itemsList =
  Collections.unmodifiableList(Arrays.asList(items));

Neither the original array values nor the public list can be modified by a client. For more details about unmodifiable wrappers, refer to OBJ56-J. Provide sensitive mutable classes with unmodifiable wrappers. This solution can also be used when the array contains mutable objects.

Exceptions

OBJ01-EX0: Fields with no associated behavior or invariants can be public.  According to Sun's Code Conventions document [Conventions 2009]:

One example of appropriate public instance variables is the case where the class is essentially a data structure, with no behavior. In other words, if you would have used a struct instead of a class (if Java supported struct), then it's appropriate to make the class's instance variables public.

OBJ01-EX1: Fields in a package-private class or in a private nested class may be pubic or protected. There is nothing inherently wrong with declaring fields to be public or protected in these cases. Eliminating accessor-methods generally improves the readability of the code, both in the class definition and in the client  [Bloch 2008].

OBJ01-EX2: Static final fields that contain or reference immutable constants may be public or protected.

Risk Assessment

Failing to limit field accessibility can defeat encapsulation, allow attackers to manipulate fields to violate class invariants, or allow these fields to be corrupted as the result of concurrent accesses from multiple threads.

Automated Detection

Detection of public and protected fields is trivial; heuristic detection of the presence or absence of accessor methods is straightforward. However, simply reporting all detected cases without suppressing those cases covered by the exceptions to this rule would produce excessive false positives. Sound detection and application of the exceptions to this rule is infeasible; however, heuristic techniques may be useful.

Related Guidelines

Bibliography

OBJ00-J. Limit the extensibility of classes and methods with invariants