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Java classes and methods may have invariants. An invariant is a property that is assumed to be true at certain points during program execution but is not formally specified as true. Invariants may be used in assert statements or may be informally specified in comments.

Method invariants can include Many methods offer invariants, which can be any or all of the guarantees made about what the method can do, requirements about the required state of the object when the method is invoked, or guarantees about the state of the object when the method completes. For instance, the % operator, which computes the remainder of a number, provides the invariant that

0 <= abs(a % b) < abs(b), for all integers a, b where b != 0

example, a method of a Date class might guarantee that 1 <= day_of_month <= 31 when the method exits.  

Class invariants Many classes also offer invariants, which are guarantees made about the state of their objects' fields upon the completion of any of their methods. For instanceexample, classes whose member fields may not be modified once they have assumed a value are called immutable classes. An important consequence of immutability is that the invariants of instances of these classes are preserved throughout their lifetimes.

Similarly, classes can rely on invariants in order to properly implement their public interfaces. These invariants might relate to the state of member fields or the implementation of member methods. Generally, classes can rely on encapsulation to help maintain these invariants, for example, by making member fields private. However, encapsulation can be incompatible with extensibility. For example, a class designer might want a method to be publicly accessible , yet rely on the particulars of its implementation when using it in another method within the class. In this case, overriding the method in a subclass can break the internal invariants of the class.Therefore, extensibility caries   Extensibility consequently carries with it two significant risks: a subclass can fail to satisfy the invariants promised to clients by its superclass, and it can break the internal invariants on which the superclass relies. For instanceexample, an immutable class that lacks the final qualifier can be extended by a malicious subclass that can modify the state of the supposedly immutable object. Furthermore, a malicious subclass object can impersonate the immutable object while actually remaining mutable. Such malicious subclasses can violate program invariants on which clients depend, consequently introducing security vulnerabilities. Note that the same can be said for a benign subclass which that mistakenly supports mutability. The These risks above relate to both benign and malicious development.

To mitigate these risks, by default classes must should be declared final by default. Developers should permit extensibility only when unless there is a perceived definite need for it and must, in the class to be extensible. In that case, developers must carefully design the class with extensibility in mind. As a specific instance of this rulerecommendation, classes that are designed to be treated as immutable either must either be declared final or must have all of their member methods and fields must be declared final or private.

In systems where code can come from mixed protection domains, some superclasses might want to permit extension by trusted subclasses while simultaneously preventing extension by untrusted code. Declaring such superclasses to be final is infeasible because it would prevent the required extension by trusted code. One commonly suggested approach is to place code at each point where the superclass can be instantiated to check that the class being instantiated is either the superclass itself or a trustworthy subclass. However, this approach is brittle and is safe only safe in Java SE 6 or higher . See rule (see OBJ11-J. Be wary of letting constructors throw exceptions for a full discussion of the issues involved).

Noncompliant Code Example (BigInteger)

The java.math.BigInteger class is itself an example of noncompliant code. It is non-final nonfinal and consequently extendable. This , which can be a problem when operating on an instance of BigInteger that was obtained from an untrusted client. For example, a malicious client could construct a spurious mutable BigInteger instance by overriding BigInteger's member functions [Bloch 2008].

The following code example demonstrates such an attack.:

BigInteger msg = new BigInteger("123");
msg = msg.modPow(exp, m);  // Always returns 1

// Malicious subclassing of java.math.BigInteger
class BigInteger extends java.math.BigInteger {
  private intbyte[] valueba;

  public BigInteger(String str) {
    super(str);
    valueba = Integer.parseInt (new java.math.BigInteger(str)).toByteArray();
  }

  public void setValue(int valueBigInteger(byte[] ba) {
    super(ba);
    this.valueba = valueba;
  }

  public @Override public java.math.BigInteger modPow(
      java.math.BigInteger exponent, java.math.BigInteger mvoid setValue(byte[] ba) {
    this.valueba = ((int) (Math.pow(this.doubleValue(),  ba;
  }

  @Override public                               exponent.doubleValue()))) % m.intValue();byte[] toByteArray() {
    return thisba;
   }
}

Unlike the benign BigInteger class, this malicious BigInteger class is clearly mutable because of the setValue() method. Furthermore, the malicious modPow() method (which overrides a benign modPow() method) is subject to precision loss. (See rules NUM00-J. Detect or prevent integer overflow, NUM08-J. Check floating-point inputs for exceptional values, NUM12-J. Ensure conversions of numeric types to narrower types do not result in lost or misinterpreted data, and NUM13-J. Avoid loss of precision when converting primitive integers to floating-point for more information.) Any code that receives an object of this class and assumes that the object is immutable will behave unexpectedly. This is particularly important because the BigInteger.modPow() method Any code that receives an object of this class and assumes that the object is immutable will behave unexpectedly, as shown by the following code:

BigInteger bi = new BigInteger("123");

bi.setValue((new BigInteger("246")).toByteArray());

System.out.println(new BigInteger(bi.toByteArray()));  

This code prints: "246", which shows that the value of the supposedly immutable BigInteger bi has been changed.

OBJ01-J. Limit accessibility of fields points out that invariants cannot be enforced for mutable objects. TSM03-J. Do not publish partially initialized objects describes object construction and visibility issues specific to mutable objects, and CON50-J. Do not assume that declaring a reference volatile guarantees safe publication of the members of the referenced object and CON52-J. Document thread-safety and use annotations where applicable discuss some concurrency issues associated with mutable objects.

Violation of this recommendation can be mitigated by treating objects from untrusted sources as potentially malicious subclasses, as directed by OBJ06-J. Defensively copy mutable inputs and mutable internal components. Complying with that rule protects you from the consequences of violating this recommendation.

This example is particularly important because the BigInteger type has several useful cryptographic applications.

Noncompliant Code Example (Security Manager)

This noncompliant code example installs proposes adding a security manager check in the constructor of the java.math.BigInteger class. The security manager denies access when it detects that a subclass without the requisite permissions is attempting to instantiate the superclass [SCG 2009]. It also compares class types, in compliance with rule OBJ09-J. Compare classes and not class names. Note that this check does not prevent malicious extensions of BigInteger, ; it instead prevents the creation of BigInteger objects from untrusted code, which also prevents creation of objects of malicious extensions of BigInteger.

package java.math;
 
// ...
 
public class BigInteger {
  public BigInteger(String str) {
    securityManagerCheck(); 

    // ...
  }

  // Check the permission needed to subclass BigInteger
  // throws a security exception if not allowed
  private void securityManagerCheck() {
    // ...
  }
}

Unfortunately, throwing an exception from the constructor of a non-final nonfinal class is insecure because it allows a finalizer attack . (See rule see OBJ11-J. Be wary of letting constructors throw exceptions.)). Furthermore, since BigInteger is Serializable, an attacker could bypass the security check by deserializing a malicious instance of BigInteger. For more information on proper deserialization, see the rule SER04-J. Do not allow serialization and deserialization to bypass the security manager.

Compliant Solution (Final)

This compliant solution prevents creation of malicious subclasses by declaring the immutable java.math.BigInteger class to be final. Although this solution would be appropriate for locally maintained code, it cannot be used in the case of java.math.BigInteger because it would require changing the Java SE API, which has already been published and must remain compatible with previous versions.

package java.math;

// ...

final class BigInteger {
  // ...
}

Compliant Solution (Java SE 6, Public and Private Constructors)

This compliant solution invokes a security manager check as a side effect of computing the Boolean value passed to a private constructor (as seen in rule OBJ11-J. Be wary of letting constructors throw exceptions). The rules for order of evaluation require that the security manager check must execute before invocation of the private constructor. Consequently, the security manager check also executes before invocation of any superclass's constructor. Note that the security manager check is made without regard to whether the object under construction has the type of the parent class or the type of a subclass (whether trusted or not).

This solution prevents the finalizer attack; it applies to Java SE 6 and later versions, where throwing an exception before the java.lang.Object constructor exits prevents execution of finalizers [SCG 2009].

package java.math;

// ...

public class BigInteger {
  public BigInteger(String str) {
    this(str, check());
  }

  private BigInteger(String str, boolean dummy) {
    // regularRegular construction goes here
  }

  private static boolean check() {
    securityManagerCheck(); 
    return true;
  }
}

Noncompliant Code Example (Data-Driven Execution)

Code in privileged blocks should be as simple as possible, both to improve reliability and to simplify security audits. Invocation of overridable methods permits modification of the code that is executed in the privileged context without modification of previously audited classes. Furthermore, calling overridable methods disperses the code over multiple classes, making it harder to determine which code must be audited. Malicious subclasses cannot directly exploit this issue because privileges are dropped as soon as unprivileged code is executed. Nevertheless, maintainers of the subclasses might unintentionally violate the requirements of the base class. For example, even when the base class's overridable method is thread-safe, a subclass might provide an implementation that lacks this property, leading to security vulnerabilities.

This noncompliant code example invokes an overridable getMethodName() method in the privileged block using the reflection mechanism.

public class MethodInvoker {
  public void invokeMethod() {
    AccessController.doPrivileged(new PrivilegedAction<Object>() {
        public Object run() {
          try {
            Class<?> thisClass = MethodInvoker.class;
            String methodName = getMethodName();
            Method method = thisClass.getMethod(methodName, null);
            method.invoke(new MethodInvoker(), null);
          } catch (Throwable t) {
            // Forward to handler
          }
          return null;
        }
      }
    );
  }

  String getMethodName() {
    return "someMethod";
  }

  public void someMethod() {
    // ...
  }

  // Other methods
}

A subclass can override getMethodName() to return a string other than "someMethod". If an object of such a subclass runs invokeMethod(), control flow will divert to a method other than someMethod().

Compliant Solution (Final)

This compliant solution declares the getMethodName() method final so that it cannot be overridden.

final String getMethodName() {
  // ...
}

Alternative approaches that prevent overriding of the getMethodName() method include declaring it as private or declaring the enclosing class as final.

Risk Assessment

Permitting a nonfinal class or method to be inherited without checking the class instance allows a malicious subclass to misuse the privileges of the class.

Automated Detection

This recommendation is not checkable because it depends on factors that are unspecified in the code, including the invariants upon which the code relies and the necessity of designating a class as extensible, among others. However, simple statistical methods might be useful to find codebases that violate this recommendation by checking whether a given codebase contains a higher-than-average number of classes left nonfinal.

Related Guidelines

Bibliography

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Image Added Image Added Image Added04. Object Orientation (OBJ)      04. Object Orientation (OBJ)      OBJ01-J. Declare data members as private and provide accessible wrapper methods