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Any software system, whether object-oriented or not, relies on the state of the system being "correct" at certain stages of its execution. To take a very simple example, when a numerical division operation is performed, the divisor must be non-zero. If this is not the case, the system may crash in an unpredictable and uncontrolled manner.

One way of indicating such requirements is to state that the system must be in some state either before or after an operation. Such a statement about the state of a software system is called an assertion.

Assertions often form the basis for software specification. In some systems, the assertions are embedded in the software as annotations or formal comments. However, it can be useful to make the assertions executable so that the correctness of the system is checked at runtime.

A simple technique is to make the code do its own self-testing using a method call and a programming languages exception mechanism. The code checks that the statement passed to it is true, thus allowing it to test that the code is behaving as expected. If the statement is false, the method uses an exception to stop processing, thereby enabling debugging.

This article looks at how assertions can be implemented within the Java programming language [1]. In the remainder of this article, we introduce the concept of assertions, how they can be used and how they can be implemented. In particular, we introduce two types of assertions: one which generates a runtime exception if it fails and one which generates a checked exception (which must be handled by the programmer) if it fails. We then provide a brief analysis of the facilities provided by this approach.

Assertions and Their Uses
An assertion is a logical statement about the state of a software system. In formal methods, such as VDM [10], software is specified by making assertions about the pre- and post-conditions of functions and operations. These assertions may be used to formally prove properties of the system, such as consistency. During the development process, the abstract specification is refined into code. The problem with this approach is that the assertions used in the specification are often lost during the refinement process and indeed, it may not be possible to express the assertions in the final implementation language.

Programming languages have been developed which enable assertions to be embedded into the code itself, for example Gypsy [5] or SPARK [3, 2]. Tools available with these language systems enable the assertions to be checked and, in some cases, proved.

Another approach is to introduce assertions into existing programming languages by using formal comments, called annotations, or by pre-processing. The C programming language has been extended with annotations[7] and Ada has been similarly extended into Anna [11]. An annotation pre-processor for C is described in Rosenblum's "A Practical Approach to Programing with Assertions" [13]. In these systems, tool support enables the assertions to be checked against the program code. However, extra-language syntax is required and, in the case of annotations, the assertions are lost when the program is compiled.

It may be better to have assertions as part of the code so that they provide a permanent defensive programming mechanism for detecting faults at runtime [12]. The exception mechanism of a programming language can be used for this [8]. In the following sections we show how this can be done for Java.

Using Assertions in Java
Assertions can be implemented in Java as part of an assertion package (available at http://www.aber.ac.uk/~jjh/Java/assertion). This package can provide assertion classes which take an assertion and throw an exception if the assertion is false. The package could also define its own exceptions, thereby allowing a programmer to catch assertion exceptions. The assertion package we have defined provides two separate assertion classes and two different types of exception:

  • Assertion - The class Assertion allows programmers to use assertions within their code.
  • CheckedAssertion - The class CheckedAssertion allows programmers to use assertions within their code but forces them to explicitly catch a CheckedAssertionException.
  • AssertionException - The AssertionException class extends the RuntimeException class so that programmers do not have to handle the exception raised.
  • CheckedAssertionException - The CheckedAssertionException class extends the Exception class so that programmers are forced to handle the exception raised.

The difference between the checked and the non-checked assertions for the developer is that they must explicitly handle any exceptions raised by the CheckedAssertion class, whereas they can choose to ignore the fact that an exception may be raised by the Assertion class.

In Java, a package is an associated group of classes and interfaces [1]. We have defined a new package assertion which holds the four classes listed above. Other classes can then use these four by importing the assertion package. An example of how these classes are used is presented in Listing 1.

This example creates a simple class Example which uses the Assertion class to handle the result of checking the relationship between a and b. The result of running this application is presented in Listing 2.

We could have caught the AssertionException within a try{} catch{} block. This construct allows a piece of code to be wrapped up within a try block. The try block indicates the code which is to be monitored for the exceptions listed in the catch expressions. The catch expressions can then be used to indicate what to do when certain classes of exception occur; for example, "resolve the problem" or "generate a warning message", etc. If no catch expressions are included the throws clause must be handled by the method definition. For example:

try { e.test(1, 2);
e.test(2, 1);
}
catch (AssertionException e) {
System.out.println("First parameter larger than second");
}

This would have allowed us to respond in an appropriate manner. Below we examine the implementation of the Assertion and AssertionException classes and their checked counterparts.

Exceptions in Java
In Java, (almost) everything is an object; therefore, exceptions are objects as well. All exceptions must extend the class Throwable or one of its subclasses [4].

The class Throwable has two subclasses, Error and Exception. Errors are unchecked exceptions. These are exceptions which your methods are not expected to deal with. The compiler therefore does not check that the methods can deal with them. In contrast, most exceptions (but not RuntimeException and its subclasses) below the class Exception are checked exceptions. These exceptions must be dealt with by your methods. The compiler thus checks to see that methods throw only exceptions which they can deal with (or are passed up to other methods to handle). For example, if we wished to raise an ArithmeticException, for a divide by zero error then we would write:

throw new ArithmeticException("Division By Zero");

In Java, you are not limited to system-provided exceptions; it is also possible to provide user-defined exceptions. This can be very useful as such an approach can allow the developer to have more control over what happens in particular circumstances. To do so, a developer must subclass the Exception class or one of its subclasses.

Assertions in Java
The Assertion class
The Assertion class (a direct subclass of Object) makes two methods publicly available for handling boolean expressions. These are:

  • assert(boolean bool) - Determines the effect of the boolean result passed to it.
  • assert(boolean bools[]) - Determines the effect of the array of expressions passed to it.
The second method is really a convenience method which repeatedly calls the first method on each element in the array.

The implementation of the assert(boolean bool) method is relatively straightforward. The method checks whether the boolean value is false or not. If it is false an exception is raised. The assert(boolean bools []) method simply calls the previous method iteratively (see Listing 3).

Switching Assertion Checking Off
In the released version of the system, the assertion checks may waste processing time. they could therefore be replaced by a null version of the method:

public static void assert (boolean bool) throws AssertionException {
return true;
}

As Java dynamically loads the appropriate .class file at runtime, we could distribute the final version of the system with an Assertion.class file based on the above implementation of assert. We would therefore not even need to recompile the system being released.

The overhead of the extra method dispatch is relatively small; of course, if it is still significant we could write a utility to find the senders of the assert method and to comment out the assertion statements.

The AssertionException class
The AssertionException class is a subclass of the RuntimeException class as illustrated in Figure 1. The definition of the AssertionException class is provided in LIsting 4. Note that it provides two constructors which are used to initialize the string displayed to the user.

Figure 1
Figure 1:

CheckedAssertion and CheckedAssertionException
These classes are essentially the same as the Assertion and AssertionException classes except that the CheckedAssertion class throws the CheckedAssertionException and the CheckedAssertionException extends the Exception class (as shown in Figure 1). For the developer, the difference between the two types of assertion checking is that any exceptions raised by the CheckedAssertion class must be handled explicitly; they can not simply be propagated out of the program.

The advantage is that the developer using the assertions can decide how much control to give to the user of a component. If they wish, they can force that user to handle the exception and thereby allow them to take charge of any problems which occur. This can result in code which is more resilient to erroneous situations.

For completeness, the two classes are presented in Listings 5 and 6.

Analysis
Design methods Using Assertions
Assertions are particularly useful for testing pre-conditions, post-conditions and class invariants. The Syntropy design method makes extensive use of such assertions [6]. It would therefore be possible to use the assertions identified during the design as the basis of the assertions to place in the Java code.

JavaBeansª
The use of assertions could be extremely useful when working with JavaBeans (the Java component model). In such situations developers produce reusable components which will be used by developers in many different ways. By using assertions, developers can ensure that the state of the Bean at any time is correct. If the state is not correct then they can take appropriate action. The assertions can therefore be used as guards against inappropriate states, parameters, etc.

Shortcomings
The simple approach to inserting assertions into Java code described here cannot provide the following, more sophisticated types of assertions:

  • Assertions that relate output values of methods to their input values: For example, suppose one defined a method which swapped the values of its two parameters.
void swap (SomeType x, SomeType y) {
SomeType z;
z = x;
x = y;
y = z;
}

One might want to assert, just before the method returns, that the new value of x is equal to the old value of y and the new value of y is equal to the old value of x. This is not possible with our assertion mechanism. Assertion mechanisms for which this is possible have additional syntax which allows the "before and after" values of variables to be distinguished.

  • Assertions which involve quantified expressions: Formal methods, and the annotations of Anna and SPARK, allow expressions to be quantified by "for all", "exists", or "not exists". So, the fact that a natural number N was prime could be asserted by saying that there did not exist numbers P and Q with 1 <p, Q < N and P*Q="N." Again, this is not possible with our assertion mechanism. Mechanisms which can handle the full power of the predicate logic must have additional syntax and be provided with sophisticated theorem proving tools.
Conclusions
Recent commentators have suggested that assertions provide a valuable defensive programming technique. One of the nine ways advocated for making code more reliable is to "use assertions liberally" [9].

We have demonstrated a simple mechanism for introducing assertions into Java programs. Two classes have been defined: Assertion and CheckedAssertion. The former allows assertions to be used very simply. Any exceptions thrown as a result of the assertions failing may be caught or propagated out of the program. The CheckedAssertion class requires the exceptions to be caught or explicitly listed in a throw clause for the methods involved. This provides greater control and "self documentation" of the assertion mechanism.

Our assertion mechanism is particularly valuable when Java is used in larger, critical applications, where developers are implementing classes for general use and for Java Beans components.

References

  1. K. Arnold and J. Gosling, The Java Programming Language, Addison Wesley, ISBN 0-201-63455-4, 1996.
  2. J.G.P. Barnes, High Integrity Ada: The SPARK Approach, Addison-Wesley, ISBN 0-201-17517-7, 1997.
  3. B.A. Carré, J.R. Garnsworthy and D.W.R Marsh, "SPARK: A Safety-Related Ada Subset", in W.J. Taylor (ed.), Ada in Transition, IOS Press, Amsterdam, 1992, pp. 31-45.
  4. P. Chan and R. Lee, The Java Class Libraries: An Annotated Reference, Addison-Wesley, 0-201-69581-2, 1996.
  5. R.M. Cohen, Proving Gypsy Programs, Ph.D. Thesis, The University of Texas at Austin, 1986.
  6. S. Cook and J. Daniels, Designing Object Oriented Systems: Object-oriented modeling with Syntropy, New York: Prentice Hall, 0-13-203860-9, 1994.
  7. D.W. Flater, Y. Yesha and E.K. Park, Extensions to the C Programming Language for Enhanced Fault Detection, Software - Practice and Experience, vol. 23, no. 6, pp. 617-628, June, 1993.
  8. P. Gaution, An Assertion Mechanism Based on Exceptions, in Proc. 4th C++ Tech. Conf., USENIX Association, pp. 245-262, August, 1992.
  9. A. Joch, How Software Doesn't Work, Byte, pp. 49-60, December, 1995.
  10. C.B. Jones, Systematic Software Development using VDM, Prentice Hall, ISBN 0-13-880733-7, 1990.
  11. D.C. Luckham, F.W. von Henke, B. Krieg-Brkner, and O.Owe, Anna - A Language for Annotating Ada Programs, Lecture Notes in Computer Science, vol. 260, Springer-Verlag, 1987.
  12. B. Meyer, Applying Design by Contract, IEEE Computing, vol. 25, pp. 40-51, October, 1992.
  13. D.S. Rosenblum, A Practical Approach to Programming with Assertions, IEEE Transactions on Software Engineering, vol. 21, no. 1, pp. 19-31, 1995.
About the Authors
Dr. John Hunt has been working in the object-orientation field since the mid-1980s in both industry and academia. He became interested in Java early in 1995 and has worked with a number of companies (including Sun) with Java. He is particularly interested in developing real world applications using Java and in issues associated with Java performance, integrity and architecture. John can be reached at [email protected]

Dr. Fred Long lectures on software engineering and formal methods. He is a visiting scientist at the Software Engineering Institute in Pittsburgh. Recently, he has been researching Java's suitability for the development of critical applications. Fred can be reached at [email protected]

	

Listing 1. 

import assertion.*; 
public class Example { 
    public static void main (String args []) { 
        Example e = new Example(); 
        e.test(1, 2); 
        e.test(2, 1); 
    } 

    public void test(int a, int b) { 
System.out.println("In test with " + a + " and " + b); 
        Assertion.assert(a < b); 
} 
} 

Listing 2. 

>java Example 
In test with 1 and 2 
In test with 2 and 1 
assertion.AssertionException: Failed assertion 
        at assertion.Assertion.assert(Assertion.java:41) 
        at Example.test(Example.java:11) 
        at Example.main(Example.java:7) 

Listing 3. 

public class Assertion { 
    /* Don't make assertion instances */ 
    Assertion () {}; 

public static void assert (boolean bool) throws AssertionException { 
        if (!bool) 
            throw new AssertionException("Failed assertion"); 
} 
  

public static void assert (boolean bools []) throws AssertionException { 
        for (int i = 0; i < bools.length; i++) { 
            assert(bools[i]); 
        } 
} 
} 

Listing 4. 

public class AssertionException extends RuntimeException { 
    AssertionException () { 
        this("Assertion exception"); 
    } 
    AssertionException (String information) { 
        super(information); 
    } 
} 

Listing 5: CheckedAssertion class. 

public class CheckedAssertion { 
/* Don't make checked assertion instances */ 
CheckedAssertion () {}; 

public static void assert (boolean bool)  
throws CheckedAssertionException { 
        if (!bool) 
            throw new CheckedAssertionException("Failed assertion"); 
    } 

public static void assert (boolean bools [])  
throws CheckedAssertionException { 
        for (int i = 0; i < bools.length; i++) { 
            assert(bools[i]); 
        } 
    } 
} 

Listing 6: CheckedAssertionException class. 


public class CheckedAssertionException extends Exception { 
    CheckedAssertionException () { 
        this("CheckedAssertion exception"); 
    } 
    CheckedAssertionException (String information) { 
        super(information); 
    } 
}
 

 

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