Jackson Structured Programming

Jackson structured programming (JSP) is a method for structured programming based on correspondences between data stream structure and program structure. JSP structures programs and data in terms of sequences, iterations and selections, and as a consequence it is applied when designing a program's detailed control structure, below the level where object-oriented methods become important.

Michael A. Jackson originally developed JSP in the 1970s. He documented the system in his 1975 book Principles of Program Design. In a 2001 conference talk, he provided a retrospective analysis of the original driving forces behind the method, and related it to subsequent software engineering developments. Jackson's aim was to make COBOL batch file processing programs easier to modify and maintain, but the method can be used to design programs for any programming language that has structured control constructs, languages such as C, Java and Perl. Despite its age, JSP is still in use and is supported by diagramming tools such as Microsoft's Visio and CASE tools such as Jackson Workbench.

Jackson Structured Programming was seen by many as related to Warnier structured programming, but the latter method focused almost exclusively on the structure of the output stream. JSP and Warnier's method both structure programs and data using only sequences, iterations and selections, so they essentially create programs that are parsers for regular expressions which simultaneously match the program's input and output data streams.

Because JSP focuses on the existing input and output data streams, designing a program using JSP is claimed to be more straightforward than with other structured programming methods, avoiding the leaps of intuition needed to successfully program using methods such as top-down decomposition.

Another consequence of JSP's focus on data streams is that it creates program designs with a very different structure to the kind created by the stepwise refinement methods of Wirth and Dijkstra. One typical feature of the structure of JSP programs is that they have several input operations distributed throughout the code in contrast to programs designed using stepwise refinement, which tend to have only one input operation. Jackson illustrates this difference in Chapter 3 of Principles of Program Design. He presents two versions of a program, one designed using JSP, the other using "traditional" methods.

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