When contemplating on introducing automated testing in legacy applications, it is easy to get bogged down in terminology; unit testing, integration testing, regression testing, black box testing, white box testing, stress testing, etc. Quite a bit of time is spent in debates on unit testing versus integration testing, I have written about this before too.

A practical way to approach testing legacy applications is to first scope out the intention behind the test. Is it to test the behavior of a particular method, an API response or how an application behaves post an HTTP form submit? Next step is to jot down what and all has to be done to enable this. For example, if a database is involved, it can be mocked or a test database with bootstrapped data can be used.

The gamut of changes needed to inject testability into an application that has never seen testing before should never be underestimated. The way you would structure testable code is vividly different from coding being incognizant of testing.

Take a look at the code below, how would you unit test getUser method without creating a database connection?

public class Foo {
    DbConnection connection = null;
    public Foo() {
        connection = <establish db connection>;
    }

    public User getUser(int id) {
        ////Query db and get user data        
        User user = new User();
        //Fill user with data from db
        return user;
    }
}

To mould this into testable code, DbConnection creation needs to be decoupled from object creation, like below:

public class Foo {
    DbConnection dbConnection = null;
    public Foo(DbConnection dbConnection) {
        this.dbConnection = dbConnection;
    }

    public User getUser(int id) {
        //Query db and get user data
        User user = new User();
        //Fill user with data from db
        return user;
    }
}

Since the DbConnection is independent of object creation, DbConnection can be mocked to unit test any method in the class. An application written without testing in mind would be replete with code like the above. Code patterns like these are one of the biggest hurdles in testing legacy applications.

Next step is to eliminate the resistance to testing. This would mean all the infrastructure and libraries needed to carry out testing are set up and a reference is readily available to follow. Bunch test cases into categories like unit tests, tests that need a mocked object, tests that need a mocked database, tests that need a database seeded with data, tests that need a web server etc. Post this, implement one test case for each of these categories. This will serve a dual purpose, the setup would be ready for each category and a reference readily available to others to emulate.

One aspect that is usually neglected is the effect of testing on the product release cycle. As a result of testing, more code, dependencies, and infrastructure is introduced which needs to be maintained. Along with working on new features, writing tests for these also has to be taken into account. While refactoring, it is not just the code that has to be refactored, even the test cases have to be refactored. This is a tradeoff between time to market, and maintainability and reliability.

Testing is no longer a chore it used to be, testing tools and frameworks have grown by leaps and bounds. With the advent of docker, headless browsers, Selenium etc; testing is very much within reach of most of the teams provided the intention is there and effort is put in.