Unit testing code that uses environment variables and system properties with fakes

I did not exactly learn this today, but I am sharing it as I have found it extremely useful when unit testing code that depends on environment or system property settings. While I am using Java as an example, the general concepts apply any where.

Problem: You have a piece of code you are unit testing that uses settings from env variables or system properties passed to the VM (System.getProperty), but you don't want the tests to be affected by the 'real' environment or system properties in the VM. So, your unit tests should not get different results or fail when the real environment changes.

Solution: There are several. But the most straightforward is to use a mocking library to mock out the environment or fake it out, whatever your prefer.

You can create a fake using a library like EasyMock, PowerMock etc. This I won't discuss in this post, since there are numerous articles for that.

Or you can write a simple class that acts as a proxy, using the proxy pattern. That is what I will show here.


 Never use calls to System.getenv() or System.gerProperty() in your code. Write an env wrapper such as this:

interface Env {
   String readEnvVar(String name);
   String readSystemProp(String name);
}

Class under test (intentionally KISS for demo - keep it stupid and simple) that uses Env interface:

    class UnderTestUsesEnv {
        private final Env env;
        public UnderTestUsesEnv(Env v) { this.env = v; }
        public boolean void isXSet() {
             if (env.readEnvVar('x')!=null) return true;
             return false;
        }
    }

Now write a real implementation of Env, and a fake one for testing (this is the part that an be replaced with a mock or fake using a third party library):

class RealEnv implements Env {
   String readEnvVar(String name) { return System.getenv(name); }
   String readSystemProp(String name) { return System.getProperty(name); }
 
}

class FakeEnv implements Env {
   private Map<String,String> envmap = new HashMap<String,String>();
   private Map<String,String> sysmap = new HashMap<String,String>();

   String readEnvVar(String name) { return envmap.get(name); }
   String readSystemProp(String name) {return sysmap.get(name); }

    // This one also has methods to set expected values
    void setEnvVar(String key, String val) { envmap.put(key,val); }
    void setSysProp(String key, String val) { sysmap.put(key,val); }
}

Now, in your tests

fakeEnv = new FakeEnv();
objUnderTest = new UnderTestUsesEnv(fakeEnv);
objUnderTest.isXSet(); // returns false


fakeEnv = new FakeEnv();
fakeEnv.setEnvVar('x',1);
objUnderTest = new UnderTestUsesEnv(fakeEnv);

objUnderTest.isXSet(); // returns true

The nice thing is that the fake does not have to set to anything if you want to simulate that the variables you care about are absent. Each unit test can configure its fake env this way. Now all your unit tests are free of real env and system property sensitivity! I use this all the time, and I love it...