How to test code dependent on environment variables using JUnit?

In a similar situation like this where I had to write Test Case which is dependent on Environment Variable, I tried following:

1. I went for System Rules as suggested by Stefan Birkner. Its use was simple. But sooner than later, I found the behavior erratic. In one run, it works, in the very next run it fails. I investigated and found that System Rules work well with JUnit 4 or higher version. But in my cases, I was using some Jars which were dependent on JUnit 3. So I skipped System Rules. More on it you can find here @Rule annotation doesn't work while using TestSuite in JUnit.
2. Next I tried to create Environment Variable through Process Builder class provided by Java. Here through Java Code we can create an environment variable, but you need to know the process or program name which I did not. Also it creates environment variable for child process, not for the main process.

I wasted a day using the above two approaches, but of no avail. Then Maven came to my rescue. We can set Environment Variables or System Properties through Maven POM file which I think best way to do Unit Testing for Maven based project. Below is the entry I made in POM file.

    <build>
      <plugins>
       <plugin>
        <groupId>org.apache.maven.plugins</groupId>
        <artifactId>maven-surefire-plugin</artifactId>
        <configuration>
          <systemPropertyVariables>
              <PropertyName1>PropertyValue1</PropertyName1>                                                          
              <PropertyName2>PropertyValue2</PropertyName2>
          </systemPropertyVariables>
          <environmentVariables>
            <EnvironmentVariable1>EnvironmentVariableValue1</EnvironmentVariable1>
            <EnvironmentVariable2>EnvironmentVariableValue2</EnvironmentVariable2>
          </environmentVariables>
        </configuration>
      </plugin>
    </plugins>
  </build>

After this change, I ran Test Cases again and suddenly all worked as expected. For reader's information, I explored this approach in Maven 3.x, so I have no idea on Maven 2.x.

The library System Lambda has a method withEnvironmentVariables for setting environment variables.

import static com.github.stefanbirkner.systemlambda.SystemLambda.*;

public void EnvironmentVariablesTest {
  @Test
  public void setEnvironmentVariable() {
    String value = withEnvironmentVariable("name", "value")
      .execute(() -> System.getenv("name"));
    assertEquals("value", value);
  }
}

For Java 5 to 7 the library System Rules has a JUnit rule called EnvironmentVariables.

import org.junit.contrib.java.lang.system.EnvironmentVariables;

public class EnvironmentVariablesTest {
  @Rule
  public final EnvironmentVariables environmentVariables
    = new EnvironmentVariables();

  @Test
  public void setEnvironmentVariable() {
    environmentVariables.set("name", "value");
    assertEquals("value", System.getenv("name"));
  }
}

Full disclosure: I'm the author of both libraries.

The usual solution is to create a class which manages the access to this environmental variable, which you can then mock in your test class.

public class Environment {
    public String getVariable() {
        return System.getenv(); // or whatever
    }
}

public class ServiceTest {
    private static class MockEnvironment {
        public String getVariable() {
           return "foobar";
        }
    }

    @Test public void testService() {
        service.doSomething(new MockEnvironment());
    }
}

The class under test then gets the environment variable using the Environment class, not directly from System.getenv().