How to go around Runtime.getRuntime() while writing JUnit?

Question

I have a class where Runtime.getRuntime() is used for executing a script from command line and getting the result for further processing.

But when I write JUnit for this class, I cannot find a way to mock/avoid this Runtime.getRuntime().exec() .

I cannot use EasyMock or PowerMock or any other mock api other than Mockito.

Please give me a way to overcome this problem, because this is affecting the code coverage.

Answer 1

You must refactor. Extract Runtime.getRuntime().exec() into a separate class:

public class Shell {

  public Process exec(String command) {
    return Runtime.getRuntime().exec(command);
  }

}

Now insted of calling getRuntime() explicitly inject Shell class somehow to your class under test:

public class Foo {

  private final Shell shell;

  public Foo(Shell shell) {
    this.shell = shell;
  }

  //...
  shell.exec(...)

}

In JUnit test simply inject mocked Shell class by passing it to constructor:

@Mock
private Shell shellMock;

new Foo(shellMock);

Sidenote: yes, I am not creating a Shell interface with one implementation. Do you mind? Mockito is not. Bonus: you can now verify if the correct process was called:

verify(shellMock).exec("/usr/bin/gimp");

Answer 2

Tomasz Nurkiewicz has provided a wonderful answer. But there is an alternative.

You can move Runtime.getRuntime() into its own method in the class that you're testing; then test it with a Mockito spy. In the Mockito spy, you can override this particular method with a version that creates a mock Runtime, then do whatever you like with it.

However, Tomasz's approach is cleaner, and I recommend it thoroughly. I just wanted to say that there IS another way to do it, without resorting to Powermock.