如果仍然可以模拟对象，为什么Service Locator不适合测试？

Question

Everyone knows that code that uses Service Locator is hard to test while Dependency Injection should be used instead.

But why Service Locator is hard to test if we can still easily mock each objects?

Consider this example (written in PHP but it could be any language)

// method we want to test that has a dependency 
public myFunction() {
    $someDependency = Registry::getService('Dependency');

    // do something with the dependendcy
    $someDependency->doSomething();

    return true;
}

If we want to test this code we can simply mock our object "Dependency", example:

 function testMyFunction() {
        $mock = \Mockery::mock('Dependency');
        $mock->shouldReceive('doSomething')->once();

        Registry::set('Dependency', $mock); // set the double

        $workerObject = new MyClass;
        $this->assertTrue( $workerObject->myFunction() );
 }

Isn't this code testable? Why Service Locator is bad in this case?

Note that we all know how bad a Service locator because it hides dependency and violets SOLID principles. But in this case i am simply referring to the testing aspect.

Answer 1

Everyone knows that code that uses Service Locator is hard to test while Dependency Injection should be used instead.

I didn't know that...

The reason that Service Locator is an anti-pattern isn't because it makes the code hard to test (it doesn't), but because it makes it hard to maintain.

Service Locator breaks encapsulation because you can use it from anywhere within your code, and a client has no chance of detecting this without reading through all the code that might potentially use it.

Dependency Injection is a better option, because it solves the same problems with fewer disadvantages.