可以模拟System.identityHashCode吗？

Question

Good day.

I have a tool which instruments objects in project by adding certain bytecode-generated toString implementation to each. The logic of generated toString is irrelevant but what important is that its return value depends on System.identityHashCode invocations, made inside.

Now I want to cover the generated logic with automated tests. I created a class for testing, generated toString for it and want to assert over its toString output. It's obvious that such test won't be reproducible, since System.identityHashCode gives different results between test runs.

Is it possible somehow to mock System.identityHashCode or stabilize the result it is given?

Answer 1

It is possible to do this using Powermock, as described here: How to Mock System.getProperty using Mockito

However, the better approach would be to create a class that wraps System and then inject the wrapper into the class you are trying to test.

Then in your unit test you can inject a test implementation of the wrapper and control the output from System.

Answer 2

I would introduce a testing seam:

@Override public String toString() {
  return toString(System.identityHashCode(object));
}

/** Visible for testing. */
String toString(int hashCode) {
  // Your implementation here.
}

This way, you can call toString(int) as many times as you want from adjacent tests without concerning yourself with PowerMock or even Mockito.

As an alternative, if System.identityHashCode is used elsewhere in your class such that results need to be consistent, you can take a replacement ToIntFunction in your constructor, and have the default implementation pass System.identityHashCode as a reference to a static method .

public YourWrapperClass(Object object) {
  this(object, System::identityHashCode);
}

/** Visible for testing. */
YourWrapperClass(Object object, ToIntFunction<Object> hashCodeFunction) {
  this.object = object;
  this.hashCodeFunction = hashCodeFunction;
}