Hamcrest：测试列表包含一个具有特定值的私有字段的项目

Question

I have this class:

public class A {
    private int x;

    public A(int x) {
        this.x = x;
    }
}

And a method in a different class I want to test:

public class B {
    public List underTest() {
        int x = doStuff();
        return Collections.singletonList(new A(x));
    }

    private int doStuff() { /* ... */ }
}

I want to test that, at the end of underTest() , the item in the returned list contains an x field equal to a certain value. I've written this:

import static org.hamcrest.MatcherAssert.assertThat;
import static org.hamcrest.Matchers.contains;
import static org.hamcrest.Matchers.hasProperty;
import static org.hamcrest.Matchers.is;

@Test
public void Test1() {
    List result = bInstance.underTest();
    assertThat(result, contains(hasProperty("x", is(1))));
}

But junit complains that item 0: No Property "x" for my test case.

How can I test this? The only thing I can think of is to add a public getter for getX() , then iterate through the returned List and check every element. Remember that, while the method returns a singletonList , the return type is just List , so it could be changed in the future to have multiple values in it.

Answer 1

I think it's first worth saying that testing class internals like this is not a good idea, except maybe in very special cases. Your tests will be brittle and changes that would normally be totally safe - ie renaming a field - may now cause your automated build to fail. You should test external behaviour , not implementation details.

It seems like you'd be better off implementing equals and hashCode in your class A , so then you can simply do:

contains(new A(1))

With that said, if you do have a genuinely good reason to do this (and such cases will be rare) then you cannot use hasProperty for this.

From the JavaDoc :

Creates a matcher that matches when the examined object has a JavaBean property with the specified name whose value satisfies the specified matcher.

I believe this implies that you would need a method named getX .

You shouldn't add such a method just for the purpose of a test, but you can write your own general-purpose Matcher implementation that will use reflection to examine the fields of the class.

Here is a sample implementation:

class ReflectiveFieldMatcher<T> extends BaseMatcher<Object>
{
    private final String fieldName;
    private final T expectedValue;

    ReflectiveFieldMatcher(final String fieldName, final T expectedValue)
    {
        this.fieldName = fieldName;
        this.expectedValue = expectedValue;
    }

    @Override
    public boolean matches(final Object obj)
    {
        for (final Field field : obj.getClass().getFields())
        {
            if (field.getName().equals(fieldName))
            {
                field.setAccessible(true);
                try
                {
                    Object value = field.get(obj);
                    return expectedValue.equals(value);
                }
                catch (final IllegalAccessException e)
                {
                    throw new RuntimeException(e);
                }
            }
        }
        return false;
    }

    @Override
    public void describeTo(final Description description)
    {
        description.appendText("Object with field '" + fieldName + "' with value: " + expectedValue);
    }
}

Your example would now look like this:

assertThat(result, contains(new ReflectiveFieldMatcher<>("x", 1)));