如何用白盒实例化一个抽象类，以用可以在其中操纵参数的测试逻辑替换方法

Question

Preamble ( please read ):

• Dealing with legacy code that I can't refactor without an act of congress (but I have the sources).
• This legacy code interacts with a lot of abstract classes (and their various Implementations)
• Mocking is not enough because I need to intercept the method arguments sent to the abstract methods and manipulate them inside my replacement (test) logic
• I know I could just extend those abstract classes on my test package and provide implementations for the methods I want to replace. However the goal of this question is to see if there's a way around it, because in the case of abstract classes that have a ton of abstract methods (think SocketChannel from NIO) this is a ton of useless boilerplate code that makes the unit test unreadable.
• I know that the legacy code is poorly designed and that this is not the way of writing clean well designed unit-tests; that's not in question. Just know that the Legacy code can't be changed easily.

The question is : How to accomplish this (using PowerMock ) without getting an exception from PowerMock saying that this class can't be instantiated because it's abstract:

@PrepareForTest({SocketChannel.class})
@RunWith(PowerMockRunner.class)
public class TestThatUsesSocketChannelChannel
{
    replace(method(SocketChannel.class, "read", ByteBuffer.class)).with((proxy, method, args) -> 
    {
        // Line below intercepts the argument and manipulates it 
        ((ByteBuffer) args[0]).clear();
    });
    // The line below throws an exception (because SocketChannel is abstract)
    SocketChannel socketChannel = Whitebox.newInstance(SocketChannel.class);
    // Once here, ideally I can continue my test
}

Answer 1

You can simply mock the abstract class by using plain Mockito:

AbstractClass theMock = mock(AbstractClass.class);

Then inject this mock of the abstract class via PowerMock.

Answer 2

Found the answer : Because SocketChannel is an abstract class, in order to use Whitebox.newInstance, there needs to be a preliminary step with ConcreteClassGenerator. This creates a run time child of SocketChannel with empty method implementations. Notice that before that I replaced the method I care for. Conclusion: this allows me to instantiate (an on-the-fly) child of an abstract class without me having to extend it explicitly. See the code above, now made to work with such intermediary step:

@PrepareForTest({SocketChannel.class})
@RunWith(PowerMockRunner.class)
public class TestThatUsesSocketChannelChannel
{
    replace(method(SocketChannel.class, "read", ByteBuffer.class)).with((proxy, method, args) -> 
    {
        // Line below intercepts the argument and manipulates it 
        ((ByteBuffer) args[0]).clear();
    });
    // THIS Fixes it: generate a an on-the-fly class that implements stub methods
    // for the ones that are abstract and then (and only then) pass that to Whitebox
    Class<?> concreteSocketChannelClass = new ConcreteClassGenerator().createConcreteSubClass(SocketChannel.class);
    SocketChannel socketChannel = (SocketChannel) Whitebox.newInstance(concreteSocketChannelClass);
    // Once here, ideally I can continue my test
}