Download private static field with JMockit?

Question

I have a class, for example:

class ClassA { private static File myDir; // myDir is created at some stage private static String findFile(final String fileName) { for (final String actualBackupFileName : myDir.list()) { if (actualBackupFileName.startsWith(removeExtensionFrom(backupFile))) { return actualBackupFileName; } } } } 

So basically, I want to test this class by mocking the File class so that when I call list () it will return a list of strings that I define in my test class.

I have the following, but it doesn’t work at the moment, maybe something obvious that I am doing wrong - I am new to JMockit - any help is greatly appreciated!

 @Mocked("list") File myDir; @Test public void testClassA() { final String[] files = {"file1-bla.txt"}; new NonStrictExpectations() {{ new File(anyString).list(); returns(files); }}; String returnedFileName = Deencapsulation.invoke(ClassA.class, "findFile","file1.txt"); // assert returnedFileName is equal to "file1-bla.txt" } 

When performing the above test, I get a NullPointerException for the myDir field in ClassA - so it looks like it is being mocked incorrectly?

Answer 1

JMockit (or any other mocking tool) does not mimic fields or variables; it mocks types (classes, interfaces, etc.). If instances of these types are stored inside the test code, it does not matter.

Test example for ClassA :

 @Test public void testClassA(@Mocked File myDir) { new Expectations() {{ myDir.list(); result = "file1-bla.txt"; }}; String returnedFileName = new ClassA().publicMethodThatCallsFindFile("file1.txt"); assertEquals("file1-bla.txt", returnedFileName); } 

The above should work. Note that testing private methods directly (or accessing private fields) is considered bad practice, so I avoided it here. In addition, it is better to avoid bullying the File class. Instead, test only your public methods and use actual files instead of taunting the file system.

Answer 2

You can use the setField method from the Deencapsulation class. Example below:

 Deencapsulation.setField(ClassA, "File", your_desired_value); 

Answer 3

try the following:

 new Expectations {{ invoke(File.class, "list", null, null); returns(files); }}