What is the alternative to autowiring while running a class through testNG?

I am validating the method testValidateABC() in ABCValidatorTest.java

ABCValidatorTest.java

public class ABCValidatorTest {

 @Test
    public void testValidateABC() {
        ABCValidator validator = new ABCValidator();
        List<String> input = Arrays.asList("0", "1234", "abcd");
        List<Boolean> expectedOutput = Arrays.asList(false, true, false);
        boolean output;

        for (int i = 0; i < input.size(); i++) {
            if (StringUtils.isNotEmpty(input.get(i))) {
                output = validator.validateABC(input.get(i));
            } else {
                output = false;
            }
            Assert.assertEquals((boolean) expectedOutput.get(i), output);

        }
    }

}

Here I am creating an object of ABCValidator and below is its class:

ABCValidator.java

public class ABCValidator {

    @Autowired(required = false)
    private ABCSearch abcRestClient;

    public boolean validateABC(String abc_code) {
        String response = abcRestClient.searchABCCodes(abc_code);
        boolean hasValidabc = true;
        if ((response.startsWith("null", 1)) || (response.equals("[]"))) {
            hasValidabc = false;
        }
        return hasValidabc;
    }
}

Here I have autowired ABCSearch to get the method searchABCCodes().

I am getting a NullPointerException when i run this since abcRestClient is not intitalized. How can I fix this without changing anything in ABCValidator.java?

The clean answer is to use constructor injection instead of field injection. (This is a good idea for a number of reasons, including that it prevents several categories of bugs.)

Then you don't need any special Spring or reflection support, just a normal mock:

@Test
public void testValidateABC() {
    ABCSearch mockRestClient = Mockito.mock(ABCSearch.class);
    ABCValidator validator = new ABCValidator(mockRestClient);
    List<String> input = Arrays.asList("0", "1234", "abcd");
    List<Boolean> expectedOutput = Arrays.asList(false, true, false);

    for (int i = 0; i < input.size(); i++) {
        when(mockRestClient.searchABCCodes(input.get(i)).thenReturn(/* the correct boolean */);
        Assert.assertEquals((boolean) expectedOutput.get(i), validator.validateABC(input.get(i)));
    }
}

Note that what you're doing here is called a parameterized test; I'm not especially familiar with TestNG but expect there is support for it. I am a fan of Spock (a test DSL that runs on top of JUnit) that makes this kind of test very clean.

If you want to write a unit test you can use Mocktio and parameterized test in Testng

@RunWith(MockitoRunner.class)
public class ABCValidatorTest {

    @Mock
    private ABCSearch search;

    @DataProvider(name = "dataSet")
    public static Object[][] createdataSet() {
        return new Object[][] { { "one", true }, { "three", true }, { "four", true }, { "five", true }, };
    }

    @Test(dataProvider = "dataSet")
    public void testValidateABC(String input, Boolean expectedResult) {
        ABCValidator validator = new ABCValidator();
        Whitebox.setInternalState(validator, "abcRestClient", search);
        output = validator.validateABC(input);
        Assert.assertEquals(expectedResult, output);
    }
}