Understanding Spring Autowired: Preventing NullPointerException Compared to Manual Instantiation

The root cause is that when Spring manages beans it automatically initializes the properties that will be used later, whereas creating objects with new requires manual initialization of dependent instances; otherwise a NullPointerException occurs.

Example 1 shows a typical Spring setup where TestService is injected with @Autowired, so Spring automatically injects TestDao into TestService:

@RestController
@RequestMapping(value = "/test")
public class TestController {
    @Autowired
    private TestService testService;

    @RequestMapping(value = "/print", method = RequestMethod.GET)
    public void test() {
        testService.test();
    }
}

@Service
public class TestService {
    @Autowired
    private TestDao testDao;

    public void test() {
        testDao.test();
    }
}

If TestService is instantiated with new, Spring will not inject TestDao, leaving it null and causing a NullPointerException. In this case you must manually create a TestDao instance, as shown in Example 2.

Example 2 demonstrates manual instantiation:

@RestController
@RequestMapping(value = "/test")
public class TestController {
    private TestService testService = new TestService();

    @RequestMapping(value = "/print", method = RequestMethod.GET)
    public void test() {
        testService.test();
    }
}

@Service
public class TestService {
    @Autowired
    private TestDao testDao;

    public void test() {
        TestDao testDao = new TestDao();
        testDao.test();
    }
}

Summary: At application startup, Spring loads and initializes components according to a defined loading chain. For instance, if component A injects B and B injects C, calling B from A will work only when Spring automatically injects the dependencies; using new bypasses this mechanism and leads to NullPointerExceptions because the nested dependencies are not initialized.