问为什么测试字符串域的单元有时会使用这个通用的数据测试代码失败？

EN

pubic class MyDtoClass implements Serializable {

    private String includeNullValue;

    public String getIncludeNullValue() {
        return includeNullValue;
    }

    public void setIncludeNullValue(String includeNullValue) {
        this.includeNullValue = includeNullValue;
    }
}

java.lang.AssertionError: includeNullValue is different expected same:<> was not:<null>
Expected :
Actual   :null

public class MyDtoClassTest extends DtoTest<MyDtoClass> {

    private static MyDtoClass myDtoClass;

    @Before
    public void setup() {
        myDtoClass = new MyDtoClass();
    }

    @Override
    protected MyDtoClass getInstance() {
        return myDtoClass;
    }
}

public abstract class DtoTest<T> {

    /** A map of default mappers for common objects. */
    private static final ImmutableMap<Class<?>, Supplier<?>> DEFAULT_MAPPERS;

    static {
        final Builder<Class<?>, Supplier<?>> mapperBuilder = ImmutableMap.builder();

        /* Primitives */
        mapperBuilder.put(int.class, () -> 0);
        mapperBuilder.put(double.class, () -> 0.0d);
        mapperBuilder.put(float.class, () -> 0.0f);
        mapperBuilder.put(long.class, () -> 0l);
        mapperBuilder.put(boolean.class, () -> true);
        mapperBuilder.put(short.class, () -> (short) 0);
        mapperBuilder.put(byte.class, () -> (byte) 0);
        mapperBuilder.put(char.class, () -> (char) 0);

        mapperBuilder.put(Integer.class, () -> Integer.valueOf(0));
        mapperBuilder.put(Double.class, () -> Double.valueOf(0.0));
        mapperBuilder.put(Float.class, () -> Float.valueOf(0.0f));
        mapperBuilder.put(Long.class, () -> Long.valueOf(0));
        mapperBuilder.put(Boolean.class, () -> Boolean.TRUE);
        mapperBuilder.put(Short.class, () -> Short.valueOf((short) 0));
        mapperBuilder.put(Byte.class, () -> Byte.valueOf((byte) 0));
        mapperBuilder.put(Character.class, () -> Character.valueOf((char) 0));

        mapperBuilder.put(BigDecimal.class, () -> BigDecimal.ONE);
        mapperBuilder.put(Date.class, () -> new Date());

        /* Collection Types. */
        mapperBuilder.put(Set.class, () -> Collections.emptySet());
        mapperBuilder.put(SortedSet.class, () -> Collections.emptySortedSet());
        mapperBuilder.put(List.class, () -> Collections.emptyList());
        mapperBuilder.put(Map.class, () -> Collections.emptyMap());
        mapperBuilder.put(SortedMap.class, () -> Collections.emptySortedMap());

        DEFAULT_MAPPERS = mapperBuilder.build();
    }

    /** The get fields to ignore and not try to test. */
    private final Set<String> ignoredGetFields;

    /**
     * A custom mapper. Normally used when the test class has abstract objects.
     */
    private final ImmutableMap<Class<?>, Supplier<?>> mappers;

    /**
     * Creates an instance of {@link DtoTest} with the default ignore fields.
     */
    protected DtoTest() {
        this(null, null);
    }

    /**
     * Creates an instance of {@link DtoTest} with ignore fields and additional custom mappers.
     *
     * @param customMappers Any custom mappers for a given class type.
     * @param ignoreFields The getters which should be ignored (e.g., "getId" or "isActive").
     */
    protected DtoTest(Map<Class<?>, Supplier<?>> customMappers, Set<String> ignoreFields) {
        this.ignoredGetFields = new HashSet<>();
        if (ignoreFields != null) {
            this.ignoredGetFields.addAll(ignoreFields);
        }
        this.ignoredGetFields.add("getClass");

        if (customMappers == null) {
            this.mappers = DEFAULT_MAPPERS;
        } else {
            final Builder<Class<?>, Supplier<?>> builder = ImmutableMap.builder();
            builder.putAll(customMappers);
            builder.putAll(DEFAULT_MAPPERS);
            this.mappers = builder.build();
        }
    }

    /**
     * Calls a getter and verifies the result is what is expected.
     *
     * @param fieldName The field name (used for error messages).
     * @param getter The get {@link Method}.
     * @param instance The test instance.
     * @param expected The expected result.
     *
     * @throws IllegalAccessException if this Method object is enforcing Java language access control and the underlying
     *             method is inaccessible.
     * @throws IllegalArgumentException if the method is an instance method and the specified object argument is not an
     *             instance of the class or interface declaring the underlying method (or of a subclass or implementor
     *             thereof); if the number of actual and formal parameters differ; if an unwrapping conversion for
     *             primitive arguments fails; or if, after possible unwrapping, a parameter value cannot be converted to
     *             the corresponding formal parameter type by a method invocation conversion.
     * @throws InvocationTargetException if the underlying method throws an exception.
     */
    private void callGetter(String fieldName, Method getter, T instance, Object expected)
            throws IllegalAccessException, IllegalArgumentException, InvocationTargetException {

        final Object getResult = getter.invoke(instance);

        if (getter.getReturnType().isPrimitive()) {
            /* Calling assetEquals() here due to autoboxing of primitive to object type. */
            assertEquals(fieldName + " is different", expected, getResult);
        } else {
            /* This is a normal object. The object passed in should be the exactly same object we get back. */
            assertSame(fieldName + " is different", expected, getResult);
        }
    }

    /**
     * Creates an object for the given {@link Class}.
     *
     * @param fieldName The name of the field.
     * @param clazz The {@link Class} type to create.
     *
     * @return A new instance for the given {@link Class}.
     *
     * @throws InstantiationException If this Class represents an abstract class, an interface, an array class, a
     *             primitive type, or void; or if the class has no nullary constructor; or if the instantiation fails
     *             for some other reason.
     * @throws IllegalAccessException If the class or its nullary constructor is not accessible.
     *
     */
    private Object createObject(String fieldName, Class<?> clazz)
            throws InstantiationException, IllegalAccessException {

      try {      
          final Supplier<?> supplier = this.mappers.get(clazz);
          if (supplier != null) {
              return supplier.get();
          }

          if (clazz.isEnum()) {
              return clazz.getEnumConstants()[0];
          }

          return clazz.newInstance();        
        } catch (IllegalAccessException | InstantiationException e) {
            throw new RuntimeException("Unable to create objects for field '" + fieldName + "'.", e);
        }
    }

    /**
     * Returns an instance to use to test the get and set methods.
     *
     * @return An instance to use to test the get and set methods.
     */
    protected abstract T getInstance();

    /**
     * Tests all the getters and setters. Verifies that when a set method is called, that the get method returns the
     * same thing. This will also use reflection to set the field if no setter exists (mainly used for user immutable
     * entities but Hibernate normally populates).
     *
     * @throws Exception If an expected error occurs.
     */
    @Test
    public void testGettersAndSetters() throws Exception {
        /* Sort items for consistent test runs. */
        final SortedMap<String, GetterSetterPair> getterSetterMapping = new TreeMap<>();

        final T instance = getInstance();

        for (final Method method : instance.getClass().getMethods()) {
            final String methodName = method.getName();

            if (this.ignoredGetFields.contains(methodName)) {
                continue;
            }

            String objectName;
            if (methodName.startsWith("get") && method.getParameters().length == 0) {
                /* Found the get method. */
                objectName = methodName.substring("get".length());

                GetterSetterPair getterSettingPair = getterSetterMapping.get(objectName);
                if (getterSettingPair == null) {
                    getterSettingPair = new GetterSetterPair();
                    getterSetterMapping.put(objectName, getterSettingPair);
                }
                getterSettingPair.setGetter(method);
            } else if (methodName.startsWith("set") && method.getParameters().length == 1) {
                /* Found the set method. */
                objectName = methodName.substring("set".length());

                GetterSetterPair getterSettingPair = getterSetterMapping.get(objectName);
                if (getterSettingPair == null) {
                    getterSettingPair = new GetterSetterPair();
                    getterSetterMapping.put(objectName, getterSettingPair);
                }
                getterSettingPair.setSetter(method);
            } else if (methodName.startsWith("is") && method.getParameters().length == 0) {
                /* Found the is method, which really is a get method. */
                objectName = methodName.substring("is".length());

                GetterSetterPair getterSettingPair = getterSetterMapping.get(objectName);
                if (getterSettingPair == null) {
                    getterSettingPair = new GetterSetterPair();
                    getterSetterMapping.put(objectName, getterSettingPair);
                }
                getterSettingPair.setGetter(method);
            }
        }

        /*
         * Found all our mappings. Now call the getter and setter or set the field via reflection and call the getting
         * it doesn't have a setter.
         */
        for (final Entry<String, GetterSetterPair> entry : getterSetterMapping.entrySet()) {
            final GetterSetterPair pair = entry.getValue();

            final String objectName = entry.getKey();
            final String fieldName = objectName.substring(0, 1).toLowerCase() + objectName.substring(1);

            if (pair.hasGetterAndSetter()) {
                /* Create an object. */
                final Class<?> parameterType = pair.getSetter().getParameterTypes()[0];
                final Object newObject = createObject(fieldName, parameterType);

                pair.getSetter().invoke(instance, newObject);

                callGetter(fieldName, pair.getGetter(), instance, newObject);
            } else if (pair.getGetter() != null) {
                /*
                 * Object is immutable (no setter but Hibernate or something else sets it via reflection). Use
                 * reflection to set object and verify that same object is returned when calling the getter.
                 */
                final Object newObject = createObject(fieldName, pair.getGetter().getReturnType());
                final Field field = instance.getClass().getDeclaredField(fieldName);
                field.setAccessible(true);
                field.set(instance, newObject);

                callGetter(fieldName, pair.getGetter(), instance, newObject);
            }
        }
    }
}