Mockito 中被 Mocked 的对象属性及方法的默认值

在 Java 测试中使用 Mockito 有段时日了，以前只是想当然的认为 Mock 的对象属性值和方法返回值都是依据同样的规则。基本类型是 0, 0.0, 或 false, 对象类型都是 null, Mock 对象的默认返回值也应该是一样的。直到最近有一天，有一个返回 Optional<String> 类型的方法，由于忘记对该方法打桩，意外的发现它返回的不是 null, 而 Optional.empty(), 因此才意识到此处定有蹊跷。着实有必要用代码验证一下 Mockito 是怎么决定属性及方法的各种返回类型的默认值的。

此次测试所用的 Mockito 版本是 mockito-core-2.12.0.

于是创建了下面一个类 MyClass 用于生成 Mock 对象，选取了一些典型的数据类型, 包括 int, Double, String, long[], Optional<String>, Collection<String>, Map<String, String>, 同时测试 Mock 对象默认的属性值与方法默认返回值。

该类的完整代码如下：

package cc.unmi;
 
import java.util.Collection;
import java.util.Map;
import java.util.Optional;
 
public class MyClass {
    public int integer;
    public Double aDouble;
    public String string;
    public long[] array;
    public Optional<String> optional;
    public Collection<String> collection;
    public Map<String, String> map;
 
    public int getInteger() {
        return 99;
    }
 
    public long[] getArray() {
        return new long[]{0};
    }
 
    public Double getDouble() {
        return 9.9;
    }
 
    public String getString() {
        return "hello";
    }
 
    public Optional<String> getOptional() {
        return null;
    }
 
    public Collection<String> getCollection() {
        return null;
    }
 
    public Map<String, String> getMap() {
        return null;
    }
}
为了认识到调用 Mock 对象时默认情况下不会调用实际方法实现，我们故意让上面的方法返回一些乱七八糟的值。

测试类 MyClassTest 的代码如下

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package cc.unmi;
 
import org.junit.Test;
import org.junit.runner.RunWith;
import org.mockito.Mockito;
import org.mockito.junit.MockitoJUnitRunner;
 
@RunWith(MockitoJUnitRunner.class)
public class MyClassTest {
 
    @Test
    public void watchMockedClass() {
        MyClass myClass = Mockito.mock(MyClass.class);
        printDefaults(myClass);
    }
 
    private void printDefaults(MyClass myClass) {
        System.out.println("fields ---- ");
        System.out.println("integer: " + myClass.integer);
        System.out.println("array: " + myClass.array);
        System.out.println("double: " + myClass.aDouble);
        System.out.println("string: " + myClass.string);
        System.out.println("optional: " + myClass.optional);
        System.out.println("collection: " + myClass.collection);
        System.out.println("map: " + myClass.map);
 
        System.out.println("\nmethods ---- ");
        System.out.println("integer: " + myClass.getInteger());
        System.out.println("array: " + myClass.getArray());
        System.out.println("double: " + myClass.getDouble());
        System.out.println("string: " + myClass.getString());
        System.out.println("optional: " + myClass.getOptional());
        System.out.println("collection: " + myClass.getCollection() + ", " + myClass.getCollection().getClass());
        System.out.println("map: " + myClass.getMap() + ", " + myClass.getMap().getClass());
    }
}

执行上面的代码输出如下:

fields ---- 
integer: 0
array: null
double: null
string: null
optional: null
collection: null
map: null

methods ---- 
integer: 0
array: null
double: 0.0
string: null
optional: Optional.empty
collection: [], class java.util.LinkedList
map: {}, class java.util.HashMap

Mockito mock 的对象属性的默认值没什么异议，与 Java 初始化对象的规则一致，基本类型的默认值是 0, 0.0, 或 false。但是对于方法默认返回值就不一样了，从上面我们看到

1. int 类型方法默认返回 0
2. long[] 类型方法默认返回 null
3. Double 类型方法默认返回 0.0
4. string 类型方法默认返回 null
5. Optional<String> 类型方法默认返回 Optional.empty
6. Collection<String> 类型方法默认返回 new LinkedList<String>(0)
7. Map<String, String> 类型方法默认返回 new HashMap<String, String>(0)

关于 Mock 对象属性的默认值可以搁一边，那么 Mockito 是如何定义 Mock 对象方法的默认返回值的呢？

通常的，我们创建一个 Mock 对象都是简单的调用 Mockito 的如下方法:

public static <T> T mock(Class<T> classToMock) {
    return mock(classToMock, withSetting());
}

再看 withSetting() 方法:

public static MockSetting withSetting() {
    return new MockSettingsImpl().defaultAnswer(RETURNS_DEFAULTS);
}

绕了一圈，基实我们默认采用的 Mock 对象的方式其实就是如下:

Mockito.mock(MyClass.class, Answers.RETURNS_DEFAULTS);

在 org.mockito.Answers 中定义了如下设定方法默认返回值的选项

1. RETURN_DEFAULTS(new GloballyConfiguredAnswer())  -- 基本对应到 ReturnsEmptyValues 实现
2. RETURNS_SMART_NULLS(new ReturnsSmartNulls())  -- 最后对应到 ReturnsMoreEmptyValues 实现
3. RETURN_MOCKS(new ReturnsMocks())
4. RETURNS_DEEP_STUBS(new ReturnsDeepStubs())
5. CALL_REAL_METHODS(new CallsRealMethods())
6. RETURNS_SELF(new TriesToReturnSelf())

所以默认情况下的 RETURNS_DEFAULTS, Mock 对象方法返回值就是由 ReturnsEmptyValues 类决定的，看这个类的注释:

Default answer of every Mockito mock.

Returns appropriate primitive for primitive-returning methods
Returns consistent values for primitive wrapper classes (e.g. int-returning method retuns 0 and Integer-returning method returns 0, too)
Returns empty collection for collection-returning methods (works for most commonly used collection types)
Returns description of mock for toString() method
Returns zero if references are equals otherwise non-zero for Comparable#compareTo(T other) method (see issue 184)
Returns null for everything else

至此，最能说明问题仍然是源代码，很想节约些篇幅，但实在是不行; 欣赏一下 ReturnsEmptyValues 的源代码吧:

public class ReturnsEmptyValues implements Answer<Object>, Serializable {
 
    private static final long serialVersionUID = 1998191268711234347L;
 
 
    /* (non-Javadoc)
     * @see org.mockito.stubbing.Answer#answer(org.mockito.invocation.InvocationOnMock)
     */
    public Object answer(InvocationOnMock invocation) {
        if (isToStringMethod(invocation.getMethod())) {
            Object mock = invocation.getMock();
            MockName name = MockUtil.getMockName(mock);
            if (name.isDefault()) {
                return "Mock for " + MockUtil.getMockSettings(mock).getTypeToMock().getSimpleName() + ", hashCode: " + mock.hashCode();
            } else {
                return name.toString();
            }
        } else if (isCompareToMethod(invocation.getMethod())) {
            //see issue 184.
            //mocks by default should return 0 if references are the same, otherwise some other value because they are not the same. Hence we return 1 (anything but 0 is good).
            //Only for compareTo() method by the Comparable interface
            return invocation.getMock() == invocation.getArgument(0) ? 0 : 1;
        }
 
        Class<?> returnType = invocation.getMethod().getReturnType();
        return returnValueFor(returnType);
    }
 
    Object returnValueFor(Class<?> type) {
        if (Primitives.isPrimitiveOrWrapper(type)) {
            return Primitives.defaultValue(type);
            //new instances are used instead of Collections.emptyList(), etc.
            //to avoid UnsupportedOperationException if code under test modifies returned collection
        } else if (type == Iterable.class) {
            return new ArrayList<Object>(0);
        } else if (type == Collection.class) {
            return new LinkedList<Object>();
        } else if (type == Set.class) {
            return new HashSet<Object>();
        } else if (type == HashSet.class) {
            return new HashSet<Object>();
        } else if (type == SortedSet.class) {
            return new TreeSet<Object>();
        } else if (type == TreeSet.class) {
            return new TreeSet<Object>();
        } else if (type == LinkedHashSet.class) {
            return new LinkedHashSet<Object>();
        } else if (type == List.class) {
            return new LinkedList<Object>();
        } else if (type == LinkedList.class) {
            return new LinkedList<Object>();
        } else if (type == ArrayList.class) {
            return new ArrayList<Object>();
        } else if (type == Map.class) {
            return new HashMap<Object, Object>();
        } else if (type == HashMap.class) {
            return new HashMap<Object, Object>();
        } else if (type == SortedMap.class) {
            return new TreeMap<Object, Object>();
        } else if (type == TreeMap.class) {
            return new TreeMap<Object, Object>();
        } else if (type == LinkedHashMap.class) {
            return new LinkedHashMap<Object, Object>();
        } else if ("java.util.Optional".equals(type.getName())) {
            return JavaEightUtil.emptyOptional();
        } else if ("java.util.OptionalDouble".equals(type.getName())) {
            return JavaEightUtil.emptyOptionalDouble();
        } else if ("java.util.OptionalInt".equals(type.getName())) {
            return JavaEightUtil.emptyOptionalInt();
        } else if ("java.util.OptionalLong".equals(type.getName())) {
            return JavaEightUtil.emptyOptionalLong();
        } else if ("java.util.stream.Stream".equals(type.getName())) {
            return JavaEightUtil.emptyStream();
        } else if ("java.util.stream.DoubleStream".equals(type.getName())) {
            return JavaEightUtil.emptyDoubleStream();
        } else if ("java.util.stream.IntStream".equals(type.getName())) {
            return JavaEightUtil.emptyIntStream();
        } else if ("java.util.stream.LongStream".equals(type.getName())) {
            return JavaEightUtil.emptyLongStream();
        }
 
        //Let's not care about the rest of collections.
        return null;
    }
}

从上可以看到所有列出的方法默认返回值的映射情况，未涉及到的就是 null.

我们还可以关注一下另一个 Answer: RETURN_SMART_NULL, 同样是看相应实现类 ReturnsMoreEmptyValues  的注解 

It's likely this implementation will be used by default by every Mockito 3.0.0 mock.
Currently used only by Mockito.RETURNS_SMART_NULLS
Current version of Mockito mocks by default use ReturnsEmptyValues

Returns appropriate primitive for primitive-returning methods
Returns consistent values for primitive wrapper classes (e.g. int-returning method returns 0 and Integer-returning method returns 0, too)
Returns empty collection for collection-returning methods (works for most commonly used collection types)
Returns empty array for array-returning methods
Returns "" for String-returning method
Returns description of mock for toString() method
Returns non-zero for Comparable#compareTo(T other) method (see issue 184)
Returns null for everything else

这还是一个面向未来(Mockito 3.0.9) 的默认的 Answer, 它与 RETURNS_DEFAULTS 有所不同的是数组，字符串不再为 null, 而是空数组和空字符串。

我们可以作一个测试，前面的 MyClassTest 代码，把构造 MyClass Mock  对象那一行从

MyClass myClass = Mockito.mock(MyClass.class);

MyClass myClass = Mockito.mock(MyClass.class, Mockito.withSettings()
    .defaultAnswer(Answers.RETURNS_SMART_NULLS).verboseLogging());

我们同时开启了调用 Mock 方法时的详细输出，重新运行后，控制台输出

fields ---- 
integer: 0
array: null
double: null
string: null
optional: null
collection: null
map: null

methods ---- 
############ Logging method invocation #1 on mock/spy ########
myClass.getInteger();
invoked: -> at cc.unmi.MyClassTest.printDefaults(MyClassTest.java:31)
has returned: "0" (java.lang.Integer)

integer: 0
############ Logging method invocation #2 on mock/spy ########
myClass.getArray();
invoked: -> at cc.unmi.MyClassTest.printDefaults(MyClassTest.java:32)
has returned: "[J@4009e306" ([J)

array: [J@4009e306
############ Logging method invocation #3 on mock/spy ########
myClass.getDouble();
invoked: -> at cc.unmi.MyClassTest.printDefaults(MyClassTest.java:33)
has returned: "0.0" (java.lang.Double)

double: 0.0
############ Logging method invocation #4 on mock/spy ########
myClass.getString();
invoked: -> at cc.unmi.MyClassTest.printDefaults(MyClassTest.java:34)
has returned: "" (java.lang.String)

string: 
############ Logging method invocation #5 on mock/spy ########
myClass.getOptional();
invoked: -> at cc.unmi.MyClassTest.printDefaults(MyClassTest.java:35)
has returned: "Optional.empty" (java.util.Optional)

optional: Optional.empty
############ Logging method invocation #6 on mock/spy ########
myClass.getCollection();
invoked: -> at cc.unmi.MyClassTest.printDefaults(MyClassTest.java:36)
has returned: "[]" (java.util.LinkedList)

############ Logging method invocation #7 on mock/spy ########
myClass.getCollection();
invoked: -> at cc.unmi.MyClassTest.printDefaults(MyClassTest.java:36)
has returned: "[]" (java.util.LinkedList)

collection: [], class java.util.LinkedList
############ Logging method invocation #8 on mock/spy ########
myClass.getMap();
invoked: -> at cc.unmi.MyClassTest.printDefaults(MyClassTest.java:37)
has returned: "{}" (java.util.HashMap)

############ Logging method invocation #9 on mock/spy ########
myClass.getMap();
invoked: -> at cc.unmi.MyClassTest.printDefaults(MyClassTest.java:37)
has returned: "{}" (java.util.HashMap)

map: {}, class java.util.HashMap

有所不同的也就是数组默认为空，字符串默认为空字符串，都不再是 null 了。

另外，剩下的几个 Answer，除了 CALL_REAL_METHODS 很容易理解(就是不 Mock 方法了)。其余三个

• RETURN_MOCKS(new ReturnsMocks())
• RETURNS_DEEP_STUBS(new ReturnsDeepStubs())
• RETURNS_SELF(new TriesToReturnSelf())

的具体用意待到有需求时再去扒它们吧。

类比于 Mockito, 我也大致测试了一下 JMockit，也有类似的行为，不在此罗列了。