技術標籤：mockito junitmockito mock void方法

建造者模式Builder是一種常用的設計模式，用於構建不同的產品類。
如有以下的Builder

package com.github.mockito.builder;
class Builder{
private String name;
private String address;
public Builder setName(String name){
this.name = name;
return this;
    }
public Builder setAddress(String address){
this.address = address;
 
return this;
    }
public String sayHello(){
return "hello";
    }
}

以下是一個呼叫的場景

package com.github.mockito.builder;

public class BuilderDemo {
public Builder builder;
public String sayHello(){
return builder.setName("name").setAddress("address").sayHello();
    }
}

建造者模式引人注目的是它標誌性的鏈式方法呼叫(Fluent API)。

package com.github.mockito.builder;

import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;
import org.mockito.InjectMocks;
import org.mockito.Mock;
import org.mockito.MockitoAnnotations;

import static org.assertj.core.api.Assertions.assertThat;
import static org.mockito.Mockito.when;
 

public class BuilderDemoLegacyTest {
@Mock
    Builder builder;

@InjectMocks
    BuilderDemo builderDemo;

@BeforeEach
public void setUp() {
        MockitoAnnotations.initMocks(this);
    }
@Test
public void testSayHelloShouldFail() {
        when(builder.sayHello()).thenReturn("hi");
        assertThat(builderDemo.sayHello()).isEqualTo("hi");
    }
}

這個用例跑失敗了

image.png

失敗的症狀是丟擲了空指標異常，根本原因是沒有正確的測試樁可供使用，因為在呼叫sayHello時，mock的builder沒有被成功匹配。
看來得根據程式碼依次來進行打樁。如下例，

    @Test
    public void testSayHelloLegacy() {
when(builder.setName("name")).thenReturn(builder);
when(builder.setAddress("address")).thenReturn(builder);
when(builder.sayHello()).thenReturn("hi");
assertThat(builderDemo.sayHello()).isEqualTo("hi");
    }

這回跑通過了

image.png

不過問題來了，為了能匹配測試樁，需要逐個對鏈式呼叫中的各個中間返回物件進行打樁。如果只是像這個案例的話，也就算了，如果碰到更為複雜的鏈式方法呼叫，打樁的程式碼就會一大片。
還好Mockito從2.0開始為這個問題提供了一個優雅的解決方法。
RETURNS_SELF(new TriesToReturnSelf())

來看一下使用之後的案例

package com.github.mockito.builder;

import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;
import org.mockito.InjectMocks;
import org.mockito.Mock;
import org.mockito.Mockito;
import org.mockito.MockitoAnnotations;

import static org.assertj.core.api.Assertions.assertThat;
import static org.mockito.Answers.RETURNS_SELF;
import static org.mockito.Mockito.when;

public class BuilderDemoTest {
@Mock(answer = RETURNS_SELF)
    Builder builder;

@InjectMocks
    BuilderDemo builderDemo;

@BeforeEach
public void setUp() {
        MockitoAnnotations.initMocks(this);
    }

@Test
public void testSayHello() {
//         when(builder.setName("name")).thenReturn(builder);
//         when(builder.setAddress("address")).thenReturn(builder);
         when(builder.sayHello()).thenReturn("hi");
        assertThat(builderDemo.sayHello()).isEqualTo("hi");
    }
}

通過 @Mock(answer = RETURNS_SELF) ，告訴Mockito來返回被mock的測試樁自身，這樣就能支援鏈式方法呼叫了。
以下是三個用例的執行結果：

感興趣的讀者可以參考Mockito的Answers列舉類，瞭解各種返回型別。

package org.mockito;
//
public enum Answers implements Answer {
    RETURNS_DEFAULTS(new GloballyConfiguredAnswer()),
    RETURNS_SMART_NULLS(new ReturnsSmartNulls()),
    RETURNS_MOCKS(new ReturnsMocks()),
    RETURNS_DEEP_STUBS(new ReturnsDeepStubs()),
    CALLS_REAL_METHODS(new CallsRealMethods()),
    RETURNS_SELF(new TriesToReturnSelf());//
}

據說RETURNS_DEEP_STUBS也有類似效果，感興趣的讀者可以親自試一試。