換種思路寫Mock，讓單元測試更簡單
阿新 • • 發佈：2020-12-10
## 開篇引入
> 單元測試中的Mock方法，通常是為了繞開那些依賴外部資源或無關功能的方法呼叫，使得測試重點能夠集中在需要驗證和保障的程式碼邏輯上。在定義Mock方法時，開發者真正關心的只有一件事："這個呼叫，在測試的時候要換成那個假的Mock方法"。
然而當下主流的Mock框架在實現Mock功能時，需要開發者操心的事情實在太多：Mock框架如何初始化、與所用的單元測試框架是否相容、要被Mock的方法是不是私有的、是不是靜態的、被Mock物件是new出來的還是注入的、怎樣把被測物件送回被測類裡...這些非關鍵的額外工作極大分散了使用Mock工具應有的樂趣。
週末，在翻github上alibaba的開源專案時，無意間看到了下面這個特立獨行的輕量Mock工具。當前知道這個工具的人應該很少，star人數28(包括本人在內)，另外我留意了一下該專案在github上第一次提交程式碼時間是2020年5月9日。
專案地址：[https://github.com/alibaba/testable-mock](https://github.com/alibaba/testable-mock)
文件：[https://alibaba.github.io/testable-mock/](https://alibaba.github.io/testable-mock/)
> 換種思路寫Mock，讓單元測試更簡單。無需初始化，不挑測試框架，甭管要換的方法是被測類的私有方法、靜態方法還是其他任何類的成員方法，也甭管要換的物件是怎麼建立的。寫好Mock方法，加個@TestableMock註解，一切統統搞定。
這是 `README` 上的描述。掃了一眼專案描述與目錄結構後，就抵制不住誘惑，快速上手玩了一下。於是，就有了這篇划水部落格，讓看到的朋友也心癢一下(●´ω｀●)。當然，最重要的是如果確實好用的話，可以在實際專案中用起來，這樣就不再反感需要Mock的單元測試了。
## 快速上手
完整程式碼見本人github：[https://github.com/itwild/less/tree/master/less-alibaba/less-testable](https://github.com/itwild/less/tree/master/less-alibaba/less-testable)
這裡有一個 `WeatherApi` 的介面，通過呼叫第三方介面查詢天氣情況，如下：
```java
import com.github.itwild.less.base.http.feign.WeatherExample;
import feign.Param;
import feign.RequestLine;
public interface WeatherApi {
@RequestLine("GET /api/weather/city/{city_code}")
WeatherExample.Response query(@Param("city_code") String cityCode);
}
```
`CityWeather` 查詢具體城市的天氣，如下：
```java
import cn.hutool.core.map.MapUtil;
import com.github.itwild.less.base.http.feign.WeatherExample;
import feign.Feign;
import feign.jackson.JacksonDecoder;
import feign.jackson.JacksonEncoder;
import java.util.HashMap;
import java.util.Map;
public class CityWeather {
private static final String API_URL = "http://t.weather.itboy.net";
private static final String BEI_JING = "101010100";
private static final String SHANG_HAI = "101020100";
private static final String HE_FEI = "101220101";
public static final Map CITY_CODE = MapUtil.builder(new HashMap())
.put(BEI_JING, "北京市")
.put(SHANG_HAI, "上海市")
.put(HE_FEI, "合肥市")
.build();
private static WeatherApi weatherApi = Feign.builder()
.encoder(new JacksonEncoder())
.decoder(new JacksonDecoder())
.target(WeatherApi.class, API_URL);
public String queryShangHaiWeather() {
WeatherExample.Response response = weatherApi.query(SHANG_HAI);
return response.getCityInfo().getCity() + ": " + response.getData().getYesterday().getNotice();
}
private String queryHeFeiWeather() {
WeatherExample.Response response = weatherApi.query(HE_FEI);
return response.getCityInfo().getCity() + ": " + response.getData().getYesterday().getNotice();
}
public static String queryBeiJingWeather() {
WeatherExample.Response response = weatherApi.query(BEI_JING);
return response.getCityInfo().getCity() + ": " + response.getData().getYesterday().getNotice();
}
public static void main(String[] args) {
CityWeather cityWeather = new CityWeather();
String shanghai = cityWeather.queryShangHaiWeather();
String hefei = cityWeather.queryHeFeiWeather();
String beijing = CityWeather.queryBeiJingWeather();
System.out.println(shanghai);
System.out.println(hefei);
System.out.println(beijing);
}
```
執行 `main` 方法，輸出如下：
```bash
上海市: 不要被陰雲遮擋住好心情
合肥市: 不要被陰雲遮擋住好心情
北京市: 陰晴之間，謹防紫外線侵擾
```
相信大多數人編寫單元測試時，遇到這種依賴第三方資源時，可能就有點反感寫單元測試了。
下面看看有了 `testable-mock` 工具，如何編寫單元測試？
`CityWeatherTest` 檔案如下：
```java
import com.alibaba.testable.core.accessor.PrivateAccessor;
import com.alibaba.testable.core.annotation.TestableMock;
import com.alibaba.testable.processor.annotation.EnablePrivateAccess;
import com.github.itwild.less.base.http.feign.WeatherExample;
import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.*;
@EnablePrivateAccess
public class CityWeatherTest {
@TestableMock(targetMethod = "query")
public WeatherExample.Response query(WeatherApi self, String cityCode) {
WeatherExample.Response response = new WeatherExample.Response();
// mock天氣介面呼叫返回的結果
response.setCityInfo(new WeatherExample.CityInfo().setCity(
CityWeather.CITY_CODE.getOrDefault(cityCode, cityCode)));
response.setData(new WeatherExample.Data().setYesterday(
new WeatherExample.Forecast().setNotice("this is from mock")));
return response;
}
CityWeather cityWeather = new CityWeather();
/**
* 測試 public方法呼叫
*/
@Test
public void test_public() {
String shanghai = cityWeather.queryShangHaiWeather();
System.out.println(shanghai);
assertEquals("上海市: this is from mock", shanghai);
}
/**
* 測試 private方法呼叫
*/
@Test
public void test_private() {
String hefei = (String) PrivateAccessor.invoke(cityWeather, "queryHeFeiWeather");
System.out.println(hefei);
assertEquals("合肥市: this is from mock", hefei);
}
/**
* 測試 靜態方法呼叫
*/
@Test
public void test_static() {
String beijing = CityWeather.queryBeiJingWeather();
System.out.println(beijing);
assertEquals("北京市: this is from mock", beijing);
}
}
```
執行單元測試，輸出如下：
```bash
合肥市: this is from mock
上海市: this is from mock
北京市: this is from mock
```
從執行結果不難發現，依賴第三方介面的 `query` 方法已經被僅僅加了個 `TestableMock` 註解的方法Mock了。也就是說達到了預期的Mock效果，而且程式碼優雅易讀。
## 實現原理
那麼，這優雅易讀的背後到底隱藏著什麼祕密呢？
相信對這方面有些瞭解的朋友或多或少也猜到了，沒錯，正是位元組碼增強技術！！！
```java
package com.alibaba.testable.agent;
import com.alibaba.testable.agent.transformer.TestableClassTransformer;
import java.lang.instrument.Instrumentation;
/**
* Agent entry, dynamically modify the byte code of classes under testing
* @author flin
*/
public class PreMain {
public static void premain(String agentArgs, Instrumentation inst) {
parseArgs(agentArgs);
inst.addTransformer(new TestableClassTransformer());
}
}
```
```java
package com.alibaba.testable.agent.handler;
import org.objectweb.asm.ClassReader;
import org.objectweb.asm.ClassWriter;
import org.objectweb.asm.Opcodes;
import org.objectweb.asm.tree.ClassNode;
import java.io.IOException;
/**
* @author flin
*/
abstract public class BaseClassHandler implements Opcodes {
public byte[] getBytes(byte[] classFileBuffer) throws IOException {
ClassReader cr = new ClassReader(classFileBuffer);
ClassNode cn = new ClassNode();
cr.accept(cn, 0);
transform(cn);
ClassWriter cw = new ClassWriter( 0);
cn.accept(cw);
return cw.toByteArray();
}
/**
* Transform class byte code
* @param cn original class node
*/
abstract protected void transform(ClassNode cn);
}
```
追一下原始碼，可見，該Mock工具藉助了ASM Core API來修改位元組碼。上面也提到了，該專案在github上開源出來的時間並不長，核心程式碼並不多，認真看應該能看懂，主要是有些朋友可能從來沒有了解過位元組碼增強技術。這裡推薦美團技術團隊的一篇位元組碼增強技術相關的文章，[https://tech.meituan.com/2019/09/05/java-bytecode-enhancement.html](https://tech.meituan.com/2019/09/05/java-bytecode-enhancement.html)，相信有了這樣的基礎，回過頭來再看看 `TestableMock` 的原始碼會輕鬆許多。
本篇部落格並不會過多探究位元組碼增強技術的細節，頂多算是拋磚引玉，目的是讓讀者知道有這麼一個優雅的Mock工具，另外位元組碼增強技術相當於是一把開啟執行時JVM的鑰匙，利用它可以動態地對執行中的程式做修改，也可以跟蹤JVM執行中程式的狀態，這樣就能在開發中減少冗餘程式碼，提高開發效率。順便提一句，我們平時使用的AOP(Cglib就是基於ASM的)也與位元組碼增強密切相關，它們實質上還是利用各種手段生成符合規範的位元組碼檔案。
雖然這篇不講修改位元組碼的操作細節，但我還是想讓讀者直觀地看到增強後的位元組碼(class檔案)是什麼樣子的，說白了就是到底把我寫的程式碼在執行時修改成了啥？？？於是，我把執行時增強過的位元組碼重新寫入了檔案，然後使用反編譯工具(拖到IDEA中即可)觀察被修改後的原始碼。
執行時(即增強後的)CityWeatherTest.class反編譯後如下：
```java
import com.alibaba.testable.core.accessor.PrivateAccessor;
import com.alibaba.testable.core.annotation.TestableMock;
import com.alibaba.testable.core.util.InvokeRecordUtil;
import com.alibaba.testable.processor.annotation.EnablePrivateAccess;
import com.github.itwild.less.base.http.feign.WeatherExample.CityInfo;
import com.github.itwild.less.base.http.feign.WeatherExample.Data;
import com.github.itwild.less.base.http.feign.WeatherExample.Forecast;
import com.github.itwild.less.base.http.feign.WeatherExample.Response;
import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.Test;
@EnablePrivateAccess
public class CityWeatherTest {
CityWeather cityWeather = new CityWeather();
public static CityWeatherTest _testableInternalRef;
public static CityWeatherTest _testableInternalRef;
public CityWeatherTest() {
}
@TestableMock(
targetMethod = "query"
)
public Response query(WeatherApi var1, String cityCode) {
InvokeRecordUtil.recordMockInvoke(new Object[]{var1, cityCode}, false);
InvokeRecordUtil.recordMockInvoke(new Object[]{var1, cityCode}, false);
Response response = new Response();
response.setCityInfo((new CityInfo()).setCity((String)CityWeather.CITY_CODE.getOrDefault(cityCode, cityCode)));
response.setData((new Data()).setYesterday((new Forecast()).setNotice("this is from mock")));
return response;
}
@Test
public void test_public() {
_testableInternalRef = this;
_testableInternalRef = this;
String shanghai = this.cityWeather.queryShangHaiWeather();
System.out.println(shanghai);
Assertions.assertEquals("上海市: this is from mock", shanghai);
}
@Test
public void test_private() {
_testableInternalRef = this;
_testableInternalRef = this;
String hefei = (String)PrivateAccessor.invoke(this.cityWeather, "queryHeFeiWeather", new Object[0]);
System.out.println(hefei);
Assertions.assertEquals("合肥市: this is from mock", hefei);
}
@Test
public void test_static() {
_testableInternalRef = this;
_testableInternalRef = this;
String beijing = CityWeather.queryBeiJingWeather();
System.out.println(beijing);
Assertions.assertEquals("北京市: this is from mock", beijing);
}
}
```
執行時(即增強後的)CityWeather.class反編譯後如下：
```java
import cn.hutool.core.map.MapUtil;
import com.github.itwild.less.base.http.feign.WeatherExample.Response;
import feign.Feign;
import feign.jackson.JacksonDecoder;
import feign.jackson.JacksonEncoder;
import java.util.HashMap;
import java.util.Map;
public class CityWeather {
private static final String API_URL = "http://t.weather.itboy.net";
private static final String BEI_JING = "101010100";
private static final String SHANG_HAI = "101020100";
private static final String HE_FEI = "101220101";
public static final Map CITY_CODE = MapUtil.builder(new HashMap()).put("101010100", "北京市").put("101020100", "上海市").put("101220101", "合肥市").build();
private static WeatherApi weatherApi = (WeatherApi)Feign.builder().encoder(new JacksonEncoder()).decoder(new JacksonDecoder()).target(WeatherApi.class, "http://t.weather.itboy.net");
public CityWeather() {
}
public String queryShangHaiWeather() {
Response response = CityWeatherTest._testableInternalRef.query(weatherApi, "101020100");
return response.getCityInfo().getCity() + ": " + response.getData().getYesterday().getNotice();
}
private String queryHeFeiWeather() {
Response response = CityWeatherTest._testableInternalRef.query(weatherApi, "101220101");
return response.getCityInfo().getCity() + ": " + response.getData().getYesterday().getNotice();
}
public static String queryBeiJingWeather() {
Response response = CityWeatherTest._testableInternalRef.query(weatherApi, "101010100");
return response.getCityInfo().getCity() + ": " + response.getData().getYesterday().getNotice();
}
public static void main(String[] args) {
CityWeather cityWeather = new CityWeather();
String shanghai = cityWeather.queryShangHaiWeather();
String hefei = cityWeather.queryHeFeiWeather();
String beijing = queryBeiJingWeather();
System.out.println(shanghai);
System.out.println(hefei);
System.out.println(beijing);
}
}
```
原來，執行時把呼叫到 `query` 方法的實現都換成了自己Mock的