1. Introduction

Spring MockMVC is the Spring MVC testing framework which performs Spring MVC request handling and response asserting via mocking request and response objects without a running HTTP server. @AutoConfigureMockMvc is used to configure and a MockMVC instance is injected into a test class. In this example, I will demonstrate Spring MockMVC fetch JSON by creating a simple Get Rest API and mocking the response via MockMVC and then converting the response string to an object via both Jackson and GSON libraries.

2. Setup

In this step, I will create a gradle project with spring-boot-starter-web, Jackson, GSON, Lombok, and Junit libraries.

build.gradle

plugins {
	id 'java'
	id 'org.springframework.boot' version '3.3.0'
	id 'io.spring.dependency-management' version '1.1.5'
}

group = 'com.zheng.demo.sbtest'
version = '0.0.1-SNAPSHOT'

java {
	toolchain {
		languageVersion = JavaLanguageVersion.of(17)
	}
}

configurations {
 compileOnly {
 extendsFrom annotationProcessor
 }
}

repositories {
	mavenCentral()
}

dependencies {
	implementation 'org.springframework.boot:spring-boot-starter-web'
	
	compileOnly 'org.projectlombok:lombok'
	annotationProcessor 'org.projectlombok:lombok'
	
	implementation 'com.google.code.gson:gson:2.11.0'
	
	testImplementation 'org.springframework.boot:spring-boot-starter-test'
	testRuntimeOnly 'org.junit.platform:junit-platform-launcher'
}

tasks.named('test') {
	useJUnitPlatform()
}

3. Get Rest API

3.1 MyRestController

In this step, I will create a MyRestController.java class which has a Get method to return a RestReturnData ResponseEntity.

MyRestController.java

package com.zheng.demo.sbtest.demo.rest;

import java.util.stream.IntStream;

import org.springframework.http.HttpStatus;
import org.springframework.http.ResponseEntity;
import org.springframework.web.bind.annotation.GetMapping;
import org.springframework.web.bind.annotation.RequestParam;
import org.springframework.web.bind.annotation.RestController;

import com.zheng.demo.sbtest.demo.data.RestReturnData;

@RestController
public class MyRestController {

	@GetMapping("/")
	public ResponseEntity<RestReturnData> getJsonResponse(@RequestParam("input") String input) {
		RestReturnData retD = new RestReturnData(IntStream.range(0, input.length()).map(n -> (n + 1) * 3).toArray(),
				input.length(), input);

		return new ResponseEntity<RestReturnData>(retD, HttpStatus.OK);
	}
}

• Line 18,19: the Get API response is a dummy data generated from the input value.

3.2 Response Data

In this step, I will create a RestReturnData.java class which is used at step 3.1.

RestReturnData.java

package com.zheng.demo.sbtest.demo.data;

import lombok.AllArgsConstructor;
import lombok.Data;
import lombok.NoArgsConstructor;

@Data
@NoArgsConstructor
@AllArgsConstructor
public class RestReturnData {
	private int[] someIntArray;
	private int someNumber;
	private String someString;
}

Note: use the Lombok annotations to reduce the boilerplate code.

3.3 Spring Boot Application

In this step, I will create a DemoApplication.java annotated with @SpringBootApplication. Note: if the project is generated from the Spring initializr, then no modification is needed.

DemoApplication.java

package com.zheng.demo.sbtest.demo;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class DemoApplication {
	public static void main(String[] args) {
		SpringApplication.run(DemoApplication.class, args);
	}
}

Start the Spring boot application and navigate to http://localhost:8080/?input=hello and capture the result in the following screenshot.

4. Test Spring MockMVC Fetch JSON

4.1 DemoApplicationTests

In this step, I will run the generated DemoApplicationTests.java to ensure that the spring boot application is wired correctly. Note: no modification is needed here.

DemoApplicationTests.java

package com.zheng.demo.sbtest.demo;

import org.junit.jupiter.api.Test;
import org.springframework.boot.test.context.SpringBootTest;

@SpringBootTest
class DemoApplicationTests {
	@Test
	void contextLoads() {
	}
}

4.2 Spring MockMVC Test

In this step, I will create a MyRestControllerTest.java class which tests the Get API created at step 3.1.

MyRestControllerTest.java

package com.zheng.demo.sbtest.demo.rest;

import static org.hamcrest.Matchers.containsString;
import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
import static org.springframework.test.web.servlet.result.MockMvcResultHandlers.print;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.content;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;

import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.web.servlet.AutoConfigureMockMvc;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.web.servlet.MockMvc;

@SpringBootTest
@AutoConfigureMockMvc
class MyRestControllerTest {

	@Autowired
	private MockMvc mockMvc;

	@Test
	void shouldReturnDefaultMessage() throws Exception {
		this.mockMvc.perform(get("/?input=hello")).andDo(print()).andExpect(status().isOk())
				.andExpect(content().string(containsString("hello")));
	}

}

• Line 16: @AutoConfigureMockMvc is configured.
• Line 20: a MockMvc is autowired by Spring.
• Line 24: the mocked instance performs the Get API and response is asserted.

Run this test and capture the output here.

MyRestControllerTest output

2024-06-18T20:56:06.922-05:00 INFO 21400 --- [demo] [ main] c.z.d.s.demo.rest.MyRestControllerTest : Started MyRestControllerTest in 1.587 seconds (process running for 2.604)

MockHttpServletRequest:
 HTTP Method = GET
 Request URI = /
 Parameters = {input=[hello]}
 Headers = []
 Body = null
 Session Attrs = {}

Handler:
 Type = com.zheng.demo.sbtest.demo.rest.MyRestController
 Method = com.zheng.demo.sbtest.demo.rest.MyRestController#getJsonResponse(String)

Async:
 Async started = false
 Async result = null

Resolved Exception:
 Type = null

ModelAndView:
 View name = null
 View = null
 Model = null

FlashMap:
 Attributes = null

MockHttpServletResponse:
 Status = 200
 Error message = null
 Headers = [Content-Type:"application/json"]
 Content type = application/json
 Body = {"someIntArray":[3,6,9,12,15],"someNumber":5,"someString":"hello"}
 Forwarded URL = null
 Redirected URL = null
 Cookies = []

• Line 6: the Get request query parameter.
• Line 34,35: the Get response Json string.

4.3 Spring MockMVC Fetch Json via Gson

In this step, I will create a TestMockMvcViaGson.java test class which converts the mocked Rest Get response into POJO via the Gson library.

TestMockMvcViaGson.java

package com.zheng.demo.sbtest.demo.rest;

import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
import static org.springframework.test.web.servlet.result.MockMvcResultHandlers.print;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;

import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.web.servlet.AutoConfigureMockMvc;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.web.servlet.MockMvc;

import com.google.gson.Gson;
import com.zheng.demo.sbtest.demo.data.RestReturnData;

@SpringBootTest
@AutoConfigureMockMvc
class TestMockMvcViaGson {

	@Autowired
	private MockMvc mockMvc;

	@Test
	void use_Gson_to_map_JsonString() {
		try {
			String resultString = mockMvc.perform(get("/").param("input", "test_user")).andDo(print())
					.andExpect(status().isOk()).andReturn().getResponse().getContentAsString();
			Gson gson = new Gson();
			RestReturnData someClass = gson.fromJson(resultString, RestReturnData.class);
			assertEquals("test_user", someClass.getSomeString());
			assertEquals(9, someClass.getSomeNumber());
			assertEquals(27, someClass.getSomeIntArray()[8]);

		} catch (Exception e) {
			e.printStackTrace();
		}
	}
}

• Line 29: create a gson instance.
• Line 30: convert the JSON string to a Java object via gson.

4.4 Spring MockMvc Fetch Json via Jackson

In this step, I will create a TestMockMvcViaJackson.java test class which converts the mocked Rest Get response to POJO via Jackson’s objectMapper.

TestMockMvcViaJackson.java

package com.zheng.demo.sbtest.demo.rest;

import static org.junit.jupiter.api.Assertions.assertEquals;
import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
import static org.springframework.test.web.servlet.result.MockMvcResultHandlers.print;
import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;

import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.autoconfigure.web.servlet.AutoConfigureMockMvc;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.test.web.servlet.MockMvc;

import com.fasterxml.jackson.databind.ObjectMapper;
import com.zheng.demo.sbtest.demo.data.RestReturnData;

@SpringBootTest
@AutoConfigureMockMvc
class TestMockMvcViaJackson {

	@Autowired
	private MockMvc mockMvc;

	@Test
	void use_ObjectMapper_to_map_JsonString() {
		try {
			String resultString = mockMvc.perform(get("/").param("input", "test_user")).andDo(print())
					.andExpect(status().isOk()).andReturn().getResponse().getContentAsString();

			RestReturnData someClass = new ObjectMapper().readValue(resultString, RestReturnData.class);
			assertEquals("test_user", someClass.getSomeString());
			assertEquals(9, someClass.getSomeNumber());
			assertEquals(27, someClass.getSomeIntArray()[8]);
		} catch (Exception e) {
			e.printStackTrace();
		}

	}

}

• Line 30: convert the Json string to a Java object via Jackson’s objectMapper.

Run the Junit tests and capture the results as the following screenshot.

5. Conclusion

In this example, I created a Rest Get API via Spring @RestController and tested it via Spring MockMVC. I also converted the Rest API’s JSON string into POJO via both Jackson and Gson libraries.

6. Download

This was an example of a gradle project.