feature: add the repo's first test suite (43 JUnit 5 tests)

src/test/java/hello/ApplicationContextLoadTest.java

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+package hello;
+
+import org.junit.jupiter.api.Test;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.context.ApplicationContext;
+import org.springframework.jdbc.core.JdbcTemplate;
+import org.springframework.web.client.RestTemplate;
+
+import hello.controller.GreetingController;
+import hello.controller.HelloController;
+import hello.controller.TopicController;
+import hello.service.TopicService;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Starts the whole application context, including the {@link org.springframework.boot.CommandLineRunner}
+ * on {@link Application}. The runner contacts a demo quote service that no longer exists; the resulting
+ * WARN must not fail startup, so reaching the assertions below is itself part of what is being verified.
+ */
+@SpringBootTest
+class ApplicationContextLoadTest {
+
+    @Autowired
+    private ApplicationContext context;
+
+    @Test
+    void contextLoads() {
+        assertThat(context).isNotNull();
+    }
+
+    @Test
+    void exposesTheExpectedBeans() {
+        assertThat(context.getBean(TopicService.class)).isNotNull();
+        assertThat(context.getBean(TopicController.class)).isNotNull();
+        assertThat(context.getBean(HelloController.class)).isNotNull();
+        assertThat(context.getBean(GreetingController.class)).isNotNull();
+        assertThat(context.getBean(RestTemplate.class)).isNotNull();
+        assertThat(context.getBean(JdbcTemplate.class)).isNotNull();
+    }
+
+    @Test
+    void commandLineRunnerPopulatedTheCustomersTable() {
+        Integer count = context.getBean(JdbcTemplate.class)
+                .queryForObject("SELECT COUNT(*) FROM customers", Integer.class);
+        assertThat(count).isEqualTo(4);
+    }
+}

src/test/java/hello/controller/GreetingControllerWebTest.java

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+package hello.controller;
+
+import com.jayway.jsonpath.JsonPath;
+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.http.MediaType;
+import org.springframework.test.web.servlet.MockMvc;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.hamcrest.Matchers.hasSize;
+import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.content;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.jsonPath;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;
+
+/**
+ * Pins the JSON contract of {@code /}: {@code {"id": <number>, "content": <string>}}.
+ */
+@SpringBootTest
+@AutoConfigureMockMvc
+class GreetingControllerWebTest {
+
+    @Autowired
+    private MockMvc mockMvc;
+
+    @Test
+    void greetsTheWorldByDefault() throws Exception {
+        mockMvc.perform(get("/"))
+                .andExpect(status().isOk())
+                .andExpect(content().contentTypeCompatibleWith(MediaType.APPLICATION_JSON))
+                .andExpect(jsonPath("$.*", hasSize(2)))
+                .andExpect(jsonPath("$.id").isNumber())
+                .andExpect(jsonPath("$.content").value("Hello, World!"));
+    }
+
+    @Test
+    void greetsTheRequestedName() throws Exception {
+        mockMvc.perform(get("/").param("name", "Devin"))
+                .andExpect(status().isOk())
+                .andExpect(jsonPath("$.content").value("Hello, Devin!"));
+    }
+
+    @Test
+    void everyGreetingGetsItsOwnId() throws Exception {
+        long first = greetingId();
+        long second = greetingId();
+
+        assertThat(second).isEqualTo(first + 1);
+    }
+
+    private long greetingId() throws Exception {
+        String json = mockMvc.perform(get("/"))
+                .andExpect(status().isOk())
+                .andReturn()
+                .getResponse()
+                .getContentAsString();
+        return JsonPath.parse(json).read("$.id", Long.class);
+    }
+}

src/test/java/hello/controller/HelloControllerWebTest.java

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+package hello.controller;
+
+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 static org.hamcrest.Matchers.containsString;
+import static org.hamcrest.Matchers.matchesRegex;
+import static org.hamcrest.Matchers.not;
+import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.content;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;
+
+/**
+ * Covers the plain-text endpoints. {@code /topic/file/operation} walks the process working directory
+ * and reads {@code temp.txt} from it, so its exact output changes whenever a file is added to or
+ * removed from the repository; only the stable parts of the response are asserted.
+ */
+@SpringBootTest
+@AutoConfigureMockMvc
+class HelloControllerWebTest {
+
+    @Autowired
+    private MockMvc mockMvc;
+
+    @Test
+    void datetimeEndpointRendersTheJavaTimeShowcase() throws Exception {
+        mockMvc.perform(get("/datetime"))
+                .andExpect(status().isOk())
+                .andExpect(content().string(containsString("Greetings from Spring Boot!")))
+                .andExpect(content().string(containsString("Datetime now is ")))
+                .andExpect(content().string(containsString("Is this a leap year ?")))
+                .andExpect(content().string(containsString("Default system zone id")))
+                .andExpect(content().string(containsString("Time in California: ")))
+                // "Datetime now is <ISO local date time>"
+                .andExpect(content().string(matchesRegex("(?s).*Datetime now is \\d{4}-\\d{2}-\\d{2}T[\\d:.]+.*")));
+    }
+
+    @Test
+    void stringOperationEndpointRendersTheStreamShowcase() throws Exception {
+        mockMvc.perform(get("/topic/string/operation"))
+                .andExpect(status().isOk())
+                .andExpect(content().string(containsString("Joining All String ID's with JOIN method: ")))
+                // The distinct-and-sorted characters and the ":"-split are both sorted, so they are
+                // stable even if another test has re-ordered the singleton's topic list.
+                .andExpect(content().string(containsString(":acgijnprstv")))
+                .andExpect(content().string(containsString("java:javascript")))
+                .andExpect(content().string(containsString("[spring]")));
+    }
+
+    @Test
+    void fileOperationEndpointRendersTheNioShowcase() throws Exception {
+        mockMvc.perform(get("/topic/file/operation"))
+                .andExpect(status().isOk())
+                .andExpect(content().string(containsString("Find all files in path and sort:")))
+                .andExpect(content().string(containsString("pom.xml")))
+                .andExpect(content().string(containsString("Read \"temp.txt\" file with stream functions")))
+                .andExpect(content().string(not(containsString("Error in IO"))))
+                .andExpect(content().string(not(containsString("IO exception"))));
+    }
+}

src/test/java/hello/controller/TopicControllerWebTest.java

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+package hello.controller;
+
+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.http.MediaType;
+import org.springframework.test.annotation.DirtiesContext;
+import org.springframework.test.web.servlet.MockMvc;
+
+import static org.hamcrest.Matchers.contains;
+import static org.hamcrest.Matchers.containsInAnyOrder;
+import static org.hamcrest.Matchers.hasItem;
+import static org.hamcrest.Matchers.hasSize;
+import static org.hamcrest.Matchers.not;
+import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.delete;
+import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.get;
+import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.post;
+import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.put;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.content;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.jsonPath;
+import static org.springframework.test.web.servlet.result.MockMvcResultMatchers.status;
+
+/**
+ * End-to-end coverage of the JSON contract of the topic endpoints.
+ *
+ * <p>{@code TopicService} is a singleton holding a mutable list, so anything that writes to it leaks
+ * into the next test. Rather than depending on test order, the two tests that mutate the list
+ * ({@code /topic} writes and the in-place sort performed by {@code /topic/sort}) drop the context
+ * afterwards with {@link DirtiesContext}; every other test can then assume the seeded state.
+ *
+ * <p>Assertions go through {@code jsonPath} rather than through model getters on purpose: the JSON
+ * field names are the actual contract and they survive the ongoing conversion of the models to records.
+ */
+@SpringBootTest
+@AutoConfigureMockMvc
+class TopicControllerWebTest {
+
+    @Autowired
+    private MockMvc mockMvc;
+
+    @Test
+    void getAllTopicsReturnsTheSeededTopics() throws Exception {
+        mockMvc.perform(get("/topic"))
+                .andExpect(status().isOk())
+                .andExpect(content().contentTypeCompatibleWith(MediaType.APPLICATION_JSON))
+                .andExpect(jsonPath("$", hasSize(3)))
+                .andExpect(jsonPath("$[*].id", containsInAnyOrder("spring", "java", "javascript")))
+                .andExpect(jsonPath("$[?(@.id=='spring')].subjectName", contains("Spring Framework")))
+                .andExpect(jsonPath("$[?(@.id=='spring')].subjectDescription", contains("Spring Framework Description")));
+    }
+
+    @Test
+    void aTopicSerialisesWithExactlyIdSubjectNameAndSubjectDescription() throws Exception {
+        mockMvc.perform(get("/topic/java"))
+                .andExpect(status().isOk())
+                .andExpect(jsonPath("$.*", hasSize(3)))
+                .andExpect(jsonPath("$.id").value("java"))
+                .andExpect(jsonPath("$.subjectName").value("Core Java"))
+                .andExpect(jsonPath("$.subjectDescription").value("Java Description"));
+    }
+
+    @Test
+    @DirtiesContext(methodMode = DirtiesContext.MethodMode.AFTER_METHOD)
+    void supportsTheFullCreateReadUpdateDeleteLifecycle() throws Exception {
+        mockMvc.perform(post("/topic")
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content("""
+                                {"id":"kotlin","subjectName":"Kotlin","subjectDescription":"Kotlin Description"}"""))
+                .andExpect(status().isOk());
+
+        mockMvc.perform(get("/topic"))
+                .andExpect(status().isOk())
+                .andExpect(jsonPath("$", hasSize(4)))
+                .andExpect(jsonPath("$[*].id", containsInAnyOrder("spring", "java", "javascript", "kotlin")));
+
+        mockMvc.perform(get("/topic/kotlin"))
+                .andExpect(status().isOk())
+                .andExpect(jsonPath("$.subjectName").value("Kotlin"))
+                .andExpect(jsonPath("$.subjectDescription").value("Kotlin Description"));
+
+        mockMvc.perform(put("/topic/kotlin")
+                        .contentType(MediaType.APPLICATION_JSON)
+                        .content("""
+                                {"id":"kotlin","subjectName":"Kotlin Updated","subjectDescription":"Updated Description"}"""))
+                .andExpect(status().isOk());
+
+        mockMvc.perform(get("/topic/kotlin"))
+                .andExpect(status().isOk())
+                .andExpect(jsonPath("$.subjectName").value("Kotlin Updated"))
+                .andExpect(jsonPath("$.subjectDescription").value("Updated Description"));
+
+        mockMvc.perform(delete("/topic/kotlin"))
+                .andExpect(status().isOk());
+
+        mockMvc.perform(get("/topic"))
+                .andExpect(status().isOk())
+                .andExpect(jsonPath("$", hasSize(3)))
+                .andExpect(jsonPath("$[*].id", not(hasItem("kotlin"))));
+    }
+
+    @Test
+    @DirtiesContext(methodMode = DirtiesContext.MethodMode.AFTER_METHOD)
+    void sortEndpointReturnsTopicsOrderedById() throws Exception {
+        mockMvc.perform(get("/topic/sort"))
+                .andExpect(status().isOk())
+                .andExpect(jsonPath("$", hasSize(3)))
+                .andExpect(jsonPath("$[*].id", contains("java", "javascript", "spring")));
+    }
+
+    @Test
+    void minimumLengthEndpointFiltersOnIdLength() throws Exception {
+        mockMvc.perform(get("/topic/minimum/length/4"))
+                .andExpect(status().isOk())
+                .andExpect(jsonPath("$[*].id", containsInAnyOrder("spring", "javascript")));
+
+        mockMvc.perform(get("/topic/minimum/length/6"))
+                .andExpect(status().isOk())
+                .andExpect(jsonPath("$[*].id", contains("javascript")));
+
+        mockMvc.perform(get("/topic/minimum/length/20"))
+                .andExpect(status().isOk())
+                .andExpect(jsonPath("$", hasSize(0)));
+    }
+}

src/test/java/hello/controller/TopicNotFoundStatusTest.java

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+package hello.controller;
+
+import org.junit.jupiter.api.Disabled;
+import org.junit.jupiter.api.Test;
+import org.springframework.beans.factory.annotation.Autowired;
+import org.springframework.boot.test.context.SpringBootTest;
+import org.springframework.boot.test.web.client.TestRestTemplate;
+import org.springframework.http.HttpStatus;
+import org.springframework.http.ResponseEntity;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Requesting an unknown topic id is the one place where the HTTP status carries real information, and
+ * MockMvc rethrows unhandled exceptions instead of running the error dispatch, so this one case is
+ * driven over a real connection.
+ *
+ * <p>{@code TopicService.getTopicWithId} calls {@code Optional.get()} on a miss, which throws
+ * {@link java.util.NoSuchElementException} and surfaces as HTTP 500. That is asserted here as the
+ * <em>current</em> behaviour; the disabled test below is the replacement for when a parallel change
+ * turns the miss into a proper 404.
+ */
+@SpringBootTest(webEnvironment = SpringBootTest.WebEnvironment.RANDOM_PORT)
+class TopicNotFoundStatusTest {
+
+    @Autowired
+    private TestRestTemplate restTemplate;
+
+    @Test
+    void unknownTopicIdCurrentlyReturnsInternalServerError() {
+        ResponseEntity<String> response = restTemplate.getForEntity("/topic/does-not-exist", String.class);
+
+        assertThat(response.getStatusCode()).isEqualTo(HttpStatus.INTERNAL_SERVER_ERROR);
+    }
+
+    @Test
+    @Disabled("Enable, and delete unknownTopicIdCurrentlyReturnsInternalServerError, once an unknown id is a 404")
+    void unknownTopicIdShouldReturnNotFound() {
+        ResponseEntity<String> response = restTemplate.getForEntity("/topic/does-not-exist", String.class);
+
+        assertThat(response.getStatusCode()).isEqualTo(HttpStatus.NOT_FOUND);
+    }
+
+    @Test
+    void knownTopicIdReturnsOkWithTheUnchangedJsonShape() {
+        ResponseEntity<String> response = restTemplate.getForEntity("/topic/spring", String.class);
+
+        assertThat(response.getStatusCode()).isEqualTo(HttpStatus.OK);
+        assertThat(response.getBody())
+                .contains("\"id\":\"spring\"")
+                .contains("\"subjectName\":\"Spring Framework\"")
+                .contains("\"subjectDescription\":\"Spring Framework Description\"");
+    }
+}