BAEL-8406: added implementation for testing a spring mvc handler interceptor

spring-boot-modules/spring-boot-mvc-5/src/main/java/com/baeldung/handlerinterceptor/DeliveryChargeInterceptor.java

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+package com.baeldung.handlerinterceptor;
+
+import java.security.SecureRandom;
+
+import org.slf4j.Logger;
+import org.slf4j.LoggerFactory;
+import org.springframework.web.method.HandlerMethod;
+import org.springframework.web.servlet.HandlerInterceptor;
+
+import org.springframework.lang.NonNull;
+
+import jakarta.servlet.http.HttpServletRequest;
+import jakarta.servlet.http.HttpServletResponse;
+
+public class DeliveryChargeInterceptor implements HandlerInterceptor {
+
+    static final String USE_V2_ATTRIBUTE = "useV2";
+
+    private final FeatureFlagService featureFlagService;
+    private final SecureRandom random = new SecureRandom();
+    private final Logger logger = LoggerFactory.getLogger(DeliveryChargeInterceptor.class);
+
+    public DeliveryChargeInterceptor(FeatureFlagService featureFlagService) {
+        this.featureFlagService = featureFlagService;
+    }
+
+    @Override
+    public boolean preHandle(@NonNull HttpServletRequest request, @NonNull HttpServletResponse response, @NonNull Object handler) {
+        if (handler instanceof HandlerMethod) {
+            int rollout = featureFlagService.rolloutPercentage();
+            boolean useV2 = rollout > 0 && random.nextInt(100) < rollout;
+            request.setAttribute(USE_V2_ATTRIBUTE, useV2);
+            logger.info("Delivery charge feature: rollout={}%, useV2={}", rollout, useV2);
+        }
+        return true;
+    }
+}

spring-boot-modules/spring-boot-mvc-5/src/main/java/com/baeldung/handlerinterceptor/DeliveryChargeService.java

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+package com.baeldung.handlerinterceptor;
+
+public interface DeliveryChargeService {
+
+    double calculateV1(String postCode);
+
+    double calculateV2(String postCode);
+
+}

spring-boot-modules/spring-boot-mvc-5/src/main/java/com/baeldung/handlerinterceptor/DeliveryChargesController.java

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+package com.baeldung.handlerinterceptor;
+
+import org.springframework.web.bind.annotation.PostMapping;
+import org.springframework.web.bind.annotation.RequestParam;
+import org.springframework.web.bind.annotation.RestController;
+
+import jakarta.servlet.http.HttpServletRequest;
+
+@RestController
+public class DeliveryChargesController {
+
+    private final DeliveryChargeService deliveryChargeService;
+
+    public DeliveryChargesController(DeliveryChargeService deliveryChargeService) {
+        this.deliveryChargeService = deliveryChargeService;
+    }
+
+    @PostMapping("/delivery-charges/calculate")
+    public double calculate(@RequestParam String postcode, HttpServletRequest request) {
+        Boolean useV2 = (Boolean) request.getAttribute(DeliveryChargeInterceptor.USE_V2_ATTRIBUTE);
+        if (Boolean.TRUE.equals(useV2)) {
+            return deliveryChargeService.calculateV2(postcode);
+        }
+        return deliveryChargeService.calculateV1(postcode);
+    }
+}

spring-boot-modules/spring-boot-mvc-5/src/main/java/com/baeldung/handlerinterceptor/FeatureFlagService.java

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+package com.baeldung.handlerinterceptor;
+
+public interface FeatureFlagService {
+    int rolloutPercentage();
+}

spring-boot-modules/spring-boot-mvc-5/src/main/java/com/baeldung/handlerinterceptor/HandlerInterceptorApp.java

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+package com.baeldung.handlerinterceptor;
+
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+
+@SpringBootApplication(scanBasePackages = "com.baeldung.handlerinterceptor")
+public class HandlerInterceptorApp {
+    public static void main(String[] args) {
+        SpringApplication.run(HandlerInterceptorApp.class, args);
+    }
+}

spring-boot-modules/spring-boot-mvc-5/src/main/java/com/baeldung/handlerinterceptor/WebMvcConfig.java

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+package com.baeldung.handlerinterceptor;
+
+import org.springframework.context.annotation.Configuration;
+import org.springframework.web.servlet.config.annotation.InterceptorRegistry;
+import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;
+
+@Configuration
+public class WebMvcConfig implements WebMvcConfigurer {
+
+    private final FeatureFlagService featureFlagService;
+
+    public WebMvcConfig(FeatureFlagService featureFlagService) {
+        this.featureFlagService = featureFlagService;
+    }
+
+    @Override
+    public void addInterceptors(InterceptorRegistry registry) {
+        registry.addInterceptor(new DeliveryChargeInterceptor(featureFlagService))
+            .addPathPatterns("/delivery-charges/**");
+    }
+}

spring-boot-modules/spring-boot-mvc-5/src/test/java/com/baeldung/handlerinterceptor/DeliveryChargeInterceptorUnitTest.java

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+package com.baeldung.handlerinterceptor;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.Mockito.when;
+import static org.springframework.test.web.servlet.request.MockMvcRequestBuilders.post;
+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.WebMvcTest;
+import org.springframework.boot.test.mock.mockito.MockBean;
+import org.springframework.test.web.servlet.MockMvc;
+
+@WebMvcTest(controllers = DeliveryChargesController.class)
+class DeliveryChargeInterceptorUnitTest {
+
+    @Autowired
+    private MockMvc mockMvc;
+
+    @MockBean
+    private DeliveryChargeService deliveryChargeService;
+
+    @MockBean
+    private FeatureFlagService featureFlagService;
+
+    @Test
+    void givenZeroRollout_whenCalculateDeliveryCharge_thenV1IsUsed() throws Exception {
+        when(featureFlagService.rolloutPercentage()).thenReturn(0);
+        when(deliveryChargeService.calculateV1("SW1A")).thenReturn(5.0);
+
+        mockMvc.perform(post("/delivery-charges/calculate").param("postcode", "SW1A"))
+            .andExpect(status().isOk())
+            .andExpect(content().string("5.0"));
+    }
+
+    @Test
+    void givenFullRollout_whenCalculateDeliveryCharge_thenV2IsUsed() throws Exception {
+        when(featureFlagService.rolloutPercentage()).thenReturn(100);
+        when(deliveryChargeService.calculateV2("SW1A")).thenReturn(3.5);
+
+        mockMvc.perform(post("/delivery-charges/calculate").param("postcode", "SW1A"))
+            .andExpect(status().isOk())
+            .andExpect(content().string("3.5"));
+    }
+
+    @Test
+    void givenPartialRollout_whenCalculateDeliveryCharge_thenBothVersionsAreUsed() throws Exception {
+        when(featureFlagService.rolloutPercentage()).thenReturn(50);
+        when(deliveryChargeService.calculateV1("SW1A")).thenReturn(5.0);
+        when(deliveryChargeService.calculateV2("SW1A")).thenReturn(3.5);
+
+        int v1Count = 0;
+        int v2Count = 0;
+
+        for (int i = 0; i < 20; i++) {
+            String response = mockMvc.perform(post("/delivery-charges/calculate").param("postcode", "SW1A"))
+                .andExpect(status().isOk())
+                .andReturn()
+                .getResponse()
+                .getContentAsString();
+            if ("5.0".equals(response)) {
+                v1Count++;
+            } else if ("3.5".equals(response)) {
+                v2Count++;
+            }
+        }
+
+        assertThat(v1Count).isGreaterThan(0);
+        assertThat(v2Count).isGreaterThan(0);
+    }
+}