Add boot42-preview module: Spring Boot 4.2.0-M2 vs 4.1.1 forwarded headers, AMQP starters, buildpack cache, RestTemplate removal warnings

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Gth99spB1CzQmBFofXmU4Z

boot42-preview/src/main/java/com/ankurm/boot42/Boot42PreviewApplication.java

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+package com.ankurm.boot42;
+
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+
+/** Minimal application. Every experiment in this module lives in src/test. */
+@SpringBootApplication
+@org.springframework.boot.context.properties.EnableConfigurationProperties(OrderProperties.class)
+public class Boot42PreviewApplication {
+}

boot42-preview/src/main/java/com/ankurm/boot42/OrderProperties.java

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+package com.ankurm.boot42;
+
+import java.util.Map;
+import java.util.Set;
+import org.springframework.boot.context.properties.ConfigurationProperties;
+
+/** A Set and a Map bound from a file where the keys are deliberately not alphabetical. */
+@ConfigurationProperties("app")
+public record OrderProperties(Set<String> regions, Map<String, Integer> limits) {
+}

boot42-preview/src/main/java/com/ankurm/boot42/RestClientReplacement.java

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+package com.ankurm.boot42;
+
+import org.springframework.context.annotation.Bean;
+import org.springframework.context.annotation.Configuration;
+import org.springframework.web.client.RestClient;
+
+/** What the deprecated RestTemplateBuilder code turns into: an injected RestClient.Builder. */
+@Configuration(proxyBeanMethods = false)
+class RestClientReplacement {
+
+	@Bean
+	RestClient inventoryClient(RestClient.Builder builder) {
+		return builder.baseUrl("http://inventory.internal").build();
+	}
+}

boot42-preview/src/main/java/com/ankurm/boot42/WhoAmIController.java

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+package com.ankurm.boot42;
+
+import jakarta.servlet.http.HttpServletRequest;
+import java.util.LinkedHashMap;
+import java.util.Map;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.RestController;
+
+/** Echoes what the servlet container believes about the request, after any forwarded-header handling. */
+@RestController
+class WhoAmIController {
+
+	@GetMapping("/whoami")
+	Map<String, Object> whoAmI(HttpServletRequest request) {
+		Map<String, Object> m = new LinkedHashMap<>();
+		m.put("scheme", request.getScheme());
+		m.put("secure", request.isSecure());
+		m.put("serverName", request.getServerName());
+		m.put("serverPort", request.getServerPort());
+		m.put("remoteAddr", request.getRemoteAddr());
+		return m;
+	}
+}