Springboot project structure with idempotency logic

.gitignore

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+target/
+.env
+.ideal
+*.iml
+.vscode/
+.DS_Store

pom.xml

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+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+
+    <groupId>com.igirepay</groupId>
+    <artifactId>idempotency-gateway</artifactId>
+    <version>1.0.0</version>
+    <packaging>jar</packaging>
+
+    <!-- This makes it a Spring Boot project -->
+    <parent>
+        <groupId>org.springframework.boot</groupId>
+        <artifactId>spring-boot-starter-parent</artifactId>
+        <version>3.2.5</version>
+        <relativePath/>
+    </parent>
+
+    <properties>
+        <java.version>17</java.version>
+    </properties>
+
+    <dependencies>
+        <!-- Gives us REST API capability (like Flask in Python) -->
+        <dependency>
+            <groupId>org.springframework.boot</groupId>
+            <artifactId>spring-boot-starter-web</artifactId>
+        </dependency>
+    </dependencies>
+
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.springframework.boot</groupId>
+                <artifactId>spring-boot-maven-plugin</artifactId>
+            </plugin>
+        </plugins>
+    </build>
+
+</project>

src/main/java/com/igirepay/Application.java

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+package com.igirepay;
+import org.springframework.boot.SpringApplication;
+import org.springframework.boot.autoconfigure.SpringBootApplication;
+
+
+@SpringBootApplication
+public class Application {
+    public static void main(String[] args) {
+        SpringApplication.run(Application.class, args);
+    }
+
+}

src/main/java/com/igirepay/controller/PaymentController.java

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+package com.igirepay.controller;
+
+import org.springframework.http.HttpStatus;
+import org.springframework.http.ResponseEntity;
+import org.springframework.web.bind.annotation.*;
+
+import java.util.Map;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.locks.ReentrantLock;
+
+@RestController
+@RequestMapping("/api")
+public class PaymentController {
+    // Stores: idempotency_key -> saved response
+    private final ConcurrentHashMap<String, ResponseEntity<Map<String, Object>>> responseCache = new ConcurrentHashMap<>();
+
+    // Stores: idempotency_key -> original request body (for conflict detection)
+    private final ConcurrentHashMap<String, Map<String, Object>> requestCache = new ConcurrentHashMap<>();
+
+    // Stores: idempotency_key -> lock (for race condition handling)
+    private final ConcurrentHashMap<String, ReentrantLock> lockMap = new ConcurrentHashMap<>();
+
+    @PostMapping("/process-payment")
+    public ResponseEntity<Map<String, Object>> processPayment(
+            @RequestHeader(value = "Idempotency-key", required = false) String idempotencyKey,
+            @RequestBody Map<String, Object> requestBody) throws InterruptedException {
+
+         //  GUARD: Missing Idempotency-Key header
+        if (idempotencyKey == null || idempotencyKey.isBlank()) {
+            return ResponseEntity
+            .status(HttpStatus.BAD_REQUEST)
+            .body(Map.of("Error", "Missing Idempotency-key header"));
+        }
+
+        // Get or create a lock for this key
+        lockMap.putIfAbsent(idempotencyKey, new ReentrantLock());
+        ReentrantLock lock = lockMap.get(idempotencyKey);
+
+        // Acquire the lock — blocks duplicate requests until we're done
+        lock.lock();
+
+        try {
+            // Case 1: Key already exists
+            if(responseCache.containsKey(idempotencyKey)){
+                //Check if request body is different
+                Map<String, Object> originalBody = requestCache.get(idempotencyKey);
+                if(!originalBody.equals(requestBody)) {
+                    return ResponseEntity
+                        .status(HttpStatus.CONFLICT)
+                        .body(Map.of("error", "Idempotency key already used for different request body"));
+                }
+
+                //same body return cached response
+                ResponseEntity<Map<String,Object>> cached = responseCache.get(idempotencyKey);
+                return ResponseEntity
+                    .status(cached.getStatusCode())
+                    .header("X-Cache-Hit", "true")
+                    .body(cached.getBody());
+            }
+
+            // Case 2: New request , process it
+            // save the request body first
+            requestCache.put(idempotencyKey, requestBody);
+
+            Thread.sleep(2000);
+
+            // EXtract amount and currency from request body
+            Object amount = requestBody.get("amount");
+            Object currency = requestBody.get("currency");
+
+            // Build the response
+            Map<String, Object> responseBody = Map.of(
+                "status", "success",
+                "message", String.format("Charged %s %s", amount, currency),
+                "idempotencyKey", idempotencyKey
+            );
+
+            ResponseEntity<Map<String, Object>> response = ResponseEntity
+                .status(HttpStatus.CREATED)
+                .body(responseBody);
+            responseCache.put(idempotencyKey, response);
+
+            return response;
+        } finally {
+            lock.unlock();
+        }
+    }
+}

src/main/resources/application.properties

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+server.port=8080