phase 5 part 4: java-spring awareness, k8s basics with validated manifests, run_examples supports single-file Java

topics/backend/java-spring/examples/java-basics/OrderDemo.java

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+// Java 基础对照示例（不依赖 Spring，只用 JDK）：record、接口 + 构造器注入、Optional、Stream、异常、并发
+// 运行（JDK 21+）：java OrderDemo.java      —— 单文件源码直接运行，不需要先 javac
+import java.math.BigDecimal;
+import java.util.List;
+import java.util.Map;
+import java.util.Optional;
+import java.util.concurrent.ConcurrentHashMap;
+import java.util.concurrent.ExecutorService;
+import java.util.concurrent.Executors;
+import java.util.concurrent.atomic.AtomicInteger;
+import java.util.stream.Collectors;
+
+public class OrderDemo {
+    // record：不可变数据类，自动生成构造器、getter、equals/hashCode/toString（类似 Go struct / Python dataclass(frozen)）
+    record Order(String id, String tenant, String status, BigDecimal amount) {}
+
+    // 接口 + 实现：Spring 中会用 @Repository 标注，由容器创建并注入
+    interface OrderRepository {
+        Optional<Order> findById(String id);
+        List<Order> findByTenant(String tenant);
+    }
+
+    static class InMemoryOrderRepository implements OrderRepository {
+        private final Map<String, Order> data = new ConcurrentHashMap<>();
+        InMemoryOrderRepository(List<Order> seed) { seed.forEach(o -> data.put(o.id(), o)); }
+        public Optional<Order> findById(String id) { return Optional.ofNullable(data.get(id)); }
+        public List<Order> findByTenant(String tenant) {
+            return data.values().stream().filter(o -> o.tenant().equals(tenant)).toList();
+        }
+    }
+
+    static class NotFoundException extends RuntimeException {
+        NotFoundException(String msg) { super(msg); }
+    }
+
+    // 构造器注入：Spring 推荐的方式（@Service），依赖是 final 的，测试时可以直接传入假实现
+    static class OrderService {
+        private final OrderRepository repo;
+        OrderService(OrderRepository repo) { this.repo = repo; }
+
+        Order get(String tenant, String id) {
+            return repo.findById(id)
+                    .filter(o -> o.tenant().equals(tenant))            // 租户隔离：别的租户的订单当作不存在
+                    .orElseThrow(() -> new NotFoundException("订单不存在: " + id));
+        }
+
+        Map<String, BigDecimal> sumByStatus(String tenant) {           // Stream 分组汇总；金额用 BigDecimal，不用 double
+            return repo.findByTenant(tenant).stream()
+                    .collect(Collectors.groupingBy(Order::status,
+                            Collectors.reducing(BigDecimal.ZERO, Order::amount, BigDecimal::add)));
+        }
+    }
+
+    public static void main(String[] args) throws Exception {
+        var repo = new InMemoryOrderRepository(List.of(
+                new Order("A001", "t1", "paid", new BigDecimal("19.90")),
+                new Order("A002", "t1", "paid", new BigDecimal("0.10")),
+                new Order("A003", "t1", "refunded", new BigDecimal("5.00")),
+                new Order("B001", "t2", "paid", new BigDecimal("99.00"))));
+        var service = new OrderService(repo);
+
+        check(service.get("t1", "A001").amount().equals(new BigDecimal("19.90")), "查询订单");
+        try {
+            service.get("t1", "B001");
+            check(false, "跨租户应当查不到");
+        } catch (NotFoundException e) {
+            System.out.println("跨租户查询被拒绝: " + e.getMessage());
+        }
+        var sums = service.sumByStatus("t1");
+        check(sums.get("paid").equals(new BigDecimal("20.00")), "BigDecimal 精确求和");
+        System.out.println("t1 按状态汇总: " + new java.util.TreeMap<>(sums));
+        System.out.println("用 double 计算金额的问题: 0.1 + 0.2 = " + (0.1 + 0.2));
+
+        // 并发：虚拟线程（JDK 21 正式特性）+ 原子计数，类比 Go 的 goroutine
+        var counter = new AtomicInteger();
+        try (ExecutorService pool = Executors.newVirtualThreadPerTaskExecutor()) {
+            for (int i = 0; i < 1000; i++) pool.submit(counter::incrementAndGet);
+        }                                                               // try-with-resources：等待全部任务完成
+        check(counter.get() == 1000, "虚拟线程并发计数");
+        System.out.println("1000 个虚拟线程计数结果: " + counter.get());
+        System.out.println("全部检查通过");
+    }
+
+    static void check(boolean ok, String name) {
+        if (!ok) throw new AssertionError("检查失败: " + name);
+    }
+}