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