Java 核心知识
Java 现代特性（17+）
// Records (immutable data classes)
public record User(Long id, String name, String email) {}
// Sealed classes
public sealed interface Shape permits Circle, Rectangle {}
public final class Circle implements Shape { }
public final class Rectangle implements Shape { }
// Pattern matching for instanceof
if (obj instanceof String s) {
System.out.println(s.toUpperCase());
}
// Switch expressions
String result = switch (status) {
case ACTIVE -> "Active";
case INACTIVE -> "Inactive";
default -> "Unknown";
};
// Text blocks
String json = """
{
"name": "John",
"age": 30
}
""";
集合与流
// Stream operations
List<String> names = users.stream()
.filter(u -> u.isActive())
.map(User::getName)
.sorted()
.collect(Collectors.toList());
// Grouping
Map<Status, List<User>> byStatus = users.stream()
.collect(Collectors.groupingBy(User::getStatus));
// Optional handling
Optional<User> user = findById(id);
String name = user.map(User::getName).orElse("Unknown");
常见模式
// Builder pattern
User user = User.builder()
.name("John")
.email("[email protected]")
.build();
// Factory method
public static User of(String name, String email) {
return new User(null, name, email);
}
// Dependency Injection (constructor)
@Service
public class UserService {
private final UserRepository repository;
public UserService(UserRepository repository) {
this.repository = repository;
}
}
异常处理
// Custom exception
public class UserNotFoundException extends RuntimeException {
public UserNotFoundException(Long id) {
super("User not found: " + id);
}
}
// Try-with-resources
try (var reader = new BufferedReader(new FileReader(file))) {
return reader.lines().collect(Collectors.toList());
}
静态分析与代码检查
官方规则参考
风格指南
主要规则类别
<plugin>
<groupId>org.apache.maven.plugins</groupId>
<artifactId>maven-checkstyle-plugin</artifactId>
<version>3.3.1</version>
<configuration>
<configLocation>google_checks.xml</configLocation>
</configuration>
</plugin>
建议启用的关键规则
| 类别 | 规则 | 工具 |
|---|
| Bug | NullPointerException 风险 | SonarJava S2259 |
| Bug | 资源泄漏 | SonarJava S2095 |
| 安全 | SQL 注入 | SonarJava S3649 |
| 安全 | 硬编码凭据 | SonarJava S2068 |
| 可维护性 | 认知复杂度 | SonarJava S3776 |
| 可维护性 | 参数过多 | Checkstyle |
生产就绪性
错误处理
// Custom exception hierarchy
public abstract class BaseException extends RuntimeException {
private final String errorCode;
private final int httpStatus;
protected BaseException(String message, String errorCode, int httpStatus) {
super(message);
this.errorCode = errorCode;
this.httpStatus = httpStatus;
}
public String getErrorCode() { return errorCode; }
public int getHttpStatus() { return httpStatus; }
}
public class EntityNotFoundException extends BaseException {
public EntityNotFoundException(String entity, Object id) {
super(
String.format("%s not found with id: %s", entity, id),
"NOT_FOUND",
404
);
}
}
// Global exception handler
@RestControllerAdvice
public class GlobalExceptionHandler {
private static final Logger log = LoggerFactory.getLogger(GlobalExceptionHandler.class);
@ExceptionHandler(BaseException.class)
public ResponseEntity<ErrorResponse> handleBaseException(BaseException ex) {
log.warn("Business error: {}", ex.getMessage());
return ResponseEntity
.status(ex.getHttpStatus())
.body(new ErrorResponse(ex.getErrorCode(), ex.getMessage()));
}
@ExceptionHandler(Exception.class)
public ResponseEntity<ErrorResponse> handleUnexpected(Exception ex) {
log.error("Unexpected error", ex);
return ResponseEntity
.status(500)
.body(new ErrorResponse("INTERNAL_ERROR", "An unexpected error occurred"));
}
}
空值安全
// Use Optional properly
public Optional<User> findById(Long id) {
return repository.findById(id);
}
// Never return null from Optional
public User getById(Long id) {
return findById(id)
.orElseThrow(() -> new EntityNotFoundException("User", id));
}
// Use @Nullable and @NonNull annotations
import org.springframework.lang.NonNull;
import org.springframework.lang.Nullable;
public User updateUser(@NonNull Long id, @Nullable String email) {
User user = getById(id);
if (email != null) {
user.setEmail(email);
}
return repository.save(user);
}
// Validation
import jakarta.validation.constraints.*;
public record CreateUserRequest(
@NotBlank @Size(min = 2, max = 100) String name,
@NotBlank @Email String email,
@NotNull @Min(0) Integer age
) {}
日志记录
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import org.slf4j.MDC;
@Service
public class UserService {
private static final Logger log = LoggerFactory.getLogger(UserService.class);
public User createUser(CreateUserRequest request) {
MDC.put("operation", "createUser");
MDC.put("email", request.email());
log.info("Creating user");
try {
User user = userRepository.save(User.from(request));
log.info("User created successfully: {}", user.getId());
return user;
} catch (Exception e) {
log.error("Failed to create user", e);
throw e;
} finally {
MDC.clear();
}
}
}
测试
// Unit test with Mockito
@ExtendWith(MockitoExtension.class)
class UserServiceTest {
@Mock
private UserRepository repository;
@InjectMocks
private UserService service;
@Test
void shouldCreateUser() {
// Given
var request = new CreateUserRequest("John", "[email protected]", 30);
var expected = new User(1L, "John", "[email protected]", 30);
when(repository.save(any())).thenReturn(expected);
// When
var result = service.createUser(request);
// Then
assertThat(result).isEqualTo(expected);
verify(repository).save(any());
}
@Test
void shouldThrowWhenUserNotFound() {
// Given
when(repository.findById(1L)).thenReturn(Optional.empty());
// When/Then
assertThatThrownBy(() -> service.getById(1L))
.isInstanceOf(EntityNotFoundException.class)
.hasMessageContaining("User not found");
}
}
// Integration test
@SpringBootTest
@AutoConfigureMockMvc
@Testcontainers
class UserControllerIT {
@Container
static PostgreSQLContainer<?> postgres = new PostgreSQLContainer<>("postgres:16-alpine");
@Autowired
private MockMvc mockMvc;
@Test
void shouldCreateUser() throws Exception {
mockMvc.perform(post("/api/users")
.contentType(MediaType.APPLICATION_JSON)
.content("""
{"name": "John", "email": "[email protected]", "age": 30}
"""))
.andExpect(status().isCreated())
.andExpect(jsonPath("$.id").exists());
}
}
性能
// Connection pooling (HikariCP)
// application.yml
spring:
datasource:
hikari:
maximum-pool-size: 10
minimum-idle: 5
connection-timeout: 30000
idle-timeout: 600000
// Caching
@Cacheable(value = "users", key = "#id")
public User findById(Long id) {
return repository.findById(id).orElseThrow();
}
@CacheEvict(value = "users", key = "#user.id")
public User save(User user) {
return repository.save(user);
}
监控指标
| 指标 | 目标 |
|---|
| 测试覆盖率 | > 80% |
| 圈复杂度 | < 10 |
| SonarQube Bug 数 | 0 |
| 安全漏洞数 | 0 |
检查清单
不适合使用此技能的情况
| 场景 | 改用 |
|---|
| Spring Boot 框架 | backend-spring-boot skill |
| Lombok 注解 | lombok skill |
| MapStruct 映射 | mapstruct skill |
| JPA/Hibernate | ORM 专用技能 |
| 使用 JUnit 进行测试 | testing-junit skill |
反模式
| 反模式 | 不良影响 | 正确做法 |
|---|
| 返回 null | NullPointerException | 使用 Optional |
| 捕获通用 Exception | 隐藏具体错误 | 捕获具体异常 |
| 不关闭资源 | 资源泄漏 | 使用 try-with-resources |
| 可变静态字段 | 线程安全问题 | 使用不可变对象或同步机制 |
| 在循环中拼接字符串 | O(n²) 性能开销 | 使用 StringBuilder |
| 空 catch 块 | 静默失败 | 记录日志或重新抛出 |
对字符串使用 == | 比较引用 | 使用 .equals() |
| 不重写 equals/hashCode | 集合行为异常 | 两者都重写，或都不重写 |
快速故障排除
| 问题 | 原因 | 解决方案 |
|---|
| NullPointerException | 访问空引用 | 检查是否为空，使用 Optional |
| ClassCastException | 类型转换错误 | 转换前使用 instanceof |
| ConcurrentModificationException | 迭代时修改集合 | 使用 Iterator.remove() |
| OutOfMemoryError | 堆内存耗尽 | 增大堆内存，修复内存泄漏 |
| StackOverflowError | 无限递归 | 添加基例，或改用迭代 |
| "Cannot find symbol" | 编译错误 | 检查导入语句和拼写 |
| 死锁 | 锁依赖形成循环 | 采用一致的加锁顺序 |
| 资源泄漏 | 未关闭流 | 使用 try-with-resources |
参考文档
