面试官视角：初中级Java开发者必备核心技能与完美应答指南0628

// 基本数据类型示例
int num = 42;                    // 基本类型
Integer boxedNum = num;          // 自动装箱
int unboxedNum = boxedNum;       // 自动拆箱

// 典型使用场景
List<Integer> numbers = new ArrayList<>();  // 泛型集合必须使用包装类
numbers.add(1);                  // 自动装箱
int first = numbers.get(0);      // 自动拆箱

-- INNER JOIN: 仅返回匹配行
SELECT users.name, orders.amount
FROM users
INNER JOIN orders ON users.id = orders.user_id;

-- LEFT JOIN: 返回左表所有行
SELECT users.name, orders.amount
FROM users
LEFT JOIN orders ON users.id = orders.user_id;

// 安全的使用方式
@Select("SELECT * FROM users WHERE name = #{name}")
User findByName(@Param("name") String name);

// 不安全的动态SQL示例(仅用于演示，不要在生产环境使用)
@Select("SELECT * FROM ${table} WHERE id = ${id}")
User findById(@Param("table") String table, @Param("id") Long id);

@Configuration
@ConditionalOnClass(MyService.class)
@EnableConfigurationProperties(MyProperties.class)
public class MyAutoConfiguration {
    
    @Bean
    @ConditionalOnMissingBean
    public MyService myService(MyProperties properties) {
        return new MyService(properties);
    }
}

// synchronized用法
public synchronized void syncMethod() {
    // 临界区代码
}

private final Object lock = new Object();
public void syncBlock() {
    synchronized(lock) {
        // 临界区代码
    }
}

// ReentrantLock用法
private final ReentrantLock reentrantLock = new ReentrantLock();
public void lockMethod() {
    reentrantLock.lock();
    try {
        // 临界区代码
    } finally {
        reentrantLock.unlock();
    }
}

public class PaymentService {
    
    @Transactional
    public void handleCallback(PaymentCallback callback) {
        // 1. 检查幂等性
        if (paymentLogRepository.existsByPaymentNo(callback.getPaymentNo())) {
            return; // 已处理过
        }
        
        // 2. 创建处理记录(唯一约束保证幂等)
        PaymentLog log = new PaymentLog(callback);
        try {
            paymentLogRepository.save(log);
        } catch (DataIntegrityViolationException e) {
            throw new IdempotentException("Duplicate payment");
        }
        
        // 3. 处理支付逻辑
        processPayment(callback);
    }
}