李玉妹当选广东省人大常委会主任 马兴瑞当选广东省长

class User {
    private Long id;
    private String name;
    
    @Override
    public boolean equals(Object o) {
        if (this == o) return true;
        if (o == null || getClass() != o.getClass()) return false;
        User user = (User) o;
        return Objects.equals(id, user.id); // 业务唯一性判断
    }
    
    @Override
    public int hashCode() {
        return Objects.hash(id); // 必须与equals保持一致
    }
}

Set<User> users = new HashSet<>();
users.add(new User(1L, "Alice"));
users.add(new User(1L, "Alice")); // 重复元素

System.out.println(users.size()); // 输出：1（正确）
// 如果未重写hashCode，输出可能为2！

public class Singleton {
    private static volatile Singleton instance;
    
    public static Singleton getInstance() {
        if (instance == null) { // 第一次检查
            synchronized (Singleton.class) {
                if (instance == null) { // 第二次检查
                    instance = new Singleton();
                }
            }
        }
        return instance;
    }
}

Map<String, Integer> map = new HashMap<>();
// 模拟hash冲突
for (int i = 0; i < 8; i++) {
    map.put("key" + i, i); 
}

// 查看内部结构
Field tableField = HashMap.class.getDeclaredField("table");
tableField.setAccessible(true);
Object[] table = (Object[]) tableField.get(map);

// 输出：桶中节点类型 = class java.util.HashMap$TreeNode
System.out.println("桶中节点类型 = " + table[0].getClass()); 

class SimpleLock extends AbstractQueuedSynchronizer {
    
    protected boolean tryAcquire(int arg) {
        return compareAndSetState(0, 1); // CAS获取锁
    }
    
    protected boolean tryRelease(int arg) {
        setState(0); // 释放锁
        return true;
    }
}

// 使用示例
SimpleLock lock = new SimpleLock();
lock.acquire(1);
try {
    // 临界区代码
} finally {
    lock.release(1);
}

# 查看线程池状态
jstack <pid> | grep 'pool-1-thread' -A 15

# 典型问题日志：
"pool-1-thread-3" #17 prio=5 os_prio=0 tid=0x00007f487c0e1000 nid=0x6d0b waiting on condition [0x00007f487b4fe000]
   java.lang.Thread.State: WAITING (parking)

[GC pause (G1 Evacuation Pause) (young), 0.0151234 secs]
   [Parallel Time: 14.5 ms, GC Workers: 8]
      [Ext Root Scanning: 1.5 ms]
      [Update RS: 0.2 ms]
      [Scan RS: 0.3 ms]
      [Code Root Scanning: 0.1 ms]
      [Object Copy: 12.2 ms]  # 对象复制耗时最长！
   [Eden: 2048.0M(2048.0M)->0.0B(2048.0M) 
    Survivors: 0.0B->1024.0M 
    Heap: 2048.0M(4096.0M)->1024.0M(4096.0M)]

# 启动参数配置
java -XX:+UseZGC \
     -Xms16g -Xmx16g \
     -XX:ConcGCThreads=4 \   # 并发GC线程数
     -XX:ZAllocationSpikeTolerance=5 \ # 分配尖峰容忍度
     -jar application.jar

@Service
public class OrderService {
    
    @Transactional(propagation = Propagation.REQUIRED)
    public void createOrder(Order order) {
        // 主事务逻辑
        inventoryService.reduceStock(order); // 嵌套事务
    }
}

@Service
public class InventoryService {
    
    @Transactional(propagation = Propagation.REQUIRES_NEW)
    public void reduceStock(Order order) {
        // 独立事务执行，不受外层事务影响
    }
}

// 生产者配置
props.put("acks", "all"); // 所有ISR确认
props.put("retries", Integer.MAX_VALUE);
props.put("enable.idempotence", true); // 幂等性

// 消费者配置
props.put("enable.auto.commit", "false");
while (true) {
    ConsumerRecords<String, String> records = consumer.poll(Duration.ofMillis(100));
    for (ConsumerRecord record : records) {
        try {
            process(record); // 业务处理
            consumer.commitSync(); // 手动提交
        } catch (Exception e) {
            storeFailedRecord(record); // 死信处理
        }
    }
}