java 中ThreadLocal 的正确用法

public class SerialNum { 
  // The next serial number to be assigned 
  private static int nextSerialNum = 0; 
 
  private static ThreadLocal serialNum = new ThreadLocal() { 
    protected synchronized Object initialValue() { 
      return new Integer(nextSerialNum++); 
    } 
  }; 
 
  public static int get() { 
    return ((Integer) (serialNum.get())).intValue(); 
  } 
} 

public class ThreadContext {
 
 private String userId;
 private Long transactionId;
 
 private static ThreadLocal threadLocal = new ThreadLocal(){
  @Override
    protected ThreadContext initialValue() {
      return new ThreadContext();
    }
 
 };
 public static ThreadContext get() {
  return threadLocal.get();
 }

 public String getUserId() {
  return userId;
 }
 public void setUserId(String userId) {
  this.userId = userId;
 }
 public Long getTransactionId() {
  return transactionId;
 }
 public void setTransactionId(Long transactionId) {
  this.transactionId = transactionId;
 }
 
}

public class HibernateUtil {
  private static Log log = LogFactory.getLog(HibernateUtil.class);
  private static final SessionFactory sessionFactory;   //定义SessionFactory
 
  static {
    try {
      // 通过默认配置文件hibernate.cfg.xml创建SessionFactory
      sessionFactory = new Configuration().configure().buildSessionFactory();
    } catch (Throwable ex) {
      log.error("初始化SessionFactory失败！", ex);
      throw new ExceptionInInitializerError(ex);
    }
  }

  //创建线程局部变量session，用来保存Hibernate的Session
  public static final ThreadLocal session = new ThreadLocal();
 
  /**
   * 获取当前线程中的Session
   * @return Session
   * @throws HibernateException
   */
  public static Session currentSession() throws HibernateException {
    Session s = (Session) session.get();
    // 如果Session还没有打开，则新开一个Session
    if (s == null) {
      s = sessionFactory.openSession();
      session.set(s);     //将新开的Session保存到线程局部变量中
    }
    return s;
  }
 
  public static void closeSession() throws HibernateException {
    //获取线程局部变量，并强制转换为Session类型
    Session s = (Session) session.get();
    session.set(null);
    if (s != null)
      s.close();
  }
}

public class ThreadLocalTest implements Runnable{
  
  ThreadLocal<Studen> studenThreadLocal = new ThreadLocal<Studen>();

  @Override
  public void run() {
    String currentThreadName = Thread.currentThread().getName();
    System.out.println(currentThreadName + " is running...");
    Random random = new Random();
    int age = random.nextInt(100);
    System.out.println(currentThreadName + " is set age: " + age);
    Studen studen = getStudent(); //通过这个方法，为每个线程都独立的new一个student对象，每个线程的的student对象都可以设置不同的值
    studen.setAge(age);
    System.out.println(currentThreadName + " is first get age: " + studen.getAge());
    try {
      Thread.sleep(500);
    } catch (InterruptedException e) {
      e.printStackTrace();
    }
    System.out.println( currentThreadName + " is second get age: " + studen.getAge());
    
  }
  
  private Studen getStudent() {
    Studen studen = studenThreadLocal.get();
    if (null == studen) {
      studen = new Studen();
      studenThreadLocal.set(studen);
    }
    return studen;
  }

  public static void main(String[] args) {
    ThreadLocalTest t = new ThreadLocalTest();
    Thread t1 = new Thread(t,"Thread A");
    Thread t2 = new Thread(t,"Thread B");
    t1.start();
    t2.start();
  }
  
}

class Studen{
  int age;
  public int getAge() {
    return age;
  }
  public void setAge(int age) {
    this.age = age;
  }
  
}