ThreadLocal的正确用法

用法一：在关联数据类中创建private static ThreadLocal
ThreaLocal的JDK文档中说明：ThreadLocal instances are typically private static fields in classes that wish to associate state with a thread。如果我们希望通过某个类将状态（例如用户ID、事务ID）与线程关联起来，那么通常在这个类中定义private static类型的ThreadLocal 实例。

例如，在下面的类中，私有静态 ThreadLocal 实例（serialNum）为调用该类的静态 SerialNum.get() 方法的每个线程维护了一个“序列号”，该方法将返回当前线程的序列号。（线程的序列号是在第一次调用 SerialNum.get() 时分配的，并在后续调用中不会更改。）

[java] view plaincopyprint?
01.public class SerialNum { 
02.    // The next serial number to be assigned  
03.    private static int nextSerialNum = 0; 
04. 
05.    private static ThreadLocal serialNum = new ThreadLocal() { 
06.        protected synchronized Object initialValue() { 
07.            return new Integer(nextSerialNum++); 
08.        } 
09.    }; 
10. 
11.    public static int get() { 
12.        return ((Integer) (serialNum.get())).intValue(); 
13.    } 
14.} 
 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();
     }
 }

【例】

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;
  }
 
}

用法二：在Util类中创建ThreadLocal
这是上面用法的扩展，即把ThreadLocal的创建放到工具类中。

【例】例如Hibernate的工具类：

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();
    }
}

用法三：在Runnable中创建ThreadLocal
 还有一种用法是在线程类内部创建ThreadLocal，基本步骤如下：

1、在多线程的类（如ThreadDemo类）中，创建一个ThreadLocal对象threadXxx，用来保存线程间需要隔离处理的对象xxx。
2、在ThreadDemo类中，创建一个获取要隔离访问的数据的方法getXxx()，在方法中判断，若ThreadLocal对象为null时候，应该new()一个隔离访问类型的对象，并强制转换为要应用的类型。
3、在ThreadDemo类的run()方法中，通过调用getXxx()方法获取要操作的数据，这样可以保证每个线程对应一个数据对象，在任何时刻都操作的是这个对象。 
 
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;
    }
   
}