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java 中ThreadLocal 的正確用法

更新時(shí)間：2017年03月27日 09:13:24 投稿：lqh

這篇文章主要介紹了java 中ThreadLocal 的正確用法的相關(guān)資料,需要的朋友可以參考下

java 中ThreadLocal 的正確用法

用法一：在關(guān)聯(lián)數(shù)據(jù)類中創(chuàng)建private static ThreadLocalThreaLocal的JDK文檔中說明：ThreadLocal instances are typically private static fields in classes that wish to associate state with a thread。如果我們希望通過某個(gè)類將狀態(tài)（例如用戶ID、事務(wù)ID）與線程關(guān)聯(lián)起來，那么通常在這個(gè)類中定義private static類型的ThreadLocal 實(shí)例。

例如，在下面的類中，私有靜態(tài) ThreadLocal 實(shí)例（serialNum）為調(diào)用該類的靜態(tài) SerialNum.get() 方法的每個(gè)線程維護(hù)了一個(gè)“序列號(hào)”，該方法將返回當(dāng)前線程的序列號(hào)。（線程的序列號(hào)是在第一次調(diào)用 SerialNum.get() 時(shí)分配的，并在后續(xù)調(diào)用中不會(huì)更改。）

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類中創(chuàng)建ThreadLocal

這是上面用法的擴(kuò)展，即把ThreadLocal的創(chuàng)建放到工具類中。

【例】例如hibernate的工具類：

public class HibernateUtil {
  private static Log log = LogFactory.getLog(HibernateUtil.class);
  private static final SessionFactory sessionFactory;   //定義SessionFactory
 
  static {
    try {
      // 通過默認(rèn)配置文件hibernate.cfg.xml創(chuàng)建SessionFactory
      sessionFactory = new Configuration().configure().buildSessionFactory();
    } catch (Throwable ex) {
      log.error("初始化SessionFactory失??！", ex);
      throw new ExceptionInInitializerError(ex);
    }
  }

  //創(chuàng)建線程局部變量session，用來保存Hibernate的Session
  public static final ThreadLocal session = new ThreadLocal();
 
  /**
   * 獲取當(dāng)前線程中的Session
   * @return Session
   * @throws HibernateException
   */
  public static Session currentSession() throws HibernateException {
    Session s = (Session) session.get();
    // 如果Session還沒有打開，則新開一個(gè)Session
    if (s == null) {
      s = sessionFactory.openSession();
      session.set(s);     //將新開的Session保存到線程局部變量中
    }
    return s;
  }
 
  public static void closeSession() throws HibernateException {
    //獲取線程局部變量，并強(qiáng)制轉(zhuǎn)換為Session類型
    Session s = (Session) session.get();
    session.set(null);
    if (s != null)
      s.close();
  }
}

用法三：在Runnable中創(chuàng)建ThreadLocal

還有一種用法是在線程類內(nèi)部創(chuàng)建ThreadLocal，基本步驟如下：

1、在多線程的類（如ThreadDemo類）中，創(chuàng)建一個(gè)ThreadLocal對(duì)象threadXxx，用來保存線程間需要隔離處理的對(duì)象xxx。

2、在ThreadDemo類中，創(chuàng)建一個(gè)獲取要隔離訪問的數(shù)據(jù)的方法getXxx()，在方法中判斷，若ThreadLocal對(duì)象為null時(shí)候，應(yīng)該new()一個(gè)隔離訪問類型的對(duì)象，并強(qiáng)制轉(zhuǎn)換為要應(yīng)用的類型。

3、在ThreadDemo類的run()方法中，通過調(diào)用getXxx()方法獲取要操作的數(shù)據(jù)，這樣可以保證每個(gè)線程對(duì)應(yīng)一個(gè)數(shù)據(jù)對(duì)象，在任何時(shí)刻都操作的是這個(gè)對(duì)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(); //通過這個(gè)方法，為每個(gè)線程都獨(dú)立的new一個(gè)student對(duì)象，每個(gè)線程的的student對(duì)象都可以設(shè)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;
  }
  
}

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