java 中ThreadLocal 的正確用法
用法一：在關聯數據類中創建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類將狀態（例如用戶ID、事務ID）與線程關聯起來，那么通常在這個類中定義private static類型的ThreadLocal 實例。
例如，在下面的類中，私有靜態 ThreadLocal 實例（serialNum）為調用該類的靜態 SerialNum.get() 方法的每個線程維護了一個“序列號”，該方法將返回當前線程的序列號。（線程的序列號是在第一次調用 SerialNum.get() 時分配的，并在后續調用中不會更改。）
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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】
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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類中創建ThreadLocal
這是上面用法的擴展，即把ThreadLocal的創建放到工具類中。
【例】例如hibernate的工具類：
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public class HibernateUtil { private static Log log = LogFactory.getLog(HibernateUtil.class); private static final SessionFactory sessionFactory; //定義SessionFactory static { try { // 通過默認配置文件hibernate.cfg.xml創建SessionFactory sessionFactory = new Configuration().configure().buildSessionFactory(); } catch (Throwable ex) { log.error("初始化SessionFactory失敗！", ex); throw new ExceptionInInitializerError(ex); } } //創建線程局部變量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，并強制轉換為Session類型 Session s = (Session) session.get(); session.set(null); if (s != null) s.close(); }} |
用法三：在Runnable中創建ThreadLocal
還有一種用法是在線程類內部創建ThreadLocal，基本步驟如下：
1、在多線程的類（如ThreadDemo類）中，創建一個ThreadLocal對象threadXxx，用來保存線程間需要隔離處理的對象xxx。
2、在ThreadDemo類中，創建一個獲取要隔離訪問的數據的方法getXxx()，在方法中判斷，若ThreadLocal對象為null時候，應該new()一個隔離訪問類型的對象，并強制轉換為要應用的類型。
3、在ThreadDemo類的run()方法中，通過調用getXxx()方法獲取要操作的數據，這樣可以保證每個線程對應一個數據對象，在任何時刻都操作的是這個對象。
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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(); //通過這個方法，為每個線程都獨立的new一個student對象，每個線程的的student對象都可以設置不同的值 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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原文鏈接：http://blog.csdn.net/vking_wang/article/details/14225379
