介紹

• ThreadLocal是一個用於創建線程局部變量的類。當前線程通過ThreadLocal的set()方法設置的變量只對當前線程可見，通過get()獲取設置的變量。

使用

• 支持泛型

ThreadLocal<String> threadLocal = new ThreadLocal<>();

• 當前線程通過ThreadLocal對象的set(value)/get()設置變量和獲取設置的變量

threadLocal.set("jinshuai");
threadLocal.get();

原理

• 每個線程Thread維護一個ThreadLocalMap<key,value>
  • key是ThreadLocal對象，value是通過set(value)設置的值
    
• 當前線程調用threadLocal.set("jinshuai");
  • 首先獲取當前線程所維護的ThreadLocalMap
  • 然後判斷當前線程是否已經創建過這個ThreadLocalMap
    • 如果已經創建，會將ThreadLocal對象當作key，和當前線程要設置的值當作value放到ThreadLocalMap。
    • 如果沒有創建，會創建並初始化一個ThreadLocalMap（類似HashMap 初始化數組長度為2的冪，設置擴充閾值...）然後同上↑

        public void set(T value) {
            // 獲取當前線程對象
            Thread t = Thread.currentThread();
            // 獲取線程的ThreadLocalMap
            ThreadLocalMap map = getMap(t);
            if (map != null)
                map.set(this, value);
            else
                createMap(t, value);
        }
        // 獲取ThreadLocalMap
        ThreadLocalMap getMap(Thread t) {
            return t.threadLocals;
        }
     
    

• 當前線程調用threadLocal.get();
  • 首先獲取當前線程所維護的ThereadLocalMap
  • 然後將ThreadLocal對象作為key獲取對應的Entry
    • 如果Entry不為空獲取Entry的value
    • 如果Entry為空直接返回一個setInitvalue()值也就是null

        public T get() {
            // 獲取當前線程對象
            Thread t = Thread.currentThread();
            // 獲取當前線程對應的ThreadLocalMap
            ThreadLocalMap map = getMap(t);
            if (map != null) {
                // 將ThreadLocal(this)作為key獲取entry
                ThreadLocalMap.Entry e = map.getEntry(this);
                if (e != null) {
                    @SuppressWarnings("unchecked")
                    // 獲取entry的value
                    T result = (T)e.value;
                    return result;
                }
            }
            // 如果entry為空
            return setInitialValue();
        }
     
    

應用場景

• 如果一個對象非線程安全，但又不想通過加鎖的方式實現線程安全，可以通過ThreadLocal.set()對象的值,比如SimpleDataFormat不是線程安全的，此時可以每個線程設置一個SimpleDataFormat對象

    private static final ThreadLocal<SimpleDateFormat> formatter = new ThreadLocal<SimpleDateFormat>(){
        @Override
        protected SimpleDateFormat initialValue()
        {
            return new SimpleDateFormat("yyyyMMdd HHmm");
        }
    };

    public String formatIt(Date date)
    {
        return formatter.get().format(date);
    }

• 當某一個變量比如User對象在多個方法中傳遞時，會變得比較亂此時可以通過ThreadLocal設置變量

  • 比如在Servlet中：

    doGet(HttpServletRequest req, HttpServletResponse resp) {
        User user = getLoggedInUser(req);
        doSomething(user)
        doSomethingElse(user)
        renderResponse(resp,user)
    }

  • 每個方法都需要一個user，不夠優雅(咳咳...)，此時可以通過設置一個ThreadLocal單例，然後set(user)：

    doGet(HttpServletRequest req, HttpServletResponse resp) {
        User user = getLoggedInUser(req);
        ThreadLocalSingleInstace.getThreadLocal().set(user)
        try {
            doSomething()
            doSomethingElse()
            renderResponse(resp)
        }
        finally {
            ThreadLocalSingleInstace.getThreadLocal().remove()
        }
    }
    // 獲取ThreadLocal單例
    class ThreadLocalSingleInstace {
        static private ThreadLocal threadLocal = new ThreadLocal<User>();
        static ThreadLocal<User> getThreadLocal() {
            return threadLocal;
        }
    }

註意

• 用完以後應該調用remove()移除設定的值，防止內存泄漏

// 會移除這個Entry
threadLocal.remove();

• 在線程池中由於線程會被復用，所以不會停止，導致每個線程的ThreadLocalMap裏的key是ThreadLocal的弱引用，GC時會將其回收，但是其對應的value一直有一個強引用不會被回收造成內存泄漏。

       /**
        * The entries in this hash map extend WeakReference, using
        * its main ref field as the key (which is always a
        * ThreadLocal object).  Note that null keys (i.e. entry.get()
        * == null) mean that the key is no longer referenced, so the
        * entry can be expunged from table.  Such entries are referred to
        * as "stale entries" in the code that follows.
        */
       static class Entry extends WeakReference<ThreadLocal<?>> {
           /** The value associated with this ThreadLocal. */
           Object value;

           Entry(ThreadLocal<?> k, Object v) {
               super(k);
               value = v;
           }
       }

參考

• https://droidyue.com/blog/2016/03/13/learning-threadlocal-in-java/
  
• http://blog.xiaohansong.com/2016/08/06/ThreadLocal-memory-leak/#
  
• https://stackoverflow.com/questions/817856/when-and-how-should-i-use-a-threadlocal-variable
  
• https://stackoverflow.com/questions/1490919/purpose-of-threadlocal

ThreadLocal使用以及原理