深入理解Java中的ThreadLocal

轉自：

http://www.java265.com/JavaCourse/202205/3370.html

下文筆者講述java中ThreadLocal的詳解說明，如下所示:

ThreadLocal簡介

ThreadLocal的功能:
   用於創建線程局部變數的類
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我們都知道我們創建的變數，可以被任何一個線程訪問和修改
但是當我們使用ThreadLocal創建的變數只能被當前線程訪問，其他線程無法訪問和修改
-----------------------------------------------------------------------------

ThreaLocal用法示例

ThreadLocal定義:
   ThreadLocal<String> threadLocalInfo = new ThreadLocal<>();

set方法 
   threadLocalInfo.set("java265.com");
get方法
   threadLocalInfo.get();

完整的示例
 private void testThreadLocal() {
    Thread t = new Thread() {
        ThreadLocal<String> threadLocalInfo = new ThreadLocal<>();

        @Override
        public void run() {
            super.run();
            threadLocalInfo.set("java265.com--我最愛的網站");
            threadLocalInfo.get();
        }
    };

    t.start();
}

ThreadLocal初始值--設置方法

ThreadLocal設置預設的get初始值
  需要重寫initialValue方法

例:
ThreadLocal<String> mThreadLocal = new ThreadLocal<String>() {
    @Override
    protected String initialValue() {
      return Thread.currentThread().getName();
    }
};

ThreadLocal的set方法原理說明

 
set方法源碼:
public void set(T value) {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null)
        map.set(this, value);
    else
        createMap(t, value);
} 

getMap: 

ThreadLocalMap getMap(Thread t) {
    return t.threadLocals;
}

class Thread implements Runnable {
    ThreadLocal.ThreadLocalMap threadLocals = null;
}

當一開始設置，即ThreadLocalMap對象未創建
則新建ThreadLocalMap對象,並設置初始值
void createMap(Thread t, T firstValue) {
    t.threadLocals = new ThreadLocalMap(this, firstValue);
}

ThreadLocal導致記憶體泄露的原因說明

  ThreadLocal實例被線程的ThreadLocalMap實例持有，也可以看成被線程持有。
  當我們使用了線程池，由於之前的線程實例處理完之後出於復用的目的依然存活
  即ThreadLocal設定的值被持有,才會導致記憶體泄露 
  ThreadLocal並不會產生記憶體泄露
  因為ThreadLocalMap在選擇key的時候
  並不是直接選擇ThreadLocal實例
   而是ThreadLocal實例的弱引用。

static class ThreadLocalMap {
/**
* 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;
        }
    }
}

ThreadLocal使用場景

 如:tractional 
    一個資料庫連接，在一個線程中只此一份，所以可使用ThreadLocal實現線程間變數共用