目標：用于每個線程資源的隔離.
當工作于多線程中的對象使用ThreadLocal 維護變量時，ThreadLocal 為 每個使用該變量的線程分配一個獨立的變量副本，
threadLocal 原理與使用
。每個線程獨立改變自己的副本，而不影響其他線程所對應的變量副本。不同于線程同步：線程同步用于解決一個變量同一時刻被多個線程共享的問題（共享一個變量）。
threadLocal 使得一個變量在多個線程中各自擁有自己的副本（實現資源的隔離）。
原理：
ThreadLocal 中有一個map, 用于存儲每一個線程的變量副本，其中map的key為 線程對象，value為對應線程的變量副本。
<code class="hljs cs">public class SimpleThreadLocal<t>{ private Map valueMap = Collections.synchronizedMap(new HashMap()); public void set(T value) { valueMap.put(Thread.currentThread(), value); } public T get() { Thread currentThread = Thread.currentThread(); T value = (T) valueMap.get(currentThread); if(value == null && !valueMap.containsKey(currentThread)) { value = initValue(); valueMap.put(currentThread, value); } return value; } public void remove() { valueMap.remove(Thread.currentThread()); } public T initValue() { return null; }}</t></code>應用場景：
web應用分層結構中：同一變量在持久化，業務，展示層傳遞時（同一線程）。
接口方法：
<code class="hljs cs">protected T initialValue(); //return the initial value for this thread-local public T get(); //return the current thread's value of this thread-local public void set(T value); //@param value the value to be stored in the current thread's copy of this thread-local. public void remove(); //Removes the current thread's value for this thread-local variable.</code>
示例：
<code class="hljs cs">ThreadLocalContext.classpublic class ThreadLocalContext { private static ThreadLocal<integer>threadLocalNum = new ThreadLocal<integer>() { @Override protected Integer initialValue() { return 0; } }; public int getNextNum() { threadLocalNum.set(threadLocalNum.get()+1); return threadLocalNum.get(); }}</integer></integer></code><code class="hljs axapta">ThreadLocalTest.classpublic class ThreadLocalTest { public static void main(String[] args) { ThreadLocalContext threadLocalContext = new ThreadLocalContext(); WorkThread workThread1 = new WorkThread(threadLocalContext); WorkThread workThread2 = new WorkThread(threadLocalContext); WorkThread workThread3 = new WorkThread(threadLocalContext); workThread1.start(); workThread2.start(); workThread3.start(); }}class WorkThread extends Thread{ private ThreadLocalContext threadLocalContext; public WorkThread(ThreadLocalContext threadLocalContext) { this.threadLocalContext = threadLocalContext; } public void run() { for(int i = 0; i < 5; i++) { System.out.println("thread[" + Thread.currentThread().getName() + "] threadLocalNum [" + threadLocalContext.getNextNum() + "]"); } }}</code>