前言
ThreadLocal ：每个线程通过此对象都会返回各自的值，互不干扰，这是因为每个线程都存着自己的一份副本。需要注意的是线程结束后，它所保存的所有副本都将进行垃圾回收（除非存在对这些副本的其他引用）
ThreadLocal的get操作是这样执行的：ThreadLocalMap map = thread.threadLocals -> return map.getEntry(threadLocal)ThreadLocal的set操作是这样执行的：ThreadLocalMap map = thread.threadLocals -> map.set(threadLocal, value)
三者的关系是：
每个Thread对应的所有ThreadLocal副本都存放在ThreadLocalMap对象中，key是ThreadLocal，value是副本数据
ThreadLocalMap对象存放在Thread对象中
通过ThreadLocal获取副本数据时，实际是通过访问Thread来获取ThreadLocalMap，再通过ThreadLocalMap获取副本数据
示例代码如下：
importjava.lang.reflect.Field; importjava.util.Arrays; importjava.util.List; importjava.util.concurrent.CountDownLatch; importjava.util.stream.Collectors; /** * @author: lihui * @date: 2020-06-01 */ publicclassThreadLocalStudy { privatestaticfinal ThreadLocal threadLocal1 =newThreadLocal<>(); privatestaticfinal ThreadLocal threadLocal2 =newThreadLocal<>(); privatestaticCountDownLatch countDownLatch1 =newCountDownLatch(2); privatestaticCountDownLatch countDownLatch2 =newCountDownLatch(1); publicstaticvoidmain(String[] args) throws NoSuchFieldException, IllegalAccessException, InterruptedException { Thread thread1 =newThread(() -> { threadLocal1.set("thread1-local1"); threadLocal2.set("thread1-local2"); countDownLatch1.countDown();try{ countDownLatch2.await(); }catch(InterruptedException e) { e.printStackTrace(); } }); Thread thread2 =newThread(() -> { threadLocal1.set("thread2-local1"); threadLocal2.set("thread2-local2"); countDownLatch1.countDown();try{ countDownLatch2.await(); }catch(InterruptedException e) { e.printStackTrace(); } }); thread1.start(); thread2.start(); countDownLatch1.await(); System.out.println(threadLocal1 +" "+ threadLocal2); printThreadLocalMapInfo(thread1); printThreadLocalMapInfo(thread2); countDownLatch2.countDown(); }/** * 输出相关信息 */ privatestaticvoidprintThreadLocalMapInfo(Thread thread) throws NoSuchFieldException, IllegalAccessException { System.out.println("====="+ thread.getName() +"====="); ObjectthreadLocalMapObject = getThreadLocalMapObject(thread); System.out.println(threadLocalMapObject); List objects = getEntryList(threadLocalMapObject); for(Objectobject : objects) { System.out.println(getEntryKey(object) +" "+ getEntryValue(object)); } }/** * 获取ThreadLocalMap对象 */ privatestaticObjectgetThreadLocalMapObject(Thread thread) throws NoSuchFieldException, IllegalAccessException { Field threadLocalsField = Thread.class.getDeclaredField("threadLocals"); threadLocalsField.setAccessible(true); returnthreadLocalsField.get(thread); }/** * 获取ThreadLocalMap对象中的所有Entry */ privatestaticList getEntryList(ObjectthreadLocalMapObject) throws NoSuchFieldException, IllegalAccessException { Field tableField = threadLocalMapObject.getClass().getDeclaredField("table"); tableField.setAccessible(true); Object[] objects = (Object[]) tableField.get(threadLocalMapObject); returnArrays.stream(objects).filter((obj) -> { returnobj !=null; }).collect(Collectors.toList()); }/** * 获取Entry的key */ privatestaticObjectgetEntryKey(Objectentry) throws NoSuchFieldException, IllegalAccessException { Field referentField = entry.getClass().getSuperclass().getSuperclass().getDeclaredField("referent"); referentField.setAccessible(true); returnreferentField.get(entry); }/** * 获取Entry的value */ privatestaticObjectgetEntryValue(Objectentry) throws NoSuchFieldException, IllegalAccessException { Field valueField = entry.getClass().getDeclaredField("value"); valueField.setAccessible(true); returnvalueField.get(entry); }}
输出结果为：
java.lang.ThreadLocal@31221be2 java.lang.ThreadLocal@377dca04
=====Thread-0=====
java.lang.ThreadLocal$ThreadLocalMap@728938a9java.lang.ThreadLocal@377dca04 thread1-local2java.lang.ThreadLocal@31221be2 thread1-local1=====Thread-1=====
java.lang.ThreadLocal$ThreadLocalMap@25f38edcjava.lang.ThreadLocal@377dca04 thread2-local2java.lang.ThreadLocal@31221be2 thread2-local1
可以看出：Thread类里面的ThreadLocalMap存储着所有ThreadLocal的副本数据。
没有通过ThreadLocal的get方式进行获取数据，而是通过实实在在的通过ThreadLocalMap对象来观察数据。
