结论：内部通过,操作当前线程的成员变量threadLocalMap，即Thread.currentThread.threadLocalMap，Map中key为当前线程的threadLocal对象(即调用add()方法的实例对象this,本文中为threadLocalUser)，value为存入对象，当存取时内部自动传入当前线程key,所以只能取到当前线程的绑定变量，从而实现线程级隔离。当线程不同时操作的thread也不同，Map当然也不同，map中的key和value也就不同了。
打个不太恰当的比喻，多个It男（Thread），每个人一个背包（threadLocalMap），包里放入自己经常要用的东西比如笔记本（key标签为"笔忘本"，value"联想电脑"，没有的话，第一次参加工作createMap要用的时候也会买了放进去），当多个人同时都接到接到任务“拿出笔记本”（并发http请求），自己在任何地方都可以把“笔忘本”拿出来用，但每个人都不会拿错。
注:map的key不是线程id或者name,而是当前线程下的ThreadLocal实例对象，之前所以这么设计，保证当前线程可以有多个ThreadLocal对象，如果key为当前线程，那就限制了当前线程只能存储一个对象
如
ThreadLocal<Student> threadLocalStudent=new ThreadLocal();
ThreadLocal<UserInfo> threadLocalUserInfo=new new ThreadLocal();
为方便分析本文以一个ThreadLocal对象来举例，并且为方便线程内跨类跨包随取随用，用自定义类MyThreadLocal来将ThreadLocal成员封装为静态对象，并为之提供访问方法set ,get
1.作用：
用于存储线程中共享变量，随用随取，如操作员信息，一处存入处处可用
2.原理如下
存：Class加载初始化时将threadlocal变量加入方法区(线程共享)，存的时候直接MyThreadLocal.threadLocal.set(new UserInfo("1001","zhangsan"));内部threadlocal通过当前线程T找到线程的变量threadLocalMap,然后put(key(线程的ThreadLocal实例对象)，value(UserInfo对象))存入Map
取：UserInfo user=MyThreadLocal.threadLocal.get();内部也是通过当前线程绑定的threadLocalMap中，get(key(当前线程的ThreadLocal实例对象))，所以只要线程相同且ThreadLocal实例ID相同，取到的值就相同，反之则不同，从而实现不同线程间隔离，同一线程中共享。
3.跟踪调试
自定义类MyThreadLocal中，threadLocal在MyThreadLocal初始化时就有值，且只初始化一次,与类相关与实例对象无关，所以任何线程进来都是一个值
在ThreadLocal的set方法源码中打断点，跟踪Thread t的结构，t 中的成员变量threadLocals即为存储Map,map中有table（Entry数组，元素Entry.key 为线程的ThreadLocal实例对象的弱引用referent，Entry.value为存入对象）,第63个元素的value就是存入对象，referent为当前线程的引用
注意：线程Id不同时，本地线程中的变量为null,必须自己用自己取
referent的类型为ThreadLocal
第63元素的referent为下图
4.多线程传传参数
先将本地变量取出，再在新线程中传入，如果要传token，只需在userInfo中添加一个字段，请求进入时在controller或者拦截器中MyThreadLocal.add(userInfo)即可后续使用。
//先取出当前值
UserInfo userInfo= MyThreadLocal.getUserInfo();
//推荐使用线程池创建新线程
new Thread(()->{ log.info("before setting,"+JSONObject.toJSONString(MyThreadLocal.getUserInfo()));//返回NULL MyThreadLocal.add(sysOperDTO); log.info("after setting,=="+JSONObject.toJSONString(MyThreadLocal.getUserInfo()));//返回对象 }).start();
UserInfo类
@Data public class UserInfo { private String threadName; private String threadId; }
MyThreadLocal
public class MyThreadLocal { private MyThreadLocal() { } private static final ThreadLocal<UserInfo> threadLocalUser = new ThreadLocal<>(); public static void add(UserInfo userInfo) { userInfo.setThreadName(Thread.currentThread().getName()); userInfo.setThreadId("threadId:"+String.valueOf(Thread.currentThread().getId())+" threadLocalUser.hashCode=="+ threadLocalUser.hashCode()); threadLocalUser.set(userInfo); } public static UserInfo getUserInfo() { return threadLocalUser.get(); } public static void remove() { threadLocalUser.remove(); } }
5.Map实际结构，是ThreadLocal的静态内部类
/** * ThreadLocalMap is a customized hash map suitable only for * maintaining thread local values. No operations are exported * outside of the ThreadLocal class. The class is package private to * allow declaration of fields in class Thread. To help deal with * very large and long-lived usages, the hash table entries use * WeakReferences for keys. However, since reference queues are not * used, stale entries are guaranteed to be removed only when * the table starts running out of space. */ 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; } } /** * The initial capacity -- MUST be a power of two. */ private static final int INITIAL_CAPACITY = 16; /** * The table, resized as necessary. * table.length MUST always be a power of two. */ private Entry[] table; /** * The number of entries in the table. */ private int size = 0; /** * The next size value at which to resize. */ private int threshold; // Default to 0 /** * Set the resize threshold to maintain at worst a 2/3 load factor. */ private void setThreshold(int len) { threshold = len * 2 / 3; } /** * Increment i modulo len. */ private static int nextIndex(int i, int len) { return ((i + 1 < len) ? i + 1 : 0); } /** * Decrement i modulo len. */ private static int prevIndex(int i, int len) { return ((i - 1 >= 0) ? i - 1 : len - 1); } /** * Construct a new map initially containing (firstKey, firstValue). * ThreadLocalMaps are constructed lazily, so we only create * one when we have at least one entry to put in it. */ ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) { table = new Entry[INITIAL_CAPACITY]; int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1); table[i] = new Entry(firstKey, firstValue); size = 1; setThreshold(INITIAL_CAPACITY); } /** * Construct a new map including all Inheritable ThreadLocals * from given parent map. Called only by createInheritedMap. * * @param parentMap the map associated with parent thread. */ private ThreadLocalMap(ThreadLocalMap parentMap) { Entry[] parentTable = parentMap.table; int len = parentTable.length; setThreshold(len); table = new Entry[len]; for (int j = 0; j < len; j++) { Entry e = parentTable[j]; if (e != null) { @SuppressWarnings("unchecked") ThreadLocal<Object> key = (ThreadLocal<Object>) e.get(); if (key != null) { Object value = key.childValue(e.value); Entry c = new Entry(key, value); int h = key.threadLocalHashCode & (len - 1); while (table[h] != null) h = nextIndex(h, len); table[h] = c; size++; } } } } /** * Get the entry associated with key. This method * itself handles only the fast path: a direct hit of existing * key. It otherwise relays to getEntryAfterMiss. This is * designed to maximize performance for direct hits, in part * by making this method readily inlinable. * * @param key the thread local object * @return the entry associated with key, or null if no such */ private Entry getEntry(ThreadLocal<?> key) { int i = key.threadLocalHashCode & (table.length - 1); Entry e = table[i]; if (e != null && e.get() == key) return e; else return getEntryAfterMiss(key, i, e); } /** * Version of getEntry method for use when key is not found in * its direct hash slot. * * @param key the thread local object * @param i the table index for key's hash code * @param e the entry at table[i] * @return the entry associated with key, or null if no such */ private Entry getEntryAfterMiss(ThreadLocal<?> key, int i, Entry e) { Entry[] tab = table; int len = tab.length; while (e != null) { ThreadLocal<?> k = e.get(); if (k == key) return e; if (k == null) expungeStaleEntry(i); else i = nextIndex(i, len); e = tab[i]; } return null; } /** * Set the value associated with key. * * @param key the thread local object * @param value the value to be set */ private void set(ThreadLocal<?> key, Object value) { // We don't use a fast path as with get() because it is at // least as common to use set() to create new entries as // it is to replace existing ones, in which case, a fast // path would fail more often than not. Entry[] tab = table; int len = tab.length; int i = key.threadLocalHashCode & (len-1); for (Entry e = tab[i]; e != null; e = tab[i = nextIndex(i, len)]) { ThreadLocal<?> k = e.get(); if (k == key) { e.value = value; return; } if (k == null) { replaceStaleEntry(key, value, i); return; } } tab[i] = new Entry(key, value); int sz = ++size; if (!cleanSomeSlots(i, sz) && sz >= threshold) rehash(); } /** * Remove the entry for key. */ private void remove(ThreadLocal<?> key) { Entry[] tab = table; int len = tab.length; int i = key.threadLocalHashCode & (len-1); for (Entry e = tab[i]; e != null; e = tab[i = nextIndex(i, len)]) { if (e.get() == key) { e.clear(); expungeStaleEntry(i); return; } } } /** * Replace a stale entry encountered during a set operation * with an entry for the specified key. The value passed in * the value parameter is stored in the entry, whether or not * an entry already exists for the specified key. * * As a side effect, this method expunges all stale entries in the * "run" containing the stale entry. (A run is a sequence of entries * between two null slots.) * * @param key the key * @param value the value to be associated with key * @param staleSlot index of the first stale entry encountered while * searching for key. */ private void replaceStaleEntry(ThreadLocal<?> key, Object value, int staleSlot) { Entry[] tab = table; int len = tab.length; Entry e; // Back up to check for prior stale entry in current run. // We clean out whole runs at a time to avoid continual // incremental rehashing due to garbage collector freeing // up refs in bunches (i.e., whenever the collector runs). int slotToExpunge = staleSlot; for (int i = prevIndex(staleSlot, len); (e = tab[i]) != null; i = prevIndex(i, len)) if (e.get() == null) slotToExpunge = i; // Find either the key or trailing null slot of run, whichever // occurs first for (int i = nextIndex(staleSlot, len); (e = tab[i]) != null; i = nextIndex(i, len)) { ThreadLocal<?> k = e.get(); // If we find key, then we need to swap it // with the stale entry to maintain hash table order. // The newly stale slot, or any other stale slot // encountered above it, can then be sent to expungeStaleEntry // to remove or rehash all of the other entries in run. if (k == key) { e.value = value; tab[i] = tab[staleSlot]; tab[staleSlot] = e; // Start expunge at preceding stale entry if it exists if (slotToExpunge == staleSlot) slotToExpunge = i; cleanSomeSlots(expungeStaleEntry(slotToExpunge), len); return; } // If we didn't find stale entry on backward scan, the // first stale entry seen while scanning for key is the // first still present in the run. if (k == null && slotToExpunge == staleSlot) slotToExpunge = i; } // If key not found, put new entry in stale slot tab[staleSlot].value = null; tab[staleSlot] = new Entry(key, value); // If there are any other stale entries in run, expunge them if (slotToExpunge != staleSlot) cleanSomeSlots(expungeStaleEntry(slotToExpunge), len); } /** * Expunge a stale entry by rehashing any possibly colliding entries * lying between staleSlot and the next null slot. This also expunges * any other stale entries encountered before the trailing null. See * Knuth, Section 6.4 * * @param staleSlot index of slot known to have null key * @return the index of the next null slot after staleSlot * (all between staleSlot and this slot will have been checked * for expunging). */ private int expungeStaleEntry(int staleSlot) { Entry[] tab = table; int len = tab.length; // expunge entry at staleSlot tab[staleSlot].value = null; tab[staleSlot] = null; size--; // Rehash until we encounter null Entry e; int i; for (i = nextIndex(staleSlot, len); (e = tab[i]) != null; i = nextIndex(i, len)) { ThreadLocal<?> k = e.get(); if (k == null) { e.value = null; tab[i] = null; size--; } else { int h = k.threadLocalHashCode & (len - 1); if (h != i) { tab[i] = null; // Unlike Knuth 6.4 Algorithm R, we must scan until // null because multiple entries could have been stale. while (tab[h] != null) h = nextIndex(h, len); tab[h] = e; } } } return i; } /** * Heuristically scan some cells looking for stale entries. * This is invoked when either a new element is added, or * another stale one has been expunged. It performs a * logarithmic number of scans, as a balance between no * scanning (fast but retains garbage) and a number of scans * proportional to number of elements, that would find all * garbage but would cause some insertions to take O(n) time. * * @param i a position known NOT to hold a stale entry. The * scan starts at the element after i. * * @param n scan control: {@code log2(n)} cells are scanned, * unless a stale entry is found, in which case * {@code log2(table.length)-1} additional cells are scanned. * When called from insertions, this parameter is the number * of elements, but when from replaceStaleEntry, it is the * table length. (Note: all this could be changed to be either * more or less aggressive by weighting n instead of just * using straight log n. But this version is simple, fast, and * seems to work well.) * * @return true if any stale entries have been removed. */ private boolean cleanSomeSlots(int i, int n) { boolean removed = false; Entry[] tab = table; int len = tab.length; do { i = nextIndex(i, len); Entry e = tab[i]; if (e != null && e.get() == null) { n = len; removed = true; i = expungeStaleEntry(i); } } while ( (n >>>= 1) != 0); return removed; } /** * Re-pack and/or re-size the table. First scan the entire * table removing stale entries. If this doesn't sufficiently * shrink the size of the table, double the table size. */ private void rehash() { expungeStaleEntries(); // Use lower threshold for doubling to avoid hysteresis if (size >= threshold - threshold / 4) resize(); } /** * Double the capacity of the table. */ private void resize() { Entry[] oldTab = table; int oldLen = oldTab.length; int newLen = oldLen * 2; Entry[] newTab = new Entry[newLen]; int count = 0; for (int j = 0; j < oldLen; ++j) { Entry e = oldTab[j]; if (e != null) { ThreadLocal<?> k = e.get(); if (k == null) { e.value = null; // Help the GC } else { int h = k.threadLocalHashCode & (newLen - 1); while (newTab[h] != null) h = nextIndex(h, newLen); newTab[h] = e; count++; } } } setThreshold(newLen); size = count; table = newTab; } /** * Expunge all stale entries in the table. */ private void expungeStaleEntries() { Entry[] tab = table; int len = tab.length; for (int j = 0; j < len; j++) { Entry e = tab[j]; if (e != null && e.get() == null) expungeStaleEntry(j); } } }
附关联知识：
ThreadLocal内存泄漏问题：https://www.cnblogs.com/xzwblog/p/7227509.html
volatile的深入理解：https://www.jianshu.com/p/4901b2fdfce1
线程池的详解：https://www.cnblogs.com/zzuli/p/9386463.html
对象的强引用弱引用：https://www.cnblogs.com/yanggb/p/10386175.html
关闭线程三种方法：https://www.cnblogs.com/liyutian/p/10196044.html
