这是一个简易版的 ThreadLocal，实现了 ThreadLocal 的 set，get，及 remove 方法。
package per.lvjc.concurrent.threadlocal; import java.util.HashMap; import java.util.Map; public class LvjcThreadLocal<T> { private Map<Thread, Map<LvjcThreadLocal, Object>> mapMap; private T initValue; public LvjcThreadLocal() { this.mapMap = new HashMap<>(); } public LvjcThreadLocal(T initValue) { this.mapMap = new HashMap<>(); this.initValue = initValue; } public void set(T value) { Thread thread = Thread.currentThread(); Map<LvjcThreadLocal, Object> currentThreadMap = mapMap.get(thread); if (currentThreadMap == null) { currentThreadMap = new HashMap<>(); mapMap.put(thread, currentThreadMap); } currentThreadMap.put(this, value); } public T get() { Thread thread = Thread.currentThread(); Map<LvjcThreadLocal, Object> currentThreadMap = mapMap.get(thread); if (currentThreadMap == null) { return initValue; } return (T) currentThreadMap.get(this); } public void remove() { Thread thread = Thread.currentThread(); Map<LvjcThreadLocal, Object> currentThreadMap = mapMap.get(thread); if (currentThreadMap != null) { currentThreadMap.remove(this); } } }就这样，没了。
set，get 的基本功能有了，还可以通过带初始值的构造方法来设置初始值，可以当做 JDK ThreadLocal 一样来用。
基本原理与 JDK ThreadLocal 一致。
如果把里面的那层 Map 替换成 ThreadLocal.ThreadLocalMap，即变成 Map<Thread, ThreadLocalMap>；
再把 ThreadLocalMap 放到 Thread 对象里面去，而不是全都放在一个 ThreadLocal 对象里面以 Thread 做映射，就变成 JDK ThreadLocal 了。
