ThreadLocal源码理解

ThreadLocal其实原理是创建了多份相同数据保存在堆内存上，每个线程的Thread类里有ThreadLocal.ThreadLocalMap threadLocals的属性来指向存位置，
所以每个线程修改都不会影响到其他线程的数据
首先说下下面用到的东西：ThreadLocalMap为ThreadLocal的静态内部类，他有Entry[]数组来存放本地变量

static class Entry extends WeakReference<ThreadLocal<?>> {            /** The value associated with this ThreadLocal. */            Object value;            Entry(ThreadLocal<?> k, Object v) {                super(k);                value = v;            }        }

ThreadLocal get方法源码 它会调用getMap(t)方法，这个getMap()方法会返回当前线程的ThreadLocalMap的实例，通过ThreadLocalMap的getEntry()方法
根据当前ThreadLoad获取到Entry后取得值。如果当前线程没有ThreadLocalMap的引用，则初始化一分数据并将引用传给当前线程。

public T get() {        Thread t = Thread.currentThread();        ThreadLocalMap map = getMap(t);        if (map != null) {            ThreadLocalMap.Entry e = map.getEntry(this);            if (e != null) {                @SuppressWarnings("unchecked")                T result = (T)e.value;                return result;            }        }        return setInitialValue();    }

ThreadLocalMap getMap(Thread t) {        return t.threadLocals;    }private T setInitialValue() {        T value = initialValue();        Thread t = Thread.currentThread();        ThreadLocalMap map = getMap(t);        if (map != null)            map.set(this, value);        else            createMap(t, value);        return value;    }

ThreadLocal set方法也是通过getMap方法获取到当前线程对应的ThreadLocalMap后设置值，如果map为空则先为线程创建ThreadLocalMap并将值放入以当前
ThreadLocal为Entry的K

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

void createMap(Thread t, T firstValue) {        t.threadLocals = new ThreadLocalMap(this, firstValue);    }