ThreadLocal这个类,定义了方法但是没有数据。
1. 它的对象作为key在使用,而map放在Thread类中;2. 这个类里面封装了操作map的方法
Thread类
public class Thread implements Runnable { ThreadLocal.ThreadLocalMap threadLocals = null; }
ThreadLocal类
public class ThreadLocal<T> { public T get() { Thread t = Thread.currentThread(); //map = t.threadLocals; ThreadLocalMap map = getMap(t); if (map != null) { //以ThreadLocal对象作key ThreadLocalMap.Entry e = map.getEntry(this); if (e != null) return (T)e.value; } return setInitialValue(); } 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; } protected T initialValue() { return null; } public void set(T value) { Thread t = Thread.currentThread(); ThreadLocalMap map = getMap(t); if (map != null) map.set(this, value); else createMap(t, value); } }
java.lang.ThreadLocal.ThreadLocalMap
public class ThreadLocal<T> { static class ThreadLocalMap { static class Entry extends WeakReference<ThreadLocal> { /** The value associated with this ThreadLocal. */ Object value; Entry(ThreadLocal k, Object v) { super(k); value = v; } } 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); } 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; } } }
ThreadLocalMap 里面的 Entry 继承了 WeakReference,为什么呢?为了方便垃圾回收
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