ThreadLocal源码分析

1.核心数据结构 ThreadLocalMap

1.静态内部类 Entry

// 继承了弱引用
static class Entry extends WeakReference<ThreadLocal<?>> {// value就是平常定义ThreadLocal中存的那个东西Object value;// key就是ThreadLocal（是弱引用）Entry(ThreadLocal<?> k, Object v) {super(k);value = v;}
}

2.真正存储数据的是table数组

2.ThreadLocal.set()方法源码详解

1.set

    public void set(T value) {// 获取当前线程Thread t = Thread.currentThread();// 获取当前线程的ThreadLocalMapThreadLocalMap map = getMap(t);// 如果map不为空，就将当前的ThreadLocal作为key，ThreadLocal存的值作为value设置到map中if (map != null) {map.set(this, value);} else {// 如果map为空，就使用当前线程和ThreadLocal存的值去创建一个mapcreateMap(t, value);}}

2.getMap

    ThreadLocalMap getMap(Thread t) {// 返回当前线程的ThreadLocalMapreturn t.threadLocals;}

3.ThreadLocalMap.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.// 存储数据的Entry数组Entry[] tab = table;// 数组的长度int len = tab.length;// 计算当前的ThreadLocal对象在Entry数组中的位置，其实跟hashmap一样，都是(n - 1) & hashint i = key.threadLocalHashCode & (len-1);// 只要目标位置不为空，就进行循环for (Entry e = tab[i];e != null;e = tab[i = nextIndex(i, len)]) {ThreadLocal<?> k = e.get();// 期间如果遇到key相同的，则替换valueif (k == key) {e.value = value;return;}// 如果遇到key为null的，就会执行替换过期数据的逻辑if (k == null) {replaceStaleEntry(key, value, i);return;}}// 如果目标位置为空，就直接将这个Entry对象放进去tab[i] = new Entry(key, value);int sz = ++size;// 如果清理过期槽位失败并且元素数量大于等于阈值（数组长度乘2/3）就进行rehash逻辑if (!cleanSomeSlots(i, sz) && sz >= threshold)rehash();
}

4.createMap

void createMap(Thread t, T firstValue) {// 使用ThreadLocal对象和value去创建一个mapt.threadLocals = new ThreadLocalMap(this, firstValue);
}

ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {// table的初始容量为16table = new Entry[INITIAL_CAPACITY];// 使用(n - 1) & hash去找到指定的table数组的位置int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);// 给指定位置的table数组设置一个Entry对象table[i] = new Entry(firstKey, firstValue);// 设置数组大小为1size = 1;// 设置阈值setThreshold(INITIAL_CAPACITY);
}

private void setThreshold(int len) {// 阈值为16 * 2/3threshold = len * 2 / 3;
}

5.rehash

private void rehash() {// 进行探测式清理工作expungeStaleEntries();// 如果清理后 size >= threshold * 3/4 就进行扩容if (size >= threshold - threshold / 4)resize();
}

6.resize

private void resize() {// 旧的Entry数组Entry[] oldTab = table;// 旧的容量int oldLen = oldTab.length;// 新的长度为旧长度的两倍int newLen = oldLen * 2;// 创建一个新的Entry数组Entry[] newTab = new Entry[newLen];int count = 0;// 遍历旧的数组，将旧数组中的元素重新hash后放到新数组中，如果哈希冲突就往后放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;
}

3.ThreadLocalMap.get()详解

1.get

public T get() {// 获取当前线程Thread t = Thread.currentThread();// 获取当前线程的ThreadLocalMapThreadLocalMap map = getMap(t);// 只要map不为空if (map != null) {// 通过当前的ThreadLocal作为key去获取到对应的ThreadLocalMap.EntryThreadLocalMap.Entry e = map.getEntry(this);// 只要不为空就返回这个valueif (e != null) {@SuppressWarnings("unchecked")T result = (T)e.value;return result;}}// 如果map为空就返回一个初始值nullreturn setInitialValue();
}

2.ThreadLocalMap.getEntry

private Entry getEntry(ThreadLocal<?> key) {// (n - 1) & hash 找到table数组的位置int i = key.threadLocalHashCode & (table.length - 1);Entry e = table[i];// 如果key相同，直接返回if (e != null && e.get() == key)return e;else// 如果key不同，就向后找，如果最后找到了就返回，找不到就返回nullreturn getEntryAfterMiss(key, i, e);
}

3.getEntryAfterMiss

        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);elsei = nextIndex(i, len);e = tab[i];}return null;}

4.ThreadLocalMap过期 key 的探测式清理流程

遍历散列数组，从开始位置向后探测清理过期数据，将过期数据的Entry设置为null。

沿途中碰到未过期的数据则将此数据rehash后重新在table数组中定位，如果定位的位置已经有了数据，则会将未过期的数据放到最靠近此位置的Entry=null的桶中，使rehash后的Entry数据距离正确的桶的位置更近一些。

5.InheritableThreadLocal原理

实现原理是子线程是通过在父线程中通过调用new Thread()方法来创建子线程，Thread#init方法在Thread的构造方法中被调用。在init方法中拷贝父线程数据到子线程中：

private void init(ThreadGroup g, Runnable target, String name,long stackSize, AccessControlContext acc,boolean inheritThreadLocals) {if (name == null) {throw new NullPointerException("name cannot be null");}if (inheritThreadLocals && parent.inheritableThreadLocals != null)this.inheritableThreadLocals =ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);this.stackSize = stackSize;tid = nextThreadID();
}