ThreadLocal源码分析

Posted 2020-11-24 z-qinfeng

tags:

篇首语：本文由小常识网(cha138.com)小编为大家整理，主要介绍了ThreadLocal源码分析相关的知识，希望对你有一定的参考价值。

1.测试代码

public class ThreadLocalTest {
    public static void main(String[] args) {
        ThreadLocal<String> threadLocal = new ThreadLocal<>();
        threadLocal.set("郑钦锋");
    }
}

   public void set(T value) {
        // 获取当前线程
        Thread t = Thread.currentThread();
        // 获取当前线程关联的ThreadLocalMap
        ThreadLocalMap map = getMap(t);
        if (map != null)
            # set值， this: 指ThreadLocal
            map.set(this, value);
        else
            createMap(t, value);
    }

技术图片

下面分析set(ThreadLocal tl,Object value)方法

  private void set(ThreadLocal<?> key, Object value) {

    # 底层使用Entry数组存储数据
    Entry[] tab = table;
    # 默认16
    int len = tab.length;
    # 计算槽位
    int i = key.threadLocalHashCode & (len-1);
    # 从数组中获取key对应槽位的值，一个Entry对象
    for (Entry e = tab[i];e != null;e = tab[i = nextIndex(i, len)]) {
        # Entry对象的key就是ThreadLocal对象
        ThreadLocal<?> k = e.get();
        # 如果从槽位中获取的Entry的k 等于 传入的key,将新值赋到这个Entry上面，然后结束set方法
        if (k == key) {
            e.value = value;
            return;
        }
        # 如果槽中Entry的key为空，最终会调用expungeStaleEntry方法移除这个槽中的数据
        if (k == null) {
            replaceStaleEntry(key, value, i);
            return;
        }
    }
    # 回到正常逻辑
    # 如果槽位i是空，创建一个Entry对象，并将这个Entry对象放到i这个槽位上
    tab[i] = new Entry(key, value);
    int sz = ++size;
    # 底层调用expungeStaleEntry方法移除槽中key(ThreadLocal)为null对应的Value值，同时清空这个槽位
    if (!cleanSomeSlots(i, sz) && sz >= threshold)
        # 扩容
        rehash();
}

看了上面代码注释，我们有必要分析以下两点：

（1）创建Entry对象的具体的过程

（2） expungeStaleEntry方法是如何清空key为null的Entry对象的

接下来先看第1点，如何创建Entry？

    static class Entry extends WeakReference<ThreadLocal<?>> {
            /** The value associated with this ThreadLocal. */
            Object value;

            Entry(ThreadLocal<?> k, Object v) {
                super(k);
                value = v;
            }
        }

代码其实很简单，就是将key交给顶级父类Reference处理，一直跟踪代码会看到下面这行代码

    private T referent;         /* Treated specially by GC */  真惨

接着看expungeStaleEntry方法，这也很简单，结合着cleanSomeSlots方法来看，直接上代码

     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];
                # 如果key为null,删除
                if (e != null && e.get() == null) {
                    n = len;
                    removed = true;
                    i = expungeStaleEntry(i);
                }
            } while ( (n >>>= 1) != 0);
            return removed;
        }

 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;
        }