标签:map 添加 private 机制 oca get hash nec table
ThreadLocal可以很方便的在一个线程运行周期内传递各种数据,而不用在调用栈的每个方法上添加新的参数
它的运行机制也比较简单,每个线程实例内部都会有一个 ThreadLocal.ThreadLocalMap threadLocals(类型具体定义在ThreadLocal.class 内部)类型的字段,在调用ThreadLocal.set的时候会将自己实例作为key和value包装到一个Entry类型实例里一起存储在threadLocals的table中,因为是数组类型,所以会根据ThreadLocal实例的hashcode和数组长度做 & 运算,计算出位置,这里解决hash冲突的方式用的是开放定址法(好像是?),在发现一个位置已经存放了数据后,会向后移动一个位置,直到那里没有被使用或者ThreadLocal的实例被回收。
private void set(ThreadLocal<?> key, Object value) { ... Entry[] tab = table; int len = tab.length; int i = key.threadLocalHashCode & (len-1); for (Entry e = tab[i]; e != null; e = tab[i = nextIndex(i, len)]) { ThreadLocal<?> k = e.get(); if (k == key) { e.value = value; return; } if (k == null) { replaceStaleEntry(key, value, i); return; } } tab[i] = new Entry(key, value); ... }
为什么ThreadLocal在table中有引用还是会被回收呢?由于在Entry类型中k是WeakReference,当ThreadLocal实例不被强引用后就会被回收,但对应Entry实例并不会被回收,依然还在table里(似乎会产生泄漏,一部分内存没有被使用也无法被回收),下面看一下threadLocals的类型定义
static class ThreadLocalMap { /** * The entries in this hash map extend WeakReference, using * its main ref field as the key (which is always a * ThreadLocal object). Note that null keys (i.e. entry.get() * == null) mean that the key is no longer referenced, so the * entry can be expunged from table. Such entries are referred to * as "stale entries" in the code that follows. */ static class Entry extends WeakReference<ThreadLocal<?>> { /** The value associated with this ThreadLocal. */ Object value; Entry(ThreadLocal<?> k, Object v) { super(k); value = v; } }
/**
* The table, resized as necessary.
* table.length MUST always be a power of two.
*/
private Entry[] table;
}
标签:map 添加 private 机制 oca get hash nec table
原文地址:https://www.cnblogs.com/zhujiayi/p/12817988.html