ThreadLocal源码解析

最新推荐文章于 2025-11-23 22:03:29 发布

原创

最新推荐文章于 2025-11-23 22:03:29 发布 · 348 阅读

0 ·

CC 4.0 BY-SA版权

文章标签：

#java #开发语言

使用ThreadLocal可以为每个线程维护一个线程变量，使用场景为线程间隔离，线程内方法共享；

原理：

Thread类中有一个实例属性ThreadLocalMap，ThreadLocalMap中存放的是Entry数组，Entry数组是ThreadLocal和Object的键值对；源码如下--

Thread类：

/*
 * ThreadLocal values pertaining to this thread. This map is maintained
 * by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals;

ThreadLocal->ThreadLocalMap->Entry类：

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;
        }
    }

ThreadLocal有一个内部静态类ThreadLocalMap, ThreadLocalMap是一个Entry数组，通过map方法来访问数据，而Entry对key（ThreadLocal实例对象）的引用使用了虚引用；这里导致key可能会在JVM判断oom之前将key回收掉，导致内存泄漏-此时key对应的ThreadLocal对象被回收变成null，而value存在Thread reference->thread->threadlocalMap->entry-value的强引用链，导致无法回收；虽然在ThreadLocal的get