ThreadLocal资源泄漏和WeakReference [英] ThreadLocal Resource Leak and WeakReference

问题描述

我对 ThreadLocal 的理解有限它有资源泄漏问题。我认为可以通过正确使用来解决这个问题。使用ThreadLocal的WeakReferences （虽然我可能误解了这一点。）我想简单地使用一个模式或示例来正确使用带有WeakReference的ThreadLocal（如果存在）。例如，在这段代码片段中会引入WeakReference吗？

My limited understanding of ThreadLocal is that it has resource leak issues. I gather this problem can be remedied through proper use of WeakReferences with ThreadLocal (although I may have misunderstood this point.) I would simply like a pattern or example for correctly using ThreadLocal with WeakReference, if one exists. For instance, in this code snippet where would the WeakReference be introduced?

static class DateTimeFormatter {
    private static final ThreadLocal<SimpleDateFormat> DATE_PARSER_THREAD_LOCAL = new ThreadLocal<SimpleDateFormat>() {
        protected SimpleDateFormat initialValue() {
            return new SimpleDateFormat("yyyy/MM/dd HH:mmz");
        }
    };
    public String format(final Date date) {
        return DATE_PARSER_THREAD_LOCAL.get().format(date);
    }
    public Date parse(final String date) throws ParseException
    {
      return DATE_PARSER_THREAD_LOCAL.get().parse(date);
    }
}

推荐答案

ThreadLocal 在内部使用 WeakReference 。如果没有强引用 ThreadLocal ，它将被垃圾收集，即使各种线程通过 ThreadLocal存储值。

ThreadLocal uses a WeakReference internally. If the ThreadLocal is not strongly referenced, it will be garbage-collected, even though various threads have values stored via that ThreadLocal.

此外， ThreadLocal 值实际存储在线程;如果线程死亡，则收集与该线程通过 ThreadLocal 关联的所有值。

Additionally, ThreadLocal values are actually stored in the Thread; if a thread dies, all of the values associated with that thread through a ThreadLocal are collected.

如果你有一个 ThreadLocal 作为最终的类成员，这是一个强引用，并且在卸载该类之前无法收集它。但这是任何类成员的工作方式，并且不被视为内存泄漏。

If you have a ThreadLocal as a final class member, that's a strong reference, and it cannot be collected until the class is unloaded. But this is how any class member works, and isn't considered a memory leak.

更新：当存储在 ThreadLocal 中的值强烈引用 ThreadLocal — sort时，引用的问题才会发挥作用循环引用。

Update: The cited problem only comes into play when the value stored in a ThreadLocal strongly references that ThreadLocal—sort of a circular reference.

在这种情况下，值（a SimpleDateFormat ）没有向后引用 ThreadLocal的。此代码中没有内存泄漏。

In this case, the value (a SimpleDateFormat), has no backwards reference to the ThreadLocal. There's no memory leak in this code.

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