ThreadLocal中的WeakReference

素颜马尾好姑娘i 2023-07-03 12:22 6阅读 0赞

Thread对象有一个变量名为threadLocals的ThreadLocalMap对象，里面定义了一个Entry数组，Entry对象弱引用了ThreadLocal。

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.
    
                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);
                int sz = ++size;
                if (!cleanSomeSlots(i, sz) && sz >= threshold)
                    rehash();
            }

if (k == null) { 
                        replaceStaleEntry(key, value, i);
                        return;
                    }

Entry弱引用了ThreadLocal，因此Entry是否存活，不会影响ThreadLocal的生命周期，ThreadLocal在没有其它对象引用后被回收，但是它对应的Entry对象中的value实际还在被Entry引用，如果不清掉Entry，value就会一直强引用，无法释放，那么就可能会内存溢出。（用完要remove()或者下次set()时会检查）

弱引用在下次GC时会被回收掉，前提条件是弱引用的对象不存在其它的强引用和软引用。

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