线程本地变量ThreadLocal

线程本地变量ThreadLocal

线程本地变量只能被本线程访问，不能被其他线程访问。

ThreadLocal相关核心类

  • Thread，有两个属性threadLocals，inheritableThreadLocals，用来保存线程本地变量，threadLocals由ThreadLocal.class维护，inheritableThreadLocals由InheritableThreadLocal.class维护

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

    /*
     * InheritableThreadLocal values pertaining to this thread. This map is
     * maintained by the InheritableThreadLocal class.
     */
    ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;

  • ThreadLocal 保存线程本地变量的工具类

  • ThreadLocal.ThreadLocalMap，保存线程本地变量的容器，K是ThreadLocal实例化对象，值为变量值，保存形式是ThreadLocal.ThreadLocalMap.Entry数组，值在数组的下标由ThreadLocal实例化对象hash后获取

  • ThreadLocal.ThreadLocalMap.Entry，保存线程本地变量K-V的类，K是弱引用

  • WeakReference，弱引用， 当对象仅被弱引用指向，不被其他强引用指向，不论内存空间是否足够，GC都回收。

常用ThreadLocal
  • ThreadLocal 操作线程本地变量
  • InheritableThreadLocal 可以读取父线程以及本线程的变量
常见问题
  • ThreadLocal可以跨线程使用吗
  • ThreadLocal会内存泄漏吗
重要源码阅读
  • inheritableThreadLocals初始化
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// Thread 类： 获取父线程
Thread parent = currentThread();
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// 如果使用inheritThreadLocals 则根据父线程本地变量创建本地线程本地变量
if (inheritThreadLocals && parent.inheritableThreadLocals != null)
    this.inheritableThreadLocals =
        ThreadLocal.createInheritedMap(parent.inheritableThreadLocals);
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// ThreadLocal类： 创建ThreadLocalMap，从父线程把变量循环放入本地线程变量
private ThreadLocalMap(ThreadLocalMap parentMap) {
    Entry[] parentTable = parentMap.table;
    int len = parentTable.length;
    setThreshold(len);
    table = new Entry[len];

    for (int j = 0; j < len; j++) {
        Entry e = parentTable[j];
        if (e != null) {
            @SuppressWarnings("unchecked")
            ThreadLocal<Object> key = (ThreadLocal<Object>) e.get();
            if (key != null) {
                Object value = key.childValue(e.value);
                Entry c = new Entry(key, value);
                int h = key.threadLocalHashCode & (len - 1);
                while (table[h] != null)
                    h = nextIndex(h, len);
                table[h] = c;
                size++;
            }
        }
    }
}
  • ThreadLocal.ThreadLocalMap的set方法
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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();
}