ThreadLocal<T>学习总结

public class ThreadLocalTest {

    /**
     * @param
     * @Author: xdj
     * @Date: 2019/4/12 10:16
     * @Description:
     * @return: void
     * ThreadLocal是一种变量类型，我们称之为“线程局部变量”
     * 每个线程访问这种变量的时候都会创建该变量的副本，这个变量副本为线程私有
     * ThreadLocal类型的变量一般用private static加以修饰
     */
    @Test
    public void test() {

         ThreadLocal threadLocal = new ThreadLocal(){
             @Override
             protected Object initialValue() {
                 return Thread.currentThread().getId();
             }
         };

//        threadLocal.set("测试threadLocal");

        System.out.println(threadLocal.get());

    }
}

学习ThreadLocal<T>，首先先从它的数据结构开始，ThreadLocal的内部类ThreadLocalMap

// 只是截取部分
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;
            }
        }

而ThreadLocalMap又存储在Thread中。

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

ThreadLocal中的几个重要方法，提供了重要的操作

get方法

这里通过getMap方法获取当前线程中的所有ThreadLocal<T>记录threadLocals，先判断是否已有值，如果没有则调用setInitialvalue（）赋值

set()方法

先判断是否有值了，如果有则覆盖，没有则创建新的Map对象。

remove方法

总结：Thread通过ThreadLocalMap来存储线程中的变量，第一次获取时需要先通过set（）将值保存进map中。通过泛型来区分给个变量的key。

参考：

　　https://www.cnblogs.com/dolphin0520/p/3920407.html