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：