ThreadLocal随笔

Posted 2021-03-19 xieyanke

?

ThreadLocal<T> 用于存储线程缓存,用简单的操作实现线程间缓存的操作,做到缓存隔离

下面贴出源码:

//如图第①步 像线程中存入123 至于数据结构怎么存 下面解释
public class App 
{
    public static ThreadLocal<Integer> threadLocal = new ThreadLocal<Integer>();

    public static void main( String[] args )
    {
        threadLocal.set(123);

        threadLocal.get();
    }
}



public void set(T value) {
	//获取当前线程
    Thread t = Thread.currentThread();

    //如图第②步 第③步 获取当前线程t中的threadLocals
    ThreadLocalMap map = getMap(t);

    if (map != null)
    	//如图第④步 向map中put键值对,键为当前threadLocal对象,值为value,
    	//至于map如何操作可近似理解为 HashMap的put操作 不是本文重点不多讲
        map.set(this, value);
    else
    	//如果当前线程threadLocals为空 则创建并以此键值对初始化
        createMap(t, value);
}
//获取线程中的threadLocals
ThreadLocalMap getMap(Thread t) {
    return t.threadLocals;
}
//创建并初始化threadLocals
void createMap(Thread t, T firstValue) {
    t.threadLocals = new ThreadLocalMap(this, firstValue);
}

以上代码简述就是 向Thread中有个threadLocals中存入<ThreadLocal, value> 的键值对,对于其工作原理基本有了大致的了解；

下面介绍线程缓存中的取值代码

贴出取值代码:

public T get() {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null) {
    	//类似HashMap取值,键为当前threadLocal对象
        ThreadLocalMap.Entry e = map.getEntry(this);
        //如果不为空则做Object的转换
        if (e != null) {
            @SuppressWarnings("unchecked")
            T result = (T)e.value;
            return result;
        }
    }
    //如果map为空则以 键值对<this,null>初始化 Thread t中属性threadLocals
    return setInitialValue();
}
//这段代码就是set的翻版 只不过value的值为null
private T setInitialValue() {
    T value = initialValue();
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null)
        map.set(this, value);
    else
        createMap(t, value);
    return value;
}

总结:ThreadLocal对象通常为全局共享对象,对于每个线程来说,都以此对象为key,