ThreadLocal随笔

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,

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