ThreadLocal VS FastThreadlocal
ThreadLocal:线程局部,但更多听到是线程局部变量，所谓局部即：单个线程内变量可共享，在并发线程中见到他的概率更多，但解决的并非线程安全问题。
This class provides thread-local variables. These variables differ from * their normal counterparts in that each thread that accesses one (via its * {@code get} or {@code set} method) has its own, independently initialized * copy of the variable. {@code ThreadLocal} instances are typically private * static fields in classes that wish to associate state with a thread (e.g., * a user ID or Transaction ID).ThreadLocal源码中写道
这个类提供线程局部变量。 这些变量与其正常的对应方式不同，因为访问一个的每个线程（通过其get或set方法）都有自己独立初始化的变量副本。 ThreadLocal实例通常是希望将状态与线程关联的类中的私有静态字段（例如，用户ID或事务ID）
1.如果多个线程访问同一个共享Threadlocal变量，是保证线程隔离的，A,B,C访问的即Threadlocal的变量副本
2.与线程状态相关，单个线程内，该线程持有该资源变量是私有的
Threadlocal并不是为了保证线程安全，而是为了在线程内方便共享变量跨方法的传递
package com.atkk.kk.netty.websocket;
public class ThreadLocalDemo { public static ThreadLocal<Order> orderThreadLocal = new ThreadLocal<>();
public static void main(String[] args) { Order order = new Order(); System.out.println(order.hashCode()); orderThreadLocal.set(order); EntityBussiness1 entityBussiness1 = new EntityBussiness1(); 其他线程下是隔离的 new Thread(()->{ EntityBussiness2 entityBussiness2 = new EntityBussiness2(); entityBussiness2.run(); }).start(); entityBussiness1.run(); }
static class EntityBussiness1{ public void run(){ Order order = orderThreadLocal.get(); if(order == null){ System.out.println("threadlocal == null in thread: " + Thread.currentThread().getName()); return; } System.out.println(order.hashCode()); } }
static class EntityBussiness2{ public void run(){ Order order = orderThreadLocal.get(); if(order == null){ System.out.println("threadlocal == null in thread: " + Thread.currentThread().getName()); return; } System.out.println(order.hashCode()); } }}
此时两个方法获取的实体类hashcode值是一致的，在新线程下不能共享变量所以第二个对象为null
205125520205125520threadlocal == null in thread: Thread-0应用场景：在Httpclient下共享返回码，cookie等等
public class HttpClient { private static ThreadLocal<Integer> httpState = new ThreadLocal<Integer>(); // http 本次请求返回的状态码 private static ThreadLocal<String> httpCookieString = new ThreadLocal<String>(); // http 本次请求返回的cookie getRs 用这个方法时有效 /** * 最好能在过滤器里面调用下该方法，可以减少内存消耗 */ public static void removeThreadLocal(){ httpState.remove(); httpCookieString.remove(); } }新听到一个组件叫BdUtil,其实现是基于Threadlocal的实现来写的，说起来项目中的应用场景不多，不适用于多线程分布式框架的传参。
FastThreadlocal 更快的Theadlocal
如果你看过Netty的源码，其中 io.netty.util.concurrent包下有一个非常类似的FastThreadlocal，其构造方法与其非常相似，但是网上所说其性能是Threadlocal的3倍，是Threadlocal的变体。
A special variant of {@link ThreadLocal} that yields
higher access performance when accessed from a
{@link FastThreadLocalThread}.private static FastThreadLocal<Integer> fastThreadLocal = new FastThreadLocal<>();
public static void main(String[] args) { //if (thread instanceof FastThreadLocalThread) 使用FastThreadLocalThread更优，普通线程也可以 new FastThreadLocalThread(() -> { for (int i = 0; i < 100; i++) { fastThreadLocal.set(i); System.out.println(Thread.currentThread().getName() + "====" + fastThreadLocal.get()); try { Thread.sleep(200); } catch (InterruptedException e) { e.printStackTrace(); } } }, "fastThreadLocal1").start();
new FastThreadLocalThread(() -> { for (int i = 0; i < 100; i++) { System.out.println(Thread.currentThread().getName() + "====" + fastThreadLocal.get()); try { Thread.sleep(200); } catch (InterruptedException e) { e.printStackTrace(); } } }, "fastThreadLocal2").start(); }Connected to the target VM, address: '127.0.0.1:60863', transport: 'socket'17:08:00.714 [main] DEBUG io.netty.util.internal.logging.InternalLoggerFactory - Using SLF4J as the default logging framework17:08:00.728 [main] DEBUG io.netty.util.internal.InternalThreadLocalMap - -Dio.netty.threadLocalMap.stringBuilder.initialSize: 102417:08:00.731 [main] DEBUG io.netty.util.internal.InternalThreadLocalMap - -Dio.netty.threadLocalMap.stringBuilder.maxSize: 4096fastThreadLocal1====0fastThreadLocal2====nullfastThreadLocal2====nullfastThreadLocal1====1fastThreadLocal1====2fastThreadLocal2====nullfastThreadLocal1====3fastThreadLocal2====nullfastThreadLocal1====4fastThreadLocal2====nullfastThreadLocal1====5FastThreadLocal只有被的线程是FastThreadLocalThread或者其子类使用的时候才会更快，吞吐量我这边测试的效果大概3倍左右，但是如果是普通线程操作FastThreadLocal其吞吐量比ThreadLocal还差！
阿里TTL异步执行上下文对象传递
https://blog.csdn.net/boonya/article/details/55096596
吾常身不离鞍，髀肉皆消。今不复骑，髀里肉生。日月若驰，老将至矣，而功业不建，是以悲耳。 -----三国志》卷三十二〈蜀书·先主备传〉
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