一、ThreadLocal介绍
ThreadLocal是用来维护本线程的变量,为每一个线程分配一个只属于该线程的对象。并不能解决共享变量的并发问题.ThreadLocal是将各线程的值存入本线程的Map中,以ThreadLocal自身作为key,需要用时获取该线程之前存入的值。
二、ThreadLocal内部设计
类图关系uml图：
首先，来看一下ThreadLocal内部提供的几个方法
/**
*用来获取当前线程在threadLocal中保存的当前线程的变量副本
*/
public T get() { }
/**
*用来设置当前线程的变量副本。将当前线程变量的副本设置到threadLocal中
*/
public void set(T value) { }
/**
*用来移除当前线程在Threadlocal存储的变量副本
*/
public void remove() { }
/**
*一般是用来在使用时进行重写的，它是一个延迟加载方法。给当前线程初始化一个初始值
*/
protected T initialValue() { }
/**
* ThreadLocalMap 是ThreadLocal的核心
* ThreadLocalMap这个类在ThreadLocal中定义，它被每一个线程所持有。每一个线程ThreadLocalMap的key存储的是ThreadLocal共享的对象,value存储的是线程变量的副本的值。
*/
static class ThreadLocalMap
下面看一下ThreadLocal类是如何为每个线程创建一个变量的副本的。
1、首先看下get方法的实现
public T get() {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
}
}
return setInitialValue();
}
首先获取当前线程对象t,然后通过getMap(t)获取每一个线程中持有的Map对象，这里的Map就是ThreadLocalMap对象。 然后接着获取下面的<key,value>键值对hreadLocalMap.Entry e = map.getEntry(this); <font color=red>这里key是this,也就是说TreadLocalMap中存储的key为ThreadLocal的实例对象，不是当前线程的t</font>。 如何获取成功，则直接返回结果。如果未获取成功则调用setInitialValue()设置初始值。
对每一句进行仔细分析:
（1）首先看一下getMap方法中做了什么:
ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}
从上面getMap中调用当前线程t,返回当前线程t中的成员变量threadLocals。那么继续去Thread类中看一下这个成员变量.
/* ThreadLocal values pertaining to this thread. This map is maintained
* by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null;
说明了ThreadLocalMap是在ThreadLocal定义，被使用在Thread类中。
再看一下ThreadLocalMap中的实现
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;
}
}
可以看到ThreadLocalMap中的Entry继承了WeakReference，并且WeakReference的key值类型是ThreadLocal
(2) 然后看一下setInitialValue()方法
首先得到一个null的初始值initialValue()，然后判断ThreadLocalMap是否为null，不为null就直接将初始值set到ThreadLocalMap中,否则为空就创建一个ThreadLocalMap并将值set到ThreadLocalMap中。
到这里ThreadLocal为每个线程创建的变量副本设计的主要原理应该清楚了:
首先每个Thread线程里面持有ThreadLocal.ThreadLocalMap成员变量,线程的副本变量就是存储在ThreadLocal.ThreadLocalMap中的,它以ThreadLocal实例为key，副本变量为value。
初始时ThreadLocal.ThreadLocalMap为空 然后在调用get和set方法的时候进行ThreadLocal.ThreadLocalMap的初始化。
然后在当前线程里面如果要使用副本，就调用TheadLocal的get方法在ThreadLocal.ThreadLocalMap中查找
三、子线程获取父线程的属性用InheritableThreadLocal<T>
上面的ThreadLocal获取父线程中的属性，是获取不到的。
如果想要在子线程中获取到父线程中的属性,就需要用到InheritableThreadLocal<T>.下面看栗子
public static void main(String[] args) {
final ThreadLocal<String> inheritableThreadLocal = new InheritableThreadLocal<String>();
final ThreadLocal<String> threadLocal = new ThreadLocal<String>();
inheritableThreadLocal.set("inheritableThreadLocal 父线程 ....");
threadLocal.set("threadLoal父线程 。。。。");
Thread inheritableThread = new Thread(new Runnable() {
public void run() {
System.out.println(inheritableThreadLocal.get());
System.out.println(threadLocal.get());
}
});
inheritableThread.start();
}
运行的结果:
inheritableThreadLocal 父线程 ....
null
四、手写ThreadLocal
package test;
import java.util.Collections;
import java.util.HashMap;
import java.util.Map;
/**
* @author td
*/
public class MyThreadLocal<T> {
private Map<Thread, T> container = Collections.synchronizedMap(new HashMap<Thread, T>());
protected T initialValue() {
return null;
}
private T initilization(Thread currentThread ) {
T t = initialValue();
container.put(currentThread, t);
return t;
}
public T get() {
Thread currentThread = Thread.currentThread();
T t = container.get(currentThread);
if (t != null) {
return t;
}
return initilization(currentThread);
}
public void remove() {
Thread currentThread = Thread.currentThread();
if (this.container.containsKey(currentThread)) {
this.container.remove(currentThread);
}
}
public void set(T t) {
Thread currentThread = Thread.currentThread();
this.container.put(currentThread, t);
}
}
五、ThreadLocal应用案例
使用ThreadLocal保存线程的上下文,获取每个线程中的用户信息
public class ThreadContext {
private static ThreadLocal<String> userResurece = new ThreadLocal<>();
public static String getUser() {
return userResurece.get();
}
public static void bindUser(String user) {
userResurece.set(user);
}
public static String unbindUser() {
String user = userResurece.get();
userResurece.remove();
return user;
}
}
新建一个Filter，将每次请求的用户信息放到线程中去
public class PerFilter extends OncePerRequestFilter {
@Override
protected void doFilterInternal(HttpServletRequest httpServletRequest, HttpServletResponse httpServletResponse, FilterChain filterChain) throws ServletException, IOException {
try {
String user = ThreadContext.getUser();
if (user == null) {
Authentication authentication = SecurityContextHolder.getContext().getAuthentication();
Object object = authentication.getPrincipal();
if (object != null) {
UserEntity userEntity = (UserEntity) object;
String value = org.apache.commons.lang3.RandomStringUtils.randomAlphabetic(5);
System.out.println(userEntity.getUuid()+"----"+Thread.currentThread().getName()+ value);
ThreadContext.bindUser(userEntity.getUsername()+"----"+Thread.currentThread().getName()+value);
}else {
ThreadContext.bindUser("testtestUser");
}
}
filterChain.doFilter(httpServletRequest, httpServletResponse);
} finally {
// ThreadContext.unbindUser();
}
}
}
在方法中获取当前用户信息
private String getUser() {
/* Authentication authentication = SecurityContextHolder.getContext().getAuthentication();
UserEntity userEntity = (UserEntity) authentication.getPrincipal();*/
return ThreadContext.getUser();
}
运行结果：
zhangsan----http-nio-8080-exec-4HFGrU
zhangsan----http-nio-8080-exec-5zkHZG
zhangsan----http-nio-8080-exec-6BIJvY
zhangsan----http-nio-8080-exec-7uuljF
zhangsan----http-nio-8080-exec-8gzRWf
zhangsan----http-nio-8080-exec-9tDszn
zhangsan----http-nio-8080-exec-10WDekP
zhangsan----http-nio-8080-exec-1GYkoP
zhangsan----http-nio-8080-exec-3oYOnk
zhangsan----http-nio-8080-exec-4HFGrU
zhangsan----http-nio-8080-exec-5zkHZG
zhangsan----http-nio-8080-exec-6BIJvY
zhangsan----http-nio-8080-exec-7uuljF
zhangsan----http-nio-8080-exec-8gzRWf
zhangsan----http-nio-8080-exec-9tDszn
zhangsan----http-nio-8080-exec-1GYkoP
zhangsan----http-nio-8080-exec-2hwdaU
从上可以发现tomcat初始化线程池为10个，后面每次获取到的都相同。
在原有的基础上增加一个子线程，在子线程中给不同的用户发送消息。使用inheritableThreadLocal
//将ThreadLocal改为InheritableThreadLocal
private static ThreadLocal<String> userResurece = new InheritableThreadLocal<>();
public static String getUser() {
return userResurece.get();
}
public static void bindUser(String user) {
userResurece.set(user);
}
public static String unbindUser() {
String user = userResurece.get();
userResurece.remove();
return user;
}
//在子线程中直接使用
Thread thread = new Thread(new Runnable() {
@Override
public void run() {
System.out.println(getUser());
}
});
thread.start();
