ThreadLocal 浅析

ThreadLocal使用场景及用法

ThreadLocal常用在框架中，如Struts中存储请求对象(包括请求参数、上下文环境变量)， 项目中存储登录的用户信息， 在一个请求的过程中设置ThreadLocal值后，执行到其他方法时， 都可以获取设置的值

class AccountContext {
    private static final ThreadLocal<Account> account = new ThreadLocal<Account>();

    public static final Account getCurrentAccount() {
        return account.get();
    }

    public static final void setCurrentAccount(Account account) {
        account.set(account);
    }

    public static final void removeCurrentAccount() {
        account.remove();
    }
}

ThreadLocal、ThreadLocal.ThreadLocalMap、Thread类的关系及存取过程

1、ThreadLocalMap是ThreadLocal类的内部类，ThreadLocalMap数据结构，类似于HashMap，存储key/value，key:ThreadLocal对象， value:ThreadLocal对象存储的目标值(即上术例子中的Account对象)

2、Thread类有一成员变量，ThreadLocal.ThreadLocalMap threadLocals = null

3、threadLocal对象在执行set、get方法时， 
a、获取当前线程，即：Thread.currentThread()
b、获取ThreadLocalMap, 即：上一步的currentThread.threadLocals

4、set方法
如果获取的ThreadLocalMap为空， 则创建ThreadLocalMap， 将创建的threadLocalMap赋值给当前线程的threadLocals，threadLocals中存储key：threadLocal对象, value：目标对象

5、get方法
从当前线程的threadLocalMap中获取key=threadLocal 对应的数据

ThreadLocal类的常用方法分析

1、set方法

public void set(T value) {
    Thread t = Thread.currentThread();//获取当前的线程
    ThreadLocalMap map = getMap(t);
    if (map != null) //判断ThreadLocalMap中是否有值，如果没有，则创建ThreadLocalMap
        map.set(this, value);
    else
        createMap(t, value);
}

ThreadLocalMap getMap(Thread t) {
    return t.threadLocals;  //threadLocals是Thread类的属性
}

public class Thread implements Runnable {
     ........
    ThreadLocal.ThreadLocalMap threadLocals = null;//threadLocals类型是：ThreadLocal.ThreadLocalMap类型变量，存储类似key，value的形式数据，key:threadLocal对象，value:真正要在threadLocal中存储的数据(即目标数据)


void createMap(Thread t, T firstValue) {
  t.threadLocals = new ThreadLocalMap(this, firstValue);//创建ThreadLocalMap对象，并将要存储的值，存储放到ThreadLocalMap中， key为ThreadLocal对象， value为thredLocal.set()方法的参数值
}

2、get方法

public T get() {
    Thread t = Thread.currentThread();
    ThreadLocalMap map = getMap(t);
    if (map != null) {
        ThreadLocalMap.Entry e = map.getEntry(this);
        if (e != null)
            return (T)e.value;
    }
    return setInitialValue();
}