ThreadLocal估计很多人都没听过,因为说实话平时写代码很少用到,我第一次看到代码里用ThreadLocal是在Looper的源码,Looper的源码中有下面这样的一个定义:
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
定义
大家可以百度看,都说的很高大上,普通的解释是:当使用ThreadLocal维护变量时，ThreadLocal为每个使用该变量的线程提供独立的变量副本，所以每一个线程都可以独立地改变自己的副本，而不会影响其它线程所对应的副本。
看完了感觉没看一样, 要想了解它其实去看他的源码一看便知,因为ThreadLocal代码也就几百行而已.
ThreadLocal类主要提供set与get方法, 我们一起来看看get与set是如何与线程关联起来的.
普通的大家想把一个类与线程关联起来,我们会采用线程号作为ID,该数据作为value,可以存进一个线程安全的map里.但是ThreadLocal不是这么干的.
ThreadLocal
我们开启源码模式,JDK6与JDK7实现有些区别,不过思想是一样的.这边使用JDK7的源码
/**
* Sets the current thread's copy of this thread-local variable
* to the specified value. Most subclasses will have no need to
* override this method, relying solely on the {@link #initialValue}
* method to set the values of thread-locals.
*
* @param value the value to be stored in the current thread's copy of
* this thread-local.
*/
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
我们看到,我们存储的数据就被map这个变量set进去了.
那么map来自于以下代码.
/**
* Get the map associated with a ThreadLocal. Overridden in
* InheritableThreadLocal.
*
* @param t the current thread
* @return the map
*/
ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}
/**
* Create the map associated with a ThreadLocal. Overridden in
* InheritableThreadLocal.
*
* @param t the current thread
* @param firstValue value for the initial entry of the map
* @param map the map to store.
*/
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}
大家发现了,这个map竟然是Thread这个类的一个变量,初始时为空,拿不到map会自动给它创建一个map.
ThreadLocalMap
可以再看看,这个ThreadLocalMap类:
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;
}
}
/**
* The initial capacity -- MUST be a power of two.
*/
private static final int INITIAL_CAPACITY = 16;
/**
* Set the value associated with key.
*
* @param key the thread local object
* @param value the value to be set
*/
private void set(ThreadLocal key, Object value) {
// We don't use a fast path as with get() because it is at
// least as common to use set() to create new entries as
// it is to replace existing ones, in which case, a fast
// path would fail more often than not.
...
if (!cleanSomeSlots(i, sz) && sz >= threshold)
rehash();
}
常看源码的同学会发现了,这个类没有实现Map的接口,但是有相似的设计,我们看到了INITIAL_CAPACITY,看到了Entry这些在HashMap源码中常见的命名方式,我们还看到了set方法中rehash这个重新hash的方法,那么总体而言,我们可以把这个类当做这个是一个K为ThreadLocal的map.
总结
知道上面两点后,我们知道了.Thread这个线程类中含有一个ThreadLocalMap变量,我们的ThreadLocal只是帮助你存对象的,真正你使用ThreadLocal存储一个与线程相关的对象时,其实这个对象已经交给了这个线程.
什么 还不相信? 那我们看Thread类的代码:
public class Thread implements Runnable {
188行处...
/* ThreadLocal values pertaining to this thread. This map is maintained
* by the ThreadLocal class. */
ThreadLocal.ThreadLocalMap threadLocals = null;
}
好了,的确Thread类有这么一个变量.
再看定义: 每一个线程都可以独立地改变自己的副本，而不会影响其它线程所对应的副本。
这次都该明白了,就是每个Thread都存储着自己的局部变量,当然不会影响另一个Thread的threadLocals变量拉.
因此我们可以使用ThreadLocal,来存储与线程绑定的数据.我们必须记住,ThreadLocal存储的变量是属于每个线程的,并不是表面上看起来只有一个.
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