一、数据结构
一个线程当中，存有一个ThreadLoacalMap变量：
Thread.java中，有：
ThreadLocal.ThreadLocalMap threadLocals = null;
ThreadLocalMap，简单来说就是一个类似Map的数据结构，它存储着键值对
ThreadLocalMap中有个Entry类
/**
* 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;
}
}
也就是说，在这个哈希map中的键值对Entry继承自ThreadLocal的弱引用，用传入的ThreadLocal的弱引用作为key，用任意对象Object作为值。
即：
未命名文件.png
这个数据结构的主要存储功能实现如下：
static class ThreadLocalMap{
private Entry[] table;
private Entry getEntry(ThreadLocal<?> key) {
int i = key.threadLocalHashCode & (table.length - 1);
Entry e = table[i];
if (e != null && e.get() == key)
return e;
else
return getEntryAfterMiss(key, i, e);
}
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.
Entry[] tab = table;
int len = tab.length;
int i = key.threadLocalHashCode & (len-1);
for (Entry e = tab[i];
e != null;
e = tab[i = nextIndex(i, len)]) {
ThreadLocal<?> k = e.get();
if (k == key) {
e.value = value;
return;
}
if (k == null) {
replaceStaleEntry(key, value, i);
return;
}
}
tab[i] = new Entry(key, value);
int sz = ++size;
if (!cleanSomeSlots(i, sz) && sz >= threshold)
rehash();
}
}
用threadlocal键的哈希code按位与上哈希表的长度得到在哈希表中的位置。从而给Entry数组table第i个位置赋值，赋为当前threadlocal为键和要存储的对象为值的Entry。
那么ThreadLocal往ThreadLocal中存储什么类型的值呢？
public class ThreadLocal<T> {
}
泛型T则为往ThreadLocalMap放的值的类型。
二、流程分析
如题目，ThreadLocal如何使得一个线程只有一个Looper?
首先看Looper.java，与ThreadLocal相关的代码：
public final class Looper {
static final ThreadLocal<Looper> sThreadLocal = new ThreadLocal<Looper>();
private static void prepare(boolean quitAllowed) {
if (sThreadLocal.get() != null) {
throw new RuntimeException("Only one Looper may be created per thread");
}
sThreadLocal.set(new Looper(quitAllowed));
}
}
可以看到，它存有一个使用ThreadLocal<Looper> 对象的get方法判断是否哈希map中已经存过Looper，有即抛异常，没有就存入一个new Looper(quitAllowed)对象。
我们来看ThreadLocal中的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) {
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
}
}
return setInitialValue();
}
首先获取当前线程的ThreadLocalMap，如果map为空，返回的就是setInitialValue()；
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;
}
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}
就是说，如果当前线程里的ThreadLocalMap为空，就创建一个ThreadLocalMap，这个map里先存上第一个键值对--即当前threadLocal为键，null为值。
setInitialValue()返回的是null。
回到ThreadLocal的get方法：
if (map != null) {
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
}
}
如果map部位空，就根据键找j键值对，找到了，就返回键值对中的值。找不到，还是会调用setInitialValue()，存上键值对--即当前threadLocal为键，null为值。
所以呢，由于一个线程跟一个ThreadLocalMap是绑定的，如果这个Map中存有当前Looper里的sThreadLocal为键的键值对，就不会再存粗Looper而会抛异常了，反之，就会存入当前Looper里的sThreadLocal为键，new Looper(quitAllowed)为值的对。
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