Android Looper对象中就有ThreadLocal的应用
public final class Looper {
static final ThreadLocal sThreadLocal = new ThreadLocal();
// 对外提供的接口
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));
}
// 对外提供的接口
public static @Nullable Looper myLooper() {
return sThreadLocal.get();
}
}
Thread类
public class Thread implements Runnable {
/**
* threadLocals和线程关联，由ThreadLocal负责维护
*/
ThreadLocal.ThreadLocalMap threadLocals = null;
// 子线程会继承父线程的inheritableThreadLocals(继承关系)
ThreadLocal.ThreadLocalMap inheritableThreadLocals = null;
}
ThreadLocal类
public class ThreadLocal {
private static final int HASH_INCREMENT = 0x61c88647;
private static AtomicInteger nextHashCode =
new AtomicInteger();
private final int threadLocalHashCode = nextHashCode();
private static int nextHashCode() {
return nextHashCode.getAndAdd(HASH_INCREMENT);
}
public ThreadLocal() {}
public void set(T value) {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
map.set(this, value);
else
createMap(t, value);
}
// 线程初始值为null
ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}
static class ThreadLocalMap {
private static final int INITIAL_CAPACITY = 16;
private Entry[] table;
private int size = 0;
private int threshold;
private void setThreshold(int len) {
threshold = len * 2 / 3;
}
// hash冲突的时候，取下一个位置值
private static int nextIndex(int i, int len) {
return ((i + 1 < len) ? i + 1 : 0);
}
private static int prevIndex(int i, int len) {
return ((i - 1 >= 0) ? i - 1 : len - 1);
}
static class Entry extends WeakReference> {
Object value;
Entry(ThreadLocal k, Object v) {
super(k);
value = v;
}
}
ThreadLocalMap(ThreadLocal firstKey, Object firstValue) {
table = new Entry[INITIAL_CAPACITY];
int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
table[i] = new Entry(firstKey, firstValue);
size = 1;
setThreshold(INITIAL_CAPACITY);
}
//【重点 set】
private void set(ThreadLocal key, Object value) {
Entry[] tab = table;
int len = tab.length;
int i = key.threadLocalHashCode & (len-1);
// hash冲突的时候，取下一个位置
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) {
// 直接替换脏数据
replaceStaleEntry(key, value, i);
return;
}
}
tab[i] = new Entry(key, value);
int sz = ++size;
if (!cleanSomeSlots(i, sz) && sz >= threshold)
rehash();
}
//【重点 get】
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);
}
// get时候发现hash冲突，需要向后寻找指定的key
private Entry getEntryAfterMiss(ThreadLocal key, int i, Entry e) {
Entry[] tab = table;
int len = tab.length;
while (e != null) {
ThreadLocal k = e.get();
if (k == key) return e;
if (k == null) expungeStaleEntry(i);
else i = nextIndex(i, len); // 寻找数组下一个Entry
e = tab[i];
}
return null;
}
// 去除key = null的节点
private int expungeStaleEntry(int staleSlot) {
Entry[] tab = table;
int len = tab.length;
// 将staleSlot节点清除
tab[staleSlot].value = null;
tab[staleSlot] = null;
size--;
Entry e;
int i;
// 继续从staleSlot开始向后寻找是否有key=null的节点
for (i = nextIndex(staleSlot, len);
(e = tab[i]) != null;
i = nextIndex(i, len)) {
ThreadLocal k = e.get();
if (k == null) {
e.value = null;
tab[i] = null;
size--;
} else {
int h = k.threadLocalHashCode & (len - 1);
if (h != i) {
tab[i] = null;
while (tab[h] != null)
h = nextIndex(h, len);
tab[h] = e;
}
}
}
return i;
}
// 【重点 扩容】
private void resize() {
Entry[] oldTab = table;
int oldLen = oldTab.length;
// 2倍容量扩容
int newLen = oldLen * 2;
Entry[] newTab = new Entry[newLen];
int count = 0;
for (int j = 0; j < oldLen; ++j) {
Entry e = oldTab[j];
if (e != null) {
ThreadLocal k = e.get();
if (k == null) {
e.value = null; // Help the GC
} else {
// 重新hash过程,冲突就寻找下一个不为null的Entry
int h = k.threadLocalHashCode & (newLen - 1);
while (newTab[h] != null)
h = nextIndex(h, newLen);
newTab[h] = e;
count++;
}
}
}
setThreshold(newLen);
size = count;
table = newTab;
}
}
// 最终会走ThreadLocalMap的getEntry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) {
T result = (T)e.value;
return result;
}
}
return setInitialValue();
}
}
ThreadLocal为什么会内存泄漏？
因为ThreadLocalMap.Entry里面的ThreadLocal被设计为弱引用类型，当发生GC的时候，key = null, 但是value却不能够被回收，
key弱引用并不是导致内存泄漏的原因，而是因为ThreadLocalMap的生命周期与当前线程一样长，并且没有手动删除对应key。
在使用完ThreadLocal时，及时调用它的的remove方法清除数据。
参考： 从源码深入详解ThreadLocal内存泄漏问题