前言
ThreadLocal是一个我了解后很喜欢的东西，这种思路使得变量的作用域限于线程内，并且不同线程具有不同的数据副本。典型应用场景即Looper，每个线程有且只有一个Looper，同时，不同线程对应不同的Looper，而Handler在创建时，就可以获得当前线程的Looper，使得回调时，也能切换回该线程。
新建对象
在使用ThreadLocal时，需要新建对象：
//ActivitayThread中也使用了ThreadLocal
private static final ThreadLocal<Intent> sCurrentBroadcastIntent = new ThreadLocal<Intent>();
ThreadLocal 是一个泛型类，并且就该类本身只有有限的几个方法，同时还有两个静态内部类，可以看出，看懂get和set方法的情况下，基本就可以了解ThreadLocal的工作原理，已添加注释解析，如下所示。
private final int threadLocalHashCode = nextHashCode();
private static AtomicInteger nextHashCode =
new AtomicInteger();
private static final int HASH_INCREMENT = 0x61c88647;
private static int nextHashCode() {
return nextHashCode.getAndAdd(HASH_INCREMENT);
}
protected T initialValue() {
return null;
}
public static <S> ThreadLocal<S> withInitial(Supplier<? extends S> supplier) {
return new SuppliedThreadLocal<>(supplier);
}
public ThreadLocal() {
}
public T get() {
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null) {
//获取对应的value
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
}
}
//懒加载，未设置过值时会调用该方法，初始化map添加该TreadLocal的一对Key:Value Value为null
return 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;
}
public void set(T value) {
//获取当前线程以及对应的map
Thread t = Thread.currentThread();
ThreadLocalMap map = getMap(t);
if (map != null)
//把当前ThreadLocal作为key
map.set(this, value);
else
//初始化
createMap(t, value);
}
public void remove() {
ThreadLocalMap m = getMap(Thread.currentThread());
if (m != null)
m.remove(this);
}
//每个线程里都保存着一个ThreadLocal.ThreadLocalMap的变量，由ThreadLocalMap维护ThreadLocal的值
ThreadLocalMap getMap(Thread t) {
return t.threadLocals;
}
void createMap(Thread t, T firstValue) {
t.threadLocals = new ThreadLocalMap(this, firstValue);
}
static ThreadLocalMap createInheritedMap(ThreadLocalMap parentMap) {
return new ThreadLocalMap(parentMap);
}
T childValue(T parentValue) {
throw new UnsupportedOperationException();
}
static final class SuppliedThreadLocal<T> extends ThreadLocal<T> {
//...
}
static class ThreadLocalMap { {
//...
}
每个Thread都会持有一个ThreadLocalMap，前面提到这是ThreadLocal的一个内部类，这是一个只用来存放ThreadLocal的HashMap类，类中维护了一个table数组，并且该类还有一个静态内部类：Entry,Entry继承于WeakReference，有一个value的字段，WeakReference持有的ThreadLocal对象当做key，value则是对应ThreadLocal的值,该类比较长，这里只给出相关代码。
static class ThreadLocalMap {
//...
/**
* The table, resized as necessary.
* table.length MUST always be a power of two.
*/
private Entry[] table;
/**
* 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;
}
}
//...
/**
* Construct a new map initially containing (firstKey, firstValue).
* ThreadLocalMaps are constructed lazily, so we only create
* one when we have at least one entry to put in it.
*/
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);
}
}
当创建一个ThreadLocal变量并且给它做get,set操作时，其实也是操作的ThreadLocalMap，所以接下来我们还需要看一下ThreadLocalMap这两个比较重要的方法，代码中直接插入注释解析。
/**
* Get the entry associated with key. This method
* itself handles only the fast path: a direct hit of existing
* key. It otherwise relays to getEntryAfterMiss. This is
* designed to maximize performance for direct hits, in part
* by making this method readily inlinable.
*
* @param key the thread local object
* @return the entry associated with key, or null if no such
*/
//通过hashCode 与 当前数组的最大下标做与逻辑，直接得到目标下标。
//判断与Key是否相等，不相等说明发生过Hash碰撞，ThreadLocalMap是开地址法，直接存入下一个空的区域。
//无法直接探测到的全部交由getEntryAfterMiss方法去处理
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);
}
/**
* Version of getEntry method for use when key is not found in
* its direct hash slot.
*
* @param key the thread local object
* @param i the table index for key's hash code
* @param e the entry at table[i]
* @return the entry associated with key, or null if no such
*/
//主要是遍历冲突后的空间里是否能找到和key相等的ThreadLocal，当遇到null时，会去删除该位置的Entry。
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);
e = tab[i];
}
return null;
}
/**
* Expunge a stale entry by rehashing any possibly colliding entries
* lying between staleSlot and the next null slot. This also expunges
* any other stale entries encountered before the trailing null. See
* Knuth, Section 6.4
*
* @param staleSlot index of slot known to have null key
* @return the index of the next null slot after staleSlot
* (all between staleSlot and this slot will have been checked
* for expunging).
*/
//删除当前位置的Entry以后会遍历该位置后的空间，确认是否是因为Hash冲突而后移了存储位置，将其纠正回原位。
//同时如果该Entry引用的ThreadLocal也已经被释放的话，删除该位置的值，直到下一个位置为null。
private int expungeStaleEntry(int staleSlot) {
Entry[] tab = table;
int len = tab.length;
// expunge entry at staleSlot
tab[staleSlot].value = null;
tab[staleSlot] = null;
size--;
// Rehash until we encounter null
Entry e;
int i;
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;
// Unlike Knuth 6.4 Algorithm R, we must scan until
// null because multiple entries could have been stale.
while (tab[h] != null)
h = nextIndex(h, len);
tab[h] = e;
}
}
}
return i;
}
理解get方法以后，set方法也很一目了然：
/**
* 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.
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();
//线性探测后如果该位置有值，且key相等，则直接覆盖value
if (k == key) {
e.value = value;
return;
}
//ThreadLocal已经被释放，且存在hash冲突或者已经平移过几位，一次性向前整理所有无用的Entry，并替换值
if (k == null) {
replaceStaleEntry(key, value, i);
return;
}
}
//新建一个Entry，同时如果已经存入的Entry数量超过threshold，会扩大table容量，删除已经释放TreadLocal的Entry
tab[i] = new Entry(key, value);
int sz = ++size;
if (!cleanSomeSlots(i, sz) && sz >= threshold)
rehash();
}
/**
* Re-pack and/or re-size the table. First scan the entire
* table removing stale entries. If this doesn't sufficiently
* shrink the size of the table, double the table size.
*/
private void rehash() {
expungeStaleEntries();
// Use lower threshold for doubling to avoid hysteresis
if (size >= threshold - threshold / 4)
resize();
}
/**
* Replace a stale entry encountered during a set operation
* with an entry for the specified key. The value passed in
* the value parameter is stored in the entry, whether or not
* an entry already exists for the specified key.
*
* As a side effect, this method expunges all stale entries in the
* "run" containing the stale entry. (A run is a sequence of entries
* between two null slots.)
*
* @param key the key
* @param value the value to be associated with key
* @param staleSlot index of the first stale entry encountered while
* searching for key.
*/
private void replaceStaleEntry(ThreadLocal<?> key, Object value,
int staleSlot) {
Entry[] tab = table;
int len = tab.length;
Entry e;
// Back up to check for prior stale entry in current run.
// We clean out whole runs at a time to avoid continual
// incremental rehashing due to garbage collector freeing
// up refs in bunches (i.e., whenever the collector runs).
int slotToExpunge = staleSlot;
//向前查找过期值，找到最前的一个不为Null但是过期的值，也有可能是staleSlot
for (int i = prevIndex(staleSlot, len);
(e = tab[i]) != null;
i = prevIndex(i, len))
if (e.get() == null)
slotToExpunge = i;
// Find either the key or trailing null slot of run, whichever
// occurs first
for (int i = nextIndex(staleSlot, len);
(e = tab[i]) != null;
i = nextIndex(i, len)) {
ThreadLocal<?> k = e.get();
// If we find key, then we need to swap it
// with the stale entry to maintain hash table order.
// The newly stale slot, or any other stale slot
// encountered above it, can then be sent to expungeStaleEntry
// to remove or rehash all of the other entries in run.
if (k == key) {
e.value = value;
tab[i] = tab[staleSlot];
tab[staleSlot] = e;
//将之前存在Hash冲突后平移存储的位置进行互调换，并且从该位置开始做废弃条目的清除
// Start expunge at preceding stale entry if it exists
if (slotToExpunge == staleSlot)
slotToExpunge = i;
cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
return;
}
// If we didn't find stale entry on backward scan, the
// first stale entry seen while scanning for key is the
// first still present in the run.
//向前查找没找到废弃Entry而向后找到时，将该位置赋值给slotToExpunge
if (k == null && slotToExpunge == staleSlot)
slotToExpunge = i;
}
//之前没有存过对应的ThreadLocal在该位置上存下Entry
// If key not found, put new entry in stale slot
tab[staleSlot].value = null;
tab[staleSlot] = new Entry(key, value);
//清除其他多余的废弃条目
// If there are any other stale entries in run, expunge them
if (slotToExpunge != staleSlot)
cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
}
/**
* Heuristically scan some cells looking for stale entries.
* This is invoked when either a new element is added, or
* another stale one has been expunged. It performs a
* logarithmic number of scans, as a balance between no
* scanning (fast but retains garbage) and a number of scans
* proportional to number of elements, that would find all
* garbage but would cause some insertions to take O(n) time.
*
* @param i a position known NOT to hold a stale entry. The
* scan starts at the element after i.
*
* @param n scan control: {@code log2(n)} cells are scanned,
* unless a stale entry is found, in which case
* {@code log2(table.length)-1} additional cells are scanned.
* When called from insertions, this parameter is the number
* of elements, but when from replaceStaleEntry, it is the
* table length. (Note: all this could be changed to be either
* more or less aggressive by weighting n instead of just
* using straight log n. But this version is simple, fast, and
* seems to work well.)
*
* @return true if any stale entries have been removed.
*/
//执行table数组长度的对数次数的expungeStaleEntry扫描
private boolean cleanSomeSlots(int i, int n) {
boolean removed = false;
Entry[] tab = table;
int len = tab.length;
do {
i = nextIndex(i, len);
Entry e = tab[i];
if (e != null && e.get() == null) {
n = len;
removed = true;
i = expungeStaleEntry(i);
}
} while ( (n >>>= 1) != 0);
return removed;
}
至此ThreadLocal的源码基本分析完毕，每个Thread自己维持自己的ThreadLocalMap，可以有多个ThreadLocal变量，并且都访问属于自己的ThreadLocalMap，有对应值的返回值，没有的通过initialValue返回null，多线程之间互相不干扰对方的数据。
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