ThreadLocal源码解读¶
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Thread ThreadLocalMap ThreadLocal 关系图¶
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ThreadLocal¶
ThreadLocal每次初始后threadLocalHashCode增加HASH_INCREMENT
public class MyThreadLocal {
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);
}
public static void main(String[] args) {
System.out.println(MyThreadLocal.nextHashCode());
System.out.println(MyThreadLocal.nextHashCode());
System.out.println(MyThreadLocal.nextHashCode());
System.out.println(MyThreadLocal.nextHashCode());
}
}
##### 输出结果
0
1640531527
-1013904242
626627285
##### 哈希算法
public class MyThreadLocal {
private final int threadLocalHashCode = nextHashCode();
private static final int INITIAL_CAPACITY = 16;
private static AtomicInteger nextHashCode = new AtomicInteger();
private static final int HASH_INCREMENT = 0x61c88647;
private static int nextHashCode() {
return nextHashCode.getAndAdd(HASH_INCREMENT);
}
public static void main(String[] args) {
System.out.println(MyThreadLocal.nextHashCode() & (INITIAL_CAPACITY - 1));
System.out.println(MyThreadLocal.nextHashCode() & (INITIAL_CAPACITY - 1));
System.out.println(MyThreadLocal.nextHashCode() & (INITIAL_CAPACITY - 1));
System.out.println(MyThreadLocal.nextHashCode() & (INITIAL_CAPACITY - 1));
}
}
##### 输出结果
0
7
14
5
##### 方法
public T get() {
Thread t = Thread.currentThread();
// 获取当前线程ThreadLocalMap
ThreadLocalMap map = getMap(t);
if (map != null) {
// 通过当前ThreadLocal的threadLocalHashCode哈希值获取Entry
ThreadLocalMap.Entry e = map.getEntry(this);
if (e != null) {
@SuppressWarnings("unchecked")
T result = (T)e.value;
return result;
}
}
return setInitialValue();
}
public void set(T value) {
Thread t = Thread.currentThread();
// 获取当前线程ThreadLocalMap
ThreadLocalMap map = getMap(t);
if (map != null)
// 通过当前ThreadLocal的threadLocalHashCode哈希值设置Entry
map.set(this, value);
else
// 当前线程没有ThreadLocalMap 场景ThreadLocalMap赋值给Tread
createMap(t, value);
}
public void remove() {
// 获取当前线程ThreadLocalMap
ThreadLocalMap m = getMap(Thread.currentThread());
if (m != null)
// 删除当前线程的ThreadLocal的entry
m.remove(this);
}
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ThreadLocalMap¶
ThreadlocalMap使用 线性探测 解决hash冲突
##### 成员变量
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;
/**
* The table, resized as necessary.
* table.length MUST always be a power of two.
*/
private Entry[] table;
/**
* The number of entries in the table.
*/
private int size = 0;
/**
* The next size value at which to resize.
*/
private int threshold; // Default to 0
##### 构造函数
ThreadLocalMap(ThreadLocal<?> firstKey, Object firstValue) {
// 初始数组化大小
table = new Entry[INITIAL_CAPACITY];
// 获取散列值
int i = firstKey.threadLocalHashCode & (INITIAL_CAPACITY - 1);
table[i] = new Entry(firstKey, firstValue);
size = 1;
// 设置阙值 len * 2 / 3;
setThreshold(INITIAL_CAPACITY);
}
##### get方法
private Entry getEntry(ThreadLocal<?> key) {
// 获取散列值
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);
}
/**
* 查找数组i之后的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)
// entrty的key被回收了
expungeStaleEntry(i);
else
// 获取下一个数组的坐标
i = nextIndex(i, len);
e = tab[i];
}
return null;
}
private int expungeStaleEntry(int staleSlot) {
ThreadLocal.ThreadLocalMap.Entry[] tab = table;
int len = tab.length;
// expunge entry at staleSlot
// 清空该数组位置的entry
tab[staleSlot].value = null;
tab[staleSlot] = null;
size--;
// Rehash until we encounter null
ThreadLocal.ThreadLocalMap.Entry e;
int i;
// 循环staleSlot之后的entry
for (i = nextIndex(staleSlot, len); (e = tab[i]) != null; i = nextIndex(i, len)) {
ThreadLocal<?> k = e.get();
// 清理没有被应用的entry
if (k == null) {
e.value = null;
tab[i] = null;
size--;
} else {
// 重新计算当前位置ThreadLocal的散列值
int h = k.threadLocalHashCode & (len - 1);
// 计算后的散列值与当前散列值不相同
if (h != i) {
// 当前位置的数组设置null
tab[i] = null;
// Unlike Knuth 6.4 Algorithm R, we must scan until
// null because multiple entries could have been stale.
// 当前h位置==null设置entey
while (tab[h] != null)
// h位置向后移动1位
h = nextIndex(h, len);
tab[h] = e;
}
}
}
return i;
}
##### set方法
private void set(ThreadLocal<?> key, Object value) {
ThreadLocal.ThreadLocalMap.Entry[] tab = table;
int len = tab.length;
// 获取散列值
int i = key.threadLocalHashCode & (len-1);
// 查询i之后entry
// tab[i]位置有数据处理
for (ThreadLocal.ThreadLocalMap.Entry e = tab[i]; e != null; e = tab[i = nextIndex(i, len)]) {
ThreadLocal<?> k = e.get();
if (k == key) {
// 之前的key在值重新赋值
e.value = value;
return;
}
// tab[i]位置没有ThreadLocal被使用了
if (k == null) {
replaceStaleEntry(key, value, i);
return;
}
}
// 该位置设置最新entrty
tab[i] = new ThreadLocal.ThreadLocalMap.Entry(key, value);
int sz = ++size;
if (!cleanSomeSlots(i, sz) && sz >= threshold)
// 重新整理table
rehash();
}
private void replaceStaleEntry(ThreadLocal<?> key, Object value, int staleSlot) {
ThreadLocal.ThreadLocalMap.Entry[] tab = table;
int len = tab.length;
ThreadLocal.ThreadLocalMap.Entry e;
int slotToExpunge = staleSlot;
// staleSlot 之前位置的entrty
for (int i = prevIndex(staleSlot, len); (e = tab[i]) != null; i = prevIndex(i, len))
if (e.get() == null)
slotToExpunge = i;
// 重新设置
for (int i = nextIndex(staleSlot, len); (e = tab[i]) != null; i = nextIndex(i, len)) {
// 找见之前设置entry设置的位置
ThreadLocal<?> k = e.get();
if (k == key) {
e.value = value;
tab[i] = tab[staleSlot];
tab[staleSlot] = e;
// Start expunge at preceding stale entry if it exists
if (slotToExpunge == staleSlot)
slotToExpunge = i;
// 清理无效数组数据
cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
return;
}
if (k == null && slotToExpunge == staleSlot)
slotToExpunge = i;
}
// If key not found, put new entry in stale slot
tab[staleSlot].value = null;
tab[staleSlot] = new ThreadLocal.ThreadLocalMap.Entry(key, value);
// If there are any other stale entries in run, expunge them
if (slotToExpunge != staleSlot)
cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
}
// 清理该tabl的数据
private int expungeStaleEntry(int staleSlot) {
ThreadLocal.ThreadLocalMap.Entry[] tab = table;
int len = tab.length;
// expunge entry at staleSlot
tab[staleSlot].value = null;
tab[staleSlot] = null;
size--;
// Rehash until we encounter null
ThreadLocal.ThreadLocalMap.Entry e;
int i;
// 清理taba[staleSlot]之后的数据
for (i = nextIndex(staleSlot, len); (e = tab[i]) != null; i = nextIndex(i, len)) {
ThreadLocal<?> k = e.get();
// tab[i]位置没有ThreadLocal被使用了
if (k == null) {
e.value = null;
tab[i] = null;
size--;
} else {
// 重新计算散列值存放到下一个位置
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;
}
/**
* 清理部分数据 n = n/2
*/
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);
}
//n = n/2
} while ( (n >>>= 1) != 0);
return removed;
}
/**
* 整理数组
*/
private void rehash() {
// 清理全部数组
expungeStaleEntries();
// Use lower threshold for doubling to avoid hysteresis
// 判断size的值是否大于阙值
if (size >= threshold - threshold / 4)
// 扩容
resize();
}
/**
* Double the capacity of the table.
*/
private void resize() {
Entry[] oldTab = table;
int oldLen = oldTab.length;
int newLen = oldLen * 2;
Entry[] newTab = new Entry[newLen];
int count = 0;
// 旧的位置拷贝到行新的位置
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 {
int h = k.threadLocalHashCode & (newLen - 1);
while (newTab[h] != null)
h = nextIndex(h, newLen);
newTab[h] = e;
count++;
}
}
}
setThreshold(newLen);
size = count;
table = newTab;
}
/**
* Expunge all stale entries in the table.
* 全部清理数组
*/
private void expungeStaleEntries() {
Entry[] tab = table;
int len = tab.length;
for (int j = 0; j < len; j++) {
Entry e = tab[j];
if (e != null && e.get() == null)
expungeStaleEntry(j);
}
}
}
##### remove方法
private void remove(ThreadLocal<?> key) {
Entry[] tab = table;
int len = tab.length;
int i = key.threadLocalHashCode & (len-1);
// 查询I之后等于key的值并清理
for (Entry e = tab[i]; e != null;e = tab[i = nextIndex(i, len)]) {
if (e.get() == key) {
e.clear();
expungeStaleEntry(i);
return;
}
}
}