静态内部类（Inner Class） 素数
为ThreadLocal定制的ThreadLocalMap 2010-10-04 16:00:17| 分类：JAVA线程安全| 标签：|字号大
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ThreadLocalMap是java.lang.ThreadLocal的静态内部类。ThreadLocalMap表面看起来至少实现类似hashmap的功能，但是仔细分析它却有下面的属性。
首先，它只是尽量的让value保持在哈希值的位置或后面，
其次，它提供了对不同key（这里是ThreadLocal）对象的但hashCode相同的存储的支持。
最后，它对Key的引用是弱引用。这个和WeakHashMap一致。但这个和hashMap有点不一样，hashMap要使用用弱引用的话，
一般是对value进行弱引用的，而不是key.
也许有一天我们在自己的程序中需要类似ThreadLocalMap的功能。
以下是从ThreadLocalMap简化而来的代码：
其中Key对应JDK中的ThreadLocal
class Key {
final int hashCode = nextHashCode();
static AtomicInteger nextHashCode =
new AtomicInteger();
static final int HASH_INCREMENT = 0x61c88647;
static int nextHashCode() {
return nextHashCode.getAndAdd(HASH_INCREMENT);
}
}
class ThreadLocalMap {
static class Entry extends WeakReference
Object value;
Entry(Key k, Object v) {
super(k);
value = v;
}
}
//初始值，必须是2的n次方
private static final int INITIAL_CAPACITY = 16;
/**
* The table, resized as necessary.
* table.length MUST always be a power of two.
*/
private Entry[] table;
private int size = 0;
/**
* The next size value at which to resize.
*/
private int threshold; // Default to 0
/**
* Set the resize threshold to maintain at worst a 2/3 load factor.
*/
private void setThreshold(int len) {
threshold = len * 2 / 3;
}
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);
}
/**
* 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(Key firstKey, Object firstValue) {
table = new Entry[INITIAL_CAPACITY];
int i = firstKey.hashCode & (INITIAL_CAPACITY - 1);
table = new Entry(firstKey, firstValue);
size = 1;
setThreshold(INITIAL_CAPACITY);
}
private Entry getEntry(Key key) {
int i = key.hashCode & (table.length - 1);
Entry e = table[i];
if (e != null && e.get() == key)
return e;
else
return getEntryAfterMiss(key, i, e);
}
private Entry getEntryAfterMiss(Key key, int i, Entry e) {
Entry[] tab = table[i];
int len = tab.length;
while (e != null) {
Key k = e.get();
if (k == key)
return e;
if (k == null)
expungeStaleEntry(i);
else
i = nextIndex(i, len);
e = tab[i];
}
return null;
}
private void set(Key key, Object value) {
Entry[] tab = table;
int len = tab.length;
int i = key.hashCode & (len-1);
for (Entry e = tab[i]; e != null; e = tab[i = nextIndex(i, len)]) {
Key k = e.get();
if (k == key) {
e.value = value;