import java.lang.ref.WeakReference;
/*
* below methods have public idenifer that can be invoked by outer
*/
public class UserDefinedMap {
// 當entry.get()==null 時，說明這個key不存在引用，可以從引用數組中移除
static class Entry extends WeakReference<Key>{
Object value;
Entry(Key k, Object v) {
// a = new WeakReference<String>(k);
// k=null;
super(k);
value = v;
}
// public String get(){
// if(a.get() != null)
// return a.get();
// return null;
// }
public Object getValue(){
return value;
}
}
private static final int INITIAL_CAPACITY = 16;// 初始化的容量
private Entry[] table;
private int size = 0; // table中存儲的數量
private int threshold; // 擴容因子，初始化爲0
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);
}
/* LocalMap(String firstKey, Object firstValue) {
table = new Entry[INITIAL_CAPACITY];
int i = firstKey.hashCode() & (INITIAL_CAPACITY - 1);
table[i] = new Entry(firstKey, firstValue);
size = 1;
setThreshold(INITIAL_CAPACITY);
}*/
UserDefinedMap() {
table = new Entry[INITIAL_CAPACITY];
// table[1] = new Entry("ab", new chunkMemory());
// table[2] = new Entry("ac", new chunkMemory());
// table[3] = new Entry("ad", new chunkMemory());
// table[4] = new Entry("ae", new chunkMemory());
// table[5] = new Entry("af", new chunkMemory());
// size = 5;
setThreshold(INITIAL_CAPACITY);
}
// 直接命中時返回，否則調用getEntryAfterMiss()方法進行尋找
public Entry getEntry(Key key) {
int i = key.hashCode() & (table.length - 1);
Entry e = table[i];
if (e != null && e.get() != null && e.get().equals(key))
return e;
else
return getEntryAfterMiss(key, i, e);
}
// 從i處理查找的值爲e，但是e.getKey()!=key
// 規定傳入的key值不爲null
private Entry getEntryAfterMiss(Key key, int i, Entry e) {
Entry[] tab = table;
int len = tab.length;
while (e != null) {
Key k = e.get();
if (k != null && k.equals(key))
return e;
if (k == null) // String虛引用可能被回收
expungeStaleEntry(i);
else
i = nextIndex(i, len);
e = tab[i];
}
/*
* 有兩種可能返回值爲null
* 1、直接定位到的位置無存儲元素
* 2、直接定位的位置不爲空，但是後續的連續元素也沒有匹配的
*/
return null;
}
public void set(Key key, Object value) {
Entry[] tab = table;
int len = tab.length;
int i = key.hashCode() & (len-1);
// 可以看出，如果此時的key值相等,則value值會覆蓋
for (Entry e = tab[i]; e != null; e = tab[i = nextIndex(i, len)]) {
Key k = e.get();
if (k != null && k.equals(key)) {
e.value = value;
return;
}
// Entry不爲null，但是k卻爲null
if (k == null) {
replaceStaleEntry(key, value, i);
return;
}
}
tab[i] = new Entry(key, value);
int sz = ++size;
if (!cleanSomeSlots(i, sz) && sz >= threshold)
rehash();
}
// Remove the entry for key.
private void remove(Key key) {
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)]) {
if (e.get() == key) {
e.clear();
expungeStaleEntry(i);
return;
}
}
}
/**
* 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.)
*
* key和value表示要放入的key值和value值
* staleSlot表示在搜索key時遇到的第一個stale Entry的位置
*
*/
private void replaceStaleEntry(Key key, Object value,int staleSlot) {
Entry[] tab = table;
int len = tab.length;
Entry e;
// We clean out whole runs at a time to avoid continual
// incremental rehashing due to garbage collector freeing
// up refs in bunches (whenever the collector runs).
/*
* 在StringMap的存儲策略中，衝突的元素一定會相鄰（trail和head的不爲空Entry也是相鄰）
* 這樣在插入時如果遇到stale Entry就應該調用這一段的元素
* 找到第一個stale Entry元素位置爲slotToExpunge
*/
int slotToExpunge = 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)) {
Key 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;
// 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.
if (k == null && slotToExpunge == staleSlot)
slotToExpunge = i;
}
// If key not found, put new entry in stale slot
tab[staleSlot].value = null;
tab[staleSlot] = new Entry(key, value);
// If there are any other stale entries in run, expunge them
if (slotToExpunge != staleSlot)
cleanSomeSlots(expungeStaleEntry(slotToExpunge), len);
}
/*
null | Entry!=null | Entry!=null | null
*/
public 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)) {
Key k = e.get();
if (k == null) {
e.value = null;
tab[i] = null;
size--;
} else {
int h = k.hashCode() & (len - 1);
/*
* 當h!=i時,說明這個元素在存儲時遇到了衝突,現在清除了這個區段的一些元素
* 本來的位置可能已經爲空了。
*/
if (h != i) {
tab[i] = null;
/*
*
* 如果產生衝突,則向後掃描，直到找到一個空的位置來放Entry。注意在判斷
* 時使用的是tab[h]而不是Entry的key
*
* 這樣可能會產生一個問題，如tab[h]的Entry的key爲空，那麼這本來就已經
* 變成了一個stale entry，e元素應該存儲到這裏。但是使用的是tab[h]!=null
* 來判斷，難道這就是註釋所說的意思：
*
* 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;
}
}// end else
}
/*
* 如上代碼段掃描從staleSlot到i的元素,將key引用爲null的從table數組中
* 移除，並且重新計算不爲空Entry的存儲位置。
*/
return i; // 此時tab[i]爲空
}
/**
* 一種移除無用Entry的策略，在添加或者移除無用Entry時被調用
*
*
* @param n scan control: <tt>log2(n)</tt> cells are scanned,
* unless a stale entry is found, in which case
* <tt>log2(table.length)-1</tt> 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.)
*
*
*
* .如果 a^x=N（a>0，且a≠1），那麼數x叫做以a爲底N的對數（logarithm）
* param i:不是stale Entry,從後一個開始掃描
* param n:log2(n)個Entry將被掃描
*
*/
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;
}
// 清除無用對象，如果不能有效縮小size數量，則進行2倍擴容
private void rehash() {
expungeStaleEntries(); // 首先掃描整個table數組清除無用的Entry
// Use lower threshold for doubling to avoid hysteresis
if (size >= threshold - threshold / 4)
resize();
}
// 擴容爲原來的2倍
private void resize() {
Entry[] oldTab = table;
int oldLen = oldTab.length;
int newLen = oldLen * 2;
Entry[] newTab = new Entry[newLen];
int count = 0; // 臨時輔助計算size
for (int j = 0; j < oldLen; ++j) { // 擴容以後需要重新定位Entry的存儲位置
Entry e = oldTab[j];
if (e != null) {
Key k = e.get();
if (k == null) {
e.value = null; // Help the GC
} else {
int h = k.hashCode() & (newLen - 1);
while (newTab[h] != null)
h = nextIndex(h, newLen);
newTab[h] = e;
count++;
}
}
}
setThreshold(newLen); // 重新計算擴容因子
size = count;
table = newTab;
}
/*
* 掃描整個table表來清除無用的Entry引用
*/
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) // e不爲空,但是已經沒有對e的引用時
expungeStaleEntry(j);
}
}
}public class chunkMemory {
private static final int SIZE = 500000;
// 屬性d使得每個Grocery對象佔用較多內存，有80K左右
private double[] d = new double[SIZE];
private String id;
public chunkMemory(String id){
this.id = id;
}
public String toString(){
return id;
}
public void finalize() {
System.out.println("Finalizing " + id);
}
}public class Key {
String id;
public Key(String id) {
this.id = id;
}
public String toString() {
return id;
}
public int hashCode() {
return id.hashCode();
}
public boolean equals(Object r) {
return (r instanceof Key) && id.equals(((Key) r).id);
}
public void finalize() {
System.out.println("Finalizing Key " + id);
}
}@Test
public void add() throws InterruptedException {
UserDefinedMap x = new UserDefinedMap();
x.set(new Key("ab"), new chunkMemory("ab1"));
x.set(new Key("ac"), new chunkMemory("ac"));
x.set(new Key("ad"), new chunkMemory("ad"));
x.set(new Key("ab"), new chunkMemory("ab2"));
Thread.currentThread().sleep(8000);
Entry value = x.getEntry(new Key("ab"));
System.out.println(value.getValue().toString());
}原本打算key爲String類型，但Entry在繼承了WeakReference<String>類後，好像在任何情況下不會進行弱引用的回收，不知道怎麼回事