缓存的实现--原理

转载：http://www.iteye.com/topic/544021

1：缓存的原理：

 

 

package cache;

/**
* <p>
* Description: 缓存
* </p>
*/
public class Cache {
private String key;// 缓存ID
private Object value;// 缓存数据
private long timeOut;// 更新时间
private boolean expired; // 是否终止

public Cache() {
super();
}

public Cache(String key, Object value, long timeOut, boolean expired) {
this.key = key;
this.value = value;
this.timeOut = timeOut;
this.expired = expired;
}

public String getKey() {
return key;
}

public long getTimeOut() {
return timeOut;
}

public Object getValue() {
return value;
}

public void setKey(String string) {
key = string;
}

public void setTimeOut(long l) {
timeOut = l;
}

public void setValue(Object object) {
value = object;
}

public boolean isExpired() {
return expired;
}

public void setExpired(boolean b) {
expired = b;
}
}

 

2：

 

 

package cache; 

import java.util.*; 

/** 
* <p>Description: 管理缓存</p> 
* 可扩展的功能：当chche到内存溢出时必须清除掉最早期的一些缓存对象，这就要求对每个缓存对象保存创建时间
* 
* @version 1.0
*/
public class CacheManager {
private static HashMap cacheMap = new HashMap(); 

//单实例构造方法
private CacheManager() {
super();
}
//获取布尔值的缓存
public static boolean getSimpleFlag(String key){
try{
return (Boolean) cacheMap.get(key);
}catch(NullPointerException e){
return false;
}
}

public static long getServerStartdt(String key){
try {
return (Long)cacheMap.get(key);
} catch (Exception ex) {
return 0;
}
}

//设置布尔值的缓存
public synchronized static boolean setSimpleFlag(String key,boolean flag){
if (flag && getSimpleFlag(key)) {//假如为真不允许被覆盖
return false;
}else{
cacheMap.put(key, flag);
return true;
}
}

public synchronized static boolean setSimpleFlag(String key,long serverbegrundt){
if (cacheMap.get(key) == null) {
cacheMap.put(key,serverbegrundt);
return true;
}else{
return false;
}
}


//得到缓存。同步静态方法
private synchronized static Cache getCache(String key) {
return (Cache) cacheMap.get(key);
}

//判断是否存在一个缓存
private synchronized static boolean hasCache(String key) {
return cacheMap.containsKey(key);
}

//清除所有缓存
public synchronized static void clearAll() {
cacheMap.clear();
}

//清除某一类特定缓存,通过遍历HASHMAP下的所有对象，来判断它的KEY与传入的TYPE是否匹配
public synchronized static void clearAll(String type) {
Iterator i = cacheMap.entrySet().iterator();
String key;
ArrayList<String> arr = new ArrayList<String>();
try {
while (i.hasNext()) {
java.util.Map.Entry entry = (java.util.Map.Entry) i.next();
key = (String) entry.getKey();
if (key.startsWith(type)) { //如果匹配则删除掉
arr.add(key);
}
}
for (int k = 0; k < arr.size(); k++) {
clearOnly(arr.get(k));
}
} catch (Exception ex) {
ex.printStackTrace();
}
}

//清除指定的缓存
public synchronized static void clearOnly(String key) {
cacheMap.remove(key);
}

//载入缓存
public synchronized static void putCache(String key, Cache obj) {
cacheMap.put(key, obj);
}

//获取缓存信息
public static Cache getCacheInfo(String key) {

if (hasCache(key)) {
Cache cache = getCache(key);
if (cacheExpired(cache)) { //调用判断是否终止方法
cache.setExpired(true);
}
return cache;
}else
return null;
}

//载入缓存信息
public static void putCacheInfo(String key, Cache obj, long dt,boolean expired) {
Cache cache = new Cache();
cache.setKey(key);
cache.setTimeOut(dt + System.currentTimeMillis()); //设置多久后更新缓存
cache.setValue(obj);
cache.setExpired(expired); //缓存默认载入时，终止状态为FALSE
cacheMap.put(key, cache);
}
//重写载入缓存信息方法
public static void putCacheInfo(String key,Cache obj,long dt){
Cache cache = new Cache();
cache.setKey(key);
cache.setTimeOut(dt+System.currentTimeMillis());
cache.setValue(obj);
cache.setExpired(false);
cacheMap.put(key,cache);
}

//判断缓存是否终止
public static boolean cacheExpired(Cache cache) {
if (null == cache) { //传入的缓存不存在
return false;
}
long nowDt = System.currentTimeMillis(); //系统当前的毫秒数
long cacheDt = cache.getTimeOut(); //缓存内的过期毫秒数
if (cacheDt <= 0||cacheDt>nowDt) { //过期时间小于等于零时,或者过期时间大于当前时间时，则为FALSE
return false;
} else { //大于过期时间 即过期
return true;
}
}

//获取缓存中的大小
public static int getCacheSize() {
return cacheMap.size();
}

//获取指定的类型的大小
public static int getCacheSize(String type) {
int k = 0;
Iterator i = cacheMap.entrySet().iterator();
String key;
try {
while (i.hasNext()) {
java.util.Map.Entry entry = (java.util.Map.Entry) i.next();
key = (String) entry.getKey();
if (key.indexOf(type) != -1) { //如果匹配则删除掉
k++;
}
}
} catch (Exception ex) {
ex.printStackTrace();
}

return k;
}

//获取缓存对象中的所有键值名称
public static ArrayList<String> getCacheAllkey() {
ArrayList a = new ArrayList();
try {
Iterator i = cacheMap.entrySet().iterator();
while (i.hasNext()) {
java.util.Map.Entry entry = (java.util.Map.Entry) i.next();
a.add((String) entry.getKey());
}
} catch (Exception ex) {} finally {
return a;
}
}

//获取缓存对象中指定类型 的键值名称
public static ArrayList<String> getCacheListkey(String type) {
ArrayList a = new ArrayList();
String key;
try {
Iterator i = cacheMap.entrySet().iterator();
while (i.hasNext()) {
java.util.Map.Entry entry = (java.util.Map.Entry) i.next();
key = (String) entry.getKey();
if (key.indexOf(type) != -1) {
a.add(key);
}
}
} catch (Exception ex) {} finally {
return a;
}
}

}

 3：

 

 

package cache;
/**
* 测试类
* @author Song Shi Chao
*
*/
class Test {
public static void main(String[] args) {
// System.out.println(CacheManager.getSimpleFlag("alksd"));
// CacheManager.putCache("abc", new Cache());
// CacheManager.putCache("def", new Cache());
// CacheManager.putCache("ccc", new Cache());
// CacheManager.clearOnly("");


Cache c = new Cache();
for (int i = 0; i < 10; i++) {
CacheManager.putCache("" + i, c);
}
System.out.println(CacheManager.getCacheSize());

// CacheManager.putCache("aaaaaaaa", c);
// CacheManager.putCache("abchcy;alskd", c);
// CacheManager.putCache("cccccccc", c);
// CacheManager.putCache("abcoqiwhcy", c);
// System.out.println("删除前的大小："+CacheManager.getCacheSize());
// CacheManager.getCacheAllkey();
// CacheManager.clearAll("aaaa");
// System.out.println("删除后的大小："+CacheManager.getCacheSize());
// CacheManager.getCacheAllkey();


}
}