缓存机制应该可以了解为：
1.通对JAVA类中使用static声明一个成员变量,使其多用户并发访问这个类时,多个类对象共享这一个静态成员变量,以达到并发时缓存对象.
2.通过java.util.Timer或后台运行的多线程代码,同样类中使用1中所说的static变量,访问情况同1,只是在这种方法中,类对象始终运行,新建类对象可能更快.
以上的所述的可以通过设计模式中Singleton模式，Prototype模式来分析一下，下面的例子也可以帮助了解这种说法。
1、CacheData.java 存放缓存数据的Bean
/** *//**
*
*/
package com.cari.web.cache;
/** *//**
* @author zsy
*
*/
public class CacheData ...{
private Object data;
private long time;
private int count;
public CacheData() ...{
}
public CacheData(Object data, long time, int count) ...{
this.data = data;
this.time = time;
this.count = count;
}
public CacheData(Object data) ...{
this.data = data;
this.time = System.currentTimeMillis();
this.count = 1;
}
public void addCount() ...{
count++;
}
public int getCount() ...{
return count;
}
public void setCount(int count) ...{
this.count = count;
}
public Object getData() ...{
return data;
}
public void setData(Object data) ...{
this.data = data;
}
public long getTime() ...{
return time;
}
public void setTime(long time) ...{
this.time = time;
}
}
2、CacheOperation.java 缓存处理类
package com.cari.web.cache;
import java.lang.reflect.Method;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Hashtable;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
/** *//**
* @author zsy
*/
public class CacheOperation ...{
private static final Log log = LogFactory.getLog(CacheOperation.class);
private static CacheOperation singleton = null;
private Hashtable cacheMap;//存放缓存数据
private ArrayList threadKeys;//处于线程更新中的key值列表
public static CacheOperation getInstance() ...{
if (singleton == null) ...{
singleton = new CacheOperation();
}
return singleton;
}
private CacheOperation() ...{
cacheMap = new Hashtable();
threadKeys = new ArrayList();
}
/** *//**
* 添加数据缓存
* 与方法getCacheData(String key, long intervalTime, int maxVisitCount)配合使用
* @param key
* @param data
*/
public void addCacheData(String key, Object data) ...{
addCacheData(key, data, true);
}
private void addCacheData(String key, Object data, boolean check) ...{
if (Runtime.getRuntime().freeMemory() < 5L*1024L*1024L) ...{//虚拟机内存小于10兆，则清除缓存
log.warn("WEB缓存：内存不足，开始清空缓存！");
removeAllCacheData();
return;
} else if(check && cacheMap.containsKey(key)) ...{
log.warn("WEB缓存：key值= " + key + " 在缓存中重复, 本次不缓存！");
return;
}
cacheMap.put(key, new CacheData(data));
}
3、缓存数据的操作
/**
* 取得缓存中的数据
* 与方法addCacheData(String key, Object data)配合使用
* @param key
* @param intervalTime 缓存的时间周期，小于等于0时不限制
* @param maxVisitCount 访问累积次数，小于等于0时不限制
* @return
*/
public Object getCacheData(String key, long intervalTime, int maxVisitCount) ...{
CacheData cacheData = (CacheData)cacheMap.get(key);
if (cacheData == null) ...{
return null;
}
if (intervalTime > 0 && (System.currentTimeMillis() - cacheData.getTime()) > intervalTime) ...{
removeCacheData(key);
return null;
}
if (maxVisitCount > 0 && (maxVisitCount - cacheData.getCount()) <= 0) ...{
removeCacheData(key);
return null;
} else ...{
cacheData.addCount();
}
return cacheData.getData();
}
/** *//**
* 当缓存中数据失效时，用不给定的方法线程更新数据
* @param o 取得数据的对像(该方法是静态方法是不用实例，则传Class实列)
* @param methodName 该对像中的方法
* @param parameters 该方法的参数列表(参数列表中对像都要实现toString方法,若列表中某一参数为空则传它所属类的Class)
* @param intervalTime 缓存的时间周期，小于等于0时不限制
* @param maxVisitCount 访问累积次数，小于等于0时不限制
* @return
*/
public Object getCacheData(Object o, String methodName,Object[] parameters,
long intervalTime, int maxVisitCount) ...{
Class oc = o instanceof Class ? (Class)o : o.getClass();
StringBuffer key = new StringBuffer(oc.getName());//生成缓存key值
key.append("-").append(methodName);
if (parameters != null) ...{
for (int i = 0; i < parameters.length; i++) ...{
if (parameters[i] instanceof Object[]) ...{
key.append("-").append(Arrays.toString((Object[])parameters[i]));
} else ...{
key.append("-").append(parameters[i]);
}
}
}
CacheData cacheData = (CacheData)cacheMap.get(key.toString());
if (cacheData == null) ...{//等待加载并返回
Object returnValue = invoke(o, methodName, parameters, key.toString());
return returnValue instanceof Class ? null : returnValue;
}
if (intervalTime > 0 && (System.currentTimeMillis() - cacheData.getTime()) > intervalTime) ...{
daemonInvoke(o, methodName, parameters, key.toString());//缓存时间超时,启动线程更新数据
} else if (maxVisitCount > 0 && (maxVisitCount - cacheData.getCount()) <= 0) ...{//访问次数超出,启动线程更新数据
daemonInvoke(o, methodName, parameters, key.toString());
} else ...{
cacheData.addCount();
}
return cacheData.getData();
}
/** *//**
* 递归调用给定方法更新缓存中数据据
* @param o
* @param methodName
* @param parameters
* @param key
* @return 若反射调用方法返回值为空则返回该值的类型
*/
private Object invoke(Object o, String methodName,Object[] parameters, String key) ...{
Object returnValue = null;
try ...{
Class[] pcs = null;
if (parameters != null) ...{
pcs = new Class[parameters.length];
for (int i = 0; i < parameters.length; i++) ...{
if (parameters[i] instanceof MethodInfo) ...{//参数类型是MethodInfo则调用该方法的返回值做这参数
MethodInfo pmi = (MethodInfo)parameters[i];
Object pre = invoke(pmi.getO(), pmi.getMethodName(), pmi.getParameters(), null);
parameters[i] = pre;
}
if (parameters[i] instanceof Class) ...{
pcs[i] = (Class)parameters[i];
parameters[i] = null;
} else ...{
pcs[i] = parameters[i].getClass();
}
}
}
Class oc = o instanceof Class ? (Class)o : o.getClass();
// Method m = oc.getDeclaredMethod(methodName, pcs);
Method m = matchMethod(oc, methodName, pcs);
returnValue = m.invoke(o, parameters);
if (key != null && returnValue != null) ...{
addCacheData(key, returnValue, false);
}
if (returnValue == null) ...{
returnValue = m.getReturnType();
}
} catch(Exception e) ...{
log.error("调用方法失败,methodName=" + methodName);
if (key != null) ...{
removeCacheData(key);
log.error("更新缓存失败，缓存key=" + key);
}
e.printStackTrace();
}
return returnValue;
}
/** *//**
* 找不到完全匹配的方法时,对参数进行向父类匹配
* 因为方法aa(java.util.List) 与 aa(java.util.ArrayList)不能自动匹配到
*
* @param oc
* @param methodName
* @param pcs
* @return
* @throws NoSuchMethodException
* @throws NoSuchMethodException
*/
private Method matchMethod(Class oc, String methodName, Class[] pcs
) throws NoSuchMethodException, SecurityException ...{
try ...{
Method method = oc.getDeclaredMethod(methodName, pcs);
return method;
} catch (NoSuchMethodException e) ...{
Method[] ms = oc.getDeclaredMethods();
aa:for (int i = 0; i < ms.length; i++) ...{
if (ms[i].getName().equals(methodName)) ...{
Class[] pts = ms[i].getParameterTypes();
if (pts.length == pcs.length) ...{
for (int j = 0; j < pts.length; j++) ...{
if (!pts[j].isAssignableFrom(pcs[j])) ...{
break aa;
}
}
return ms[i];
}
}
}
throw new NoSuchMethodException();
}
}
/** *//**
* 新启线程后台调用给定方法更新缓存中数据据
* @param o
* @param methodName
* @param parameters
* @param key
*/
private void daemonInvoke(Object o, String methodName,Object[] parameters, String key) ...{
if (!threadKeys.contains(key)) ...{
InvokeThread t = new InvokeThread(o, methodName, parameters, key);
t.start();
}
}
/** *//**
* 些类存放方法的主调对像,名称及参数数组
* @author zsy
*
*/
public class MethodInfo ...{
private Object o;
private String methodName;
private Object[] parameters;
public MethodInfo(Object o, String methodName,Object[] parameters) ...{
this.o = o;
this.methodName = methodName;
this.parameters = parameters;
}
public String getMethodName() ...{
return methodName;
}
public void setMethodName(String methodName) ...{
this.methodName = methodName;
}
public Object getO() ...{
return o;
}
public void setO(Object o) ...{
this.o = o;
}
public Object[] getParameters() ...{
return parameters;
}
public void setParameters(Object[] parameters) ...{
this.parameters = parameters;
}
public String toString() ...{
StringBuffer str = new StringBuffer(methodName);
if (parameters != null) ...{
str.append("(");
for (int i = 0; i < parameters.length; i++) ...{
if (parameters[i] instanceof Object[]) ...{
str.append(Arrays.toString((Object[])parameters[i])).append(",");
} else ...{
str.append(parameters[i]).append(",");
}
}
str.append(")");
}
return str.toString();
}
}
/** *//**
* 线程调用方法
* @author zsy
*
*/
private class InvokeThread extends Thread ...{
private Object o;
private String methodName;
private Object[] parameters;
private String key;
public InvokeThread(Object o, String methodName,Object[] parameters, String key) ...{
this.o = o;
this.methodName = methodName;
this.parameters = parameters;
this.key = key;
}
public void run() ...{
threadKeys.add(key);
invoke(o, methodName, parameters, key);
threadKeys.remove(key);
}
}
/** *//**
* 移除缓存中的数据
* @param key
*/
public void removeCacheData(String key) ...{
cacheMap.remove(key);
}
/** *//**
* 移除所有缓存中的数据
*
*/
public void removeAllCacheData() ...{
cacheMap.clear();
}
public String toString() ...{
StringBuffer sb = new StringBuffer("************************ ");
sb.append("正在更新的缓存数据： ");
for (int i = 0; i < threadKeys.size(); i++) ...{
sb.append(threadKeys.get(i)).append(" ");
}
sb.append("当前缓存大小：").append(cacheMap.size()).append(" ");
sb.append("************************");
return sb.toString();
}
}
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