java之将数据库数据读取到内存中

1 为了将配置信息或数据库的信息读取到内存中。可以程序启动时，启动一个servlet。在web.xml中进行配置其0 表示优先级为最大,这样就在程序启动时去加载servlet类。然后程序自动调用该servlet类的init方法
如：
  initServlet
  com.servlet.InitServlet
  0
 


2 创建一个初始化加载的servlet时。需要在该servlet的init方法中进行初始化的工作就是把数据库的配置信息读取到内存中。
如：public class InitServlet extends HttpServlet{


@Override
public void init() throws ServletException {
// TODO Auto-generated method stub
//super.init();
System.out.println("初始化servlet的init方法");

//servlet加载完后才能使用ServletActionContext类  而此时servlet还没加载完。故无法使用ServletActionContext类  。能使用this.getServletContext()
ServletContext servletContext=this.getServletContext();//ServletActionContext.getServletContext();  

ApplicationContext applicationContext=WebApplicationContextUtils.getWebApplicationContext(servletContext);

SpringFactory.getInstance().setApplicationContext(applicationContext);

new ConfigFactory().loadSystemConfig();
new Thread(new ConfigThread()).start();
}
}


3 在servlet的init方法中，可以通过this.getServletContext()来得到servletContext对象。然后通过WebApplicationContextUtils.getWebApplicationContext(servletContext)来得到ApplicationContext对象。得到applicationContext对象的目的是为了得到bean对象（spring配置文件中的bean对象）
如：public class InitServlet extends HttpServlet{


@Override
public void init() throws ServletException {
// TODO Auto-generated method stub
//super.init();
System.out.println("初始化servlet的init方法");

//servlet加载完后才能使用ServletActionContext类  而此时servlet还没加载完。故无法使用ServletActionContext类  。能使用this.getServletContext()
ServletContext servletContext=this.getServletContext();//ServletActionContext.getServletContext();  

ApplicationContext applicationContext=WebApplicationContextUtils.getWebApplicationContext(servletContext);

SpringFactory.getInstance().setApplicationContext(applicationContext);

new ConfigFactory().loadSystemConfig();
new Thread(new ConfigThread()).start();
}
}


特别说明啊。在servlet的init方法中，在servlet还没加载完的时候。不能通过ServletActionContext.getServletContext()来得到servletContext对象。程序会报错。只能在servlet类的init方法中通过this.getServletContext()方法来得到servletContext对象。


4 在servlet的init方法中时通过线程来读取数据库信息然后保存在内存中的(先读取一次。然后通过线程一直循环来读取)。
如：new ConfigFactory().loadSystemConfig();//先读取一次


//然后线程一直循环来读取
public class ConfigThread implements Runnable{


public void run() {
// TODO Auto-generated method stub
while(true){
try {
Thread.sleep(1000*60);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
new ConfigFactory().loadSystemConfig();

}
}


}


5 配置保存在内存中的类。用map来存。
如：public class ConfigFactory {


private SysConfigService sysConfigService=(SysConfigService)SpringFactory.getInstance().getBean(SpringBean.SYS_CONFIG_SERVICE);

private static Map sysConfigMap=new HashMap();

public void loadSystemConfig(){
List sysConfigLists=sysConfigService.getALlSysConfig();
if(sysConfigLists.size()>0){
for(int i=0;i SysConfig sysConfig=sysConfigLists.get(i);
String name=sysConfig.getName();
String value=sysConfig.getValue();
System.out.println("添加了"+name+","+value);
sysConfigMap.put(name,value);
}
}
}
}






例子：web.xml:

xmlns="http://java.sun.com/xml/ns/javaee" 
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 
xsi:schemaLocation="http://java.sun.com/xml/ns/javaee 
http://java.sun.com/xml/ns/javaee/web-app_2_5.xsd">
 
    index.jsp
 
 
  contextConfigLocation
  /WEB-INF/applicationContext-*.xml
 


 
  org.springframework.web.context.ContextLoaderListener
 
 


 
  struts2
  org.apache.struts2.dispatcher.FilterDispatcher
 
 
  struts2
  /*
 
 
 
  initServlet
  com.servlet.InitServlet
  0
 







//InitServlet.java:


package com.servlet;


import java.io.IOException;


import javax.servlet.ServletContext;
import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;


import org.apache.struts2.ServletActionContext;
import org.springframework.context.ApplicationContext;
import org.springframework.web.context.support.WebApplicationContextUtils;


import com.common.SpringFactory;
import com.factory.ConfigFactory;
import com.thread.ConfigThread;


public class InitServlet extends HttpServlet{




@Override
public void init() throws ServletException {
// TODO Auto-generated method stub
//super.init();
System.out.println("初始化servlet的init方法");

//servlet加载完后才能使用ServletActionContext类  而此时servlet还没加载完。故无法使用ServletActionContext类  。能使用this.getServletContext()
ServletContext servletContext=this.getServletContext();//ServletActionContext.getServletContext();  

ApplicationContext applicationContext=WebApplicationContextUtils.getWebApplicationContext(servletContext);

SpringFactory.getInstance().setApplicationContext(applicationContext);

new ConfigFactory().loadSystemConfig();
new Thread(new ConfigThread()).start();
}


@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
// TODO Auto-generated method stub
//super.doGet(req, resp);
this.doPost(req, resp);
}


@Override
protected void doPost(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
// TODO Auto-generated method stub
//super.doPost(req, resp);
//
}




}






//ConfigFactory.java:


public class ConfigFactory {


private SysConfigService sysConfigService=(SysConfigService)SpringFactory.getInstance().getBean(SpringBean.SYS_CONFIG_SERVICE);

private static Map sysConfigMap=new HashMap();

public void loadSystemConfig(){
List sysConfigLists=sysConfigService.getALlSysConfig();
if(sysConfigLists.size()>0){
for(int i=0;i SysConfig sysConfig=sysConfigLists.get(i);
String name=sysConfig.getName();
String value=sysConfig.getValue();
System.out.println("添加了"+name+","+value);
sysConfigMap.put(name,value);
}
}
}
}




//ConfigThread.java:


package com.thread;


import com.factory.ConfigFactory;


public class ConfigThread implements Runnable{


public void run() {
// TODO Auto-generated method stub
while(true){
try {
Thread.sleep(1000*60);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
new ConfigFactory().loadSystemConfig();

}
}


}




//SpringFactory.java:


package com.common;


import javax.servlet.ServletContext;


import org.apache.struts2.ServletActionContext;
import org.springframework.context.ApplicationContext;
import org.springframework.web.context.support.WebApplicationContextUtils;


public class SpringFactory {


private ApplicationContext applicationContext;

private static SpringFactory springFactory=null;


private SpringFactory() {
super();
// TODO Auto-generated constructor stub
//ServletContext servletContext=ServletActionContext.getServletContext();
//applicationContext=WebApplicationContextUtils.getWebApplicationContext(servletContext);
}

public static SpringFactory getInstance(){
if(springFactory==null){
springFactory=new SpringFactory();
}
return springFactory;
}






public Object getBean(String name){
return applicationContext.getBean(name);
}


public ApplicationContext getApplicationContext() {
return applicationContext;
}


public void setApplicationContext(ApplicationContext applicationContext) {
this.applicationContext = applicationContext;
}


}




SpringBean.java:


package com.common;


public class SpringBean {


public static final String MENU_SERVICE="menuService";
public static final String SYS_CONFIG_SERVICE="sysConfigService";
}