①servlet单例模式以及该单例类何时才会毁掉②服务器为每个用户创建线程，线程何时终止问题

经测试，每次请求结束后，单例类中的静态属性还在，即使关闭窗口，单例类的静态属性依然存在，单例类的静态属性的生命周期与tomcat有关，当tomcat关闭的时候，

单例类的静态属性才会消失。。 

线程何时终止：由于session的存在，线程的生命周期或许与session的生命周期有关。。

测试单例类的代码：

package com.zjlolife.util;

import java.util.HashMap;
import java.util.Map;

import org.dom4j.Document;
import org.dom4j.DocumentException;
import org.dom4j.Element;
import org.dom4j.io.SAXReader;

import com.zjlolife.domain.User;

public class BeanFactory {

private Document doc;

private Map beans = new HashMap();

private static BeanFactory instance = new BeanFactory();

private BeanFactory() {

try {

doc = new SAXReader().read(Thread.currentThread()
.getContextClassLoader()
.getResourceAsStream("application-beans.xml"));

} catch (DocumentException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
}

public static BeanFactory getInstance() {
System.out.println(instance);
return instance;
}

@SuppressWarnings("unchecked")
public Object getBean(Class c) {
Object object = null;

if (beans.containsKey(c.getName())) {
object = beans.get(c.getName());
return object;
} else {

Element elt = (Element) doc.selectObject("//bean[@id=\""
+ c.getName() + "\"]");
try {

object = Class.forName(elt.attributeValue("class"))
.newInstance();
beans.put(c.getName(), object);
} catch (Exception e) {
// TODO Auto-generated catch block
e.printStackTrace();

}
}

return object;
}

public static void main(String [] args) {
User user = (User)BeanFactory.getInstance().getBean(User.class);
System.out.println(user);
}
}

<?xml version="1.0" encoding="UTF-8"?>
<beans>
<bean id="com.zjlolife.domain.User" class="com.zjlolife.domain.User" />

</beans>

package com.zjlolife.domain;

public class User {

private String id;

private String name;

public String getId() {
return id;
}

public void setId(String id) {
this.id = id;
}

public String getName() {
return name;
}

public void setName(String name) {
this.name = name;
}

}

package com.zjlolife.web;

import java.io.IOException;
import java.io.PrintWriter;

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

import com.zjlolife.domain.User;
import com.zjlolife.util.BeanFactory;

public class TestServlet extends HttpServlet {

public TestServlet() {
super();
}

public void doGet(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {
User user =(User)BeanFactory.getInstance().getBean(User.class);
System.out.println(user);
}

public void doPost(HttpServletRequest request, HttpServletResponse response)
throws ServletException, IOException {

this.doGet(request, response);
}

}

测试效果：

只要tomcat没关闭，每个用户访问的BeanFactory的实例instance都是同一对象，因此BeanFactory中的User也是同一对象。。。

其实这笔记可以不做的！！！