如何在servlet中使用多線程，求解？？？

要求如下：

在一個web Service項目中，使用servlet程式設計，如何防止Tomcat被非正常關閉而導緻伺服器端的servletContext資料丢失？

本人思路如下：

在該項目初始化時，啟動一個線程，每隔一個小時就将servletContext的值存儲到一個txt檔案中核心代碼如下：

public class ServletContextThread extends HttpServlet implements Runnable{

//servletContext的值已先在該工程的其他處設定好了，結構為(name:---:nums,value:---:0)，該線程已在初始化時就啟動了

public void run() {

}

public void init() throws ServletException {

FileWriter fw = null;

BufferedWriter bw = null;

String path = this.getServletContext().getRealPath("recorder.txt");

try {

while(true){

Thread.sleep(60*1000);

String nums = this.getServletContext().getAttribute("nums").toString();

fw = new FileWriter(path);

bw = new BufferedWriter(fw);

bw.write(nums);

}

} catch (InterruptedException e) {

e.printStackTrace();

} catch (IOException e) {

e.printStackTrace();

}finally{

try {

bw.close();

fw.close();

} catch (IOException e) {

e.printStackTrace();

}

一直說}

但是它總是說 bw.close();處空指針異常，求解？？？？？？？？？？？？？？？？

2013-9-11 16:51:35 org.apache.catalina.core.ApplicationContext log

嚴重: Servlet servletContextBiz threw unload() exception

javax.servlet.ServletException: Servlet.destroy() for servlet servletContextBiz threw exception

at org.apache.catalina.core.StandardWrapper.unload(StandardWrapper.java:1401)

at org.apache.catalina.core.StandardWrapper.stop(StandardWrapper.java:1727)

at org.apache.catalina.core.StandardContext.stop(StandardContext.java:4488)

at org.apache.catalina.startup.HostConfig.checkResources(HostConfig.java:1097)

at org.apache.catalina.startup.HostConfig.check(HostConfig.java:1203)

at org.apache.catalina.startup.HostConfig.lifecycleEvent(HostConfig.java:293)

at org.apache.catalina.util.LifecycleSupport.fireLifecycleEvent(LifecycleSupport.java:120)

at org.apache.catalina.core.ContainerBase.backgroundProcess(ContainerBase.java:1307)

at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.processChildren(ContainerBase.java:1571)

at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.processChildren(ContainerBase.java:1580)

at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.run(ContainerBase.java:1560)

at java.lang.Thread.run(Unknown Source)

Caused by: java.lang.NullPointerException

at com.lx.biz.servletContextBiz.destroy(servletContextBiz.java:35)

at org.apache.catalina.core.StandardWrapper.unload(StandardWrapper.java:1382)

... 11 more

2013-9-11 16:51:36 org.apache.catalina.core.ApplicationContext log

嚴重: StandardWrapper.Throwable

java.lang.NullPointerException

at com.lx.biz.ServletContextThread.init(ServletContextThread.java:45)

at javax.servlet.GenericServlet.init(GenericServlet.java:212)

at org.apache.catalina.core.StandardWrapper.loadServlet(StandardWrapper.java:1161)

at org.apache.catalina.core.StandardWrapper.load(StandardWrapper.java:981)

at org.apache.catalina.core.StandardContext.loadOnStartup(StandardContext.java:4044)

at org.apache.catalina.core.StandardContext.start(StandardContext.java:4350)

at org.apache.catalina.startup.HostConfig.checkResources(HostConfig.java:1105)

at org.apache.catalina.startup.HostConfig.check(HostConfig.java:1203)

at org.apache.catalina.startup.HostConfig.lifecycleEvent(HostConfig.java:293)

at org.apache.catalina.util.LifecycleSupport.fireLifecycleEvent(LifecycleSupport.java:120)

at org.apache.catalina.core.ContainerBase.backgroundProcess(ContainerBase.java:1307)

at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.processChildren(ContainerBase.java:1571)

at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.processChildren(ContainerBase.java:1580)

at org.apache.catalina.core.ContainerBase$ContainerBackgroundProcessor.run(ContainerBase.java:1560)

at java.lang.Thread.run(Unknown Source)

2013-9-11 16:51:36 org.apache.catalina.core.StandardContext loadOnStartup

嚴重: Servlet /counter threw load() exception