學習Servlet時遇到的bug。

問題描述：
java.lang.NumberFormatException: For input string: “null”

在開發中你是否遇到過這樣的問題，不管請求到的值是什麼都能進入不為null或”“的判斷中，如下例：

ArrayList<Items> itemlist = itemDao.getViewList(list);
                if(itemlist!=null&&itemlist.size()>0)//big bug
                {
                   System.
 
out.println("itemlist.size="+itemlist.size());
                   for(Items i:itemlist)
                   {

執行時itemlist總能進入if條件判斷，並且你會得到錯誤提示如下：

at java.lang.NumberFormatException.forInputString(NumberFormatException.java:65)
    at java.lang.Integer.parseInt(Integer.java:580)
    at java.lang.Integer
 
.parseInt(Integer.java:615)
    at dao.ItemsDAO.getViewList(ItemsDAO.java:127)
    at org.apache.jsp.details_jsp._jspService(details_jsp.java:303)
    at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
    at org.apache
 
.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:443)
    at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:385)
    at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:329)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:230)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:165)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:192)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:165)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:198)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:96)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:474)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:140)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:79)
    at org.apache.catalina.valves.AbstractAccessLogValve.invoke(AbstractAccessLogValve.java:624)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:87)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:349)
    at org.apache.coyote.http11.Http11Processor.service(Http11Processor.java:783)
    at org.apache.coyote.AbstractProcessorLight.process(AbstractProcessorLight.java:66)
    at org.apache.coyote.AbstractProtocol$ConnectionHandler.process(AbstractProtocol.java:798)
    at org.apache.tomcat.util.net.NioEndpoint$SocketProcessor.doRun(NioEndpoint.java:1434)
    at org.apache.tomcat.util.net.SocketProcessorBase.run(SocketProcessorBase.java:49)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Thread.java:745)

產生問題的原因：錯誤的理解了提示 For input string: “null”
這句話的意思不是說輸入的是空值，而是說你輸入的是String型別的字串”null“

解決辦法：

ArrayList<Items> itemlist = itemDao.getViewList(list);
                if(itemlist!=null&&itemlist.size()>0&&!"null".equals(itemlist))//big bug!!!!
                {
                   System.out.println("itemlist.size="+itemlist.size());
                   for(Items i:itemlist)
                   {

只需要加上一條不為字串”null“的判斷即可。

照著教程敲有bug。。。把原始碼拿過來執行還有bug。。。真是想掀桌子(╯‵□′)╯︵┻━┻