Question

我在web.xml配置文件中添加了一些参数，如下所示：

  <context-param>
    <param-name>service1</param-name>
    <param-value>http://www.example.com/example2.html</param-value>
  </context-param>
  <context-param>
    <param-name>service2</param-name>
    <param-value>http://www.example.com/example2.html</param-value>
  </context-param>
  ...

我尝试在我的servlet中获取参数，特别是在我的服务方法中：

protected void service(HttpServletRequest request, HttpServletResponse response) 
throws ServletException, IOException {
    System.out.println(this.getServletContext().getInitParameter("service1"));
    ...

但是在运行时我有一个NullPointerException ...如何获取web.xml中包含的参数值？

这是堆栈跟踪：

GRAVE: Servlet.service() for servlet DispatcherServlet threw exception
java.lang.NullPointerException
    at javax.servlet.GenericServlet.getServletContext(GenericServlet.java:160)
    at it.servlethope.DispatcherServlet.service(DispatcherServlet.java:66)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.tuckey.web.filters.urlrewrite.RuleChain.handleRewrite(RuleChain.java:176)
    at org.tuckey.web.filters.urlrewrite.RuleChain.doRules(RuleChain.java:145)
    at org.tuckey.web.filters.urlrewrite.UrlRewriter.processRequest(UrlRewriter.java:92)
    at org.tuckey.web.filters.urlrewrite.UrlRewriteFilter.doFilter(UrlRewriteFilter.java:381)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:235)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:127)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:298)
    at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:852)
    at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:588)
    at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:489)
    at java.lang.Thread.run(Unknown Source)

DispatcherServlet.java:66是我尝试getInitParameter（）的行

Answer 1

如果调用getServletContext()会抛出NPE，则意味着ServletConfig为空。

因此，您已在自己的servlet中覆盖了init(ServletConfig config)，并且未按照Javadoc正确调用super.init(config)：

由servlet容器调用，以向servlet指示servlet正在投入使用。请参阅Servlet.init(javax.servlet.ServletConfig)。

此实现存储从servlet容器接收的ServletConfig对象供以后使用。 覆盖此方法时，请致电super.init(config) 。

相应修复：

public void init(ServletConfig config) throws ServletException {
    // Do your thing here.
    // ...
    // And then finally:
    super.init(config);
}

或者只使用无init()的无参数ServletConfig方法，这样您就不必担心了。

Java Servlet：getInitParameter在Service（）中不起作用

1 个答案: