java加载资源包会产生nullpointerexception

Question

我有以下文件结构：

projectname/war/i18n/strings.properties
projectname/war/i18n/strings_de.properties
projectname/war/i18n/strings_fr.properties
projectname/war/i18n/strings_es.properties
projectname/war/i18n/strings_it.properties

所以默认为英语。要访问这些文件，我需要执行以下操作：

public void doGet( HttpServletRequest request, HttpServletResponse response ) throws ServletException, IOException
    {
        String lang = request.getParameter("lang");
        Locale locale = null;
        if( lang.equals("DE"))
            locale = new Locale("de");
        else if( lang.equals("FR"))
            locale = new Locale("fr");
        else if( lang.equals("IT"))
            locale = new Locale("it");
        else if( lang.equals("ES"))
            locale = new Locale("es");

        URL[] urls = {new File("i18n/").toURI().toURL()};
        ClassLoader loader = new URLClassLoader(urls);
        ResourceBundle strings = ResourceBundle.getBundle("strings", locale, loader ); 
        request.setAttribute("strings", strings );

然而，当运行这段代码时，我发现

java.lang.NullPointerException
    at java.util.ResourceBundle.getBundleImpl(ResourceBundle.java:1252)
    at java.util.ResourceBundle.getBundle(ResourceBundle.java:1028)
    at de.projectname.server.pages.MyServlet.doGet(MyServlet.java:41)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:487)
    at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:362)
    at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
    at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:181)
    at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:729)
    at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:405)
    at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
    at org.mortbay.jetty.handler.RequestLogHandler.handle(RequestLogHandler.java:49)
    at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
    at org.mortbay.jetty.Server.handle(Server.java:324)
    at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:505)
    at org.mortbay.jetty.HttpConnection$RequestHandler.headerComplete(HttpConnection.java:829)
    at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:513)
    at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:211)
    at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:380)
    at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:395)
    at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:488)

为什么

编辑：这也不起作用：

locale = null;
ResourceBundle strings = ResourceBundle.getBundle("strings", locale );

同样的错误

Answer 1

区域设置不能为null，否则在尝试加载资源包时将获得该NPE

如果名为lang的请求参数不适合If-then决策中的某个区域设置，它将为null

修改代码，使其始终具有值，例如：

public void doGet( HttpServletRequest request, HttpServletResponse response ) throwsServletException, IOException
{
    String lang = request.getParameter("lang");

    // Default is english
    Locale locale = Locale.ENGLISH;

    if( lang.equals("DE"))
        locale = new Locale("de");
    else if( lang.equals("FR"))
        locale = new Locale("fr");
    else if( lang.equals("IT"))
        locale = new Locale("it");
    else if( lang.equals("ES"))
        locale = new Locale("es");
    ResourceBundle strings = ResourceBundle.getBundle("/i18n/strings", locale);
    // etc. etc.
}