java - 需要解析httpServletRequest时出现ClassCastException

Question

我从 tomcat 运行我的代码但出现错误

HashMap<String, Object> params;
params = (HashMap) HttpUtils.getParamsFromRequest(request);

HttpUtils 代码如下

    public static Map<String, ? extends Object> getParamsFromRequest(HttpServletRequest httpServletRequest) {
    return httpServletRequest.getParameterMap();
}

但是得到这样的异常

java.lang.ClassCastException: org.apache.catalina.util.ParameterMap cannot be cast to java.util.HashMap
at se.mindspot.tender.backend.servlet.service.file.FileServiceServlet.doGet(FileServiceServlet.java:47)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:622)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:729)
at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:292)
at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:207)
at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)

Answer 1

Map是接口(interface)，HashMap是具体实现。 ParameterMap实现了 Map 但没有子类 HashMap 所以它不能cast 到 HashMap。正确的做法通常是

Map<String, Object> params;
params = HttpUtils.getParamsFromRequest(request);

和program to the Map interface .但是，您可以这样做

params = new HashMap<>(HttpUtils.getParamsFromRequest(request));

如果您需要一个HashMap并且有一些其他类型的Map，但这涉及到一个副本。