java - 无法写入 JSON 通用对象

Question

当我尝试返回通用对象时，出现异常:

@RequestMapping(value="/administration/get_stat_all") 
public @ResponseBody
List<StatAllBean<String>>  get_stat_all(..) {

List<StatAllBean<String>> all_stats = new ArrayList<StatAllBean<String>>();
....
return all_stats ;

这是我的对象:

public class StatAllBean<T> {

    public Map<T, Long> totalMap ;
    public Date date;

异常(exception):

19:14:56.366 [http-nio-8080-exec-9] DEBUG org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver - Resolving exception from handler [<java.lang.reflect.MalformedParameterizedTypeException>]: org.springframework.http.converter.HttpMessageNotWritableException: Could not write JSON: (was java.lang.NullPointerException) (through reference chain: java.util.ArrayList[0]); nested exception is org.codehaus.jackson.map.JsonMappingException: (was java.lang.NullPointerException) (through reference chain: java.util.ArrayList[0])

Answer 1

all_stats 列表是一个空列表，您需要检查:

return ( all_stats.isEmpty() ? null : all_stats );