JSONObject.toJSONString() 空值处理

/**
         * 指定排除属性过滤器和包含属性过滤器
         * 指定排除属性过滤器：转换成JSON字符串时，排除哪些属性
         * 指定包含属性过滤器：转换成JSON字符串时，包含哪些属性
         */
        String[] excludeProperties = {"country", "city"};
        String[] includeProperties = {"id", "username", "mobile"};
        PropertyPreFilters filters = new PropertyPreFilters();
        PropertyPreFilters.MySimplePropertyPreFilter excludefilter = filters.addFilter();
        excludefilter.addExcludes(excludeProperties);
        PropertyPreFilters.MySimplePropertyPreFilter includefilter = filters.addFilter();
        includefilter.addIncludes(includeProperties);


        /**
         * 情况一：默认忽略值为null的属性
         */
        jsonUser = JSONObject.toJSONString(user, SerializerFeature.PrettyFormat);
        System.out.println("情况一:
" + jsonUser);

        /**
         * 情况二：包含值为null的属性
         */
        jsonUser = JSONObject.toJSONString(user, SerializerFeature.PrettyFormat, SerializerFeature.WriteMapNullValue);
        System.out.println("情况二:
" + jsonUser);

        /**
         * 情况三：默认忽略值为null的属性，但是排除country和city这两个属性
         */
        jsonUser = JSONObject.toJSONString(user, excludefilter, SerializerFeature.PrettyFormat);
        System.out.println("情况三:
" + jsonUser);

        /**
         * 情况四：包含值为null的属性，但是排除country和city这两个属性
         */
        jsonUser = JSONObject.toJSONString(user, excludefilter, SerializerFeature.PrettyFormat, SerializerFeature.WriteMapNullValue);
        System.out.println("情况四:
" + jsonUser);

        /**
         * 情况五：默认忽略值为null的属性，但是包含id、username和mobile这三个属性
         */
        jsonUser = JSONObject.toJSONString(user, includefilter, SerializerFeature.PrettyFormat);
        System.out.println("情况五:
" + jsonUser);

        /**
         * 情况六：包含值为null的属性，但是包含id、username和mobile这三个属性
         */
        jsonUser = JSONObject.toJSONString(user, includefilter, SerializerFeature.PrettyFormat, SerializerFeature.WriteMapNullValue);