JSON.toJSONString的坑

1.问题
 在使用fastjson中的JSON.toJSONString方法将对象转换成json字符串的时候，发现有些字段没有了。如：

public static void main(String[] args) {
        Mapmap=new HashMap<>();
        map.put("id","1");
        map.put("name",null);
        System.out.println(JSON.toJSONString(map));
    }
   
 输出结果： {"id":"1"}

输出结果中没有了 "name"这个属性了；

2.解决方式
加SerializerFeature.WriteMapNullValue)

public static void main(String[] args) {
        Mapmap=new HashMap<>();
        map.put("id","1");
        map.put("name",null);
        System.out.println(JSON.toJSONString(map,SerializerFeature.WriteMapNullValue));
    }


输出结果： {"name":null,"id":"1"}

3.原因

转json串，默认设置了许多序列化配置：

4.源码

/*
 * Copyright 1999-2101 Alibaba Group.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.alibaba.fastjson.serializer;

/**
 * @author wenshao[szujobs@hotmail.com]
 */
public enum SerializerFeature {
    QuoteFieldNames,
    /**
     * 
     */
    UseSingleQuotes,
    /**
     * 
     */
    WriteMapNullValue,
    /**
     * 用枚举toString()值输出
     */
    WriteEnumUsingToString,
    /**
     * 用枚举name()输出
     */
    WriteEnumUsingName,
    /**
     * 
     */
    UseISO8601DateFormat,
    /**
     * @since 1.1
     */
    WriteNullListAsEmpty,
    /**
     * @since 1.1
     */
    WriteNullStringAsEmpty,
    /**
     * @since 1.1
     */
    WriteNullNumberAsZero,
    /**
     * @since 1.1
     */
    WriteNullBooleanAsFalse,
    /**
     * @since 1.1
     */
    SkipTransientField,
    /**
     * @since 1.1
     */
    SortField,
    /**
     * @since 1.1.1
     */
    @Deprecated
    WriteTabAsSpecial,
    /**
     * @since 1.1.2
     */
    PrettyFormat,
    /**
     * @since 1.1.2
     */
    WriteClassName,

    /**
     * @since 1.1.6
     */
    DisableCircularReferenceDetect, // 32768

    /**
     * @since 1.1.9
     */
    WriteSlashAsSpecial,

    /**
     * @since 1.1.10
     */
    BrowserCompatible,

    /**
     * @since 1.1.14
     */
    WriteDateUseDateFormat,

    /**
     * @since 1.1.15
     */
    NotWriteRootClassName,

    /**
     * @since 1.1.19
     * @deprecated
     */
    DisableCheckSpecialChar,

    /**
     * @since 1.1.35
     */
    BeanToArray,

    /**
     * @since 1.1.37
     */
    WriteNonStringKeyAsString,
    
    /**
     * @since 1.1.42
     */
    NotWriteDefaultValue,
    
    /**
     * @since 1.2.6
     */
    BrowserSecure,
    
    /**
     * @since 1.2.7
     */
    IgnoreNonFieldGetter,
    
    /**
     * @since 1.2.9
     */
    WriteNonStringValueAsString,
    
    /**
     * @since 1.2.11
     */
    IgnoreErrorGetter
    ;

    SerializerFeature(){
        mask = (1 << ordinal());
    }

    public final int mask;

    public final int getMask() {
        return mask;
    }

    public static boolean isEnabled(int features, SerializerFeature feature) {
        return (features & feature.mask) != 0;
    }
    
    public static boolean isEnabled(int features, int fieaturesB, SerializerFeature feature) {
        int mask = feature.mask;
        
        return (features & mask) != 0 || (fieaturesB & mask) != 0;
    }

    public static int config(int features, SerializerFeature feature, boolean state) {
        if (state) {
            features |= feature.mask;
        } else {
            features &= ~feature.mask;
        }

        return features;
    }
    
    public static int of(SerializerFeature[] features) {
        if (features == null) {
            return 0;
        }
        
        int value = 0;
        
        for (SerializerFeature feature: features) {
            value |= feature.mask;
        }
        
        return value;
    }
    
    public final static SerializerFeature[] EMPTY = new SerializerFeature[0];
}