Springboot返回数据给前端-参数为null处理

转：https://www.pianshen.com/article/950119559/

1.返回对象参数为null时，该参数选择显示或者不显示

　　在返回参数给前端的时候，有些参数的值为null的时候，我们可以设置需要显示或者不需要显示

1.1设置返回为null的参数

　　在配置文件中加入配置

spring.jackson.default-property-inclusion: always

1.2设置不返回为null的参数

　　在配置文件中加入配置

spring.jackson.default-property-inclusion: non_null

2.返回参数为null时，转换为空字符串或者空数组

　　在返回数据时，前端不想要null值，需要我们把字符串、数字类型的null转换为空字符串，把集合类型的null转换为[]

2.1创建类MyNullArrayJsonSerializer

　　该类会将字段类型为集合类型的null值处理成空集合

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.SerializerProvider;

import java.io.IOException;

/**
 * 处理数组类型的null值
 */
public class MyNullArrayJsonSerializer extends JsonSerializer {

    @Override
    public void serialize(Object value, JsonGenerator jgen, SerializerProvider provider) throws IOException, JsonProcessingException {
        if (value == null) {
            jgen.writeStartArray();
            jgen.writeEndArray();
        }
    }
}

2.2 创建类MyNullJsonSerializer

　　该类会将字段类型为字符串、int等类型的null值处理成空字符串

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.SerializerProvider;

import java.io.IOException;

/**
 * 处理字符串等类型的null值
 */
public class MyNullJsonSerializer extends JsonSerializer{

    @Override
    public void serialize(Object o, JsonGenerator jsonGenerator, SerializerProvider serializerProvider)
            throws IOException, JsonProcessingException {
        jsonGenerator.writeString("");
    }
}

2.3创建类MyBeanSerializerModifier

　　该类控制将null值处理成空集合还是空字符串

package com.answer.waka.framework.platform.configuration;

import com.fasterxml.jackson.databind.BeanDescription;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.SerializationConfig;
import com.fasterxml.jackson.databind.ser.BeanPropertyWriter;
import com.fasterxml.jackson.databind.ser.BeanSerializerModifier;

import java.util.List;
import java.util.Set;

public class MyBeanSerializerModifier extends BeanSerializerModifier {

    //  数组类型
    private JsonSerializer _nullArrayJsonSerializer = new MyNullArrayJsonSerializer();
    // 字符串等类型
    private JsonSerializer _nullJsonSerializer = new MyNullJsonSerializer();

    @Override
    public List<BeanPropertyWriter> changeProperties(SerializationConfig config, BeanDescription beanDesc,
                                                     List beanProperties) {
        //循环所有的beanPropertyWriter
        for (int i = 0; i < beanProperties.size(); i++) {
            BeanPropertyWriter writer = (BeanPropertyWriter) beanProperties.get(i);
            //判断字段的类型，如果是array，list，set则注册nullSerializer
            if (isArrayType(writer)) {
                //给writer注册一个自己的nullSerializer
                writer.assignNullSerializer(this._nullArrayJsonSerializer);
            } else {
                writer.assignNullSerializer(this._nullJsonSerializer);
            }
        }
        return beanProperties;
    }

    //判断是什么类型
    protected boolean isArrayType(BeanPropertyWriter writer) {
        Class clazz = writer.getPropertyType();
        return clazz.isArray() || clazz.equals(List.class) || clazz.equals(Set.class);
    }

}

2.4配置类

import com.fasterxml.jackson.databind.ObjectMapper;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Primary;
import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;

/**
 * 处理返回值中的null值
 */
@Configuration
public class JsonConfig {

    @Bean
    public MappingJackson2HttpMessageConverter mappingJacksonHttpMessageConverter() {

        final MappingJackson2HttpMessageConverter converter = new MappingJackson2HttpMessageConverter();

        ObjectMapper mapper = converter.getObjectMapper();

        // 为mapper注册一个带有SerializerModifier的Factory，此modifier主要做的事情为：当序列化类型为array，list、set时，当值为空时，序列化成[]
        mapper.setSerializerFactory(mapper.getSerializerFactory().withSerializerModifier(new MyBeanSerializerModifier()));

        return converter;
    }

}

这样子处理后发现一个问题，Map格式数据返回，里面的日期全部成为了时间戳