Jackson序列化json时null转成空串或空对象

在项目中可能会遇到需要将null转""，可以通过以下方法解决。

一：添加JacksonConfig 配置

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializationFeature;
import com.fasterxml.jackson.databind.SerializerProvider;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Primary;
import org.springframework.http.converter.json.Jackson2ObjectMapperBuilder;

import java.io.IOException;

@Configuration
public class JacksonConfig {

@Bean
@Primary
@ConditionalOnMissingBean(ObjectMapper.class)
public ObjectMapper jacksonObjectMapper(Jackson2ObjectMapperBuilder builder) {
ObjectMapper objectMapper = builder.createXmlMapper(false).build();
// 关键代码-解决No serializer found for class java.lang.Object异常
objectMapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false);
//关键代码-将null转为空字符串、空对象
objectMapper.getSerializerProvider().setNullValueSerializer(new JsonSerializer<Object>() {
@Override
public void serialize(Object o, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException {
jsonGenerator.writeString("");//空串
//jsonGenerator.writeObject(new Object());//空对象
}
});
return objectMapper;
}

}

二、注意

1、关键代码分析

objectMapper.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false)

此行代码的作用是解决下面的异常：

com.fasterxml.jackson.databind.exc.InvalidDefinitionException: No serializer found for class java.lang.Object and no properties discovered to create BeanSerializer (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) (through reference chain: cloudpc.datasource.common.response.Result["data"])

2、 @ConditionalOnMissingBean

它是修饰bean的一个注解，主要实现的是，当你的bean被注册之后，如果而注册相同类型的bean，就不会成功，它会保证你的bean只有一个，即你的实例只有一个，当你注册多个相同的bean时，会出现异常，以此来告诉开发人员。

一般来说，对于自定义的配置类，我们应该加上@ConditionalOnMissingBean注解，以避免多个配置同时注入的风险。