Spring boot 配置自定义Json类型转换

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zhangyue

## 项目中遇到一个问题，就是由于我们数据库表ID使用分布式唯一算法生成的Long类型，所以有很长（19位数字），导致转成json传至前端js使用时报错，因为js的数字类型最大只能表示15个数字长度，见[Javascript 的number对象说明](http://www.w3school.com.cn/js/js_obj_number.asp)

## 解决方案：使用Spring自定义的Json序列化，将过长的Long类型转成String类型给到js使用，这样就不会有问题了

**中途遇到了许多坑，在这里记录一下**

# 局部配置某个字段的序列化
1. 实现抽象接口`JsonSerializer`自定义序列化类
```
public class MyLongConverter extends JsonSerializer<Long> {
    @Override
    public void serialize(Long value, JsonGenerator gen, SerializerProvider serializers) throws IOException, JsonProcessingException {
        if (value.toString().length() > 12) {
            gen.writeString(value.toString());
            log.info("the Long value is to long. will convert to String");
        } else {
            gen.writeNumber(value);
        }
    }
}
```
2. 使用`@JsonSerialize(using = CustomLongConverter.class)`注解到指定字段上
```
    @JsonSerialize(using = CustomLongConverter.class)
    private Long storageId;
```

# 全局配置序注册自定义的序列化类

## 使用在`@Configration`类中使用`@Bean`配置

1. 定义一个继承制StdSerializer类的一个自定义序列化
```
public class CustomLongConverter extends StdSerializer<Long> {

public CustomLongConverter() {
        super(Long.class);
    }

@Override
    public void serialize(Long value, JsonGenerator gen, SerializerProvider serializers) throws IOException, JsonProcessingException {
        if (value.toString().length() > 12) {
            gen.writeString(value.toString());
            log.info("the Long value is to long. will convert to String");
        } else {
            gen.writeNumber(value);
        }
    }
}
```
2. 注册到ObjectMapper中
- 参考官方文档[ Customize the Jackson ObjectMapper](https://docs.spring.io/spring-boot/docs/current/reference/html/howto-spring-mvc.html#howto-customize-the-jackson-objectmapper)
- 参考文章[How to customise the Jackson JSON mapper implicitly used by Spring Boot?
](https://stackoverflow.com/questions/28324352/how-to-customise-the-jackson-json-mapper-implicitly-used-by-spring-boot)
- 参考[自定义Jackson ObjectMapper把Long型转化为String类型](http://blog.csdn.net/L_Sail/article/details/70217393)
- 参考[Java Web程序中利用Spring框架返回JSON格式的日期](http://www.jb51.net/article/85093.htm)
Spring boot 默认使用`Jackson2ObjectMapperBuilder` 来构建ObjectMapper，如果想添加一些自定义的序列化只需定义`Jackson2ObjectMapperBuilderCustomizer`的Bean就可以将自定义的序列化注册到全局的ObjectMapper中， 在`@Configration`配置类中加入
```
@Bean
    public Jackson2ObjectMapperBuilderCustomizer initJackson() {
        Jackson2ObjectMapperBuilderCustomizer c = new Jackson2ObjectMapperBuilderCustomizer() {
            @Override
            public void customize(Jackson2ObjectMapperBuilder builder) {
                //自定义Long类型转换 超过12个数字用String格式返回，由于js的number只能表示15个数字
                builder.serializerByType(Long.class,new CustomLongConverter());
                builder.serializerByType(Long.TYPE,new CustomLongConverter());

builder.serializationInclusion(JsonInclude.Include.NON_NULL);//不包含为空的字段
                builder.serializationInclusion(JsonInclude.Include.NON_EMPTY);//不包含空字符串字段
                builder.indentOutput(true).dateFormat(new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"));
            }
        };
        return c;
    }
```
如果只是想在Mvc中的@ResponBody中生效，则可以用以下配置
```
public class Bootstrap extends WebMvcConfigurerAdapter {

public static void main(String[] args) {

SpringApplication.run(Bootstrap.class, args);
        log.info("application start success!");
    }

@Override
    public void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
        Jackson2ObjectMapperBuilder builder = new Jackson2ObjectMapperBuilder();
        builder.serializationInclusion(JsonInclude.Include.NON_NULL);
        //builder.serializationInclusion(JsonInclude.Include.NON_EMPTY);
        builder.indentOutput(true).dateFormat(new SimpleDateFormat("yyyy-MM-dd HH:mm:ss"));

//自定义Long类型转换 超过12个数字用String格式返回，由于js的number只能表示15个数字
        builder.serializerByType(Long.class,new CustomLongConverter());
        builder.serializerByType(Long.TYPE,new CustomLongConverter());
        converters.add(0,new MappingJackson2HttpMessageConverter(builder.build()));
    }
}

```

如果想完全替换ObjectMapper序列化器,可以自定义`Jackson2ObjectMapperBuilder` 的Bean并且将Bean的优先级设置为`@Primary`,但是我们这里不需要，只需要注册上自定义的序列化就可以了

## xml的配置注册方式
1.  在spring上下文中加入一下配置
```
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
	   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
	   xmlns:context="http://www.springframework.org/schema/context"
	   xmlns:aop="http://www.springframework.org/schema/aop" xmlns:tx="http://www.springframework.org/schema/tx"
	   xmlns:mvc="http://www.springframework.org/schema/mvc"
	   xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-4.1.xsd
    http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-4.1.xsd   
    http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-4.1.xsd   
	http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop-4.1.xsd http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc.xsd">

<mvc:annotation-driven>
		<mvc:message-converters register-defaults="false">
			<bean id="converter" class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter">
				<property name="objectMapper">
					<bean class="org.springframework.http.converter.json.Jackson2ObjectMapperFactoryBean">
						<property name="serializers">
							<array>
								
								<bean class="com.baibei.portal.center.util.CustomLongConverter"/>
							</array>
						</property>
						<property name="dateFormat">
							<bean class="java.text.SimpleDateFormat">
								<constructor-arg type="java.lang.String" value="yyyy-MM-dd HH:mm:ss"/>
							</bean>
						</property>
					</bean>
				</property>
			</bean>
		</mvc:message-converters>
	</mvc:annotation-driven>
</beans>

```
> 注意：，这一有一个坑，当xml的上下文中有多余的 `<mvc:annotation-driven>`,它将生成默认生成一个优先级较高的相关配置interceptor，http message converter,所以如果使用xml配置，要将其包含在`<mvc:annotation-driven ></mvc:annotation-driven>`并且将其他多余的`<mvc:annotation-driven>`去掉，否则这些配置将被覆盖失效

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