如何使用 Jackson 和 Spring Boot 提供自定义反序列化器 [英] How to provide a custom deserializer with Jackson and Spring Boot

问题描述

我有以下三个应用程序:

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项目 1 保留

• 业务逻辑(Spring Cloud 函数)
• 一个接口IDemoEntity

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项目 2

• AWS 特定的处理程序
• IDemoEntity 的一种实现，带有 DynamoDB 特定的注释
• 项目基于 Spring Boot

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项目 3

• IDemoEntity 的一种实现，带有 CosmosDB 注释
• 特定于 Azure 的处理程序

项目 1 的类如下所示:

公共接口 IDemoEntity {字符串 getName();void setName(String name);}@成分公共类 StoreFunction 实现 Consumer>{@覆盖公共无效接受(消息 t){System.out.println("存储实体" + t.getPayload().getName());返回;}}

对于项目 2，IDemoEntity 的实现如下所示:

@DynamoDBTable(tableName = "DemoEntity")公共类 DynamoDemoEntity 实现 IDemoEntity {私人字符串名称；@覆盖@DynamoDBHashKey公共字符串 getName() {返回名称；}@覆盖公共无效集名称(字符串名称){this.name = 名称；}}

对于项目 3，IDemoEntity 的实现看起来类似于 DynamoDemoEntity，但带有 CosmosDB 注释.

结构可能看起来有点复杂，但想法如下:

1. 一次性实现业务逻辑和数据模型(在项目 1 中)(利用 Spring Cloud 功能)
2. 仅为每个平台实现一个包装器项目(我从项目 2 中的 AWS Lambda 开始，但用于 Azure 的项目 3 看起来类似)，以及特定于平台的东西(例如实体实现，它需要特定于数据库的注释)
3. 使用项目 1 作为依赖项编译特定于平台的项目(例如 AWS Lambda 的项目 2)

我已经试过了，设置基本有效.但是，有一个大问题:

在调用上面的 StoreFunction 时，Jackson 抛出以下异常:

Caused by: com.fasterxml.jackson.databind.exc.InvalidDefinitionException: 无法构造`de.margul.awstutorials.springcloudfunction.logic.IDemoEntity` 的实例(没有创建者，如默认构造，存在):抽象类型要么需要映射到具体类型，要么有自定义反序列化器，要么包含额外的类型信息在 [来源: (String)"{"name": "Detlef"}";行:1，列:1]在 com.fasterxml.jackson.databind.exc.InvalidDefinitionException.from(InvalidDefinitionException.java:67)在 com.fasterxml.jackson.databind.DeserializationContext.reportBadDefinition(DeserializationContext.java:1452)在 com.fasterxml.jackson.databind.DeserializationContext.handleMissingInstantiator(DeserializationContext.java:1028)在 com.fasterxml.jackson.databind.deser.AbstractDeserializer.deserialize(AbstractDeserializer.java:265)在 com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4013)在 com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3004)在 de.margul.awstutorials.springcloudfunction.aws.handler.RestSpringBootApiGatewayRequestHandler.deserializeBody(RestSpringBootApiGatewayRequestHandler.java:57)... 3个

这是有道理的，因为 Jackson 不知道，如果 IDemoEntity 它将反序列化接收到的 JSON，那么对于哪个实现.

现在最简单的方法是在 IDemoEntity 上放置一个 @JsonDeserialize(as = DynamoDemoEntity.class).

然而，这会破坏我的完整结构:项目 1 将没有信息，它是用哪个平台特定的项目编译的.

任何想法，我如何提供自定义反序列化器(例如作为 Spring bean)，但不在项目 1 中进行特定于平台的修改?

解决方案

首先，您需要创建您的自定义 DynamoDemoEntityDeserializer，如下所示:

class DynamoDemoEntityDeserializer 扩展 JsonDeserializer{@覆盖公共 DynamoDemoEntity 反序列化(JsonParser p，DeserializationContext ctxt)抛出 IOException，JsonProcessingException {//返回 DynamoDemoEntity 实例；}}

然后你可以像下面这样创建com.fasterxml.jackson.databind.Module的bean:

@Bean公共模块 dynamoDemoEntityDeserializer() {SimpleModule 模块 = new SimpleModule();module.addDeserializer(IDemoEntity.class, new DynamoDemoEntityDeserializer());返回模块；}

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任何 com.fasterxml.jackson.databind.Module 类型的 bean 都会自动注册到自动配置的 Jackson2ObjectMapperBuilder 并应用于它创建的任何 ObjectMapper 实例.当您向应用程序添加新功能时，这提供了一种用于贡献自定义模块的全局机制.

来源:howto-customize-the-jackson-objectmapper

I have the following three applications:

Project 1 holds

• Business logic (Spring Cloud Function)
• An interface IDemoEntity

Project 2

• AWS-specific handler
• One implementation of IDemoEntity, with DynamoDB-specific annotations
• The project is based on Spring Boot

Project 3

• One implementation of IDemoEntity, with CosmosDB annotation
• Azure-specific handler

The classes of project 1 look like this:

public interface IDemoEntity {
    String getName();
    void setName(String name);
}

@Component
public class StoreFunction implements Consumer<Message<IDemoEntity>> {

    @Override
    public void accept(Message<IDemoEntity> t) {

        System.out.println("Stored entity " + t.getPayload().getName());
        return;
    }
}

For project 2, the implementation of IDemoEntity looks like this:

@DynamoDBTable(tableName = "DemoEntity")
public class DynamoDemoEntity implements IDemoEntity {

    private String name;
    @Override
    @DynamoDBHashKey
    public String getName() {

        return name;
    }

    @Override
    public void setName(String name) {

        this.name = name;
    }    
}

For project 3, the implementation of IDemoEntity would look similar to DynamoDemoEntity, but with CosmosDB annotations.

The structure might look a bit complicated, but the idea is the following:

1. Implement business logic and data model one time (in project 1) (leveraging Spring Cloud Function)
2. Implement just a wrapper project for each platform (I'm starting with AWS Lambda in project 2, but project 3 for Azure would look similar), as well as platform-specific things (like entity implementation, which needs DB-specific annotations)
3. Compile the platform-specific project (e. g. project 2 for AWS Lambda) with project 1 as dependency

I've tried it, and the setup works basically. However, there is one big problem:

When calling the StoreFunction above, Jackson throws the following exception:

Caused by: com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Cannot construct instance of `de.margul.awstutorials.springcloudfunction.logic.IDemoEntity` (no Creators, like default construct, exist): abstract types either need to be mapped to concrete types, have custom deserializer, or contain additional type information
 at [Source: (String)"{"name": "Detlef"}"; line: 1, column: 1]
    at com.fasterxml.jackson.databind.exc.InvalidDefinitionException.from(InvalidDefinitionException.java:67)
    at com.fasterxml.jackson.databind.DeserializationContext.reportBadDefinition(DeserializationContext.java:1452)
    at com.fasterxml.jackson.databind.DeserializationContext.handleMissingInstantiator(DeserializationContext.java:1028)
    at com.fasterxml.jackson.databind.deser.AbstractDeserializer.deserialize(AbstractDeserializer.java:265)
    at com.fasterxml.jackson.databind.ObjectMapper._readMapAndClose(ObjectMapper.java:4013)
    at com.fasterxml.jackson.databind.ObjectMapper.readValue(ObjectMapper.java:3004)
    at de.margul.awstutorials.springcloudfunction.aws.handler.RestSpringBootApiGatewayRequestHandler.deserializeBody(RestSpringBootApiGatewayRequestHandler.java:57)
    ... 3 more

That makes sense, because Jackson does not know, to which implementation if IDemoEntity it shall deserialize the received JSON.

The easiest way now would be to place a @JsonDeserialize(as = DynamoDemoEntity.class) on IDemoEntity.

However, that would break my complete structure: Project 1 shall have no information, which platform-specific project it is compiled with.

Any ideas, how I could provide a custom deserializer (e. g. as Spring bean), but without making platform-specific modifications in project 1?

解决方案

Firstly, you need to create your custom DynamoDemoEntityDeserializer like below:

class DynamoDemoEntityDeserializer extends JsonDeserializer<DynamoDemoEntity> {
    @Override
    public DynamoDemoEntity deserialize(JsonParser p, DeserializationContext ctxt)
                throws IOException, JsonProcessingException {
       // return DynamoDemoEntity instance;
    }
}

Then you can create bean of com.fasterxml.jackson.databind.Module like below:

@Bean
public Module dynamoDemoEntityDeserializer() {
    SimpleModule module = new SimpleModule();
    module.addDeserializer(IDemoEntity.class, new DynamoDemoEntityDeserializer());
    return module;
}

Any beans of type com.fasterxml.jackson.databind.Module are automatically registered with the auto-configured Jackson2ObjectMapperBuilder and are applied to any ObjectMapper instances that it creates. This provides a global mechanism for contributing custom modules when you add new features to your application.

Source: howto-customize-the-jackson-objectmapper

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