# 3. The Boot 4 starter split, and the exchange() trap

[Prev: RestClient with Basic Auth, two ways](02-restclient-basic-auth-patterns.md) | [README](../README.md)

Source: [`pom.xml`](../pom.xml).

## `spring-boot-starter-restclient` is not optional in Boot 4

The original article's `spring-boot-starter-web` dependency was, on Boot 3, sufficient to get an
auto-configured `RestTemplateBuilder` bean for free. On Boot 4.1, HTTP client support moved into
its own starter, confirmed the same way every version fact in this repository is confirmed --
build a throwaway project and read the real dependency tree:

```
$ mvn dependency:tree   # against spring-boot-starter-parent:4.1.1 + spring-boot-starter-webmvc only
```

`spring-boot-starter-webmvc` alone does **not** pull in `spring-boot-restclient`. Leave
`spring-boot-starter-restclient` off this module's `pom.xml` and the auto-configured
`RestClient.Builder` this chapter's code depends on is simply not there -- a `NoSuchBeanDefinitionException`
at startup, not a subtle behavioural difference. This module declares it explicitly.

## The trap this module deliberately avoids

`RestClient` has its own `exchange()` method, and it means something different from
`RestTemplate.exchange()`: **RestClient's `exchange()` disables the default status handlers**, so a
4xx or 5xx response is silently returned to you instead of thrown. Every example in this module
uses `retrieve()` for exactly that reason -- `retrieve()` keeps the throw-on-4xx/5xx behaviour this
Basic Auth demo relies on (see [`docs/output/03-restclient-no-credentials-401.txt`](output/03-restclient-no-credentials-401.txt)).
A team that mechanically renames `restTemplate.exchange(...)` call sites to
`restClient.exchange(...)` during a migration ships code that stops noticing failed requests. The
[RestTemplate to RestClient migration guide](https://ankurm.com/resttemplate-to-restclient-migration-guide/)
covers this trap, the full method-mapping table, and the three behavioural differences that matter
in more depth than a Basic Auth-focused rewrite has room for.

## Going deeper

- [RestTemplate to RestClient Migration Guide](https://ankurm.com/resttemplate-to-restclient-migration-guide/) -- the exchange() trap, timeouts, and the full mapping table
- [Spring Boot 4 HTTP Service Clients (@HttpExchange)](https://ankurm.com/spring-boot-4-http-service-clients/) -- the declarative layer built on top of RestClient
- Prev: [RestClient with Basic Auth, two ways](02-restclient-basic-auth-patterns.md)