The Problem

After upgrading to Spring Boot 3.5.3, developers noticed that Spring started preferring XML over JSON when no Content-Type header was explicitly provided.

Inspecting the registered HttpMessageConverters revealed this order:

ByteArrayHttpMessageConverter
StringHttpMessageConverter
ResourceHttpMessageConverter
AllEncompassingFormHttpMessageConverter
MappingJackson2XmlHttpMessageConverter
MappingJackson2HttpMessageConverter

Since XML converter appears before JSON, the framework defaults to XML, leading to surprising results:

• Default error pages returned as XML
• @ControllerAdvice responses serialized as XML
• RestTemplate POSTs defaulting to XML instead of JSON

This behavior wasn’t present in Spring Boot 3.2.x, making the upgrade seem like a breaking change.

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🧠 Why It Happens

Spring Boot auto-registers message converters based on what’s found on the classpath.

• If jackson-dataformat-xml is present, Spring automatically configures MappingJackson2XmlHttpMessageConverter.
• Converters are then ordered, and XML ends up ahead of JSON in some cases.

In the reported case, the root cause was introducing JasperReports, which brought in:

com.fasterxml.jackson.dataformat:jackson-dataformat-xml

That dependency enabled XML support unintentionally. Without XML on the classpath, JSON is always preferred.

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Solutions

1. Remove or Exclude XML Support

If your application doesn’t need XML at all, the cleanest fix is to exclude jackson-dataformat-xml from your dependencies.

Maven example:

<dependency>
  <groupId>net.sf.jasperreports</groupId>
  <artifactId>jasperreports</artifactId>
  <version>...</version>
  <exclusions>
    <exclusion>
      <groupId>com.fasterxml.jackson.dataformat</groupId>
      <artifactId>jackson-dataformat-xml</artifactId>
    </exclusion>
  </exclusions>
</dependency>

Gradle example:

implementation("net.sf.jasperreports:jasperreports") {
    exclude group: "com.fasterxml.jackson.dataformat", module: "jackson-dataformat-xml"
}

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2. Force JSON as Default on the Server

If XML must remain available but JSON should be the default, configure Spring MVC:

# application.properties
spring.mvc.contentnegotiation.default-content-type=application/json

This ensures that when the client doesn’t send a header, JSON is returned instead of XML.

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3. Enforce JSON in RestTemplate

On the client side, if you use RestTemplate, you can override the default converters:

@Bean
public RestTemplate restTemplate(RestTemplateBuilder builder) {
    return builder
            .messageConverters(new MappingJackson2HttpMessageConverter())
            .build();
}

Alternatively, add a ClientHttpRequestInterceptor that injects Content-Type: application/json into outgoing requests.

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Final Root Cause

In the real case, the developer confirmed:

“Seems you found the issue, we introduced JasperReports, which brought in jackson-dataformat-xml, and Spring started using it.”

https://stackoverflow.com/questions/79762083/spring-changed-converters-order-prefering-xml

So the unexpected behavior wasn’t a Spring Boot bug, but a dependency side-effect.

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Conclusion

If you suddenly see XML responses in a Spring Boot app after upgrading:

• Check your classpath for jackson-dataformat-xml.
• If you don’t need XML, exclude the dependency.
• If you do, configure spring.mvc.contentnegotiation.default-content-type=application/json to keep JSON as the default.

This ensures consistent behavior and prevents XML from taking over your APIs unintentionally.