Grails 8 Upgrade

What I Do

• Guide upgrades of Grails applications from Grails 7.x to Grails 8.
• Turn the Grails 8 upgrade guide into a practical migration checklist.
• Identify Spring Boot 4.1, Spring Framework 7, Jackson 3, Gradle platform, Micronaut, Hibernate, TagLib, testing, validation, and content negotiation changes that can break an application.
• Keep migration work focused on public application behavior, not internal Grails framework implementation details.

When to Use Me

Activate this skill when:

• Updating an application to Grails 8.
• Reviewing a Grails 8 upgrade branch or pull request.
• Fixing tests, build failures, runtime failures, or behavior changes after moving from Grails 7.x to Grails 8.
• Deciding whether an upgrade should remain on Hibernate 5 or opt in to Hibernate 7.
• Updating application build files, configuration, custom Spring Boot integration, TagLibs, JSON rendering, or content negotiation behavior for Grails 8.

Primary Sources

Use these repository docs as the source of truth before changing an application:

When checking published docs, use explicit Grails 8 URLs such as https://grails.apache.org/docs/8.0.0-M1/guide/upgrading.html or https://grails.apache.org/docs/snapshot/guide/upgrading.html. Do not assume https://grails.apache.org/docs/latest/guide/upgrading.html points at Grails 8.

Upgrade Baseline

Start every Grails 8 upgrade by checking these platform requirements:

• Use JDK 21 or later to build and run ordinary Grails 8 applications.
• Use JDK 25 or later if the application uses grails-micronaut, micronaut-http-client, or other Micronaut features.
• Update the Gradle wrapper to the Grails 8 managed line. Current 8.0.x snapshot docs use Gradle 9.7.1, while milestone docs may show an earlier Gradle 9.x version.
• Expect Spring Boot 4.1.x, Spring Framework 7.0.x, Spring Security 7.1.x, Spring Data 2026.0.x, Micrometer 1.17.x, Jackson 3.1.x, Tomcat 11.0.x, and Jakarta Servlet 6.1.
• Keep using jakarta.* APIs. Do not reintroduce javax.* packages.
• Add runtimeOnly 'org.springframework.boot:spring-boot-properties-migrator' temporarily during the migration, boot once, fix reported configuration properties, then remove it.

Migration Workflow

Use this order so failures are isolated and reversible:

1. Move the build, CI, and deployment runtime to the required JDK.
2. Update the Grails version and regenerate or compare a fresh Grails 8 application for build-file conventions.
3. Boot with the Spring Boot properties migrator and fix configuration warnings.
4. Fix dependency management and starter changes before code changes.
5. Compile and fix direct Spring Boot, Spring Framework, Jackson, Hibernate, and testing API removals.
6. Run application tests and manually exercise public behavior that depends on rendering, validation, content negotiation, persistence, and custom Spring integration.
7. Remove temporary migration dependencies and compatibility toggles once the application is clean.

Build and Dependency Management

Grails 8 no longer applies the io.spring.dependency-management plugin by default.

• Grails now uses Gradle native platform() dependency management with the Grails BOM.
• The Grails Gradle Plugin auto-applies the selected Grails BOM to declarable configurations.
• Existing version overrides in gradle.properties and ext['property.version'] still work through the bundled org.apache.grails.gradle.bom-property-overrides plugin.
• Replace grails { springDependencyManagement = false } with grails { bom = null } for new builds that intentionally opt out.
• If Spring DM was used to pin arbitrary dependencies, replace it with direct dependencies or Gradle resolutionStrategy.force where a transitive version must be forced.
• If strict BOM behavior is required, explicitly use enforcedPlatform("org.apache.grails:grails-bom:$grailsVersion").

Spring Boot 4 renamed common starters:

For WAR deployment to an external servlet container, replace providedRuntime 'org.springframework.boot:spring-boot-starter-tomcat' with providedRuntime 'org.springframework.boot:spring-boot-starter-tomcat-runtime'.

Undertow is not available in Grails 8 until Undertow supports Servlet 6.1. Use Tomcat or Jetty.

Spring Retry is no longer managed by Spring Boot. If the application directly uses @Retryable, @EnableRetry, or @Recover, add implementation 'org.springframework.retry:spring-retry'.

Spring Boot 4 and Spring Framework 7 Code Changes

Check for direct imports or references to Spring Boot auto-configuration classes. Spring Boot 4 split the old monolithic auto-configure module into domain modules, and many classes moved.

Examples:

// Before
import org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration

// After
import org.springframework.boot.mongodb.autoconfigure.MongoAutoConfiguration

If the application contributes an EnvironmentPostProcessor, update both imports and META-INF/spring.factories keys from org.springframework.boot.env.EnvironmentPostProcessor to org.springframework.boot.EnvironmentPostProcessor.

Other common Spring changes:

• Rename HttpStatus.MOVED_TEMPORARILY to HttpStatus.FOUND.
• Remove HandlerAdapter.getLastModified overrides.
• Replace use of removed Spring theme infrastructure such as ThemeSource, Theme, SimpleTheme, and SessionThemeResolver.
• Replace SecurityProperties.DEFAULT_FILTER_ORDER with -100 only when a direct constant replacement is needed, and prefer fluent HttpSecurity configuration long term.
• Update embedded server imports such as TomcatServletWebServerFactory to the new Spring Boot 4 packages.
• Add spring-boot-tomcat explicitly if application code directly references Tomcat classes.
• Prefer JSpecify nullability annotations for new Spring-facing code.

Configuration Changes

Review application configuration after booting with the Spring Boot properties migrator.

• Rename Spring Boot MongoDB auto-configuration properties from spring.data.mongodb.* to spring.mongodb.*.
• Do not change Grails GORM MongoDB mongodb.* properties for this Spring Boot rename.
• Jackson configuration under spring.jackson.* was reorganized. Watch for migrator output such as spring.jackson.read.* moving to spring.jackson.json.read.* and spring.jackson.write.* moving to spring.jackson.json.write.*.
• DevTools live reload is disabled by default. Set spring.devtools.livereload.enabled: true under the development environment if needed.
• Spring Boot now writes build info to META-INF/build-info.properties by default.
• Liveness and readiness probes are enabled by default on the health endpoint. Disable through management.endpoint.health.probes.enabled: false only if required.

Micronaut Integration

Micronaut-enabled Grails 8 applications have stricter requirements than ordinary Grails 8 applications.

• Run Micronaut-enabled applications on JDK 25 or later.
• Apply a Micronaut-compatible Grails BOM as enforcedPlatform, not plain platform.
• Use grails-micronaut-bom for the default Micronaut setup.
• Use grails-hibernate5-micronaut-bom for Micronaut with Hibernate 5.
• Use grails-hibernate7-micronaut-bom for Micronaut with Hibernate 7.
• Remove explicit BootArchive.loaderImplementation = LoaderImplementation.CLASSIC; Spring Boot 4 removed the CLASSIC loader.
• Do not disable grails { micronautAutoSetup = false } unless the build intentionally owns all Micronaut annotation processors and BOM validation.

Example:

dependencies {
    implementation enforcedPlatform("org.apache.grails:grails-micronaut-bom:$grailsVersion")
    implementation 'org.apache.grails:grails-micronaut'
}

Jackson and JSON Rendering

Grails 8 follows Spring Boot 4 and Jackson 3.

• Jackson annotations remain under com.fasterxml.jackson.annotation.*.
• Jackson databind moves from com.fasterxml.jackson.databind.* to tools.jackson.databind.*.
• The auto-configured mapper is tools.jackson.databind.json.JsonMapper, not Jackson 2 ObjectMapper.
• Prefer JsonMapper.builder().build() for direct mapper construction.
• Jackson 3 exceptions are unchecked JacksonException types.
• Default dates are ISO-8601 strings instead of numeric timestamps.
• To keep Jackson 2 defaults temporarily, set spring.jackson.use-jackson2-defaults: true.

Spring Boot helper renames include:

JSON views now use groovy.json.JsonGenerator. If custom JSON view converters used Grails-specific generator classes, migrate them to groovy.json.JsonGenerator.Converter and rename the service loader file to src/main/resources/META-INF/services/groovy.json.JsonGenerator$Converter.

Enum serialization changed:

• SimpleEnumMarshaller is the default for JSON and XML enum serialization.
• Remove explicit configuration that opted into simple enum formatting.
• Rendering a single enum with render(MyEnum.VALUE as JSON) now throws ConverterException; render an object, map, or explicit string instead.

Hibernate and GORM

Do not assume a Grails 8 upgrade requires Hibernate 7.

• Grails 8 supports Hibernate 5 by default.
• Opt in to Hibernate 7 only when the application is ready for Hibernate ORM 7 changes.
• If application code directly imports Spring ORM Hibernate classes, replace org.springframework.orm.hibernate5 with org.grails.orm.hibernate.support.hibernate5 for Hibernate 5 or org.grails.orm.hibernate.support.hibernate7 for Hibernate 7.

Hibernate 7 opt-in example:

dependencies {
    implementation enforcedPlatform("org.apache.grails:grails-hibernate7-bom:$grailsVersion")
    implementation 'org.apache.grails:grails-hibernate7'
}

If moving from Hibernate 5 to Hibernate 7, audit direct Hibernate API use:

Also check for:

• Removed annotations such as @Where, @WhereJoinTable, @Proxy, @LazyCollection, @Persister, @SelectBeforeUpdate, and @Loader.
• Removed CascadeType.SAVE_UPDATE; use CascadeType.ALL, CascadeType.PERSIST, or CascadeType.MERGE as appropriate.
• Detached refresh() or lock() calls, which now throw IllegalArgumentException.
• Native SQL query results now returning java.time date/time types instead of java.sql date/time types.
• StatelessSession participating in second-level cache by default. Set CacheMode.IGNORE for intentional cache bypass.
• DDL differences for character columns, Oracle floating and timestamp columns, SQL Server timestamp precision, and MySQL or MariaDB array columns.
• Hibernate 6 intermediate changes: HQL join result typing, 1-based ordinal parameters, legacy Criteria API removal, boolean type mapping converters, and continued jakarta.persistence usage.

GORM behavior changes:

• Unconstrained persistent domain properties are nullable by default in Grails 8.
• Declare nullable: false on required domain properties, or set grails.gorm.default.nullable: false to restore the previous application-wide default.
• Command object fields are unaffected and remain required by default.
• GORM dynamic methods such as save(), delete(), get(), load(), and merge() are not affected by Hibernate Session API removals.

Web Layer and Content Negotiation

Grails 8 supplies MIME type defaults from the framework.

• New applications no longer need to declare the full grails.mime.types block.
• An existing grails.mime.types block still replaces the defaults.
• Set grails.mime.mergeDefaults: true when adding custom MIME types while keeping built-in defaults.

The HTTP Accept header is honored for all clients by default, including browsers.

• Browser page loads still usually negotiate HTML because modern browsers rank text/html highest.
• Browser fetch() or XMLHttpRequest calls requesting JSON now receive JSON without relying on X-Requested-With.
• respond actions without an HTML view may now error for browser requests because browsers negotiate HTML. Add a GSP view, use render, or scope formats with responseFormats or respond(..., formats: ...).
• To restore the old browser-ignore behavior, set grails.mime.disable.accept.header.userAgents explicitly.

TagLibs and Tests

Grails 8 recommends method-based TagLib handlers while keeping closure-based tags supported.

• Conventional method signatures def tag(Map attrs), def tag(Closure body), and def tag(Map attrs, Closure body) are discovered automatically.
• Zero-argument method tags and method tags with named parameters must use @grails.gsp.Tag.
• Annotate public helper methods with @grails.gsp.NotATag when they must not be exposed as tags.
• A Map parameter named attrs receives the full attributes map.
• A Closure parameter named body receives the tag body.
• In Grails 8, grails { preserveParameterNames = true } is the default for Groovy compilation.

Test changes:

• Remove custom purgeTagLibMetaClass properties or getters from TagLib specs.
• TagLib metadata cleanup happens automatically through the web test infrastructure.
• Mocked tag libraries are cleared after each feature method. If a spec implements GrailsWebUnitTest and calls mockTagLib directly, move the call from setupSpec() to setup().
• @SpringBootTest no longer auto-configures MockMvc, WebClient, or TestRestTemplate. Add @AutoConfigureMockMvc, @AutoConfigureWebClient, or @AutoConfigureTestRestTemplate as needed.
• Replace removed @MockBean and @SpyBean with @MockitoBean and @MockitoSpyBean from org.springframework.test.context.bean.override.mockito.
• Remove explicit MockitoTestExecutionListener registration.
• Update TestRestTemplate imports to org.springframework.boot.resttestclient.

Plugin Compatibility

Check plugin compatibility before treating a Grails 8 runtime failure as an application bug.

• Some plugins may not yet support Spring Boot 4 or Spring Framework 7.
• The Grails 8 upgrade guide calls out grails-spring-security and SiteMesh 3 as known compatibility risks at the time of writing.
• Applications using grails-sitemesh3 should stay on the grails-layout plugin until SiteMesh 3 compatibility is available.
• Check the Grails issue tracker and each plugin release notes before upgrading plugin-dependent applications.

Verification Checklist

Run verification through public application behavior, not only compilation.

• ./gradlew clean check for the application, or the closest module-specific equivalent when the full suite is too expensive.
• Start the application with the target JDK and no Spring Boot properties migrator warnings.
• Exercise login, main pages, JSON endpoints, browser fetch() flows, file uploads, custom withFormat or respond actions, TagLib-rendered views, and persistence paths.
• Run database migration diffs before enabling Hibernate 7 schema update behavior in any environment with existing data.
• Re-run affected integration tests after changing Spring Boot test annotations, mocked beans, content negotiation, or persistence behavior.
• Remove temporary migration toggles once equivalent permanent changes are in place.

Common Fix Patterns

// Required domain property after Grails 8 nullable-default change
class Book {
    String title

    static constraints = {
        title nullable: false
    }
}

# Restore legacy required-by-default domain validation temporarily
grails:
    gorm:
        default:
            nullable: false

# Add a custom MIME type while keeping Grails 8 defaults
grails:
    mime:
        mergeDefaults: true
        types:
            custom: application/vnd.example+json

// Spring Boot 4 test bean override
import org.springframework.test.context.bean.override.mockito.MockitoBean

@SpringBootTest
class BookServiceSpec extends Specification {
    @MockitoBean BookRepository bookRepository
}

Pitfalls to Avoid

• Do not upgrade to Hibernate 7 just because the application is upgrading to Grails 8.
• Do not leave the Spring Boot properties migrator or Jackson 2 compatibility toggles in the final build without a clear follow-up task.
• Do not rely on compilation alone. Grails 8 changes default validation and content negotiation behavior that may only show up through real requests.
• Do not keep an old full grails.mime.types block if the intent is to extend the new defaults. Use grails.mime.mergeDefaults.
• Do not keep purgeTagLibMetaClass in tests. It is removed.
• Do not use javax.* imports. Grails 8 continues the Jakarta baseline.
• Do not ignore plugin compatibility. Spring Boot 4 and Spring Framework 7 removals often surface first through plugins.