Clarify the contract of features and capabilities

- introduce enums for core extensions
- validate duplicate features

core/deployment/src/main/java/io/quarkus/deployment/Capabilities.java

@@ -3,47 +3,17 @@
 import java.util.Set;
 
 import io.quarkus.builder.item.SimpleBuildItem;
+import io.quarkus.deployment.builditem.CapabilityBuildItem;
 
 /**
- * The list of capabilities.
+ * This build items holds the set of registered capabilities.
+ * 
+ * @see CapabilityBuildItem
  */
 public final class Capabilities extends SimpleBuildItem {
 
-    public static final String AGROAL = "io.quarkus.agroal";
-    public static final String CDI_ARC = "io.quarkus.cdi";
-    public static final String SERVLET = "io.quarkus.servlet";
-    public static final String TRANSACTIONS = "io.quarkus.transactions";
-    public static final String JACKSON = "io.quarkus.jackson";
-    public static final String JSONB = "io.quarkus.jsonb";
-    public static final String REST_JACKSON = "io.quarkus.rest.jackson";
-    public static final String REST_JSONB = "io.quarkus.rest.jsonb";
-    public static final String RESTEASY_JSON_EXTENSION = "io.quarkus.resteasy-json";
-    public static final String RESTEASY = "io.quarkus.resteasy";
-    public static final String JWT = "io.quarkus.jwt";
-    public static final String TIKA = "io.quarkus.tika";
-    public static final String MONGODB_PANACHE = "io.quarkus.mongodb.panache";
-    public static final String FLYWAY = "io.quarkus.flyway";
-    public static final String LIQUIBASE = "io.quarkus.liquibase";
-    public static final String SECURITY = "io.quarkus.security";
-    public static final String SECURITY_ELYTRON_OAUTH2 = "io.quarkus.elytron.security.oauth2";
-    public static final String SECURITY_ELYTRON_JDBC = "io.quarkus.elytron.security.jdbc";
-    public static final String SECURITY_ELYTRON_LDAP = "io.quarkus.elytron.security.ldap";
-    public static final String SECURITY_JPA = "io.quarkus.security.jpa";
-    public static final String QUARTZ = "io.quarkus.quartz";
-    public static final String METRICS = "io.quarkus.metrics";
-    public static final String RESTEASY_MUTINY_EXTENSION = "io.quarkus.resteasy-mutiny";
-    public static final String CONTAINER_IMAGE_JIB = "io.quarkus.container-image-jib";
-    public static final String CONTAINER_IMAGE_DOCKER = "io.quarkus.container-image-docker";
-    public static final String CONTAINER_IMAGE_S2I = "io.quarkus.container-image-s2i";
-    public static final String HIBERNATE_ORM = "io.quarkus.hibernate-orm";
-    public static final String SMALLRYE_OPENTRACING = "io.quarkus.opentracing";
-
     private final Set<String> capabilities;
 
-    public boolean isCapabilityPresent(String capability) {
-        return capabilities.contains(capability);
-    }
-
     public Capabilities(Set<String> capabilities) {
         this.capabilities = capabilities;
     }
@@ -52,4 +22,20 @@ public Set<String> getCapabilities() {
         return capabilities;
     }
 
+    public boolean isPresent(Capability capability) {
+        return isPresent(capability.getName());
+    }
+
+    public boolean isPresent(String capability) {
+        return capabilities.contains(capability);
+    }
+
+    public boolean isMissing(String capability) {
+        return !isPresent(capability);
+    }
+
+    public boolean isMissing(Capability capability) {
+        return isMissing(capability.getName());
+    }
+
 }

core/deployment/src/main/java/io/quarkus/deployment/Capability.java

@@ -0,0 +1,58 @@
+package io.quarkus.deployment;
+
+/**
+ * Represents a capability of a core extension.
+ */
+public enum Capability {
+
+    /**
+     * A datasource connection pool implementation
+     */
+    AGROAL,
+    /**
+     * JSR 365 compatible contexts and dependency injection
+     */
+    CDI,
+    /**
+     * Java Servlet API
+     */
+    SERVLET,
+    /**
+     * Java Transaction API (JTA)
+     */
+    TRANSACTIONS,
+    JACKSON,
+    JSONB,
+    REST_JACKSON,
+    REST_JSONB,
+    RESTEASY,
+    RESTEASY_JSON,
+    RESTEASY_MUTINY,
+    JWT,
+    TIKA,
+    MONGODB_PANACHE,
+    FLYWAY,
+    LIQUIBASE,
+    SECURITY,
+    SECURITY_ELYTRON_OAUTH2,
+    SECURITY_ELYTRON_JDBC,
+    SECURITY_ELYTRON_LDAP,
+    SECURITY_JPA,
+    QUARTZ,
+    METRICS,
+    CONTAINER_IMAGE_JIB,
+    CONTAINER_IMAGE_DOCKER,
+    CONTAINER_IMAGE_S2I,
+    HIBERNATE_ORM,
+    SMALLRYE_OPENTRACING,
+    ;
+
+    /**
+     * 
+     * @return the name
+     */
+    public String getName() {
+        return "io.quarkus." + toString().toLowerCase().replace("_", ".");
+    }
+
+}

core/deployment/src/main/java/io/quarkus/deployment/Feature.java

@@ -0,0 +1,123 @@
+package io.quarkus.deployment;
+
+import io.quarkus.deployment.builditem.FeatureBuildItem;
+
+/**
+ * Represents a feature provided by a core extension.
+ * 
+ * @see FeatureBuildItem
+ */
+public enum Feature {
+
+    AGROAL,
+    AMAZON_DYNAMODB,
+    AMAZON_LAMBDA,
+    AMAZON_S3,
+    AMAZON_SNS,
+    AMAZON_SQS,
+    AMAZON_SES,
+    AMAZON_KMS,
+    ARTEMIS_CORE,
+    ARTEMIS_JMS,
+    CACHE,
+    CDI,
+    CONFIG_YAML,
+    CONSUL_CONFIG,
+    ELASTICSEARCH_REST_CLIENT,
+    FLYWAY,
+    GRPC_CLIENT,
+    GRPC_SERVER,
+    HIBERNATE_ORM,
+    HIBERNATE_ORM_PANACHE,
+    HIBERNATE_ORM_PANACHE_KOTLIN,
+    HIBERNATE_ORM_REST_DATA_PANACHE,
+    HIBERNATE_VALIDATOR,
+    HIBERNATE_SEARCH_ELASTICSEARCH,
+    INFINISPAN_CLIENT,
+    INFINISPAN_EMBEDDED,
+    JAEGER,
+    JDBC_DERBY,
+    JDBC_H2,
+    JDBC_POSTGRESQL,
+    JDBC_MARIADB,
+    JDBC_MSSQL,
+    JDBC_MYSQL,
+    JGIT,
+    JSCH,
+    KAFKA_STREAMS,
+    KEYCLOAK_AUTHORIZATION,
+    KOTLIN,
+    KUBERNETES,
+    KUBERNETES_CLIENT,
+    LIQUIBASE,
+    LOGGING_GELF,
+    MAILER,
+    MONGODB_CLIENT,
+    MONGODB_PANACHE,
+    MUTINY,
+    NARAYANA_JTA,
+    NARAYANA_STM,
+    REACTIVE_PG_CLIENT,
+    REACTIVE_MYSQL_CLIENT,
+    NEO4J,
+    OIDC,
+    OPTAPLANNER,
+    OPTAPLANNER_JACKSON,
+    OPTAPLANNER_JSONB,
+    PICOCLI,
+    QUTE,
+    RESTEASY,
+    RESTEASY_JACKSON,
+    RESTEASY_JAXB,
+    RESTEASY_JSONB,
+    RESTEASY_MUTINY,
+    RESTEASY_QUTE,
+    REST_CLIENT,
+    REST_CLIENT_JACKSON,
+    REST_CLIENT_JAXB,
+    REST_CLIENT_JSONB,
+    SCALA,
+    SCHEDULER,
+    SECURITY,
+    SECURITY_JDBC,
+    SECURITY_LDAP,
+    SECURITY_JPA,
+    SECURITY_PROPERTIES_FILE,
+    SECURITY_OAUTH2,
+    SERVLET,
+    SMALLRYE_CONTEXT_PROPAGATION,
+    SMALLRYE_FAULT_TOLERANCE,
+    SMALLRYE_HEALTH,
+    SMALLRYE_JWT,
+    SMALLRYE_METRICS,
+    SMALLRYE_OPENAPI,
+    SMALLRYE_OPENTRACING,
+    SMALLRYE_REACTIVE_MESSAGING,
+    SMALLRYE_REACTIVE_MESSAGING_KAFKA,
+    SMALLRYE_REACTIVE_MESSAGING_AMQP,
+    SMALLRYE_REACTIVE_MESSAGING_MQTT,
+    SMALLRYE_REACTIVE_STREAMS_OPERATORS,
+    SMALLRYE_REACTIVE_TYPE_CONVERTERS,
+    SMALLRYE_GRAPHQL,
+    SPRING_DI,
+    SPRING_WEB,
+    SPRING_DATA_JPA,
+    SPRING_SECURITY,
+    SPRING_BOOT_PROPERTIES,
+    SPRING_CACHE,
+    SPRING_CLOUD_CONFIG_CLIENT,
+    SWAGGER_UI,
+    TIKA,
+    UNDERTOW_WEBSOCKETS,
+    VAULT,
+    VERTX,
+    VERTX_WEB,
+    VERTX_GRAPHQL,
+    WEBJARS_LOCATOR,
+    ;
+
+    public String getName() {
+        return toString().toLowerCase().replace("_", "-");
+    }
+
+}

core/deployment/src/main/java/io/quarkus/deployment/builditem/CapabilityBuildItem.java

@@ -2,16 +2,31 @@
 
 import io.quarkus.builder.item.MultiBuildItem;
 import io.quarkus.deployment.Capabilities;
+import io.quarkus.deployment.Capability;
 
 /**
- * Registers an internal feature.
+ * Represents a technical capability that can be queried by other extensions.
+ * <p>
+ * Build steps can inject {@link Capabilities} - a convenient build item that holds the set of registered capabilities.
+ * <p>
+ * An extension may provide multiple capabilities. Multiple extensions can provide the same capability. By default, capabilities
+ * are not displayed to users.
+ * <p>
+ * Capabilities should follow the naming conventions of Java packages; e.g. {@code io.quarkus.security.jpa}. Capabilities
+ * provided by core extensions should be listed in the {@link Capability} enum and their name should always start with the
+ * {@code io.quarkus} prefix.
  * 
- * @see Capabilities#isCapabilityPresent(String)
+ * @see Capabilities
+ * @see Capability
  */
 public final class CapabilityBuildItem extends MultiBuildItem {
 
     private final String name;
 
+    public CapabilityBuildItem(Capability capability) {
+        this(capability.getName());
+    }
+
     public CapabilityBuildItem(String name) {
         if (name == null) {
             throw new IllegalArgumentException("name cannot be null");

core/deployment/src/main/java/io/quarkus/deployment/builditem/ExtensionSslNativeSupportBuildItem.java

@@ -1,11 +1,16 @@
 package io.quarkus.deployment.builditem;
 
 import io.quarkus.builder.item.MultiBuildItem;
+import io.quarkus.deployment.Feature;
 
 public final class ExtensionSslNativeSupportBuildItem extends MultiBuildItem {
 
     private String extension;
 
+    public ExtensionSslNativeSupportBuildItem(Feature feature) {
+        this(feature.getName());
+    }
+
     public ExtensionSslNativeSupportBuildItem(String extension) {
         this.extension = extension;
     }