Make server build fail when duplicate route and fail handler. (#6499)

### Motivation:
`RejectRouterHandler.FAIL` does not work as intended.

### Modifications:
- The call site that calls `rejectionHandler.handleDuplicateRoute(virtualHost, route, existingRoute)` catch`IllegalStateException` and throw it.
- Fix false alert in `ContextPathTest`. (`ContextPathTest` already has duplicated route with `RejectRouterHandler.FAIL`, but it just warn not fail. )

### Result:
- If there are duplicated routes and `RejectRouterHandler.FAIL` is configured, 
Server build will be failed.
- Close #6498

Author: chickenchickenlove

core/src/main/java/com/linecorp/armeria/server/DuplicateRouteException.java

@@ -0,0 +1,39 @@
+/*
+ * Copyright 2025 LINE Corporation
+ *
+ * LINE Corporation licenses this file to you under the Apache License,
+ * version 2.0 (the "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at:
+ *
+ *   https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ */
+
+package com.linecorp.armeria.server;
+
+import com.linecorp.armeria.common.annotation.UnstableApi;
+
+/**
+ * A {@link RuntimeException} thrown when attempting to register a route
+ * that conflicts with an existing route for the same path.
+ *
+ * <p>This exception is raised when multiple routes share an identical path
+ * and thus cannot be uniquely distinguished.</p>
+ */
+@UnstableApi
+public final class DuplicateRouteException extends RuntimeException {
+
+    private static final long serialVersionUID = 4679512839761213302L;
+
+    /**
+     * Creates a new instance with the error message.
+     */
+    public DuplicateRouteException(String errorMsg) {
+        super(errorMsg);
+    }
+}

core/src/main/java/com/linecorp/armeria/server/RejectedRouteHandler.java

@@ -60,18 +60,18 @@ public interface RejectedRouteHandler {
     };
 
     /**
-     * A {@link RejectedRouteHandler} that raises an {@link IllegalStateException} for a problematic
+     * A {@link RejectedRouteHandler} that raises an {@link DuplicateRouteException} for a problematic
      * {@link Route}.
      */
     RejectedRouteHandler FAIL = (virtualHost, route, existingRoute) -> {
         final String a = route.toString();
         final String b = existingRoute.toString();
         final String hostnamePattern = virtualHost.hostnamePattern();
         if (a.equals(b)) {
-            throw new IllegalStateException(
+            throw new DuplicateRouteException(
                     "Virtual host '" + hostnamePattern + "' has a duplicate route: " + a);
         } else {
-            throw new IllegalStateException(
+            throw new DuplicateRouteException(
                     "Virtual host '" + hostnamePattern + "' has routes with a conflict: " +
                     a + " vs. " + b);
         }

core/src/main/java/com/linecorp/armeria/server/Routers.java

@@ -72,6 +72,8 @@ static Router<ServiceConfig> ofVirtualHost(VirtualHost virtualHost, Iterable<Ser
         final BiConsumer<Route, Route> rejectionConsumer = (route, existingRoute) -> {
             try {
                 rejectionHandler.handleDuplicateRoute(virtualHost, route, existingRoute);
+            } catch (DuplicateRouteException duplicateRouteException) {
+                throw duplicateRouteException;
             } catch (Exception e) {
                 logger.warn("Unexpected exception from a {}:",
                             RejectedRouteHandler.class.getSimpleName(), e);

core/src/test/java/com/linecorp/armeria/server/ContextPathTest.java

@@ -142,13 +142,7 @@ protected void configure(ServerBuilder sb) throws Exception {
               .get("/route1")
               .build((ctx, req) -> HttpResponse.of("route1"))
               .serviceUnder("/serviceUnder1", (ctx, req) -> HttpResponse.of("serviceUnder1"))
-              .serviceUnder("/serviceUnder2", serviceWithRoutes)
-              // virtual host decorator
-              .service("/decorated1", (ctx, req) -> HttpResponse.of(500))
-              .decorator("/decorated1", (delegate, ctx, req) -> HttpResponse.of("decorated1"))
-              .routeDecorator()
-              .path("/decorated2")
-              .build((delegate, ctx, req) -> HttpResponse.of("decorated2"));
+              .serviceUnder("/serviceUnder2", serviceWithRoutes);
             sb.decorator(LoggingService.newDecorator());
             sb.rejectedRouteHandler(RejectedRouteHandler.FAIL);
         }

core/src/test/java/com/linecorp/armeria/server/RejectRouterHandlerTest.java

@@ -0,0 +1,121 @@
+/*
+ * Copyright 2025 LINE Corporation
+ *
+ * LINE Corporation licenses this file to you under the Apache License,
+ * version 2.0 (the "License"); you may not use this file except in compliance
+ * with the License. You may obtain a copy of the License at:
+ *
+ *   https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
+ * WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
+ * License for the specific language governing permissions and limitations
+ * under the License.
+ */
+
+package com.linecorp.armeria.server;
+
+import static org.assertj.core.api.Assertions.assertThatCode;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+import java.util.List;
+
+import org.junit.jupiter.api.Test;
+
+import com.google.common.collect.ImmutableList;
+
+import com.linecorp.armeria.common.HttpResponse;
+
+class RejectRouterHandlerTest {
+
+    @Test
+    void when_duplicate_route_exists_then_server_builder_should_ignore_or_warn() {
+        final List<RejectedRouteHandler> handlers = ImmutableList.of(
+                RejectedRouteHandler.DISABLED,
+                RejectedRouteHandler.WARN
+        );
+
+        for (RejectedRouteHandler handler : handlers) {
+            assertThatCode(
+                    () -> duplicateRouteServerBuilder()
+                            .rejectedRouteHandler(handler)
+                            .build()
+            ).doesNotThrowAnyException();
+        }
+    }
+
+    @Test
+    void when_duplicate_route_exists_then_server_builder_should_throw_error() {
+        assertThatThrownBy(
+                () -> duplicateRouteServerBuilder()
+                        .rejectedRouteHandler(RejectedRouteHandler.FAIL)
+                        .build()
+        ).isInstanceOf(DuplicateRouteException.class);
+    }
+
+    @Test
+    void when_duplicate_route_exists_then_virtual_host_builder_should_ignore_or_warn() {
+        final List<RejectedRouteHandler> handlers = ImmutableList.of(
+                RejectedRouteHandler.DISABLED,
+                RejectedRouteHandler.WARN
+        );
+
+        for (RejectedRouteHandler handler : handlers) {
+            assertThatCode(
+                    () -> duplicatedRouteVirtualHostBuilder()
+                            .rejectedRouteHandler(handler)
+                            .and()
+                            .build()
+            ).doesNotThrowAnyException();
+        }
+    }
+
+    @Test
+    void when_duplicate_route_exists_then_virtual_host_builder_should_throw_error() {
+        assertThatThrownBy(
+                () -> duplicatedRouteVirtualHostBuilder()
+                        .rejectedRouteHandler(RejectedRouteHandler.FAIL)
+                        .and()
+                        .build()
+        ).isInstanceOf(DuplicateRouteException.class);
+    }
+
+    @Test
+    void fail_route_handler_do_not_consider_decorator_in_same_path() {
+        assertThatCode(() -> serverBuilderWithDecorator()
+                .rejectedRouteHandler(RejectedRouteHandler.FAIL)
+                .build()).doesNotThrowAnyException();
+
+        assertThatCode(() -> virtualServerBuilderWithDecorator()
+                .rejectedRouteHandler(RejectedRouteHandler.FAIL)
+                .and()
+                .build()).doesNotThrowAnyException();
+    }
+
+    private VirtualHostBuilder duplicatedRouteVirtualHostBuilder() {
+        return Server.builder()
+                     .virtualHost("foo.com")
+                     .service("/foo", (ctx, req) -> HttpResponse.of("ok"))
+                     .service("/foo", (ctx, req) -> HttpResponse.of("duplicate"));
+    }
+
+    private VirtualHostBuilder virtualServerBuilderWithDecorator() {
+        return Server.builder()
+                     .virtualHost("foo.com")
+                     .service("/foo", (ctx, req) -> HttpResponse.of("ok"))
+                     .decorator("/foo", (delegate, ctx, req) -> HttpResponse.of("duplicate"));
+    }
+
+    private ServerBuilder duplicateRouteServerBuilder() {
+        return Server.builder()
+                     .service("/foo", (ctx, req) -> HttpResponse.of("ok"))
+                     .service("/foo", (ctx, req) -> HttpResponse.of("duplicate"));
+    }
+
+    private ServerBuilder serverBuilderWithDecorator() {
+        return Server.builder()
+                     .service("/foo", (ctx, req) -> HttpResponse.of("ok"))
+                     .decorator("/foo", (delegate, ctx, req) -> HttpResponse.of("duplicate"));
+    }
+}