fix(config): reject a solr.url SolrJ cannot connect to at bind time

src/main/java/org/apache/solr/mcp/server/config/SolrConfigurationProperties.java

@@ -16,8 +16,10 @@
  */
 package org.apache.solr.mcp.server.config;
 
+import java.net.URI;
 import org.jspecify.annotations.Nullable;
 import org.springframework.boot.context.properties.ConfigurationProperties;
+import org.springframework.util.StringUtils;
 
 /**
  * Spring Boot Configuration Properties record for Apache Solr connection
@@ -101,12 +103,17 @@
  * that requires Solr connection information.
  *
  * <p>
- * <strong>Validation Considerations:</strong>
+ * <strong>Validation:</strong>
  *
  * <p>
- * While basic validation is handled by the configuration system, additional URL
- * validation and normalization occurs in the {@link SolrConfig} class during
- * SolrClient bean creation.
+ * {@code url} must be an absolute {@code http} or {@code https} URL with a
+ * host. The compact constructor enforces this at bind time, so a misconfigured
+ * deployment fails at startup with an actionable message instead of at first
+ * request with an opaque SolrJ error. {@code localhost:8983} (scheme omitted)
+ * is the easy mistake: {@code java.net.URI} parses it as scheme
+ * {@code localhost} with no host, and path normalization in {@link SolrConfig}
+ * would otherwise concatenate it without noticing. Path normalization (adding
+ * {@code /solr/}) still happens in {@link SolrConfig}.
  *
  * <p>
  * <strong>Optional Basic Authentication:</strong>
@@ -120,7 +127,8 @@
  * header is attached and the client behaves as before.
  *
  * @param url
- *            the base URL of the Apache Solr server (required, non-null)
+ *            the base URL of the Apache Solr server; an absolute http(s) URL
+ *            with a host
  * @param username
  *            the HTTP Basic Authentication username (optional; required
  *            together with {@code password} to enable auth)
@@ -133,4 +141,33 @@
  */
 @ConfigurationProperties(prefix = "solr")
 public record SolrConfigurationProperties(String url, @Nullable String username, @Nullable String password) {
+
+	private static final String HTTP = "http";
+
+	private static final String HTTPS = "https";
+
+	public SolrConfigurationProperties {
+		if (!isAbsoluteHttpUrlWithHost(url)) {
+			throw new IllegalArgumentException("solr.url must be an absolute http or https URL including a host, "
+					+ "for example http://localhost:8983/solr/ (was: '" + url + "')");
+		}
+	}
+
+	/**
+	 * The binder hands over whatever was configured, including nothing at all, so
+	 * this is the one place a null or blank value can arrive.
+	 */
+	private static boolean isAbsoluteHttpUrlWithHost(@Nullable String url) {
+		if (!StringUtils.hasText(url)) {
+			return false;
+		}
+		URI uri;
+		try {
+			uri = URI.create(url);
+		} catch (IllegalArgumentException ex) {
+			return false;
+		}
+		String scheme = uri.getScheme();
+		return (HTTP.equals(scheme) || HTTPS.equals(scheme)) && StringUtils.hasText(uri.getHost());
+	}
 }

src/test/java/org/apache/solr/mcp/server/config/SolrConfigurationPropertiesTest.java

@@ -0,0 +1,93 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF 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
+ *
+ *     http://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 org.apache.solr.mcp.server.config;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatThrownBy;
+
+import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.condition.DisabledInNativeImage;
+import org.junit.jupiter.params.ParameterizedTest;
+import org.junit.jupiter.params.provider.ValueSource;
+import org.springframework.boot.context.properties.EnableConfigurationProperties;
+import org.springframework.boot.test.context.runner.ApplicationContextRunner;
+
+/**
+ * {@code solr.url} is validated where the binder hands it over, in the record's
+ * compact constructor, so a misconfigured deployment dies at startup with an
+ * actionable message instead of at first request with an opaque SolrJ error.
+ *
+ * <p>
+ * {@code URI.create("localhost:8983")} happily parses as scheme
+ * {@code localhost} with no host, and {@link SolrConfig} would normalize it by
+ * string concatenation without noticing; that is the case that motivates the
+ * check.
+ */
+class SolrConfigurationPropertiesTest {
+
+	@ParameterizedTest
+	@ValueSource(
+			strings = {"http://localhost:8983", "http://localhost:8983/", "http://localhost:8983/solr",
+					"http://localhost:8983/solr/", "https://solr.internal:8983/custom/solr/",
+					"https://solr.example.com", "http://solr:8983/solr/"})
+	void acceptsAbsoluteHttpUrlsWithAHost(String url) {
+		assertThat(new SolrConfigurationProperties(url, null, null).url()).isEqualTo(url);
+	}
+
+	@ParameterizedTest
+	@ValueSource(
+			strings = {"localhost:8983", "solr.example.com", "/solr", "ftp://solr.example.com/solr", "file:///var/solr",
+					"not a url", "http://", "", "   "})
+	void rejectsUrlsSolrJCannotConnectTo(String url) {
+		assertThatThrownBy(() -> new SolrConfigurationProperties(url, null, null))
+				.isInstanceOf(IllegalArgumentException.class)
+				.hasMessageContaining("solr.url must be an absolute http or https URL including a host")
+				.hasMessageContaining("http://localhost:8983/solr/");
+	}
+
+	/** The binder passes null when the property is absent altogether. */
+	@Test
+	void rejectsAMissingUrl() {
+		assertThatThrownBy(() -> new SolrConfigurationProperties(null, null, null))
+				.isInstanceOf(IllegalArgumentException.class)
+				.hasMessageContaining("solr.url must be an absolute http or https URL including a host");
+	}
+
+	@Test
+	@DisabledInNativeImage
+	void bindingFailsAtStartupWhenTheSchemeIsMissing() {
+		contextRunner().withPropertyValues("solr.url=localhost:8983").run(context -> {
+			assertThat(context).hasFailed();
+			assertThat(context).getFailure().rootCause().hasMessageContaining("solr.url must be an absolute");
+		});
+	}
+
+	@Test
+	@DisabledInNativeImage
+	void bindingSucceedsForAnAbsoluteUrl() {
+		contextRunner().withPropertyValues("solr.url=http://localhost:8983/solr/")
+				.run(context -> assertThat(context).hasNotFailed());
+	}
+
+	private static ApplicationContextRunner contextRunner() {
+		return new ApplicationContextRunner().withUserConfiguration(PropertiesOnly.class);
+	}
+
+	@EnableConfigurationProperties(SolrConfigurationProperties.class)
+	static class PropertiesOnly {
+	}
+}