SOLR-16959: Make CoresContainer class configurable

solr/core/src/java/org/apache/solr/core/CoreContainer.java

@@ -377,21 +377,24 @@ public CoreContainer(Path solrHome, Properties properties) {
    * @see #load()
    */
   public CoreContainer(NodeConfig config) {
-    this(config, new CorePropertiesLocator(config.getCoreRootDirectory()));
+    this(config, CoresLocator.instantiate(config));
   }
 
   public CoreContainer(NodeConfig config, boolean asyncSolrCoreLoad) {
-    this(config, new CorePropertiesLocator(config.getCoreRootDirectory()), asyncSolrCoreLoad);
+    this(config, CoresLocator.instantiate(config), asyncSolrCoreLoad);
   }
 
   /**
-   * Create a new CoreContainer using the given configuration and locator. The container's cores are
-   * not loaded.
+   * Create a new CoreContainer using the given configuration and locator.
+   *
+   * <p>The container's cores are not loaded. This constructor should be used only in tests, as it
+   * overrides {@link CoresLocator}'s instantiation process.
    *
    * @param config a ConfigSolr representation of this container's configuration
    * @param locator a CoresLocator
    * @see #load()
    */
+  @VisibleForTesting
   public CoreContainer(NodeConfig config, CoresLocator locator) {
     this(config, locator, false);
   }

solr/core/src/java/org/apache/solr/core/CoresLocator.java

@@ -16,6 +16,7 @@
  */
 package org.apache.solr.core;
 
+import java.lang.reflect.Constructor;
 import java.util.List;
 
 /** Manage the discovery and persistence of core definitions across Solr restarts */
@@ -72,4 +73,26 @@ public interface CoresLocator {
    * @return a list of all CoreDescriptors found
    */
   public List<CoreDescriptor> discover(CoreContainer cc);
+
+  /**
+   * Returns a new instance of {@link CoresLocator}, depending on provided config.
+   *
+   * @param nodeConfig Solr configuration.
+   */
+  static CoresLocator instantiate(NodeConfig nodeConfig) {
+    final String coresLocatorClass = nodeConfig.getCoresLocatorClass();
+    if (coresLocatorClass != null) {
+      try {
+        Class<? extends CoresLocator> clazz =
+            nodeConfig.getSolrResourceLoader().findClass(coresLocatorClass, CoresLocator.class);
+        Constructor<? extends CoresLocator> constructor = clazz.getConstructor(NodeConfig.class);
+        return constructor.newInstance(nodeConfig);
+      } catch (Exception e) {
+        throw new RuntimeException(
+            "create CoresLocator instance failed, coresLocatorClass=" + coresLocatorClass, e);
+      }
+    } else {
+      return new CorePropertiesLocator(nodeConfig.getCoreRootDirectory());
+    }
+  }
 }

solr/core/src/java/org/apache/solr/core/NodeConfig.java

@@ -79,6 +79,7 @@ public class NodeConfig {
   private final UpdateShardHandlerConfig updateShardHandlerConfig;
 
   private final String configSetServiceClass;
+  private final String coresLocatorClass;
 
   private final String coreAdminHandlerClass;
 
@@ -154,6 +155,7 @@ private NodeConfig(
       Set<Path> allowPaths,
       List<String> allowUrls,
       String configSetServiceClass,
+      String coresLocatorClass,
       String modules,
       Set<String> hiddenSysProps) {
     // all Path params here are absolute and normalized.
@@ -190,6 +192,7 @@ private NodeConfig(
     this.allowPaths = allowPaths;
     this.allowUrls = allowUrls;
     this.configSetServiceClass = configSetServiceClass;
+    this.coresLocatorClass = coresLocatorClass;
     this.modules = modules;
     this.hiddenSysProps = hiddenSysProps;
     this.hiddenSysPropPattern =
@@ -258,6 +261,10 @@ public String getConfigSetServiceClass() {
     return this.configSetServiceClass;
   }
 
+  public String getCoresLocatorClass() {
+    return this.coresLocatorClass;
+  }
+
   public String getNodeName() {
     return nodeName;
   }
@@ -593,6 +600,7 @@ public static class NodeConfigBuilder {
     private PluginInfo shardHandlerFactoryConfig;
     private UpdateShardHandlerConfig updateShardHandlerConfig = UpdateShardHandlerConfig.DEFAULT;
     private String configSetServiceClass;
+    private String coresLocatorClass;
     private String coreAdminHandlerClass = DEFAULT_ADMINHANDLERCLASS;
     private Map<String, String> coreAdminHandlerActions = Collections.emptyMap();
     private String collectionsAdminHandlerClass = DEFAULT_COLLECTIONSHANDLERCLASS;
@@ -814,6 +822,11 @@ public NodeConfigBuilder setConfigSetServiceClass(String configSetServiceClass)
       return this;
     }
 
+    public NodeConfigBuilder setCoresLocatorClass(String coresLocatorClass) {
+      this.coresLocatorClass = coresLocatorClass;
+      return this;
+    }
+
     /**
      * Set list of modules to add to class path
      *
@@ -903,6 +916,7 @@ public NodeConfig build() {
           allowPaths,
           allowUrls,
           configSetServiceClass,
+          coresLocatorClass,
           modules,
           resolveHiddenSysPropsFromSysPropOrEnvOrDefault(hiddenSysProps));
     }

solr/core/src/java/org/apache/solr/core/SolrXmlConfig.java

@@ -348,6 +348,9 @@ private static NodeConfig fillSolrSection(NodeConfig.NodeConfigBuilder builder,
               case "configSetService":
                 builder.setConfigSetServiceClass(it.txt());
                 break;
+              case "coresLocator":
+                builder.setCoresLocatorClass(it.txt());
+                break;
               case "coreRootDirectory":
                 builder.setCoreRootDirectory(it.txt());
                 break;

solr/core/src/test/org/apache/solr/core/CoresLocatorTest.java

@@ -0,0 +1,54 @@
+/*
+ * 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.core;
+
+import static org.junit.Assert.assertSame;
+import static org.junit.Assert.assertTrue;
+
+import org.apache.solr.util.NoCoresLocator;
+import org.junit.Rule;
+import org.junit.Test;
+import org.junit.rules.TemporaryFolder;
+
+/** Unit tests for {@link CoresLocator}. */
+public class CoresLocatorTest {
+
+  @Rule public TemporaryFolder temporaryFolder = new TemporaryFolder();
+
+  @Test
+  public void testInstantiateDefaultCoresLocator() throws Exception {
+    String solrXml = "<?xml version=\"1.0\" encoding=\"UTF-8\" ?><solr></solr>";
+    NodeConfig nodeConfig = SolrXmlConfig.fromString(temporaryFolder.newFolder().toPath(), solrXml);
+    CoresLocator coresLocator = CoresLocator.instantiate(nodeConfig);
+
+    assertTrue(coresLocator instanceof CorePropertiesLocator);
+  }
+
+  @Test
+  public void testInstantiateCustomCoresLocator() throws Exception {
+    String solrXml =
+        "<?xml version=\"1.0\" encoding=\"UTF-8\" ?>\n"
+            + "<solr>\n"
+            + "<str name=\"coresLocator\">org.apache.solr.util.NoCoresLocator</str>\n"
+            + "</solr>";
+    NodeConfig nodeConfig = SolrXmlConfig.fromString(temporaryFolder.newFolder().toPath(), solrXml);
+    CoresLocator coresLocator = CoresLocator.instantiate(nodeConfig);
+
+    assertTrue(coresLocator instanceof NoCoresLocator);
+    assertSame(nodeConfig, ((NoCoresLocator) coresLocator).getNodeConfig());
+  }
+}

solr/solr-ref-guide/modules/configuration-guide/pages/configuring-solr-xml.adoc

@@ -99,6 +99,18 @@ For example, `<str name="configSetService">com.myorg.CustomConfigSetService</str
 +
 If this attribute isn't set, Solr uses the default `configSetService`, with zookeeper aware of `org.apache.solr.cloud.ZkConfigSetService`, without zookeeper aware of `org.apache.solr.core.FileSystemConfigSetService`.
 
+`coresLocator`::
++
+[%autowidth,frame=none]
+|===
+|Optional |Default: `org.apache.solr.core.CoresPropertiesLocator`
+|===
++
+This attribute does not need to be set.
++
+If used, this attribute should be set to the FQN (Fully qualified name) of a class that inherits from `CoresLocator`, and you must provide a constructor with one parameter which the type is `org.apache.solr.core.NodeConfig`.
+For example, `<str name="coresLocator">com.myorg.CustomCoresLocator</str>`.
+
 `adminHandler`::
 +
 [%autowidth,frame=none]

solr/test-framework/src/java/org/apache/solr/util/NoCoresLocator.java

@@ -0,0 +1,40 @@
+/*
+ * 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.util;
+
+import java.util.List;
+import org.apache.solr.core.CoreContainer;
+import org.apache.solr.core.CoreDescriptor;
+import org.apache.solr.core.NodeConfig;
+
+/** A {@link org.apache.solr.core.CoresLocator} that locates no cores. */
+public class NoCoresLocator extends ReadOnlyCoresLocator {
+  private final NodeConfig nodeConfig;
+
+  public NoCoresLocator(NodeConfig nodeConfig) {
+    this.nodeConfig = nodeConfig;
+  }
+
+  public NodeConfig getNodeConfig() {
+    return nodeConfig;
+  }
+
+  @Override
+  public List<CoreDescriptor> discover(CoreContainer cc) {
+    return List.of();
+  }
+}