Fix potential NPE in getJdkHome() with null guards

src/main/java/org/apache/maven/plugins/toolchain/jdk/SelectJdkToolchainMojo.java

@@ -262,8 +262,17 @@ private boolean matches(String key, String reqVal, String tcVal) {
     }
 
     private String getJdkHome(ToolchainPrivate toolchain) {
-        return ((Xpp3Dom) toolchain.getModel().getConfiguration())
-                .getChild("jdkHome")
-                .getValue();
+        ToolchainModel model = toolchain.getModel();
+        if (model == null) {
+            return null;
+        }
+        Object configuration = model.getConfiguration();
+        if (configuration instanceof Xpp3Dom) {
+            Xpp3Dom child = ((Xpp3Dom) configuration).getChild("jdkHome");
+            if (child != null) {
+                return child.getValue();
+            }
+        }
+        return null;
     }
 }

src/test/java/org/apache/maven/plugins/toolchain/jdk/SelectJdkToolchainMojoTest.java

@@ -0,0 +1,107 @@
+/*
+ * 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.maven.plugins.toolchain.jdk;
+
+import java.lang.reflect.Method;
+import java.util.Map;
+
+import org.apache.maven.toolchain.ToolchainPrivate;
+import org.apache.maven.toolchain.model.ToolchainModel;
+import org.codehaus.plexus.util.xml.Xpp3Dom;
+import org.junit.jupiter.api.Test;
+
+import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNull;
+
+class SelectJdkToolchainMojoTest {
+
+    @Test
+    void testGetJdkHomeWithNonXpp3DomConfiguration() throws Exception {
+        ToolchainModel model = new ToolchainModel();
+        model.setConfiguration("string-config");
+        ToolchainPrivate toolchain = createStub(model);
+        assertNull(invokeGetJdkHome(toolchain));
+    }
+
+    @Test
+    void testGetJdkHomeWithNullModel() throws Exception {
+        ToolchainPrivate toolchain = createStub(null);
+        assertNull(invokeGetJdkHome(toolchain));
+    }
+
+    @Test
+    void testGetJdkHomeWithNullConfiguration() throws Exception {
+        ToolchainModel model = new ToolchainModel();
+        model.setConfiguration(null);
+        ToolchainPrivate toolchain = createStub(model);
+        assertNull(invokeGetJdkHome(toolchain));
+    }
+
+    @Test
+    void testGetJdkHomeWithMissingJdkHomeChild() throws Exception {
+        Xpp3Dom config = new Xpp3Dom("configuration");
+        ToolchainModel model = new ToolchainModel();
+        model.setConfiguration(config);
+        ToolchainPrivate toolchain = createStub(model);
+        assertNull(invokeGetJdkHome(toolchain));
+    }
+
+    @Test
+    void testGetJdkHomeWithValidJdkHome() throws Exception {
+        Xpp3Dom config = new Xpp3Dom("configuration");
+        Xpp3Dom jdkHome = new Xpp3Dom("jdkHome");
+        jdkHome.setValue("/path/to/jdk");
+        config.addChild(jdkHome);
+        ToolchainModel model = new ToolchainModel();
+        model.setConfiguration(config);
+        ToolchainPrivate toolchain = createStub(model);
+        assertEquals("/path/to/jdk", invokeGetJdkHome(toolchain));
+    }
+
+    private String invokeGetJdkHome(ToolchainPrivate toolchain) throws Exception {
+        SelectJdkToolchainMojo mojo = new SelectJdkToolchainMojo();
+        Method method = SelectJdkToolchainMojo.class.getDeclaredMethod("getJdkHome", ToolchainPrivate.class);
+        method.setAccessible(true);
+        return (String) method.invoke(mojo, toolchain);
+    }
+
+    private ToolchainPrivate createStub(ToolchainModel model) {
+        return new ToolchainPrivate() {
+            @Override
+            public ToolchainModel getModel() {
+                return model;
+            }
+
+            @Override
+            public boolean matchesRequirements(Map<String, String> requirements) {
+                return false;
+            }
+
+            @Override
+            public String getType() {
+                return "jdk";
+            }
+
+            @Override
+            public String findTool(String toolName) {
+                return null;
+            }
+        };
+    }
+}