Use ClassUtils.forName() in FastReaderBuilder for consistency

FastReaderBuilder.findClass() was using data.getClassLoader().loadClass()
directly, bypassing ClassUtils.forName() which is used everywhere else in
the codebase. This change aligns FastReaderBuilder with the standard class
loading path and adds tests for class loading behavior.

Author: iemejia

lang/java/avro/src/main/java/org/apache/avro/io/FastReaderBuilder.java

@@ -52,6 +52,7 @@
 import org.apache.avro.reflect.ReflectionUtil;
 import org.apache.avro.specific.SpecificData;
 import org.apache.avro.specific.SpecificRecordBase;
+import org.apache.avro.util.ClassUtils;
 import org.apache.avro.util.Utf8;
 import org.apache.avro.util.WeakIdentityHashMap;
 import org.apache.avro.util.internal.Accessor;
@@ -446,8 +447,8 @@ private FieldReader getTransformingStringReader(String valueClass, FieldReader s
 
   private Optional<Class<?>> findClass(String clazz) {
     try {
-      return Optional.of(data.getClassLoader().loadClass(clazz));
-    } catch (ReflectiveOperationException e) {
+      return Optional.of(ClassUtils.forName(data.getClassLoader(), clazz));
+    } catch (ClassNotFoundException | SecurityException e) {
       return Optional.empty();
     }
   }

lang/java/avro/src/test/java/org/apache/avro/io/TestFastReaderBuilderClassLoading.java

@@ -0,0 +1,121 @@
+/*
+ * 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
+ *
+ *     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 org.apache.avro.io;
+
+import static org.junit.jupiter.api.Assertions.*;
+
+import java.io.ByteArrayInputStream;
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+import java.net.URI;
+import java.util.Collections;
+
+import org.apache.avro.Schema;
+import org.apache.avro.generic.GenericData;
+import org.apache.avro.generic.GenericDatumReader;
+import org.apache.avro.generic.GenericDatumWriter;
+import org.apache.avro.generic.GenericRecord;
+import org.apache.avro.generic.GenericRecordBuilder;
+import org.apache.avro.specific.SpecificData;
+import org.apache.avro.util.ClassSecurityValidator;
+import org.apache.avro.util.ClassSecurityValidator.ClassSecurityPredicate;
+import org.junit.jupiter.api.AfterEach;
+import org.junit.jupiter.api.BeforeEach;
+import org.junit.jupiter.api.Test;
+
+/**
+ * Tests that FastReaderBuilder.findClass() routes class loading through
+ * ClassUtils.forName(), so that ClassSecurityValidator is applied consistently.
+ */
+public class TestFastReaderBuilderClassLoading {
+
+  private static final String TEST_VALUE = "https://example.com";
+
+  private ClassSecurityPredicate originalValidator;
+
+  @BeforeEach
+  public void saveValidator() {
+    originalValidator = ClassSecurityValidator.getGlobal();
+  }
+
+  @AfterEach
+  public void restoreValidator() {
+    ClassSecurityValidator.setGlobal(originalValidator);
+  }
+
+  /**
+   * When the validator blocks a class referenced by java-class, FastReaderBuilder
+   * must NOT instantiate it. The value should be returned as a plain string.
+   */
+  @Test
+  void blockedClassIsNotInstantiated() {
+    // Block java.net.URI
+    ClassSecurityValidator.setGlobal(ClassSecurityValidator.composite(ClassSecurityValidator.DEFAULT_TRUSTED_CLASSES,
+        ClassSecurityValidator.builder().add("org.apache.avro.util.Utf8").build()));
+
+    GenericRecord result = readWithJavaClass("java.net.URI");
+
+    assertNotNull(result.get("value"));
+    assertFalse(result.get("value") instanceof URI, "Blocked class should not be instantiated");
+  }
+
+  /**
+   * When the validator trusts a class referenced by java-class, FastReaderBuilder
+   * should instantiate it normally.
+   */
+  @Test
+  void trustedClassIsInstantiated() {
+    ClassSecurityValidator.setGlobal(ClassSecurityValidator.composite(ClassSecurityValidator.DEFAULT_TRUSTED_CLASSES,
+        ClassSecurityValidator.builder().add("java.net.URI").add("org.apache.avro.util.Utf8").build()));
+
+    GenericRecord result = readWithJavaClass("java.net.URI");
+
+    assertInstanceOf(URI.class, result.get("value"));
+    assertEquals(URI.create(TEST_VALUE), result.get("value"));
+  }
+
+  /**
+   * Encode a string, then read it back through FastReaderBuilder with the given
+   * java-class.
+   */
+  private GenericRecord readWithJavaClass(String javaClass) {
+    try {
+      Schema stringSchema = Schema.create(Schema.Type.STRING);
+      stringSchema.addProp(SpecificData.CLASS_PROP, javaClass);
+      stringSchema.addProp(GenericData.STRING_PROP, GenericData.StringType.String.name());
+
+      Schema recordSchema = Schema.createRecord("TestRecord", null, "test", false);
+      recordSchema.setFields(Collections.singletonList(new Schema.Field("value", stringSchema, null, null)));
+
+      // Encode
+      GenericRecord record = new GenericRecordBuilder(recordSchema).set("value", TEST_VALUE).build();
+      ByteArrayOutputStream out = new ByteArrayOutputStream();
+      Encoder encoder = EncoderFactory.get().binaryEncoder(out, null);
+      new GenericDatumWriter<GenericRecord>(recordSchema).write(record, encoder);
+      encoder.flush();
+
+      // Decode with fast reader enabled
+      GenericData data = new GenericData();
+      data.setFastReaderEnabled(true);
+      GenericDatumReader<GenericRecord> reader = new GenericDatumReader<>(recordSchema, recordSchema, data);
+      return reader.read(null, DecoderFactory.get().binaryDecoder(new ByteArrayInputStream(out.toByteArray()), null));
+    } catch (IOException e) {
+      return fail("Unexpected IOException during encode/decode", e);
+    }
+  }
+}

lang/java/avro/src/test/java/org/apache/avro/util/TestClassSecurityValidator.java

@@ -97,6 +97,29 @@ public void forbiddenClass(String className) {
     assertEquals("Not inner", e.getMessage());
   }
 
+  @Test
+  void testClassUtilsEnforcesValidator() {
+    ClassSecurityValidator.setGlobal(ClassSecurityValidator.builder().add("java.lang.String").build());
+
+    assertThrows(SecurityException.class, () -> ClassUtils.forName("java.net.URI"),
+        "ClassUtils.forName should reject classes not in the trusted set");
+
+    assertDoesNotThrow(() -> ClassUtils.forName("java.lang.String"),
+        "ClassUtils.forName should allow classes in the trusted set");
+  }
+
+  @Test
+  void testDirectLoadClassDoesNotUseValidator() throws ClassNotFoundException {
+    ClassSecurityValidator.setGlobal(ClassSecurityValidator.builder().add("java.lang.String").build());
+
+    ClassLoader cl = Thread.currentThread().getContextClassLoader();
+    Class<?> loaded = cl.loadClass("java.net.URI");
+    assertNotNull(loaded, "Direct ClassLoader.loadClass() loads any class regardless of the validator");
+
+    assertThrows(SecurityException.class, () -> ClassUtils.forName("java.net.URI"),
+        "ClassUtils.forName correctly applies the validator");
+  }
+
   @Test
   void testBuildComplexPredicate() {
     ClassSecurityValidator.setGlobal(ClassSecurityValidator.composite(