AVRO-4225: Fix ClassCastException in FastReaderBuilder for java-class attribute

When using GenericDatumReader with schemas containing java-class
attributes on string fields, FastReaderBuilder.getTransformingStringReader()
was casting stringReader.read() directly to String, but GenericData
returns Utf8, causing ClassCastException.

Fix by explicitly handling both Utf8 and String types, consistent with
the rest of the Avro codebase.

Author: santosh-d3vpl3x

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

@@ -437,7 +437,11 @@ private FieldReader getTransformingStringReader(String valueClass, FieldReader s
       Function<String, ?> transformer = findClass(valueClass)
           .map(clazz -> ReflectionUtil.getConstructorAsFunction(String.class, clazz)).orElse(null);
       if (transformer != null) {
-        return (old, decoder) -> transformer.apply((String) stringReader.read(null, decoder));
+        return (old, decoder) -> {
+          Object value = stringReader.read(null, decoder);
+          String stringValue = value instanceof Utf8 ? ((Utf8) value).toString() : (String) value;
+          return transformer.apply(stringValue);
+        };
       }
     }
 

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

@@ -0,0 +1,116 @@
+/*
+ * 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.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertNotNull;
+
+import java.io.ByteArrayOutputStream;
+import java.io.IOException;
+
+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.junit.jupiter.api.Test;
+
+/**
+ * Tests for FastReaderBuilder behavior with schemas containing "java-class"
+ * attributes.
+ */
+public class FastReaderBuilderJavaClassTest {
+
+  /**
+   * Tests that GenericDatumReader can deserialize records with string fields that
+   * have a "java-class" attribute (e.g., BigDecimal).
+   *
+   * This test reproduces a bug where
+   * FastReaderBuilder.getTransformingStringReader() casts the result of
+   * stringReader.read() directly to String, but in GenericData mode the reader
+   * returns Utf8, causing a ClassCastException.
+   */
+  @Test
+  void genericDatumReaderWithJavaClassAttribute() throws IOException {
+    // Schema with a string field that has "java-class": "java.math.BigDecimal"
+    // This is a common pattern for representing decimal values as strings
+    String schemaJson = "{\n" + "  \"type\": \"record\",\n" + "  \"name\": \"TestRecord\",\n" + "  \"fields\": [\n"
+        + "    {\"name\": \"id\", \"type\": \"string\"},\n" + "    {\"name\": \"price\", \"type\": [\"null\", {\n"
+        + "      \"type\": \"string\",\n" + "      \"java-class\": \"java.math.BigDecimal\"\n" + "    }]}\n" + "  ]\n"
+        + "}";
+
+    Schema schema = new Schema.Parser().parse(schemaJson);
+
+    GenericRecord record = new GenericData.Record(schema);
+    record.put("id", "123");
+    record.put("price", "-0.0002");
+
+    ByteArrayOutputStream out = new ByteArrayOutputStream();
+    GenericDatumWriter<GenericRecord> writer = new GenericDatumWriter<>(schema);
+    BinaryEncoder encoder = EncoderFactory.get().binaryEncoder(out, null);
+    writer.write(record, encoder);
+    encoder.flush();
+
+    byte[] serialized = out.toByteArray();
+
+    // Deserialize using GenericDatumReader (which uses FastReaderBuilder by
+    // default)
+    GenericDatumReader<GenericRecord> reader = new GenericDatumReader<>(schema);
+    BinaryDecoder decoder = DecoderFactory.get().binaryDecoder(serialized, null);
+
+    // AVRO-4225 this should not throw ClassCastException: Utf8 cannot be cast
+    // to String
+    GenericRecord result = reader.read(null, decoder);
+
+    assertNotNull(result);
+    assertEquals("123", result.get("id").toString());
+    assertEquals("-0.0002", result.get("price").toString());
+  }
+
+  /**
+   * Tests that GenericDatumReader can deserialize records with a direct string
+   * field (not in a union) that has a "java-class" attribute.
+   */
+  @Test
+  void genericDatumReaderWithDirectJavaClassString() throws IOException {
+    String schemaJson = "{\n" + "  \"type\": \"record\",\n" + "  \"name\": \"TestRecord\",\n" + "  \"fields\": [\n"
+        + "    {\"name\": \"amount\", \"type\": {\n" + "      \"type\": \"string\",\n"
+        + "      \"java-class\": \"java.math.BigDecimal\"\n" + "    }}\n" + "  ]\n" + "}";
+
+    Schema schema = new Schema.Parser().parse(schemaJson);
+
+    GenericRecord record = new GenericData.Record(schema);
+    record.put("amount", "123.45");
+
+    ByteArrayOutputStream out = new ByteArrayOutputStream();
+    GenericDatumWriter<GenericRecord> writer = new GenericDatumWriter<>(schema);
+    BinaryEncoder encoder = EncoderFactory.get().binaryEncoder(out, null);
+    writer.write(record, encoder);
+    encoder.flush();
+
+    byte[] serialized = out.toByteArray();
+
+    GenericDatumReader<GenericRecord> reader = new GenericDatumReader<>(schema);
+    BinaryDecoder decoder = DecoderFactory.get().binaryDecoder(serialized, null);
+
+    GenericRecord result = reader.read(null, decoder);
+
+    assertNotNull(result);
+    assertEquals("123.45", result.get("amount").toString());
+  }
+}