[WIP] Configure the right FieldExtractor based on the type of items in FlatFileItemWriterBuilder

This is an attempt to get the generic type information
at runtime from the builder and use it to detect if
items type is a record or a regular class.

This seems impossible due to type erasure.

Issue spring-projects#4161

Author: fmbenhassine

spring-batch-infrastructure/src/main/java/org/springframework/batch/item/file/builder/FlatFileItemWriterBuilder.java

@@ -15,6 +15,9 @@
  */
 package org.springframework.batch.item.file.builder;
 
+import java.lang.reflect.ParameterizedType;
+import java.lang.reflect.Type;
+import java.lang.reflect.TypeVariable;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.List;
@@ -31,8 +34,16 @@
 import org.springframework.batch.item.file.transform.FieldExtractor;
 import org.springframework.batch.item.file.transform.FormatterLineAggregator;
 import org.springframework.batch.item.file.transform.LineAggregator;
+import org.springframework.batch.item.file.transform.RecordFieldExtractor;
+import org.springframework.cglib.core.TypeUtils;
+import org.springframework.core.GenericTypeResolver;
+import org.springframework.core.ParameterizedTypeReference;
+import org.springframework.core.ResolvableType;
 import org.springframework.core.io.WritableResource;
 import org.springframework.util.Assert;
+import org.springframework.util.ClassUtils;
+import org.springframework.util.ObjectUtils;
+import org.springframework.util.ReflectionUtils;
 
 /**
  * A builder implementation for the {@link FlatFileItemWriter}
@@ -46,7 +57,7 @@
  */
 public class FlatFileItemWriterBuilder<T> {
 
-	protected Log logger = LogFactory.getLog(getClass());
+	protected static Log logger = LogFactory.getLog(FlatFileItemWriterBuilder.class);
 
 	private WritableResource resource;
 
@@ -372,21 +383,41 @@ public FormatterLineAggregator<T> build() {
 			formatterLineAggregator.setMaximumLength(this.maximumLength);
 
 			if (this.fieldExtractor == null) {
-				BeanWrapperFieldExtractor<T> beanWrapperFieldExtractor = new BeanWrapperFieldExtractor<>();
-				beanWrapperFieldExtractor.setNames(this.names.toArray(new String[this.names.size()]));
+				FieldExtractor<T> fieldExtractor = null;
+				logger.info("No field extractor specified, trying to introspect item type from generic type information");
+				Class<T> itemType = null;
 				try {
-					beanWrapperFieldExtractor.afterPropertiesSet();
-				}
-				catch (Exception e) {
+					itemType = getItemType();
+					if (itemType != null && itemType.isRecord()) {
+						fieldExtractor = new RecordFieldExtractor<>(itemType);
+					}
+					else {
+						BeanWrapperFieldExtractor<T> beanWrapperFieldExtractor = new BeanWrapperFieldExtractor<>();
+						beanWrapperFieldExtractor.setNames(this.names.toArray(new String[this.names.size()]));
+						beanWrapperFieldExtractor.afterPropertiesSet();
+						fieldExtractor = beanWrapperFieldExtractor;
+					}
+				} catch (Exception e) {
 					throw new IllegalStateException("Unable to initialize FormatterLineAggregator", e);
 				}
-				this.fieldExtractor = beanWrapperFieldExtractor;
+				this.fieldExtractor = fieldExtractor;
 			}
 
 			formatterLineAggregator.setFieldExtractor(this.fieldExtractor);
 			return formatterLineAggregator;
 		}
 
+		private Class<T> getItemType() throws ClassNotFoundException {
+			TypeVariable<? extends Class<? extends FormattedBuilder>>[] typeParameters = this.getClass()
+					.getTypeParameters();
+			if (typeParameters != null && typeParameters.length > 0) {
+				TypeVariable<? extends Class<? extends FormattedBuilder>> typeParameter = typeParameters[0];
+				String typeName = typeParameter.getName();
+				return (Class<T>) ClassUtils.forName(typeName, this.getClass().getClassLoader());
+			}
+			return null;
+		}
+
 	}
 
 	/**
@@ -453,21 +484,41 @@ public DelimitedLineAggregator<T> build() {
 			}
 
 			if (this.fieldExtractor == null) {
-				BeanWrapperFieldExtractor<T> beanWrapperFieldExtractor = new BeanWrapperFieldExtractor<>();
-				beanWrapperFieldExtractor.setNames(this.names.toArray(new String[this.names.size()]));
+				FieldExtractor<T> fieldExtractor = null;
+				logger.info("No field extractor specified, trying to introspect item type from generic type information");
+				Class<T> itemType = null;
 				try {
-					beanWrapperFieldExtractor.afterPropertiesSet();
-				}
-				catch (Exception e) {
+					itemType = getItemType();
+					if (itemType != null && itemType.isRecord()) {
+						fieldExtractor = new RecordFieldExtractor<>(itemType);
+					}
+					else {
+						BeanWrapperFieldExtractor<T> beanWrapperFieldExtractor = new BeanWrapperFieldExtractor<>();
+						beanWrapperFieldExtractor.setNames(this.names.toArray(new String[this.names.size()]));
+						beanWrapperFieldExtractor.afterPropertiesSet();
+						fieldExtractor = beanWrapperFieldExtractor;
+					}
+				} catch (Exception e) {
 					throw new IllegalStateException("Unable to initialize DelimitedLineAggregator", e);
 				}
-				this.fieldExtractor = beanWrapperFieldExtractor;
+				this.fieldExtractor = fieldExtractor;
 			}
 
 			delimitedLineAggregator.setFieldExtractor(this.fieldExtractor);
 			return delimitedLineAggregator;
 		}
 
+		private Class<T> getItemType() throws ClassNotFoundException {
+			TypeVariable<? extends Class<? extends DelimitedBuilder>>[] typeParameters = this.getClass()
+					.getTypeParameters();
+			if (typeParameters != null && typeParameters.length > 0) {
+				TypeVariable<? extends Class<? extends DelimitedBuilder>> typeParameter = typeParameters[0];
+				String typeName = typeParameter.getName();
+				return (Class<T>) ClassUtils.forName(typeName, this.getClass().getClassLoader());
+			}
+			return null;
+		}
+
 	}
 
 	/**

spring-batch-infrastructure/src/test/java/org/springframework/batch/item/file/builder/FlatFileItemWriterBuilderTests.java

@@ -22,12 +22,15 @@
 import java.nio.charset.Charset;
 import java.util.Arrays;
 
+import org.junit.Assert;
 import org.junit.Test;
 
 import org.springframework.batch.item.ExecutionContext;
-
 import org.springframework.batch.item.file.FlatFileItemWriter;
+import org.springframework.batch.item.file.transform.BeanWrapperFieldExtractor;
+import org.springframework.batch.item.file.transform.DelimitedLineAggregator;
 import org.springframework.batch.item.file.transform.PassThroughLineAggregator;
+import org.springframework.batch.item.file.transform.RecordFieldExtractor;
 import org.springframework.core.io.FileSystemResource;
 import org.springframework.core.io.Resource;
 import org.springframework.core.io.WritableResource;
@@ -236,6 +239,69 @@ public void testFlagsWithEncoding() throws Exception {
 		validateBuilderFlags(writer, encoding);
 	}
 
+	@Test
+	public void testSetupWithRecordItemType() throws IOException {
+		// given
+		WritableResource output = new FileSystemResource(File.createTempFile("foo", "txt"));
+		record Person(int id, String name) {
+		}
+
+		// when
+		FlatFileItemWriter<Person> writer = new FlatFileItemWriterBuilder<Person>().name("personWriter")
+				.resource(output).delimited().names("id", "name").build();
+
+		// then
+		Object lineAggregator = ReflectionTestUtils.getField(writer, "lineAggregator");
+		Assert.assertNotNull(lineAggregator);
+		Assert.assertTrue(lineAggregator instanceof DelimitedLineAggregator);
+		Object fieldExtractor = ReflectionTestUtils.getField(lineAggregator, "fieldExtractor");
+		Assert.assertNotNull(fieldExtractor);
+		Assert.assertTrue(fieldExtractor instanceof RecordFieldExtractor);
+	}
+
+	@Test
+	public void testSetupWithClassItemType() throws IOException {
+		// given
+		WritableResource output = new FileSystemResource(File.createTempFile("foo", "txt"));
+		class Person {
+
+			int id;
+
+			String name;
+
+		}
+
+		// when
+		FlatFileItemWriter<Person> writer = new FlatFileItemWriterBuilder<Person>().name("personWriter")
+				.resource(output).delimited().names("id", "name").build();
+
+		// then
+		Object lineAggregator = ReflectionTestUtils.getField(writer, "lineAggregator");
+		Assert.assertNotNull(lineAggregator);
+		Assert.assertTrue(lineAggregator instanceof DelimitedLineAggregator);
+		Object fieldExtractor = ReflectionTestUtils.getField(lineAggregator, "fieldExtractor");
+		Assert.assertNotNull(fieldExtractor);
+		Assert.assertTrue(fieldExtractor instanceof BeanWrapperFieldExtractor);
+	}
+
+	@Test
+	public void testSetupWithNoGeneticType() throws IOException {
+		// given
+		WritableResource output = new FileSystemResource(File.createTempFile("foo", "txt"));
+
+		// when
+		FlatFileItemWriter writer = new FlatFileItemWriterBuilder().name("personWriter")
+				.resource(output).delimited().names("id", "name").build();
+
+		// then
+		Object lineAggregator = ReflectionTestUtils.getField(writer, "lineAggregator");
+		Assert.assertNotNull(lineAggregator);
+		Assert.assertTrue(lineAggregator instanceof DelimitedLineAggregator);
+		Object fieldExtractor = ReflectionTestUtils.getField(lineAggregator, "fieldExtractor");
+		Assert.assertNotNull(fieldExtractor);
+		Assert.assertTrue(fieldExtractor instanceof BeanWrapperFieldExtractor);
+	}
+
 	private void validateBuilderFlags(FlatFileItemWriter<Foo> writer, String encoding) {
 		assertFalse((Boolean) ReflectionTestUtils.getField(writer, "saveState"));
 		assertTrue((Boolean) ReflectionTestUtils.getField(writer, "append"));