Fix JsonFileItemWriter to produce valid JSON when append allowed

Closes GH-5272

Signed-off-by: Yanming Zhou <zhouyanming@gmail.com>

Author: quaff

spring-batch-infrastructure/src/main/java/org/springframework/batch/infrastructure/item/json/JsonFileItemWriter.java

@@ -16,9 +16,14 @@
 
 package org.springframework.batch.infrastructure.item.json;
 
+import java.io.File;
+import java.io.IOException;
+import java.io.RandomAccessFile;
 import java.util.Iterator;
 
 import org.springframework.batch.infrastructure.item.Chunk;
+import org.springframework.batch.infrastructure.item.ExecutionContext;
+import org.springframework.batch.infrastructure.item.ItemStreamException;
 import org.springframework.batch.infrastructure.item.support.AbstractFileItemWriter;
 import org.springframework.core.io.WritableResource;
 import org.springframework.util.Assert;
@@ -46,6 +51,7 @@
  * @param <T> type of object to write as json representation
  * @author Mahmoud Ben Hassine
  * @author Jimmy Praet
+ * @author Yanming Zhou
  * @since 4.1
  */
 public class JsonFileItemWriter<T> extends AbstractFileItemWriter<T> {
@@ -58,6 +64,8 @@ public class JsonFileItemWriter<T> extends AbstractFileItemWriter<T> {
 
 	private JsonObjectMarshaller<T> jsonObjectMarshaller;
 
+	private boolean hasExistingItems;
+
 	/**
 	 * Create a new {@link JsonFileItemWriter} instance.
 	 * @param resource to write json data to
@@ -91,10 +99,27 @@ public void setJsonObjectMarshaller(JsonObjectMarshaller<T> jsonObjectMarshaller
 		this.jsonObjectMarshaller = jsonObjectMarshaller;
 	}
 
+	@Override
+	public void open(ExecutionContext executionContext) throws ItemStreamException {
+		super.open(executionContext);
+		if (this.append && this.resource != null && this.resource.exists()) {
+			try {
+				this.hasExistingItems = reopen(this.resource.getFile());
+			}
+			catch (IOException ex) {
+				throw new ItemStreamException(ex.getMessage(), ex);
+			}
+		}
+	}
+
 	@SuppressWarnings("DataFlowIssue")
 	@Override
 	public String doWrite(Chunk<? extends T> items) {
 		StringBuilder lines = new StringBuilder();
+		if (this.hasExistingItems) {
+			lines.append(JSON_OBJECT_SEPARATOR).append(this.lineSeparator);
+			this.hasExistingItems = false;
+		}
 		Iterator<? extends T> iterator = items.iterator();
 		if (!items.isEmpty() && state.getLinesWritten() > 0) {
 			lines.append(JSON_OBJECT_SEPARATOR).append(this.lineSeparator);
@@ -109,4 +134,19 @@ public String doWrite(Chunk<? extends T> items) {
 		return lines.toString();
 	}
 
+	private boolean reopen(File file) throws IOException {
+		long length = file.length();
+		// try to delete JSON_OBJECT_SEPARATOR + lineSeparator + JSON_ARRAY_STOP
+		long pos = length - (2 + this.lineSeparator.length());
+		if (pos <= 0) {
+			return false;
+		}
+		try (RandomAccessFile raf = new RandomAccessFile(file, "rw")) {
+			raf.setLength(pos);
+			// file content is not empty or empty JSON array
+			// (JSON_ARRAY_START + 2 * lineSeparator + JSON_ARRAY_STOP + lineSeparator)
+			return length > 2 + 3L * this.lineSeparator.length();
+		}
+	}
+
 }

spring-batch-infrastructure/src/test/java/org/springframework/batch/infrastructure/item/json/JsonFileItemWriterTests.java

@@ -25,18 +25,18 @@
 import org.mockito.Mock;
 import org.mockito.Mockito;
 import org.mockito.junit.jupiter.MockitoExtension;
+import tools.jackson.databind.json.JsonMapper;
 
 import org.springframework.batch.infrastructure.item.Chunk;
 import org.springframework.batch.infrastructure.item.ExecutionContext;
-import org.springframework.batch.infrastructure.item.json.JsonFileItemWriter;
-import org.springframework.batch.infrastructure.item.json.JsonObjectMarshaller;
 import org.springframework.core.io.FileSystemResource;
 import org.springframework.core.io.WritableResource;
 
-import static org.junit.jupiter.api.Assertions.assertThrows;
+import static org.junit.jupiter.api.Assertions.*;
 
 /**
  * @author Mahmoud Ben Hassine
+ * @author Yanming Zhou
  */
 @ExtendWith(MockitoExtension.class)
 class JsonFileItemWriterTests {
@@ -72,4 +72,38 @@ void itemsShouldBeMarshalledToJsonWithTheJsonObjectMarshaller() throws Exception
 		Mockito.verify(this.jsonObjectMarshaller).marshal("bar");
 	}
 
+	@Test
+	void appendAllowed() throws Exception {
+		JsonFileItemWriter<String> writer = new JsonFileItemWriter<>(this.resource,
+				new JacksonJsonObjectMarshaller<>());
+		writer.setAppendAllowed(true);
+
+		writer.open(new ExecutionContext());
+		writer.close();
+
+		resourceShouldContains();
+
+		writer.open(new ExecutionContext());
+		writer.write(Chunk.of("aaa"));
+		writer.write(Chunk.of("bbb"));
+		writer.close();
+
+		resourceShouldContains("aaa", "bbb");
+
+		writer.open(new ExecutionContext());
+		writer.close();
+
+		resourceShouldContains("aaa", "bbb");
+
+		writer.open(new ExecutionContext());
+		writer.write(Chunk.of("ccc"));
+		writer.close();
+
+		resourceShouldContains("aaa", "bbb", "ccc");
+	}
+
+	private void resourceShouldContains(String... array) throws Exception {
+		assertArrayEquals(array, new JsonMapper().readValue(this.resource.getContentAsByteArray(), String[].class));
+	}
+
 }