appendAllowed(true) breaks JSON array when the writer is opened/written/closed multiple times

[banseok1216]

Bug description
When org.springframework.batch.infrastructure.item.json.JsonFileItemWriter is configured with setAppendAllowed(true) and used in multiple runs (open → write → close) against the same target file, the resulting file is not a valid JSON array. The second run appends data after the closing bracket (]) instead of inserting it inside the array with a comma.

Environment
Spring Batch 6.0.2

Steps to reproduce
Even with appendAllowed(true), after multiple runs the final file should remain a valid JSON array, and in the reproduction test below ["foo", "bar"] should be read correctly.

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package org.springframework.batch.infrastructure.item.json;

import java.io.File;
import java.nio.file.Files;

import tools.jackson.databind.json.JsonMapper;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.extension.ExtendWith;
import org.mockito.Mock;
import org.mockito.Mockito;
import org.mockito.junit.jupiter.MockitoExtension;

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.assertArrayEquals;
import static org.junit.jupiter.api.Assertions.assertThrows;

class JsonFileItemWriterTests {

	private WritableResource resource;

	@BeforeEach
	void setUp() throws Exception {
		File file = Files.createTempFile("test", "json").toFile();
		this.resource = new FileSystemResource(file);
	}

	@Test
	void appendAllowedShouldNotCorruptJsonArray() throws Exception {
		JsonFileItemWriter<String> writer = new JsonFileItemWriter<>(this.resource, new JacksonJsonObjectMarshaller<>());
		writer.setAppendAllowed(true);

		writer.open(new ExecutionContext());
		writer.write(Chunk.of("foo"));
		writer.close();

		writer.open(new ExecutionContext());
		writer.write(Chunk.of("bar"));
		writer.close();

		String[] result = readJsonArrayOfStrings();
		assertArrayEquals(new String[] { "foo", "bar" }, result);
	}

	private String[] readJsonArrayOfStrings() throws Exception {
		File file = this.resource.getFile();
		String raw = Files.readString(file.toPath());
		System.out.println(raw);
		return new JsonMapper().readValue(raw, String[].class);
	}

}

current json structure

[
 "foo"
]
 "bar"
]

Expected behavior

expect json structure

[
 "foo",
 "bar"
]

[quaff]

It requires deleting ] before writing new items which violates append semantics, I doubt that JsonFileItemWriter should supports this feature since JSON is not appendable.

[banseok1216]

I agree. It's ambiguous.

It's confusing because JsonFileItemWriter inherits setAppendAllowed(true).

appendAllowed should be explicitly rejected as an unsupported feature, or the semantics should be changed.

[fmbenhassine]

I don't see this as a violation of appending logic. We should be able to append a json element in the file, even if the content is wrapped in an array. Same thinking about an XML file (Spring Batch should be able to append an element before the closing the tag).

This is a bug to me and should be fixed.

[quaff]

I created #5273 to fix.