In unit tests, why does mocked s3Object.getObjectContent() return empty after first call?

I have made the following JUnit5/Mockito Test code an easy self-contained copy & paste;

import com.amazonaws.services.s3.model.S3Object;
import com.amazonaws.services.s3.model.S3ObjectInputStream;
import com.fasterxml.jackson.databind.ObjectMapper;
import lombok.Builder;
import lombok.Data;
import org.apache.http.client.methods.HttpRequestBase;
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.mockito.junit.jupiter.MockitoSettings;
import org.mockito.quality.Strictness;
import java.io.BufferedReader;
import java.io.ByteArrayInputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.nio.charset.StandardCharsets;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.stream.Collectors;
@ExtendWith(MockitoExtension.class)
@MockitoSettings(strictness = Strictness.WARN)
public class S3GetObjectContentTest {
private static final ObjectMapper OBJECT_MAPPER = new ObjectMapper();
@Mock private HttpRequestBase httpRequestBase;
@Mock private S3Object s3Object;
private final LinkedHashSet<Pet> s3FileContents = new LinkedHashSet<>();
@BeforeEach
void setup() throws IOException {
s3FileContents.clear();
s3FileContents.addAll(
List.of(
Pet.builder().name("Jerry").type("mouse").build(),
Pet.builder().name("Tom").type("cat").build(),
Pet.builder().name("Roger").type("bunny").build()));
Mockito.when(s3Object.getObjectContent())
.thenReturn(
new S3ObjectInputStream(
new ByteArrayInputStream(OBJECT_MAPPER.writeValueAsBytes(s3FileContents)),
httpRequestBase));
}
@Test
public void doTest() {
for(int n=0; n < 3; n++){
try (final InputStreamReader streamReader =
new InputStreamReader(s3Object.getObjectContent(), StandardCharsets.UTF_8);
final BufferedReader reader = new BufferedReader(streamReader)) {
String text = reader.lines().collect(Collectors.joining());
System.out.println("reader-content: "+ (text.isEmpty() ? "\"\"" : text));
} catch (IOException e) {
throw new RuntimeException(e);
}
}
}
@Data
@Builder
public static class Pet {
public final String name;
public final String type;
}
}

When the above Test code is run, the following output is expected;

reader-content: [{"name":"Jerry","type":"mouse"},{"name":"Tom","type":"cat"},{"name":"Roger","type":"bunny"}]
reader-content: [{"name":"Jerry","type":"mouse"},{"name":"Tom","type":"cat"},{"name":"Roger","type":"bunny"}]
reader-content: [{"name":"Jerry","type":"mouse"},{"name":"Tom","type":"cat"},{"name":"Roger","type":"bunny"}]

However the actual output seen is;

reader-content: [{"name":"Jerry","type":"mouse"},{"name":"Tom","type":"cat"},{"name":"Roger","type":"bunny"}]
reader-content: ""
reader-content: ""

<h1>Question:</h1>

Why is the Mock for s3Object.getObjectContent() kicking-in only on the first call and barfing with empty afterwards?

<h3>NOTE:</h3>
Experimentally (never ideal), I observed that I only get the expected result when I put the mock inside the for...loop... Suppose this might just give some extra context

  @Test
public void doTest() throws JsonProcessingException {
for(int n=0; n < 3; n++){
// ========================== Experimental (Not practical) ==========================
Mockito.when(s3Object.getObjectContent())
.thenReturn(
new S3ObjectInputStream(
new ByteArrayInputStream(OBJECT_MAPPER.writeValueAsBytes(s3FileContents)),
httpRequestBase));
// ==================================================================================
try (final InputStreamReader streamReader =
new InputStreamReader(s3Object.getObjectContent(), StandardCharsets.UTF_8);
final BufferedReader reader = new BufferedReader(streamReader)) {
String text = reader.lines().collect(Collectors.joining());
System.out.println("reader-content: "+ (text.isEmpty() ? "\"\"" : text));
} catch (IOException e) {
throw new RuntimeException(e);
}
}
}

I suspect that after first run, the stream is closed so you don’t get any more data out of it. Your beforeEach method only runs once, not three times. You could solve this by moving the “when” statement from setup method into the for loop.

Well, turns out the solution wasn't really far-from-home 😊.

Simply had to mock s3Object.getObjectContent() based on invocation as follows;

Mockito.when(s3Object.getObjectContent())
.thenAnswer(invocation -> {
return new S3ObjectInputStream(
new ByteArrayInputStream(OBJECT_MAPPER.writeValueAsBytes(s3FileContents)),
httpRequestBase);
});

Shout out to Tofig Hasanov for the quick insight that helped me fix this.