Spring MVC - HTTP message converter

Quite often you need to provide users with the same data, but in different forms, like JSON, PDF, XLS, etc. If your application is Spring Framework based, this task can be achieved using HTTP message converters.

HTTP message converters are applied when HTTP request (or its parts) needs to be converted into type required for handler method argument (see: Handler methods - method arguments), or when value returned by handler method needs to be converted somehow to create HTTP response (see: Handler methods - Return values).

Spring Framework provides you with a set of predefined HTTP message converters, for ex. for byte arrays, JSON, etc. - This set can be modified or extended to your needs.
 
In this post we will focus on converting value returned from handler method into desired form, using example provided by me (see below for the link to the source code repository).

 Suppose that we have a controller returning some Team data, like this (yes, I know, I've ignored team Id)

@RestController
public class TeamDetailsController {

    @GetMapping("/teams/{teamId}/")
    public Team read() {
        final Set<TeamMember> members = new LinkedHashSet<>();
        members.add(new TeamMember("Albert Einstein", LocalDate.of(1879, 3, 14)));
        members.add(new TeamMember("Benjamin Franklin", LocalDate.of(1706, 1, 17)));
        members.add(new TeamMember("Isaac Newton", LocalDate.of(1643, 1, 4)));
        return new Team(members);
    }

}

In our example, handler method response will be, by default, converted into JSON:

{
  "members": [
    {
      "dateOfBirth": "1879-03-14",
      "name": "Albert Einstein"    },
    {
      "dateOfBirth": "1706-01-17",
      "name": "Benjamin Franklin"    },
    {
      "dateOfBirth": "1643-01-04",
      "name": "Isaac Newton"    }
  ]
}

If we would like to convert the data returned by the handler into XLS file, we can simply define a bean being HTTP message converter implementation, which will be activated by the HTTP Accept header:

@Service
public class TeamToXlsConverter extends AbstractHttpMessageConverter<Team> {

    private static final MediaType EXCEL_TYPE = MediaType.valueOf("application/vnd.ms-excel");

    TeamToXlsConverter() {
        super(EXCEL_TYPE);
    }

    @Override
    protected Team readInternal(final Class<? extends Team> clazz, final HttpInputMessage inputMessage) throws IOException, HttpMessageNotReadableException {
        return null;
    }

    @Override
    protected boolean supports(final Class<?> clazz) {
        return (Team.class == clazz);
    }

    @Override
    protected void writeInternal(final Team team, final HttpOutputMessage outputMessage) throws IOException, HttpMessageNotWritableException {
        try (final Workbook workbook = new HSSFWorkbook()) {
            final Sheet sheet = workbook.createSheet();
            int rowNo = 0;
            for (final TeamMember member : team.getMembers()) {
                final Row row = sheet.createRow(rowNo++);
                row.createCell(0)
                   .setCellValue(member.getName());
            }
            workbook.write(outputMessage.getBody());
        }
    }

}

You have to keep in mind that in our example, defined HTTP message converter will be applied always when the handler method returns value of type Team (see supports method), and HTTP Accept header matches "application/vnd.ms-excel". In this case, XLS file generated by the HTTP message converter is returned instead of JSON representation of Team.

Few links for the dessert:

•  GitHub Repository

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Follow-ups:

This article has been republished on DZone's Java Zone (10/19/2019) and on Java Code Geeks (10/21/2019).

JDBC - Emulating a sequence

Probably each of us encountered this problem at least once in the programmer's life - how to emulate a database sequence? Below you may find my variation of this problem's solution.

Suppose that we have an interface defining the desired API for returning a sequence of integer numbers:

public interface Sequences {

    int nextValue(String sequenceName) throws SQLException;

}

and the implementation of this API in the following form:

class SequencesService implements Sequences {

    private static final String SQL_QUERY =
        "SELECT SEQ_NAME, SEQ_VALUE FROM SEQUENCE WHERE SEQ_NAME = ? FOR UPDATE";

    private final DataSource dataSource;

    SequencesService(final DataSource dataSource) {
        this.dataSource = dataSource;
    }

    @Override
    public int nextValue(final String sequenceName) throws SQLException {
        final long threadId = Thread.currentThread().getId();

        try (final Connection connection = dataSource.getConnection()) {
            connection.setAutoCommit(false);
            try (final PreparedStatement statement =
                     connection.prepareStatement(
                         SQL_QUERY, TYPE_SCROLL_SENSITIVE, CONCUR_UPDATABLE)) {
                statement.setString(1, sequenceName);
                try (final ResultSet resultSet = statement.executeQuery()) {
                    System.out.println(
                        String.format("[%d] - select for update", threadId));
                    int nextValue = 1;
                    if (resultSet.next()) {
                        nextValue = 1 + resultSet.getInt(2);
                        resultSet.updateInt(2, nextValue);
                        resultSet.updateRow();
                    } else {
                        resultSet.moveToInsertRow();
                        resultSet.updateString(1, sequenceName);
                        resultSet.updateInt(2, nextValue);
                        resultSet.insertRow();
                    }
                    System.out.println(
                        String.format("[%d] - next val: %d", threadId, nextValue));
                    return nextValue;
                }
            } finally {
                System.out.println(String.format("[%d] - commit", threadId));
                connection.commit();
            }
        }
    }

}

You have to forgive me two things :) - the println usage, which I added for generating some visual feedback ;) and a lack of detailed explanation how this solution works ;) I'll just mention that the clue is the way prepared statement is created, and the result set handling: updateRow / moveToInsertRow / insertRow usage ;) (see the links at the bottom of this post for the details).

I wrote simple test case to observe and verify this code, something like:

@Autowired
private Sequences sequences;

private Callable<Integer> callable() {
    return () -> {
        System.out.println(String.format("[%d] - starting", Thread.currentThread().getId()));
        return sequences.nextValue("My Sequence");
    };
}

@Test
public void test() throws Exception {
    final ExecutorService executor = Executors.newFixedThreadPool(3);
    final CompletionService<Integer> completion = new ExecutorCompletionService<>(executor);

    for (int i = 0; i < 3; i++) {
        completion.submit(callable());
    }
    
    for (int completed = 1; completed <= 3; completed++) {
        final Future<Integer> result = completion.take();
        System.out.println(String.format("Result %d - %d", completed, result.get()));
        assertEquals(Integer.valueOf(completed), result.get());
    }
}

When run, the above code, the output will be something like this (threads' IDs in the brackets):
[16] - starting
[18] - starting
[17] - starting
[17] - select for update
[17] - next val: 1
[17] - commit
[18] - select for update
Result 1 - 1
[18] - next val: 2
[18] - commit
[16] - select for update
[16] - next val: 3
[16] - commit
Result 2 - 2
Result 3 - 3

This code is just for demonstration purposes :) - if you want to do something similar in your project, it's probable that you will rather use for ex. Spring Framework's @Transactional annotation, instead of manual transactions handling, or even JPA delegating this work to JDBC. For example in Hibernate you may do it somehow like this:

import org.hibernate.Session;
...

entityManager.unwrap(Session.class)
                      .doReturningWork(connection -> { ... code derived from my example ... });

Few links for the dessert:

• Updating Rows in ResultSet Objects (JDBC) 
• Inserting Rows in ResultSet Objects (JDBC)
• Declarative transaction management and Using @Transactional (Spring Framework)
• ReturningWork (JPA, Hibernate)  

... and I almost forgot ;) - GitHub repository holding all my code expriments for this post

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Follow-ups:

 

This article has been republished on DZone's Database Zone (08/21/2019).