How to stream JSON result with Jackson in Vert.x (java)

I assume you are generating huge JSON documents as for the small ones string output is good enough: objectMapper.writeValue(<String>, result);

There's a problem with streams. ObjectMapper doesn't know the result size and you will end up with the exception:

java.lang.IllegalStateException: You must set the Content-Length header to be the total size of the message body BEFORE sending any data if you are not using HTTP chunked encoding.        at org.vertx.java.core.http.impl.DefaultHttpServerResponse.write(DefaultHttpServerResponse.java:474)

So in your example I would use temporary files for JSON output and then flush them into response (I haven't tested the code)

File tmpFile = File.createTempFile("tmp", ".json");mapper.writeValue(tmpFile, result);req.response().sendFile(tmpFile.getAbsolutePath(), (result) -> tmpFile.delete());

In case you know content length initially you can use the following code to map OutputStream with WriteStream

import org.vertx.java.core.buffer.Buffer;import org.vertx.java.core.streams.WriteStream;import java.io.IOException;import java.io.OutputStream;public class OutputWriterStream extends OutputStream {    public WriteStream writeStream;    public Runnable closeHandler;    @Override    public void write(int b) throws IOException {        throw new UnsupportedOperationException();    }    @Override    public void write(byte[] b, int off, int len) throws IOException {        if (off == 0 && len == b.length) {            writeStream.write(new Buffer(b));            return;        }        byte[] bytes = new byte[len];        System.arraycopy(b, off, bytes, 0, len);        writeStream.write(new Buffer(bytes));    }    @Override    public void write(byte[] b) throws IOException {        writeStream.write(new Buffer(b));    }    @Override    public void close() throws IOException {        closeHandler.run();    }}

This might be a bit better (and updated for Vertx3) answer:

import io.vertx.core.file.AsyncFile;import io.vertx.core.buffer.Buffer;import io.vertx.core.http.HttpServerResponse;import io.vertx.core.streams.WriteStream;import java.io.IOException;import java.io.OutputStream;public class OutputWriterStream extends OutputStream {    public OutputWriterStream(final WriteStream response) {        this.response = response;        this.buffer = new byte[8192];    }    @Override    public synchronized void write(final int b) throws IOException {        buffer[counter++] = (byte) b;        if (counter >= buffer.length) {            flush();        }    }    @Override    public void flush() throws IOException {        super.flush();        if (counter > 0) {            byte[] remaining = buffer;            if (counter < buffer.length) {                remaining = new byte[counter];                System.arraycopy(buffer, 0, remaining, 0, counter);            }            response.write(Buffer.buffer(remaining));            counter = 0;        }    }    @Override    public void close() throws IOException {        flush();        super.close();        if (response instanceof HttpServerResponse) {            try {                response.end();            }            catch (final IllegalStateException ignore) {            }        }        else if (response instanceof AsyncFile) {            ((AsyncFile) response).close();        }    }    private final WriteStream<Buffer> response;    private final byte[] buffer;    private int counter = 0;}