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DWR is easy Ajax for Java. It makes it simple to call Java code directly from Javascript. It gets rid of almost all the boilerplate code between the web browser and your Java code. This version 4.0.2 works with Jakarta Servlet 4.0.2.

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package org.directwebremoting.util;

import java.io.IOException;
import java.io.Writer;

import jakarta.servlet.ServletOutputStream;
import jakarta.servlet.WriteListener;

/**
 * This is not the evil hack you are looking for.
 * @author Joe Walker [joe at getahead dot ltd dot uk]
 */
public final class WriterOutputStream extends ServletOutputStream {

    /**
     * Constructor that allows us to specify how strings are created
     * @param writer The stream that we proxy to
     * @param encoding The string encoding of data that we write to the stream
     */
    public WriterOutputStream(Writer writer, String encoding) {
        this.writer = writer;
        this.encoding = encoding;
    }

    @Override public void print(String s) throws IOException {writer.write(s);}

    /** @return true */
    @Override public boolean isReady() {return true;}

    /** Does nothing. */
    @Override public void setWriteListener(WriteListener writeListener) {}

    @Override public void write(byte[] ba) throws IOException {
        if (encoding == null) {
            writer.write(new String(ba));
        } else {
            writer.write(new String(ba, encoding));
        }
    }

    @Override public void write(byte[] ba, int off, int len) throws IOException {
        if (encoding == null) {
            writer.write(new String(ba, off, len));
        } else {
            writer.write(new String(ba, off, len, encoding));
        }
    }

    @Override public synchronized void write(int bite) throws IOException {
        buffer[0] = (byte) bite;
        write(buffer);
    }

    @Override public void close() throws IOException {
        if (writer != null) {
            writer.close();
            writer = null;
            encoding = null;
        }
    }

    @Override public void flush() throws IOException {writer.flush();}

    /**
     * The destination of all our printing
     */
    private Writer writer;

    /**
     * What string encoding should we use
     */
    private String encoding = null;

    /**
     * Buffer for write(int)
     */
    private final byte[] buffer = new byte[1];
}