CapturingResponseWrapper.java

import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.OutputStreamWriter;
import java.io.PrintWriter;

import jakarta.servlet.ServletOutputStream;
import jakarta.servlet.WriteListener;
import jakarta.servlet.http.HttpServletResponse;
import jakarta.servlet.http.HttpServletResponseWrapper;

class CapturingResponseWrapper extends HttpServletResponseWrapper {

    private final ByteArrayOutputStream capture;
    private ServletOutputStream output;
    private PrintWriter writer;

    public CapturingResponseWrapper(HttpServletResponse response) {
        super(response);
        capture = new ByteArrayOutputStream(response.getBufferSize());
    }

    @Override
    public ServletOutputStream getOutputStream() {
        if (writer != null) {
            throw new IllegalStateException("getWriter() has already been called on this response.");
        }

        if (output == null) {
            output = new ServletOutputStream() {
                @Override public boolean isReady() {
                    return true;
                }

                @Override public void setWriteListener(WriteListener writeListener) {

                }

                @Override
                public void write(int b) throws IOException {
                    capture.write(b);
                }

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

                @Override
                public void close() throws IOException {
                    capture.close();
                }
            };
        }

        return output;
    }

    @Override
    public PrintWriter getWriter() throws IOException {
        if (output != null) {
            throw new IllegalStateException("getOutputStream() has already been called on this response.");
        }

        if (writer == null) {
            writer = new PrintWriter(new OutputStreamWriter(capture, getCharacterEncoding()));
        }

        return writer;
    }

    @Override
    public void flushBuffer() throws IOException {
        super.flushBuffer();

        if (writer != null) {
            writer.flush();
        } else if (output != null) {
            output.flush();
        }
    }

    public byte[] getCaptureAsBytes() throws IOException {
        if (writer != null) {
            writer.close();
        } else if (output != null) {
            output.close();
        }

        return capture.toByteArray();
    }

    public String getCaptureAsString() throws IOException {
        return new String(getCaptureAsBytes(), getCharacterEncoding());
    }

}

LiveReloadScriptFilter.java

package com.maciejwalkowiak.just.spring.boot.livereload.jakarta;

import java.io.IOException;

import jakarta.servlet.Filter;
import jakarta.servlet.FilterChain;
import jakarta.servlet.ServletException;
import jakarta.servlet.ServletRequest;
import jakarta.servlet.ServletResponse;
import jakarta.servlet.http.HttpServletResponse;

import org.springframework.http.MediaType;

class LiveReloadScriptFilter implements Filter {

    private final String scriptSnippet;

    LiveReloadScriptFilter(int liveReloadPort, int justServerPort) {
        this.scriptSnippet = String.format(
                "<script src=\"http://localhost:%d/livereload/livereload.js?port=%d&host=localhost\"></script>", justServerPort,
                liveReloadPort);
    }

    @Override
    public void doFilter(ServletRequest request, ServletResponse response, FilterChain filterChain)
            throws IOException, ServletException {
        CapturingResponseWrapper capturingResponseWrapper = new CapturingResponseWrapper((HttpServletResponse) response);
        filterChain.doFilter(request, capturingResponseWrapper);

        String contentType = capturingResponseWrapper.getContentType();
        String content = capturingResponseWrapper.getCaptureAsString();
        if ((contentType != null) && MediaType.TEXT_HTML.isCompatibleWith(MediaType.parseMediaType(contentType))) {
            String alreadyFilteredAttributeName = this.getAlreadyFilteredAttributeName();
            boolean hasAlreadyFilteredAttribute = request.getAttribute(alreadyFilteredAttributeName) != null;

            if (!hasAlreadyFilteredAttribute) {
                request.setAttribute(alreadyFilteredAttributeName, Boolean.TRUE);
                response.getWriter().write(content + this.scriptSnippet);
            } else {
                response.getWriter().write(content);
            }
        } else {
            response.getWriter().write(content);
        }
    }

    protected String getAlreadyFilteredAttributeName() {
        String name = this.getClass().getName();
        return name + ".FILTERED";
    }

}