Gzip Servlet Filter in Spring MVC

在这篇博客中，我将解释如何在Spring Web应用程序中使用GZip servlet过滤器来加速页面加载。GZip servlet过滤器用于压缩JSP数据（html，JS，CSS）并发送到浏览器，然后浏览器将自动解压缩它。 这加快了页面加载比较没有压缩数据发送到浏览器。

How it works
在呈现jsp页面之前，过滤器将通过名为ACCEPT_ENCODING的标头获取浏览器属性。 因此，如果浏览器支持GZip编码格式，filter会将servletResponse封装到我们的自定义封装器中，并将封装后的响应发送给浏览器，然后浏览器将对其进行解压缩。 如果浏览器不支持Gzip编码格式，那么我们的过滤器将发送未包装的响应。
每当请求发送到服务器时，GZip servlet过滤器会封装HttpServletResponse并覆盖ServletResponse的getWritter（）和getOutputStream（）方法，以便在响应中写入压缩的字符串。

Here is the Code

GZIPFilter.java

public class GZIPFilter implements Filter {

    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {
        HttpServletRequest httpRequest = (HttpServletRequest) request;
        HttpServletResponse httpResponse = (HttpServletResponse) response;
        String acceptEncoding = httpRequest.getHeader(HttpHeaders.ACCEPT_ENCODING);
        if (acceptEncoding != null) {
            System.out.println("Processing GZIPFilter");
            //searching for 'gzip' in ACCEPT_ENCODING header
            if (acceptEncoding.indexOf("gzip") >= 0) {
                GZIPResponseWrapper gzipResponse = new GZIPResponseWrapper(httpResponse);
               chain.doFilter(request, gzipResponse);
               gzipResponse.finish();
               return;
            }
        }
        chain.doFilter(request, response);
    }
}

现在这里是ServletResponseWrapper的代码，它覆盖ServletResponse的方法来编写压缩的响应。

GZIPResponseWrapper.java

public class GZIPResponseWrapper extends HttpServletResponseWrapper {

    private GZIPResponseStream gzipStream;
    private ServletOutputStream outputStream;
    private PrintWriter printWriter;

    public GZIPResponseWrapper(HttpServletResponse response) {
        super(response);
        response.addHeader(HttpHeaders.CONTENT_ENCODING, "gzip");
    }

    public void finish() throws IOException {
        if (printWriter != null) {
            printWriter.close();
        }
        if (outputStream != null) {
            outputStream.close();
        }
        if (gzipStream != null) {
            gzipStream.close();
        }
    }

    @Override
    public void flushBuffer() throws IOException {
        if (printWriter != null) {
                printWriter.flush();
        }
        if (outputStream != null) {
            outputStream.flush();
        }
        super.flushBuffer();
    }

    @Override
    public ServletOutputStream getOutputStream() throws IOException {
        if (printWriter != null) {
            throw new IllegalStateException("printWriter already defined");
        }
        if (outputStream == null) {
            initGzip();
            outputStream = gzipStream;
        }
        return outputStream;
    }

    @Override
    public PrintWriter getWriter() throws IOException {
        if (outputStream != null) {
            throw new IllegalStateException("printWriter already defined");
        }
        if (printWriter == null) {
            initGzip();
            printWriter = new PrintWriter(new OutputStreamWriter(gzipStream, getResponse().getCharacterEncoding()));
        }
        System.out.println("GZIPFilter is Working fine!!!!!");
        return printWriter;
    }

    /* Creates a new output stream for writing compressed data in
    * the GZIP file format. */
    private void initGzip() throws IOException {
        gzipStream = new GZIPResponseStream(getResponse().getOutputStream());
    }
}

GZIPResponseStream.java

public class GZIPResponseStream extends ServletOutputStream {
    GZIPOutputStream gzipStream;
    final AtomicBoolean open = new AtomicBoolean(true);
    OutputStream output;

    public GZIPResponseStream(OutputStream output) throws IOException {
        this.output = output;
        gzipStream = new GZIPOutputStream(output);
    }

    @Override
    public void close() throws IOException {
        if (open.compareAndSet(true, false)) {
            gzipStream.close();
        }
    }

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

    @Override
    public void write(byte b[]) throws IOException {
        write(b, 0, b.length);
    }

    @Override
    public void write(byte b[], int off, int len) throws IOException {
        if (!open.get()) {
             throw new IOException("Stream closed!");
        }
        gzipStream.write(b, off, len);
    }

    @Override
    public void write(int b) throws IOException {
        if (!open.get()) {
            throw new IOException("Stream closed!");
        }
        gzipStream.write(b);
    }   
}