how modify the response body with java filter?

Question

I want to perform some filter logic to HTTP responses (which are in json format).

I had successfully change the response body, but when the (string) size of the body changes: I am getting missing the last characters.

To make it simpler, I had created a simple Spring Boot application, with only Web dependency for my rest controller.

My Rest Controller

@RestController
@RequestMapping("/home/")
public class RestControllerHome {

@GetMapping (produces=MediaType.APPLICATION_JSON_VALUE)
public String home() {
        return "{ \"name\" : \"Peter\" }";
  }
}

My Filter

@Component
public class MyFilter implements Filter {

@Override
public void destroy() { }

@Override
public void doFilter(ServletRequest request, ServletResponse response, FilterChain filterChain) throws IOException, ServletException {

    HtmlResponseWrapper capturingResponseWrapper = new HtmlResponseWrapper((HttpServletResponse) response);
    filterChain.doFilter(request, capturingResponseWrapper);        
    if (response.getContentType() != null && response.getContentType().contains("application/json")) {
        String content = capturingResponseWrapper.getCaptureAsString();

        // This code works fine
        //response.getWriter().write(content.toUpperCase());

        // This code doesn't works because the content size is changed
        response.getWriter().write("{ \"name\" : \"************r\" }");

    }
}

@Override
public void init(FilterConfig arg0) throws ServletException {  }    
}

HttpServletResponseWrapper // capture the response before it is written

public class HtmlResponseWrapper extends HttpServletResponseWrapper {

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

public HtmlResponseWrapper(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) {
        // inner class - lets the wrapper manipulate the response 
        output = new ServletOutputStream() {
            @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();
            }

            @Override
            public boolean isReady() {
                return false;
            }

            @Override
            public void setWriteListener(WriteListener arg0) {
            }
        };
    }

    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());
}

}

In my doFilter method, the following code ...

// This code works fine
response.getWriter().write(content.toUpperCase());

// This code doesn't works because the content size is changed
//response.getWriter().write("{ \"name\" : \"************r\" }");

... gives my the following output : {"NAME": "PETER"} Which tell me, that the code is working properly.

But, in reality I want to change the body content ...

// This code works fine
//response.getWriter().write(content.toUpperCase());

// This code doesn't works because the content size is changed
response.getWriter().write("{ \"name\" : \"************r\" }");

... and the previous code, is giving me an incomplete text body as output: **{ "name" : "**********

What am I doing wrong? My app have a bigger json body, and a little more complex logic in the filter. But, if I dont get this working I am not being able to make the rest of my code work. Please, help.

I took the Filter and HttpServletResponseWrapper from https://www.leveluplunch.com/java/tutorials/034-modify-html-response-using-filter/

Solution

Thanks to the help of JBNizet, I found that the solution was to add the Content Lenght:

String newContent = "{ \"name\" : \"************r\" }";
response.setContentLength(newContent .length());
response.getWriter().write(newContent);

Comments

[pkgajulapalli]

I'm getting getWriter() has already been called for this response error with this solution. Was able to work with medium.com/@sportans300/…?