How to Fix `java.lang.IllegalStateException: getReader() has already been called for this request` in Java Servlets?

What causes the IllegalStateException when calling getReader() on a Servlet Request?

java.lang.IllegalStateException: getReader() has already been called for this request

The `java.lang.IllegalStateException: getReader() has already been called for this request` error occurs when you attempt to read the request body multiple times in a Java Servlet. This happens because the servlet container allows the request body to be read only once using the `getReader()` or `getInputStream()` methods. If you try to read it again, you'll encounter this exception. To fix this issue, you can wrap the `HttpServletRequest` and cache the input stream or reader for multiple accesses.

import javax.servlet.*;
import javax.servlet.http.*;
import java.io.*;

public class MyHttpServletRequestWrapper extends HttpServletRequestWrapper {
    private byte[] cachedBody;

    public MyHttpServletRequestWrapper(HttpServletRequest request) throws IOException {
        super(request);
        InputStream is = request.getInputStream();
        this.cachedBody = is.readAllBytes();
    }

    @Override
    public BufferedReader getReader() throws UnsupportedEncodingException {
        return new BufferedReader(new InputStreamReader(new ByteArrayInputStream(cachedBody), getCharacterEncoding()));
    }

    @Override
    public ServletInputStream getInputStream() throws IOException {
        return new CachedBodyServletInputStream(cachedBody);
    }
}

class CachedBodyServletInputStream extends ServletInputStream {
    private ByteArrayInputStream inputStream;

    public CachedBodyServletInputStream(byte[] body) {
        this.inputStream = new ByteArrayInputStream(body);
    }

    @Override
    public boolean isFinished() {
        return inputStream.available() == 0;
    }

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

    @Override
    public void setReadListener(ReadListener readListener) {
        // Not needed for this implementation
    }

    @Override
    public int read() throws IOException {
        return inputStream.read();
    }
}

Causes

• Calling `getReader()` more than once on the same request object.
• Using filters or servlets that internally read the request body without proper handling.
• Attempting to use both `getReader()` and `getInputStream()` on the same request.

Solutions

• Implement a custom `HttpServletRequestWrapper` that caches the request body.
• Read the request body once and store it in a variable to use for later access.
• Ensure that any filters or servlets do not consume the request body if they intend to pass it along.

Mistake: Not wrapping the request properly before reading the body.

Solution: Always use a custom wrapper to cache the request body.

Mistake: Calling both `getReader()` and `getInputStream()` in the same filter or servlet.

Solution: Decide on one method to read the request body and stick with it.

Mistake: Not handling exceptions that may occur while reading the body.

Solution: Add appropriate error handling in your wrapper implementation.