Java 讀取 HTTP 請求 Body 完整代碼示例

HTTP 請求由 Header 和 Body 組成。Servlet 中通過 HttpServletRequest.getReader() (字符流) 或 getInputStream() (字節流) 讀取 Body。本文提供完整工具方法，含編碼處理、大數據流式讀取、Spring MVC 替代方案與常見坑點。

原生 Servlet 讀取 Body

方法 1：BufferedReader 逐行讀取 (適用文本、JSON、表單)

public static String readBodyAsString(HttpServletRequest request) throws IOException {
    StringBuilder sb = new StringBuilder();
    try (BufferedReader reader = request.getReader()) { // 自動按 request 編碼解碼
        char[] buffer = new char[1024];
        int len;
        while ((len = reader.read(buffer)) != -1) {
            sb.append(buffer, 0, len);
        }
    }
    return sb.toString();
}

注意：

• getReader() 和 getInputStream() **互斥**，只能調用其中之一，否則拋 IllegalStateException。
• 默認編碼 ISO-8859-1，需確保客戶端發送 Content-Type: application/json; charset=UTF-8 或在 Filter 中設置 request.setCharacterEncoding("UTF-8")。

方法 2：InputStream 讀取字節 (適用二進制、文件、自定義編碼)

public static byte[] readBodyAsBytes(HttpServletRequest request) throws IOException {
    try (InputStream is = request.getInputStream();
         ByteArrayOutputStream baos = new ByteArrayOutputStream()) {
        byte[] buffer = new byte[4096];
        int len;
        while ((len = is.read(buffer)) != -1) {
            baos.write(buffer, 0, len);
        }
        return baos.toByteArray();
    }
}

// 轉字符串 (指定編碼)
String body = new String(readBodyAsBytes(request), StandardCharsets.UTF_8);

方法 3：Servlet 3.1+ 非阻塞讀取 (高併發、長連接)

@WebServlet(urlPatterns = "/async", asyncSupported = true)
public class AsyncBodyServlet extends HttpServlet {
    @Override
    protected void doPost(HttpServletRequest req, HttpServletResponse resp) {
        req.startAsync();
        ReadListener listener = new ReadListener() {
            private final StringBuilder sb = new StringBuilder();
            @Override
            public void onDataAvailable() throws IOException {
                try (BufferedReader reader = req.getReader()) {
                    char[] buf = new char[1024];
                    int len;
                    while (req.isReady() && (len = reader.read(buf)) != -1) {
                        sb.append(buf, 0, len);
                    }
                }
            }
            @Override
            public void onAllDataRead() {
                String body = sb.toString();
                // 處理 body
                req.getAsyncContext().complete();
            }
            @Override
            public void onError(Throwable t) {
                log.error("讀取失敗", t);
                req.getAsyncContext().complete();
            }
        };
        req.getInputStream().setReadListener(listener);
    }
}

Spring MVC 簡化讀取

常見坑點與解決方案

可重複讀取請求包裝器 (生產必備)

public class ContentCachingRequestWrapper extends HttpServletRequestWrapper {
    private final byte[] cachedBody;

    public ContentCachingRequestWrapper(HttpServletRequest request) throws IOException {
        super(request);
        // 讀取並緩存
        try (InputStream is = request.getInputStream();
             ByteArrayOutputStream baos = new ByteArrayOutputStream()) {
            byte[] buf = new byte[1024];
            int len;
            while ((len = is.read(buf)) != -1) baos.write(buf, 0, len);
            cachedBody = baos.toByteArray();
        }
    }

    @Override
    public ServletInputStream getInputStream() {
        return new ServletInputStream() {
            private final ByteArrayInputStream bais = new ByteArrayInputStream(cachedBody);
            @Override public int read() { return bais.read(); }
            @Override public boolean isFinished() { return bais.available() == 0; }
            @Override public boolean isReady() { return true; }
            @Override public void setReadListener(ReadListener l) { throw new UnsupportedOperationException(); }
        };
    }

    @Override
    public BufferedReader getReader() {
        return new BufferedReader(new InputStreamReader(getInputStream(), 
            StandardCharsets.UTF_8));
    }
}

// Filter 中包裝
@WebFilter("/*")
public class RequestCachingFilter implements Filter {
    @Override
    public void doFilter(ServletRequest req, ServletResponse resp, FilterChain chain) 
            throws IOException, ServletException {
        if (req instanceof HttpServletRequest) {
            HttpServletRequest httpReq = (HttpServletRequest) req;
            // 僅對有 Body 的請求包裝 (POST/PUT/PATCH)
            if ("POST".equalsIgnoreCase(httpReq.getMethod()) 
                || "PUT".equalsIgnoreCase(httpReq.getMethod())
                || "PATCH".equalsIgnoreCase(httpReq.getMethod())) {
                chain.doFilter(new ContentCachingRequestWrapper(httpReq), resp);
                return;
            }
        }
        chain.doFilter(req, resp);
    }
}

總結

1. 原生 Servlet 用 request.getReader() 讀文本，getInputStream() 讀二進制，**二選一**。
2. Spring MVC 優先用 @RequestBody 自動綁定，原始報文用 @RequestBody String。
3. 攔截器/過濾器需多次讀取 Body 時，**必須**用 ContentCachingRequestWrapper 包裝。
4. 編碼統一 UTF-8：Filter 設置、客戶端 Header、資料庫連接、JVM 文件編碼。
5. 大數據場景採用流式處理或非阻塞 IO，避免全量載入內存。