Request IO流無法重複讀取的原因和解決方法

Request IO流只能讀取一次的原因和解決方法

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探索路線

只能讀一次的原因

首先要知道爲什麼httpServletRequest的流只能讀取一次。

調用httpServletRequest.getInputStream()可以看到獲取的流類型爲ServletInputStream，繼承InputStream。

下面複習下InputStream,InputStream的read方法內部有一個postion，標誌當前流讀取到的位置，每讀取一次，位置就會移動一次，如果讀到最後，read()會返回-1，標誌已經讀取完了。如果想要重新讀取則需要重寫reset()方法，當然能否reset是有條件的，它取決於markSupported()是否返回true。

在InputStream源碼中默認不實現reset()，並且markSupported()默認返回false：

public synchronized void reset() throws IOException {
    // 調用重新讀取則拋出異常
    throw new IOException("mark/reset not supported");
}
public boolean markSupported() {
    // 不支持重新讀取
    return false;
}

而查看ServletInputStream源碼可以發現，該類沒有重寫mark(),reset()以及markSupported()，因此Request IO流無法重複讀取

public abstract class ServletInputStream extends InputStream {
    protected ServletInputStream() {
    }

    public int readLine(byte[] b, int off, int len) throws IOException {
        if (len <= 0) {
            return 0;
        } else {
            int count = 0;

            int c;
            while((c = this.read()) != -1) {
                b[off++] = (byte)c;
                ++count;
                if (c == 10 || count == len) {
                    break;
                }
            }

            return count > 0 ? count : -1;
        }
    }

    public abstract boolean isFinished();

    public abstract boolean isReady();

    public abstract void setReadListener(ReadListener var1);
}

使用HttpServletRequestWrapper

既然ServletInputStream不支持重新讀寫，那麼爲什麼不把流讀出來後用容器存儲起來，後面就可以多次利用了。

所幸Java提供了一個請求包裝器 ：HttpServletRequestWrapper基於裝飾者模式實現了HttpServletRequest介面，只需要繼承該類並實現你想要重新定義的方法即可。

/**
 * 代碼來自網絡，侵刪
 */
public class BodyReaderHttpServletRequestWrapper  extends HttpServletRequestWrapper {

    private final byte[] body;

    BodyReaderHttpServletRequestWrapper(HttpServletRequest request) {
        super(request);
        String sessionStream = getBodyString(request);
        body = sessionStream.getBytes(Charset.forName("UTF-8"));
    }

    /**
     * 獲取請求Body
     *
     * @param request 請求
     * @return Body字符串
     */
    private String getBodyString(final ServletRequest request) {
        StringBuilder sb = new StringBuilder();
        InputStream inputStream = null;
        BufferedReader reader = null;
        try {
            inputStream = cloneInputStream(request.getInputStream());
            reader = new BufferedReader(new InputStreamReader(inputStream, Charset.forName("UTF-8")));
            String line = "";
            while ((line = reader.readLine()) != null) {
                sb.append(line);
            }
        }
        catch (IOException e) {
            e.printStackTrace();
        }
        finally {
            if (inputStream != null) {
                try {
                    inputStream.close();
                }
                catch (IOException e) {
                    e.printStackTrace();
                }
            }
            if (reader != null) {
                try {
                    reader.close();
                }
                catch (IOException e) {
                    e.printStackTrace();
                }
            }
        }
        return sb.toString();
    }

    /**
     * 複製輸入流
     * @param inputStream 請求輸入流
     * @return 複製出來的輸入流
     */
    private InputStream cloneInputStream(ServletInputStream inputStream) {
        ByteArrayOutputStream byteArrayOutputStream = new ByteArrayOutputStream();
        byte[] buffer = new byte[1024];
        int len;
        try {
            while ((len = inputStream.read(buffer)) > -1) {
                byteArrayOutputStream.write(buffer, 0, len);
            }
            byteArrayOutputStream.flush();
        }
        catch (IOException e) {
            e.printStackTrace();
        }
        return new ByteArrayInputStream(byteArrayOutputStream.toByteArray());
    }

    @Override
    public BufferedReader getReader() {
        return new BufferedReader(new InputStreamReader(getInputStream()));
    }

    @Override
    public ServletInputStream getInputStream() {

        final ByteArrayInputStream bais = new ByteArrayInputStream(body);

        return new ServletInputStream() {

            @Override
            public int read() {
                return bais.read();
            }

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

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

            @Override
            public void setReadListener(ReadListener readListener) {
            }
        };
    }
}

並且在讀取請求流的過濾器中使用該包裝器：

@Override
public void doFilter(ServletRequest servletRequest, ServletResponse servletResponse, FilterChain filterChain) throws IOException, ServletException {
    BodyReaderHttpServletRequestWrapper myRequestWrapper;
    HttpServletRequest httpServletRequest = (HttpServletRequest) servletRequest;

    // 封裝請求參數
    myRequestWrapper = new BodyReaderHttpServletRequestWrapper(httpServletRequest);
    // 從請求包裝器中取出json值
    int index = new Gson().fromJson(servletRequest.getReader(), InteractionData.class).getIndex();
    // 從請求包裝器拿出Session屬性user
    User user = (User) myRequestWrapper.getSession().getAttribute("user");
    
    // 將請求包裝器傳遞下
    filterChain.doFilter(myRequestWrapper, servletResponse);
}

這時候就在後面的代碼中再次調用流了