Request IO流无法重复读取的原因和解决方法

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Request IO流只能读取一次的原因和解决方法

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探索路线

只能读一次的原因

首先要知道为什么httpServletRequest的流只能读取一次。

调用httpServletRequest.getInputStream()可以看到获取的流类型为ServletInputStream，继承InputStream。

下面复习下InputStream,InputStream的read方法内部有一个postion，标志当前流读取到的位置，每读取一次，位置就会移动一次，如果读到最后，read()会返回-1，标志已经读取完了。如果想要重新读取则需要重写reset()方法，当然能否reset是有条件的，它取决于markSupported()是否返回true。

在InputStream源码中默认不实现reset()，并且markSupported()默认返回false：

public synchronized void reset() throws IOException {
    // 调用重新读取则抛出异常
    throw new IOException("mark/reset not supported");
}
public boolean markSupported() {
    // 不支持重新读取
    return false;
}

而查看ServletInputStream源码可以发现，该类没有重写mark(),reset()以及markSupported()，因此Request IO流无法重复读取

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不支持重新读写，那么为什么不把流读出来后用容器存储起来，后面就可以多次利用了。

所幸Java提供了一个请求包装器 ：HttpServletRequestWrapper基于装饰者模式实现了HttpServletRequest介面，只需要继承该类并实现你想要重新定义的方法即可。

/**
 * 代码来自网络，侵删
 */
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"));
    }

    /**
     * 获取请求Body
     *
     * @param request 请求
     * @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();
    }

    /**
     * 复制输入流
     * @param inputStream 请求输入流
     * @return 复制出来的输入流
     */
    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) {
            }
        };
    }
}

并且在读取请求流的过滤器中使用该包装器：

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

    // 封装请求参数
    myRequestWrapper = new BodyReaderHttpServletRequestWrapper(httpServletRequest);
    // 从请求包装器中取出json值
    int index = new Gson().fromJson(servletRequest.getReader(), InteractionData.class).getIndex();
    // 从请求包装器拿出Session属性user
    User user = (User) myRequestWrapper.getSession().getAttribute("user");
    
    // 将请求包装器传递下
    filterChain.doFilter(myRequestWrapper, servletResponse);
}

这时候就在后面的代码中再次调用流了