读取流两次

Question

你怎么读两次相同的输入流？有可能以某种方式复制它吗？

我需要从网上获取图像，在本地保存，然后返回保存的图像。我只是认为使用相同的流而不是为下载的内容启动新流然后再次读取它会更快。

Answer 1

您可以使用org.apache.commons.io.IOUtils.copy将InputStream的内容复制到字节数组，然后使用ByteArrayInputStream从字节数组中重复读取。例如。：

ByteArrayOutputStream baos = new ByteArrayOutputStream();
org.apache.commons.io.IOUtils.copy(in, baos);
byte[] bytes = baos.toByteArray();

// either
while (needToReadAgain) {
    ByteArrayInputStream bais = new ByteArrayInputStream(bytes);
    yourReadMethodHere(bais);
}

// or
ByteArrayInputStream bais = new ByteArrayInputStream(bytes);
while (needToReadAgain) {
    bais.reset();
    yourReadMethodHere(bais);
}

Answer 2

根据InputStream的来源，您可能无法重置它。您可以使用mark()检查是否支持reset()和markSupported()。

如果是，您可以在InputStream上调用reset()返回到开头。如果没有，则需要再次从源读取InputStream。

Answer 3

如果你的InputStream支持使用mark，那么你可以mark()你的inputStream然后reset()它。如果你的InputStrem不支持mark，那么你可以使用java.io.BufferedInputStream类，所以你可以将你的流嵌入一个像这样的BufferedInputStream

    InputStream bufferdInputStream = new BufferedInputStream(yourInputStream);
    bufferdInputStream.mark(some_value);
    //read your bufferdInputStream 
    bufferdInputStream.reset();
    //read it again

Answer 4

您可以使用PushbackInputStream包装输入流。 PushbackInputStream允许读取（“回写”）已读取的字节，因此您可以这样做：

public class StreamTest {
  public static void main(String[] args) throws IOException {
    byte[] bytes = new byte[] { 1, 2, 3, 4, 5, 6, 7, 8, 9 };

    InputStream originalStream = new ByteArrayInputStream(bytes);

    byte[] readBytes = getBytes(originalStream, 3);
    printBytes(readBytes); // prints: 1 2 3

    readBytes = getBytes(originalStream, 3);
    printBytes(readBytes); // prints: 4 5 6

    // now let's wrap it with PushBackInputStream

    originalStream = new ByteArrayInputStream(bytes);

    InputStream wrappedStream = new PushbackInputStream(originalStream, 10); // 10 means that maximnum 10 characters can be "written back" to the stream

    readBytes = getBytes(wrappedStream, 3);
    printBytes(readBytes); // prints 1 2 3

    ((PushbackInputStream) wrappedStream).unread(readBytes, 0, readBytes.length);

    readBytes = getBytes(wrappedStream, 3);
    printBytes(readBytes); // prints 1 2 3


  }

  private static byte[] getBytes(InputStream is, int howManyBytes) throws IOException {
    System.out.print("Reading stream: ");

    byte[] buf = new byte[howManyBytes];

    int next = 0;
    for (int i = 0; i < howManyBytes; i++) {
      next = is.read();
      if (next > 0) {
        buf[i] = (byte) next;
      }
    }
    return buf;
  }

  private static void printBytes(byte[] buffer) throws IOException {
    System.out.print("Reading stream: ");

    for (int i = 0; i < buffer.length; i++) {
      System.out.print(buffer[i] + " ");
    }
    System.out.println();
  }


}

请注意，PushbackInputStream存储字节的内部缓冲区，因此它确实在内存中创建了一个缓冲区“保持写回”的缓冲区。

知道了这种方法，我们可以进一步将它与FilterInputStream结合起来。 FilterInputStream将原始输入流存储为委托。这允许创建新的类定义，允许自动“读取”原始数据。这个类的定义如下：

public class TryReadInputStream extends FilterInputStream {
  private final int maxPushbackBufferSize;

  /**
  * Creates a <code>FilterInputStream</code>
  * by assigning the  argument <code>in</code>
  * to the field <code>this.in</code> so as
  * to remember it for later use.
  *
  * @param in the underlying input stream, or <code>null</code> if
  *           this instance is to be created without an underlying stream.
  */
  public TryReadInputStream(InputStream in, int maxPushbackBufferSize) {
    super(new PushbackInputStream(in, maxPushbackBufferSize));
    this.maxPushbackBufferSize = maxPushbackBufferSize;
  }

  /**
   * Reads from input stream the <code>length</code> of bytes to given buffer. The read bytes are still avilable
   * in the stream
   *
   * @param buffer the destination buffer to which read the data
   * @param offset  the start offset in the destination <code>buffer</code>
   * @aram length how many bytes to read from the stream to buff. Length needs to be less than
   *        <code>maxPushbackBufferSize</code> or IOException will be thrown
   *
   * @return number of bytes read
   * @throws java.io.IOException in case length is
   */
  public int tryRead(byte[] buffer, int offset, int length) throws IOException {
    validateMaxLength(length);

    // NOTE: below reading byte by byte instead of "int bytesRead = is.read(firstBytes, 0, maxBytesOfResponseToLog);"
    // because read() guarantees to read a byte

    int bytesRead = 0;

    int nextByte = 0;

    for (int i = 0; (i < length) && (nextByte >= 0); i++) {
      nextByte = read();
      if (nextByte >= 0) {
        buffer[offset + bytesRead++] = (byte) nextByte;
      }
    }

    if (bytesRead > 0) {
      ((PushbackInputStream) in).unread(buffer, offset, bytesRead);
    }

    return bytesRead;

  }

  public byte[] tryRead(int maxBytesToRead) throws IOException {
    validateMaxLength(maxBytesToRead);

    ByteArrayOutputStream baos = new ByteArrayOutputStream(); // as ByteArrayOutputStream to dynamically allocate internal bytes array instead of allocating possibly large buffer (if maxBytesToRead is large)

    // NOTE: below reading byte by byte instead of "int bytesRead = is.read(firstBytes, 0, maxBytesOfResponseToLog);"
    // because read() guarantees to read a byte

    int nextByte = 0;

    for (int i = 0; (i < maxBytesToRead) && (nextByte >= 0); i++) {
      nextByte = read();
      if (nextByte >= 0) {
        baos.write((byte) nextByte);
      }
    }

    byte[] buffer = baos.toByteArray();

    if (buffer.length > 0) {
      ((PushbackInputStream) in).unread(buffer, 0, buffer.length);
    }

    return buffer;

  }

  private void validateMaxLength(int length) throws IOException {
    if (length > maxPushbackBufferSize) {
      throw new IOException(
        "Trying to read more bytes than maxBytesToRead. Max bytes: " + maxPushbackBufferSize + ". Trying to read: " +
        length);
    }
  }

}

这个类有两种方法。一个用于读入现有缓冲区（定义类似于调用InputStream类的public int read(byte b[], int off, int len)）。第二个返回新缓冲区（如果要读取的缓冲区大小未知，则可能更有效）。

现在让我们看看我们的课程：

public class StreamTest2 {
  public static void main(String[] args) throws IOException {
    byte[] bytes = new byte[] { 1, 2, 3, 4, 5, 6, 7, 8, 9 };

    InputStream originalStream = new ByteArrayInputStream(bytes);

    byte[] readBytes = getBytes(originalStream, 3);
    printBytes(readBytes); // prints: 1 2 3

    readBytes = getBytes(originalStream, 3);
    printBytes(readBytes); // prints: 4 5 6

    // now let's use our TryReadInputStream

    originalStream = new ByteArrayInputStream(bytes);

    InputStream wrappedStream = new TryReadInputStream(originalStream, 10);

    readBytes = ((TryReadInputStream) wrappedStream).tryRead(3); // NOTE: no manual call to "unread"(!) because TryReadInputStream handles this internally
    printBytes(readBytes); // prints 1 2 3

    readBytes = ((TryReadInputStream) wrappedStream).tryRead(3); 
    printBytes(readBytes); // prints 1 2 3

    readBytes = ((TryReadInputStream) wrappedStream).tryRead(3);
    printBytes(readBytes); // prints 1 2 3

    // we can also call normal read which will actually read the bytes without "writing them back"
    readBytes = getBytes(wrappedStream, 3);
    printBytes(readBytes); // prints 1 2 3

    readBytes = getBytes(wrappedStream, 3);
    printBytes(readBytes); // prints 4 5 6

    readBytes = ((TryReadInputStream) wrappedStream).tryRead(3); // now we can try read next bytes
    printBytes(readBytes); // prints 7 8 9

    readBytes = ((TryReadInputStream) wrappedStream).tryRead(3); 
    printBytes(readBytes); // prints 7 8 9


  }



}

Answer 5

如果您正在使用InputStream的实现，您可以检查InputStream#markSupported()的结果，告诉您是否可以使用mark() / reset()方法。

如果您在阅读时可以标记流，请调用reset()返回开始。

如果你不能，你将不得不再次打开一个流。

另一种解决方案是将InputStream转换为字节数组，然后根据需要迭代数组。你可以在这篇文章Convert InputStream to byte array in Java中找到几个使用第三方库的解决方案。注意，如果读取的内容太大，您可能会遇到一些内存问题。

最后，如果您需要阅读图像，请使用：

BufferedImage image = ImageIO.read(new URL("http://www.example.com/images/toto.jpg"));

使用ImageIO#read(java.net.URL)还允许您使用缓存。

Answer 6

将inputstream转换为字节，然后将其传递给savefile函数，然后将其组合到inputstream中。同样在原始函数中使用字节用于其他任务

Answer 7

怎么样：

if (stream.markSupported() == false) {

        // lets replace the stream object
        ByteArrayOutputStream baos = new ByteArrayOutputStream();
        IOUtils.copy(stream, baos);
        stream.close();
        stream = new ByteArrayInputStream(baos.toByteArray());
        // now the stream should support 'mark' and 'reset'

    }

Answer 8

如果有人在Spring Boot应用程序中运行，并且您想要读取RestTemplate的响应主体（这就是我想要读取两次流的原因），那么有一种干净的（呃）方法。

首先，您需要使用Spring的StreamUtils将流复制到String：

String text = StreamUtils.copyToString(response.getBody(), Charset.defaultCharset()))

但那还不是全部。您还需要使用可以为您缓冲流的请求工厂，如下所示：

ClientHttpRequestFactory factory = new BufferingClientHttpRequestFactory(new SimpleClientHttpRequestFactory());
RestTemplate restTemplate = new RestTemplate(factory);

或者，如果你正在使用工厂bean，那么（这是Kotlin但是）：

@Bean
@Scope(ConfigurableBeanFactory.SCOPE_PROTOTYPE)
fun createRestTemplate(): RestTemplate = RestTemplateBuilder()
  .requestFactory { BufferingClientHttpRequestFactory(SimpleClientHttpRequestFactory()) }
  .additionalInterceptors(loggingInterceptor)
  .build()

资料来源：https://objectpartners.com/2018/03/01/log-your-resttemplate-request-and-response-without-destroying-the-body/

Answer 9

将InputStream分成两部分，同时避免将所有数据加载到内存中，然后单独处理它们：

创建几个OutputStream，恰恰是：PipedOutputStream
将每个PipedOutputStream与PipedInputStream连接，这些PipedInputStream是返回的InputStream。
使用刚创建的OutputStream连接源InputStream。因此，从采购InputStream读取的所有内容都将用OutputStream编写。没有必要实现它，因为它已经在TeeInputStream（commons.io）中完成。
在一个单独的线程中读取整个源inputStream，并隐式地将输入数据传输到目标inputStreams。 public static final List<InputStream> splitInputStream(InputStream input) throws IOException { Objects.requireNonNull(input); PipedOutputStream pipedOut01 = new PipedOutputStream(); PipedOutputStream pipedOut02 = new PipedOutputStream(); List<InputStream> inputStreamList = new ArrayList<>(); inputStreamList.add(new PipedInputStream(pipedOut01)); inputStreamList.add(new PipedInputStream(pipedOut02)); TeeOutputStream tout = new TeeOutputStream(pipedOut01, pipedOut02); TeeInputStream tin = new TeeInputStream(input, tout, true); Executors.newSingleThreadExecutor().submit(tin::readAllBytes); return Collections.unmodifiableList(inputStreamList); }

请注意在使用后关闭inputStreams，并关闭运行的线程：TeeInputStream.readAllBytes()

如果你需要将它分成多个InputStream，而不是只有两个。在前面的代码片段中替换类TeeOutputStream用于您自己的实现，它将封装List<OutputStream>并覆盖OutputStream接口：

public final class TeeListOutputStream extends OutputStream {
    private final List<? extends OutputStream> branchList;

    public TeeListOutputStream(final List<? extends OutputStream> branchList) {
        Objects.requireNonNull(branchList);
        this.branchList = branchList;
    }

    @Override
    public synchronized void write(final int b) throws IOException {
        for (OutputStream branch : branchList) {
            branch.write(b);
        }
    }

    @Override
    public void flush() throws IOException {
        for (OutputStream branch : branchList) {
            branch.flush();
        }
    }

    @Override
    public void close() throws IOException {
        for (OutputStream branch : branchList) {
            branch.close();
        }
    }
}

读取流两次

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读取流两次

问题描述 投票：102回答：9

9个回答

最新问题

问题描述投票：102回答：9