ByteArrayOutputStream buffer = (ByteArrayOutputStream) aOutputStream;
byte[] bytes = buffer.toByteArray();
InputStream inputStream = new ByteArrayInputStream(bytes);

库io-extras可能很有用。例如，如果你想使用GZIPOutputStream gzip一个InputStream，并且你希望它同步发生(使用默认的缓冲区大小8192):

InputStream is = ...
InputStream gz = IOUtil.pipe(is, o -> new GZIPOutputStream(o));

请注意，该库具有100%的单元测试覆盖率(当然，这是值得的!)，并且位于Maven Central上。Maven依赖项是:

<dependency>
  <groupId>com.github.davidmoten</groupId>
  <artifactId>io-extras</artifactId>
  <version>0.1</version>
</dependency>

一定要查看更新的版本。

老帖子，但可能会帮助别人，使用这种方式:

OutputStream out = new ByteArrayOutputStream();
...
out.write();
...
ObjectInputStream ois = new ObjectInputStream(new ByteArrayInputStream(out.toString().getBytes()));

由于输入和输出流只是起点和终点，解决方案是将数据临时存储在字节数组中。因此，您必须创建中间ByteArrayOutputStream，从中创建字节[]，用作新ByteArrayInputStream的输入。

public void doTwoThingsWithStream(InputStream inStream, OutputStream outStream){ 
  //create temporary bayte array output stream
  ByteArrayOutputStream baos = new ByteArrayOutputStream();
  doFirstThing(inStream, baos);
  //create input stream from baos
  InputStream isFromFirstData = new ByteArrayInputStream(baos.toByteArray()); 
  doSecondThing(isFromFirstData, outStream);
}

希望能有所帮助。

Though you cannot convert an OutputStream to an InputStream, java provides a way using PipedOutputStream and PipedInputStream that you can have data written to a PipedOutputStream to become available through an associated PipedInputStream. Sometime back I faced a similar situation when dealing with third party libraries that required an InputStream instance to be passed to them instead of an OutputStream instance. The way I fixed this issue is to use the PipedInputStream and PipedOutputStream. By the way they are tricky to use and you must use multithreading to achieve what you want. I recently published an implementation on github which you can use. Here is the link . You can go through the wiki to understand how to use it.

似乎有许多链接和其他类似的东西，但没有使用管道的实际代码。使用java.io.PipedInputStream和java.io.PipedOutputStream的优点是不会额外消耗内存。ByteArrayOutputStream.toByteArray()返回原始缓冲区的副本，因此这意味着无论内存中有什么，现在都有它的两个副本。然后写入InputStream意味着现在有了数据的三个副本。

使用lambdas的代码(从评论中向@John Manko致敬):

PipedInputStream in = new PipedInputStream();
final PipedOutputStream out = new PipedOutputStream(in);
// in a background thread, write the given output stream to the
// PipedOutputStream for consumption
new Thread(() -> {originalOutputStream.writeTo(out);}).start();

@John Manko注意到的一件事是，在某些情况下，当您无法控制OutputStream的创建时，您可能会在创建者过早地清理OutputStream对象的情况下结束。如果你正在获取ClosedPipeException，那么你应该尝试反向构造函数:

PipedInputStream in = new PipedInputStream(out);
new Thread(() -> {originalOutputStream.writeTo(out);}).start();

注意，您也可以为下面的示例反转构造函数。

也感谢@AlexK纠正我开始一个线程，而不是仅仅启动一个Runnable。

使用try-with-resources的代码:

// take the copy of the stream and re-write it to an InputStream
PipedInputStream in = new PipedInputStream();
    new Thread(new Runnable() {
        public void run () {
            // try-with-resources here
            // putting the try block outside the Thread will cause the
            // PipedOutputStream resource to close before the Runnable finishes
            try (final PipedOutputStream out = new PipedOutputStream(in)) {
                // write the original OutputStream to the PipedOutputStream
                // note that in order for the below method to work, you need
                // to ensure that the data has finished writing to the
                // ByteArrayOutputStream
                originalByteArrayOutputStream.writeTo(out);
            }
            catch (IOException e) {
                // logging and exception handling should go here
            }
        }
    }).start();

我写的原始代码:

// take the copy of the stream and re-write it to an InputStream
PipedInputStream in = new PipedInputStream();
final PipedOutputStream out = new PipedOutputStream(in);
new Thread(new Runnable() {
    public void run () {
        try {
            // write the original OutputStream to the PipedOutputStream
            // note that in order for the below method to work, you need
            // to ensure that the data has finished writing to the
            // ByteArrayOutputStream
            originalByteArrayOutputStream.writeTo(out);
        }
        catch (IOException e) {
            // logging and exception handling should go here
        }
        finally {
            // close the PipedOutputStream here because we're done writing data
            // once this thread has completed its run
            if (out != null) {
                // close the PipedOutputStream cleanly
                out.close();
            }
        }   
    }
}).start();

这段代码假设原始ByteArrayOutputStream是一个ByteArrayOutputStream，因为它通常是唯一可用的输出流，除非您正在写入文件。这样做的好处是，因为它在一个单独的线程中，所以它也是并行工作的，所以无论消耗你的输入流的是什么，它也会从你的旧输出流中流出。这是有益的，因为缓冲区可以保持更小，您将有更少的延迟和内存使用。

如果你没有ByteArrayOutputStream，那么你将不得不使用java.io.OutputStream类中的write()方法之一或子类中可用的其他方法之一，而不是使用writeTo()。