easystream开源库直接支持将OutputStream转换为InputStream: http://io-tools.sourceforge.net/easystream/tutorial/tutorial.html

// create conversion
final OutputStreamToInputStream<Void> out = new OutputStreamToInputStream<Void>() {
    @Override
    protected Void doRead(final InputStream in) throws Exception {
           LibraryClass2.processDataFromInputStream(in);
           return null;
        }
    };
try {   
     LibraryClass1.writeDataToTheOutputStream(out);
} finally {
     // don't miss the close (or a thread would not terminate correctly).
     out.close();
}

他们还列出了其他选项:http://io-tools.sourceforge.net/easystream/outputstream_to_inputstream/implementations.html

Write the data the data into a memory buffer (ByteArrayOutputStream) get the byteArray and read it again with a ByteArrayInputStream. This is the best approach if you're sure your data fits into memory. Copy your data to a temporary file and read it back. Use pipes: this is the best approach both for memory usage and speed (you can take full advantage of the multi-core processors) and also the standard solution offered by Sun. Use InputStreamFromOutputStream and OutputStreamToInputStream from the easystream library.

如果你想从一个InputStream生成一个OutputStream，有一个基本的问题。写入OutputStream的方法会阻塞，直到完成为止。因此，当编写方法完成时，结果是可用的。这有两个后果:

如果只使用一个线程，则需要等待所有内容写入(因此需要将流数据存储在内存或磁盘中)。 如果希望在数据完成之前访问数据，则需要第二个线程。

变体1可以使用字节数组或字段实现。 变体1可以使用pipies实现(直接或额外的抽象-例如RingBuffer或来自其他注释的谷歌库)。

事实上，在标准java中，没有其他方法可以解决这个问题。每个解决方案都是其中一个的实现。

有一个概念叫做“延续”(详见维基百科)。在这种情况下，这基本上意味着:

有一个特殊的输出流，它需要一定数量的数据 如果达到数量，则流将控制权交给对应的特殊输入流 输入流在读取数据之前提供可用的数据量，在读取之后，它将控制传递回输出流

虽然有些语言内置了这个概念，但对于java，您需要一些“魔法”。例如apache中的“commons-javaflow”实现了这样的java。缺点是这需要在构建时进行一些特殊的字节码修改。因此，将所有的东西都放在一个带有自定义构建脚本的额外库中是有意义的。

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()));

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。

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

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.