How to clone an InputStream?

Question

I have a InputStream that I pass to a method to do some processing. I will use the same InputStream in other method, but after the first processing, the InputStream appears be closed inside the method.

How I can clone the InputStream to send to the method that closes him? There is another solution?

EDIT: the methods that closes the InputStream is an external method from a lib. I dont have control about closing or not.

private String getContent(HttpURLConnection con) {
    InputStream content = null;
    String charset = "";
    try {
        content = con.getInputStream();
        CloseShieldInputStream csContent = new CloseShieldInputStream(content);
        charset = getCharset(csContent);            
        return  IOUtils.toString(content,charset);
    } catch (Exception e) {
        System.out.println("Error downloading page: " + e);
        return null;
    }
}

private String getCharset(InputStream content) {
    try {
        Source parser = new Source(content);
        return parser.getEncoding();
    } catch (Exception e) {
        System.out.println("Error determining charset: " + e);
        return "UTF-8";
    }
}

Answer 1

If you are using apache.commons you may copy streams using IOUtils .

You can use following code:

InputStream = IOUtils.toBufferedInputStream(toCopy);

Here is the full example suitable for your situation:

public void cloneStream() throws IOException{
    InputStream toCopy=IOUtils.toInputStream("aaa");
    InputStream dest= null;
    dest=IOUtils.toBufferedInputStream(toCopy);
    toCopy.close();
    String result = new String(IOUtils.toByteArray(dest));
    System.out.println(result);
}

This code requires some dependencies:

MAVEN

<dependency>
    <groupId>commons-io</groupId>
    <artifactId>commons-io</artifactId>
    <version>2.4</version>
</dependency>

GRADLE

'commons-io:commons-io:2.4'

Here is the DOC reference for this method:

Fetches entire contents of an InputStream and represent same data as result InputStream. This method is useful where,

Source InputStream is slow. It has network resources associated, so we cannot keep it open for long time. It has network timeout associated.

You can find more about IOUtils here: http://commons.apache.org/proper/commons-io/javadocs/api-2.4/org/apache/commons/io/IOUtils.html#toBufferedInputStream(java.io.InputStream)

Answer 2

Cloning an input stream might not be a good idea, because this requires deep knowledge about the details of the input stream being cloned. A workaround for this is to create a new input stream that reads from the same source again.

So using some Java 8 features this would look like this:

public class Foo {

    private Supplier<InputStream> inputStreamSupplier;

    public void bar() {
        procesDataThisWay(inputStreamSupplier.get());
        procesDataTheOtherWay(inputStreamSupplier.get());
    }

    private void procesDataThisWay(InputStream) {
        // ...
    }

    private void procesDataTheOtherWay(InputStream) {
        // ...
    }
}

This method has the positive effect that it will reuse code that is already in place - the creation of the input stream encapsulated in inputStreamSupplier. And there is no need to maintain a second code path for the cloning of the stream.

On the other hand, if reading from the stream is expensive (because a it's done over a low bandwith connection), then this method will double the costs. This could be circumvented by using a specific supplier that will store the stream content locally first and provide an InputStream for that now local resource.

Answer 3

If the data read from the stream is large, I would recommend using a TeeInputStream from Apache Commons IO. That way you can essentially replicate the input and pass a t'd pipe as your clone.