How to convert an InputStream to a DataHandler?

问题

I'm working on a java web application in which files will be stored in a database. Originally we retrieved files already in the DB by simply calling getBytes on our result set:

byte[] bytes = resultSet.getBytes(1);
...

This byte array was then converted into a DataHandler using the obvious constructor:

dataHandler=new DataHandler(bytes,"application/octet-stream");

This worked great until we started trying to store and retrieve larger files. Dumping the entire file contents into a byte array and then building a DataHandler out of that simply requires too much memory.

My immediate idea is to retrieve a stream of the data in the database with getBinaryStream and somehow convert that InputStream into a DataHandler in a memory-efficient way. Unfortunately it doesn't seem like there's a direct way to convert an InputStream into a DataHandler. Another idea I've been playing with is reading chunks of data from the InputStream and writing them to the OutputStream of the DataHandler. But... I can't find a way to create an "empty" DataHandler that returns a non-null OutputStream when I call getOutputStream...

Has anyone done this? I'd appreciate any help you can give me or leads in the right direction.

回答1:

My approach would be to write a custom class implementing DataSource that wraps your InputStream. Then create the DataHandler giving it the created DataSource.

回答2:

An implementation of answer from "Kathy Van Stone":

At first create helper class, which create DataSource from InputStream:

public class InputStreamDataSource implements DataSource {
    private InputStream inputStream;

    public InputStreamDataSource(InputStream inputStream) {
        this.inputStream = inputStream;
    }

    @Override
    public InputStream getInputStream() throws IOException {
        return inputStream;
    }

    @Override
    public OutputStream getOutputStream() throws IOException {
        throw new UnsupportedOperationException("Not implemented");
    }

    @Override
    public String getContentType() {
        return "*/*";
    }

    @Override
    public String getName() {
        return "InputStreamDataSource";
    }
}

And then you can create DataHandler from InputStream:

DataHandler dataHandler = new DataHandler(new InputStreamDataSource(inputStream))

imports:

import javax.activation.DataSource;
import java.io.OutputStream;
import java.io.InputStream;

回答3:

I also ran into this issue. If your source data is a byte[] Axis already has a class that wraps the InputStream and creates a DataHandler object. Here is the code

//this constructor takes byte[] as input
ByteArrayDataSource rawData= new ByteArrayDataSource(resultSet.getBytes(1));
DataHandler data= new DataHandler(rawData);
yourObject.setData(data);

Related imports

import javax.activation.DataHandler;
import org.apache.axiom.attachments.ByteArrayDataSource;

Hope it helps!

回答4:

Note that the getInputStream of the DataSource must return a new InputStream everytime called. This means, you need to copy somewhere 1st. For more info, see http://bugs.sun.com/bugdatabase/view_bug.do?bug_id=4267294

回答5:

(bugs_)code doesn't works for me. I use DataSource to create Attachments to email (from objects that have inputStream and name) and content of attachments lost. Looks like Stefan is right and new inputStream must be returned every time. At least in my specific case. Next implementation deals with problem:

public class InputStreamDataSource implements DataSource {

    ByteArrayOutputStream buffer = new ByteArrayOutputStream();
    private final String name;

    public InputStreamDataSource(InputStream inputStream, String name) {
        this.name = name;
        try {
            int nRead;
            byte[] data = new byte[16384];
            while ((nRead = inputStream.read(data, 0, data.length)) != -1) {
                buffer.write(data, 0, nRead);
            }

            buffer.flush();
            inputStream.close();
        } catch (IOException e) {
            e.printStackTrace();
        }

    }

    @Override
    public String getContentType() {
        return new MimetypesFileTypeMap().getContentType(name);
    }

    @Override
    public InputStream getInputStream() throws IOException {
        return new ByteArrayInputStream(buffer.toByteArray());
    }

    @Override
    public String getName() {
        return name;
    }

    @Override
    public OutputStream getOutputStream() throws IOException {
        throw new IOException("Read-only data");
    }

}

回答6:

I've meet situation, when InputStream requested from DataSource twice: using Logging Handler together with MTOM feature. With this proxy stream solution my implementation works fine:

import org.apache.commons.io.input.CloseShieldInputStream;
import javax.activation.DataHandler;
import javax.activation.DataSource;
...

private static class InputStreamDataSource implements DataSource {
    private InputStream inputStream;

    @Override
    public InputStream getInputStream() throws IOException {
        return new CloseShieldInputStream(inputStream);
    }

    @Override
    public OutputStream getOutputStream() throws IOException {
        throw new UnsupportedOperationException("Not implemented");
    }

    @Override
    public String getContentType() {
        return "application/octet-stream";
    }

    @Override
    public String getName() {
        return "";
    }
}

回答7:

Here is an answer for specifically working with the Spring Boot org.springframework.core.io.Resource object which is I think how a lot of us are getting here. Note that you might need to modify the content type in the code below as I'm inserting a png file into an html formatted email.

Note: As others have mentioned, merely attaching an InputStream isn't enough as it gets used multiple times, just mapping through to Resource.getInputStream() does the trick.

public class SpringResourceDataSource implements DataSource {
    private Resource resource;

    public SpringResourceDataSource(Resource resource) {
        this.resource = resource;
    }

    @Override
    public InputStream getInputStream() throws IOException {
        return resource.getInputStream();
    }

    @Override
    public OutputStream getOutputStream() throws IOException {
        throw new UnsupportedOperationException("Not implemented");
    }

    @Override
    public String getContentType() {
        return "image/png";
    }

    @Override
    public String getName() {
        return "SpringResourceDataSource";
    }
}

Usage of the class looks like this:

    PathMatchingResourcePatternResolver pathMatchingResourcePatternResolver = new PathMatchingResourcePatternResolver();
    Resource logoImage = pathMatchingResourcePatternResolver.getResource("/static/images/logo.png");
    MimeBodyPart logoBodyPart = new MimeBodyPart();
    DataSource logoFileDataSource = new SpringResourceDataSource(logoImage);


    logoBodyPart.setDataHandler(new DataHandler(logoFileDataSource));

来源：https://stackoverflow.com/questions/2830561/how-to-convert-an-inputstream-to-a-datahandler

标签

java

inputstream

outputstream