Convert InputStream to OutputStream

June 07, 2020 No comments Java IO Java Conversions InputStream OutputStream

1. Introduction

In this article, we are going to present several ways to convert InputStream to OutpuStream in Java. To perform this operation we will use classes available in plain Java and libraries such as Guava and Apache Commons IO.

For more Java I/O related articles check the following links:

2. Copy InputStream to OutputStream using OutputStream.write(...) method

Below Java 8 we can simply rewrite bytes from InputStream to OutputStream using read(...) and write(...) method like in the following example:

package com.frontbackend.java.io.conversions.frominputstream.tooutputstream;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;

public class InputStreamToOutputStreamUsingWrite {

    public static void main(String[] args) throws IOException {
        try (InputStream in = new FileInputStream("/tmp/input.txt");
             OutputStream out = new FileOutputStream("/tmp/output.txt")) {

            int length;
            byte[] bytes = new byte[1024];

            while ((length = in.read(bytes)) != -1) {
                out.write(bytes, 0, length);
            }
        }
    }
}

In this approach, we used read(...) method available on InputStream to read bytes from the byte-based stream. In a loop, we write read bytes directly to the OutputStream using write(...) method.

3. Convert InputStream to OutputStream using InputString.transferTo(...) method available in JDK 9

Let's take a look at the next example that makes the use of the transferTo(...) method available from Java 9. 

package com.frontbackend.java.io.conversions.frominputstream.tooutputstream;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;

public class InputStreamToOutputStreamUsingTransferTo {

    public static void main(String[] args) throws IOException {
        try (InputStream in = new FileInputStream("/tmp/input.txt");
             OutputStream out = new FileOutputStream("/tmp/output.txt")) {

            in.transferTo(out);
        }
    }
}

Using InputStream.transferTo(...) allows us to transfer bytes from InputStream to the OutputStream in a one line of code.

4. InputStream to OutputStream using Apache Commons IO library

The Apache Commons IO library was designed with IO operations in mind. It comes with many utility methods, and we can use one of them to copy bytes from a byte-based stream to the OutputStream.

package com.frontbackend.java.io.conversions.frominputstream.tooutputstream;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;

import org.apache.commons.io.IOUtils;

public class InputStreamToOutputStreamUsingIOUtils {

    public static void main(String[] args) throws IOException {
        try (InputStream in = new FileInputStream("/tmp/input.txt"); 
             OutputStream out = new FileOutputStream("/tmp/output.txt")) {

            IOUtils.copy(in, out);
        }
    }
}

In this example, we make use of IOUtils.copy(...) method that copy bytes from InputStream to the OutputStream transparently.

5. Convert InputStream to OutputStream with Guava

Guava is another library that we can use to write bytes from InputStream to the OutputStream. Let's check the following example:

package com.frontbackend.java.io.conversions.frominputstream.tooutputstream;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;

import com.google.common.io.ByteStreams;

public class InputStreamToOutputStreamUsingGuava {

    public static void main(String[] args) throws IOException {
        try (InputStream in = new FileInputStream("/tmp/input.txt"); 
             OutputStream out = new FileOutputStream("/tmp/output.txt")) {

            ByteStreams.copy(in, out);
        }
    }
}

Guava provides ByteStreams.copy(...) method very similar to Apache Commons IOs IOUtils.copy(...). The logic behind these two methods is the same.

6. Conclusion

In this article, we presented various ways to convert InputStream to the OutputStream. This operation comes down to read bytes from the InputStream and write them to the OutputStream in the same order.

Examples used in this article are available under our GitHub repository.

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