Converting InputStream to String | Java IO tutorial InputStream to String Conversion Example | Java interview topic

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Converting InputStream to String in Java has become very easy after introduction of Scanner class in Java 5 and due to development of several open source libraries like Apache commons IOUtils and Google Open source guava-libraries which provides excellent support to convert InputStream to String in Java program. we often need to convert InputStream to String  while working in Java for example if you are reading XML files from InputStream and later performing XSLT transformation on it or if InputStream is reading data from text file or text input Source and we either want to log  Strings in log file or want to operate on whole String. Before Java 5 you would have to write lots of boiler plate code to read String line by line or byte by byte depending upon whether you are using either BufferedReader or not but as I said since JDK 5 added Scanner for reading input, its fairly easy to convert InputStream into String.


Before Converting any InputStream or ByteStream into String don't forget to provide character encoding or charSet which tells Java Which characters to expect from those streams of bytes. in the absence of correct character encoding you might alter the output because same bytes can be used to represent different character in different encoding. Another thing to keep in mind is that if you don't provide character encoding, Default character encoding in Java will be used which can be specified from System property "file.encoding" or "UTF-8" if file.encoding is not specified. In this Java tutorial we will see 5 different example of converting InputStream to String in Java both by using standard JDK libraries and using open source libraries.

How to convert InputStream to String in Java – 5 Examples
here are different ways to convert InputStream to String in Java, first we will see most simple way of reading InputStream as String.

InputStream to String -  Using Java 5 Scanner
Convert InputStream to String in Java - 5 Example tutorialjava.util.Scanner has constructor which accept an InputStream, a character encoding and a delimiter to read String from InputStream. Here we have used delimiter as "\A" which is boundary match for beginning of  the input as declared in java.util.regex.Pattern and that's why Scanner is returning whole String form InputStream. I frequently use this technique to read input from user in Java using System.in which is most common example of InputStream in Java, but as demonstrated here this can also be used to read text file in Java.

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.util.Scanner;

/**
 * Java program example to demonstrate How to convert InputStream into String by using JDK
 * Scanner utility. This program will work Java 5 onwards as Scanner was added in Java 5.
 */       
public class InputStreamTest {

    public static void main(String args[]) throws FileNotFoundException {
        FileInputStream fis = new FileInputStream("c:/sample.txt");
        String inputStreamString = new Scanner(fis,"UTF-8").useDelimiter("\\A").next();
        System.out.println(inputStreamString);
    }
}

Output:
This String is read from InputStream by changing InputStream to String in Java.

If you want to see how an incorrect character encoding completely changes the String just change the character encoding form "UTF-8" to "UTF-16" and you see Chinese(may be) characters instead of English.

Output:
?????????????!???????????!??????^(4)????????

Convert InputStream to String - Plain old JDK4 Example
If you still need to do it on plain old Java on JDK4 without including any additional dependency in your PATH and Classpath than here is quick example using BufferedReader in Java, Remember you can also do this by reading byte by byte from InputStream but that's very slow so consider using BufferedReader for better performance even if you code on JDK4 . For more performance tips see my post 4 JDBC performance tips in Java program. Let’s see example of converting InputStream to String using BufferedReader in Java.

/**
 * Java program to demonstrate How to read InputStream as String
 * using BufferedReader and StringBuilder in Java.
 * This is old and standard way of converting an InputStream into String in Java
 */
public static void main(String args[]) throws FileNotFoundException, UnsupportedEncodingException, IOException {
        FileInputStream fis = new FileInputStream("c:/sample.txt");
        StringBuilder inputStringBuilder = new StringBuilder();
        BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(fis, "UTF-8"));
        String line = bufferedReader.readLine();
        while(line != null){
            inputStringBuilder.append(line);inputStringBuilder.append('\n');
            line = bufferedReader.readLine();
        }
        System.out.println(inputStringBuilder.toString());

}
Output:
This String is read from InputStream by changing InputStream to String in Java.
second line


This is good plain old Java method of converting InputStream to String without adding any extra dependency but remember its converting \r to \n because we are reading line by line, which in most cases fine.

Read InputStream to String - Using Apache IOUtils library
As I said earlire there are many open source library in Java which makes coding lot more easier than any other language. Here is code example for How to convert InputStream to String in Java using Apache IOUtils

/**
 * Example of How to read InputStream into String by using Apache IOUtils library.
 * Nice and clean way of getting InputStream as String in Java
 */

FileInputStream fis = new FileInputStream("c:/sample.txt");
String StringFromInputStream = IOUtils.toString(fis, "UTF-8");
System.out.println(StringFromInputStream);

Isn't it a compact way of converting InputStream to String in Java, just one line of code and that does take care of character encoding as well..

InputStream to String - Using Google's guava-libraries
Google has open source its own set of Java libraries they use for Java development inside Google. it has lots of utility function and also complements Apache commons package. Here is a quick way of converting InputStream to String using Google libraries:

String stringFromStream = CharStreams.toString(new InputStreamReader(fis, "UTF-8"));

Regarding dependency, You need to include guava library i.e. guava-11.0.1.jar in your project classpath. here is full code example:

import com.google.common.io.CharStreams;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStreamReader;
import java.io.UnsupportedEncodingException;

/**
 * How to convert InputStream into String in Java using Google's Guava library
 */
public class InputStreamToString{

    public static void main(String args[]) throws UnsupportedEncodingException, IOException {
        FileInputStream fis = new FileInputStream("c:/sample.txt");
        String stringFromStream = CharStreams.toString(new InputStreamReader(fis, "UTF-8"));
        System.out.println(stringFromStream);
    }   
}

How to read InputStream into String with IOUtils.copy and StringWriter class
java.io.StringWriter is another convenient way of reading writing Strings and by using IOUtils.copy() you can copy contents form InputStream to reader, Here is a complete code example of reading InputStream as String using StringWriter:

import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.StringWriter;
import java.io.UnsupportedEncodingException;
import org.apache.commons.io.IOUtils;

/**
  * Java Program to demonstrate reading of InputStream as String
  * using StringWriter and Apache IOUtils
  */
public class InputStreamToStringConversion{

    public static void main(String args[]) throws FileNotFoundException, UnsupportedEncodingException, IOException {
        FileInputStream fis = new FileInputStream("c:/sample.txt");
        StringWriter writer = new StringWriter();
        String encoding = "UTF-8";
        IOUtils.copy(fis, writer, encoding);
        System.out.println(writer.toString());
    }
}

That's all on converting InputStream to String in Java by using standard core java library and by using open source Apache commons IOUtils and Google's guava libraries. Don't forget Character encoding when converting bytes to String which is case here also make a convenient choice as sometime adding additional library for one functionality is not the best option. let us know if you know any other way of converting InputStream to String in Java.