Java - 从Reader到String

In this quick tutorial we’re going to convert a Reader into a String using plain Java, Guava and the Apache Commons IO library.

This article is part of the “Java – Back to Basic” series here on Baeldung.

1. With Java

Let’s start with a simple Java solution that reads characters sequentially from the Reader:

@Test
public void givenUsingPlainJava_whenConvertingReaderIntoStringV1_thenCorrect() 
  throws IOException {
    StringReader reader = new StringReader("text");
    int intValueOfChar;
    String targetString = "";
    while ((intValueOfChar = reader.read()) != -1) {
        targetString += (char) intValueOfChar;
    }
    reader.close();
}

If there is a lot of content to read, a bulk-read solution will work better:

@Test
public void givenUsingPlainJava_whenConvertingReaderIntoStringV2_thenCorrect() 
  throws IOException {
    Reader initialReader = new StringReader("text");
    char[] arr = new char[8 * 1024];
    StringBuilder buffer = new StringBuilder();
    int numCharsRead;
    while ((numCharsRead = initialReader.read(arr, 0, arr.length)) != -1) {
        buffer.append(arr, 0, numCharsRead);
    }
    initialReader.close();
    String targetString = buffer.toString();
}

2. With Guava

Guava provides a utility that can do the conversion directly:

@Test
public void givenUsingGuava_whenConvertingReaderIntoString_thenCorrect() 
  throws IOException {
    Reader initialReader = CharSource.wrap("With Google Guava").openStream();
    String targetString = CharStreams.toString(initialReader);
    initialReader.close();
}

3. With Commons IO

Same with Apache Commons IO – there is an IO utility capable of performing the direct conversion:

@Test
public void givenUsingCommonsIO_whenConvertingReaderIntoString_thenCorrect() 
  throws IOException {
    Reader initialReader = new StringReader("With Apache Commons");
    String targetString = IOUtils.toString(initialReader);
    initialReader.close();
}

And there you have it – 4 ways of transforming a Reader into a plain String. Make sure to check out the sample over on GitHub.