1. Introduction
In this quick tutorial, we’ll see how we use Java to append data to the content of a file – in a few simple ways.
Let’s start with how we can do this using core Java’s FileWriter.
2. Using FileWriter
Here’s a simple test – reading an existing file, appending some text, and then making sure that got appended correctly:
@Test
public void whenAppendToFileUsingFileWriter_thenCorrect()
throws IOException {
FileWriter fw = new FileWriter(fileName, true);
BufferedWriter bw = new BufferedWriter(fw);
bw.write("Spain");
bw.newLine();
bw.close();
assertThat(getStringFromInputStream(
new FileInputStream(fileName)))
.isEqualTo("UK\r\n" + "US\r\n" + "Germany\r\n" + "Spain\r\n");
}
Note that FileWriter’s constructor accepts a boolean marking if we want to append data to an existing file.
If we set it to false, then the existing content will be replaced.
3. Using FileOutputStream
Next – let’s see how we can do the same operation – using FileOutputStream:
@Test
public void whenAppendToFileUsingFileOutputStream_thenCorrect()
throws Exception {
FileOutputStream fos = new FileOutputStream(fileName, true);
fos.write("Spain\r\n".getBytes());
fos.close();
assertThat(StreamUtils.getStringFromInputStream(
new FileInputStream(fileName)))
.isEqualTo("UK\r\n" + "US\r\n" + "Germany\r\n" + "Spain\r\n");
}
Similarly, the FileOutputStream constructor accepts a boolean that should be set to true to mark that we want to append data to an existing file.
4. Using java.nio.file
Next – we can also append content to files using functionality in java.nio.file – which was introduced in JDK 7:
@Test
public void whenAppendToFileUsingFiles_thenCorrect()
throws IOException {
String contentToAppend = "Spain\r\n";
Files.write(
Paths.get(fileName),
contentToAppend.getBytes(),
StandardOpenOption.APPEND);
assertThat(StreamUtils.getStringFromInputStream(
new FileInputStream(fileName)))
.isEqualTo("UK\r\n" + "US\r\n" + "Germany\r\n" + "Spain\r\n");
}
5. Using Guava
To start using Guava, we need to add its dependency to our pom.xml:
<dependency>
<groupId>com.google.guava</groupId>
<artifactId>guava</artifactId>
<version>31.0.1-jre</version>
</dependency>
Now, let’s see how we can start using Guava to append content to an existing file:
@Test
public void whenAppendToFileUsingFileWriter_thenCorrect()
throws IOException {
File file = new File(fileName);
CharSink chs = Files.asCharSink(
file, Charsets.UTF_8, FileWriteMode.APPEND);
chs.write("Spain\r\n");
assertThat(StreamUtils.getStringFromInputStream(
new FileInputStream(fileName)))
.isEqualTo("UK\r\n" + "US\r\n" + "Germany\r\n" + "Spain\r\n");
}
6. Using Apache Commons IO FileUtils
Finally – let’s see how we can append content to an existing file using Apache Commons IO FileUtils.
First, let’s add the Apache Commons IO dependency to our pom.xml:
<dependency>
<groupId>commons-io</groupId>
<artifactId>commons-io</artifactId>
<version>2.15.1</version>
</dependency>
Now, let’s see a quick example that demonstrates appending content to an existing file using FileUtils:
@Test
public void whenAppendToFileUsingFiles_thenCorrect()
throws IOException {
File file = new File(fileName);
FileUtils.writeStringToFile(
file, "Spain\r\n", StandardCharsets.UTF_8, true);
assertThat(StreamUtils.getStringFromInputStream(
new FileInputStream(fileName)))
.isEqualTo("UK\r\n" + "US\r\n" + "Germany\r\n" + "Spain\r\n");
}
7. Conclusion
In this article, we’ve seen how we can append content in multiple ways.
The full implementation of this tutorial can be found over on GitHub.