We can use the Guava library for appending to a file.

We need this Guava dependency:

<dependency>
    <groupId>com.google.guava</groupId>
    <artifactId>guava</artifactId>
    <version>22.0</version>
</dependency>

Java append to file with Guava

In the example, we append to a file with Guava's CharSink class.

import com.google.common.base.Charsets;
import com.google.common.io.CharSink;
import com.google.common.io.FileWriteMode;
import com.google.common.io.Files;
import java.io.File;
import java.io.IOException;

public class JavaAppendFileGuava {

    public static void main(String[] args) throws IOException {

        String fileName = "src/main/resources/sample.txt";

        File file = new File(fileName);

        CharSink chs = Files.asCharSink(file, Charsets.UTF_8, FileWriteMode.APPEND);
        chs.write(" Appended Data \n");
    }
}

The third parameter of Files.asCharSink() specifies the file writing mode; with FileWriteMode.APPEND option the file is opened for writing.

CharSink chs = Files.asCharSink(file, Charsets.UTF_8, FileWriteMode.APPEND);