1. Overview
In this article, we’ll cover common ways of copying files in Java.
First, we’ll use the standard IO and NIO.2 APIs, and two external libraries: commons-io and guava.
2. IO API (Before JDK7)
First of all, to copy a file with java.io API, we’re required to open a stream, loop through the content and write it out to another stream:
@Test
public void givenIoAPI_whenCopied_thenCopyExistsWithSameContents()
throws IOException {
File copied = new File("src/test/resources/copiedWithIo.txt");
try (
InputStream in = new BufferedInputStream(
new FileInputStream(original));
OutputStream out = new BufferedOutputStream(
new FileOutputStream(copied))) {
byte[] buffer = new byte[1024];
int lengthRead;
while ((lengthRead = in.read(buffer)) > 0) {
out.write(buffer, 0, lengthRead);
out.flush();
}
}
assertThat(copied).exists();
assertThat(Files.readAllLines(original.toPath())
.equals(Files.readAllLines(copied.toPath())));
}
Quite a lot of work to implement such basic functionality.
Luckily for us, Java has improved its core APIs and we have a simpler way of copying files using NIO.2 API.
3. NIO.2 API (JDK7)
Using NIO.2 can significantly increase file copying performance since the NIO.2 utilizes lower-level system entry points.
Let’s take a closer look at how the Files.copy() method works.
The copy() method gives us the ability to specify an optional argument representing a copy option. By default, copying files and directories won’t overwrite existing ones, nor will it copy file attributes.
This behavior can be changed using the following copy options:
- REPLACE_EXISTING – replace a file if it exists
- COPY_ATTRIBUTES – copy metadata to the new file
- NOFOLLOW_LINKS – shouldn’t follow symbolic links
The NIO.2 Files class provides a set of overloaded copy() methods for copying files and directories within the file system.
Let’s take a look at an example using copy() with two Path arguments:
@Test
public void givenNIO2_whenCopied_thenCopyExistsWithSameContents()
throws IOException {
Path copied = Paths.get("src/test/resources/copiedWithNio.txt");
Path originalPath = original.toPath();
Files.copy(originalPath, copied, StandardCopyOption.REPLACE_EXISTING);
assertThat(copied).exists();
assertThat(Files.readAllLines(originalPath)
.equals(Files.readAllLines(copied)));
}
Note that directory copies are shallow, meaning that files and sub-directories within the directory are not copied.
4. Apache Commons IO
Another common way to copy a file with Java is by using the commons-io library.
First, we need to add the dependency:
<dependency>
<groupId>commons-io</groupId>
<artifactId>commons-io</artifactId>
<version>2.15.1</version>
</dependency>
The latest version can be downloaded from Maven Central.
Then, to copy a file we just need to use the copyFile() method defined in the FileUtils class. The method takes a source and a target file.
Let’s take a look at a JUnit test using the copyFile() method:
@Test
public void givenCommonsIoAPI_whenCopied_thenCopyExistsWithSameContents()
throws IOException {
File copied = new File(
"src/test/resources/copiedWithApacheCommons.txt");
FileUtils.copyFile(original, copied);
assertThat(copied).exists();
assertThat(Files.readAllLines(original.toPath())
.equals(Files.readAllLines(copied.toPath())));
}
5. Guava
Finally, we’ll take a look at Google’s Guava library.
Again, if we want to use Guava, we need to include the dependency:
<dependency>
<groupId>com.google.guava</groupId>
<artifactId>guava</artifactId>
<version>31.0.1-jre</version>
</dependency>
The latest version can be found on Maven Central.
And here’s the Guava’s way of copying a file:
@Test
public void givenGuava_whenCopied_thenCopyExistsWithSameContents()
throws IOException {
File copied = new File("src/test/resources/copiedWithGuava.txt");
com.google.common.io.Files.copy(original, copied);
assertThat(copied).exists();
assertThat(Files.readAllLines(original.toPath())
.equals(Files.readAllLines(copied.toPath())));
}
6. Conclusion
In this article, we explored the most common ways to copy a file in Java.
The full implementation of this article can be found over on Github.
