如何用java复制文件

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.