Getting a File’s Mime Type in Java

1. Overview

In this tutorial, we’ll take a look at various strategies for getting MIME types of a file. We’ll look at ways to extend the MIME types available to the strategies, wherever applicable.

We’ll also point out where we should favor one strategy over the other.

2. Using Java 7

Let’s start with Java 7 – which provides the method Files.probeContentType(path) for resolving the MIME type:

@Test
public void whenUsingJava7_thenSuccess() {
    Path path = new File("product.png").toPath();
    String mimeType = Files.probeContentType(path);
 
    assertEquals(mimeType, "image/png");
}

This method makes use of the installed FileTypeDetector implementations to probe the MIME type. It invokes the probeContentType of each implementation to resolve the type.

Now, if the file is recognized by any of the implementations, the content type is returned. However, if that doesn’t happen, a system-default file type detector is invoked.

However, the default implementations are OS-specific and might fail depending on the OS that we are using.

In addition to that, it’s also important to note that the strategy will fail if the file isn’t present in the filesystem. Furthermore, if the file doesn’t have an extension, it will fail.

 3. Using URLConnection

URLConnection provides several APIs for detecting MIME types of a file. Let’s briefly explore each of them.

3.1. Using getContentType()

We can use the getContentType() method of URLConnection to retrieve a file’s MIME type:

@Test
public void whenUsingGetContentType_thenSuccess(){
    File file = new File("product.png");
    URLConnection connection = file.toURL().openConnection();
    String mimeType = connection.getContentType();
 
    assertEquals(mimeType, "image/png");
}

However, a major drawback of this approach is that it’s very slow.

3.2. Using guessContentTypeFromName()

Next, let’s see how we can make use of the guessContentTypeFromName() for the purpose:

@Test
public void whenUsingGuessContentTypeFromName_thenSuccess(){
    File file = new File("product.png");
    String mimeType = URLConnection.guessContentTypeFromName(file.getName());
 
    assertEquals(mimeType, "image/png");
}

This method makes use of the internal FileNameMap to resolve the MIME type from the extension.

We also have the option of using guessContentTypeFromStream() instead, which uses the first few characters of the input stream, to determine the type.

3.3. Using getFileNameMap()

A faster way to obtain the MIME type using URLConnection is using the getFileNameMap() method:

@Test
public void whenUsingGetFileNameMap_thenSuccess(){
    File file = new File("product.png");
    FileNameMap fileNameMap = URLConnection.getFileNameMap();
    String mimeType = fileNameMap.getContentTypeFor(file.getName());
 
    assertEquals(mimeType, "image/png");
}

The method returns the table of MIME types used by all instances of URLConnection. This table is then used to resolve the input file type.

The built-in table of MIME types is very limited when it comes to URLConnection.

By default, the class uses the content-types.properties file in JRE_HOME/lib. We can, however, extend it, by specifying a user-specific table using the content.types.user.table property:

System.setProperty("content.types.user.table","<path-to-file>");

4. Using MimeTypesFileTypeMap

MimeTypesFileTypeMap resolves MIME types by using the file’s extension. This class came with Java 6 and hence comes very handy when we’re working with JDK 1.6.

Now let’s see how to use it:

@Test
public void whenUsingMimeTypesFileTypeMap_thenSuccess() {
    File file = new File("product.png");
    MimetypesFileTypeMap fileTypeMap = new MimetypesFileTypeMap();
    String mimeType = fileTypeMap.getContentType(file.getName());
 
    assertEquals(mimeType, "image/png");
}

Here, we can either pass the name of the file or the File instance itself as the parameter to the function. However, the function with File instance as the parameter internally calls the overloaded method that accepts the filename as the parameter.

Internally, this method looks up a file called mime.types for the type resolution. It’s very important to note that the method searches for the file in a specific order:

1. Programmatically added entries to the MimetypesFileTypeMap instance
2. .mime.types in the user’s home directory
3. <java.home>/lib/mime.types
4. resources named META-INF/mime.types
5. resource named META-INF/mimetypes.default (usually found only in the activation.jar file)

However, if no file is found, it will return application/octet-stream as the response.

5. Using jMimeMagic

jMimeMagic is a restrictively licensed library that we can use to obtain the MIME type of a file.

Let’s start by configuring the Maven dependency:

<dependency>
    <groupId>net.sf.jmimemagic</groupId>
    <artifactId>jmimemagic</artifactId>
    <version>0.1.5</version>
</dependency>

We can find the latest version of this library on Maven Central.

Next, we’ll explore how to work with the library:

@Test    
public void whenUsingJmimeMagic_thenSuccess() {
    File file = new File("product.png");
    Magic magic = new Magic();
    MagicMatch match = magic.getMagicMatch(file, false);
 
    assertEquals(match.getMimeType(), "image/png");
}

This library can work with a stream of data and hence doesn’t require the file to be present in the file system.

6. Using Apache Tika

Apache Tika is a toolset that detects and extracts metadata and text from a variety of files. It has a rich and powerful API and comes with tika-core, which we can use to detect the MIME type of file.

Let’s begin by configuring the Maven dependency:

<dependency>
    <groupId>org.apache.tika</groupId>
    <artifactId>tika-core</artifactId>
    <version>1.18</version>
</dependency>

Next, we’ll make use of the detect() method to resolve the type:

@Test
public void whenUsingTika_thenSuccess() {
    File file = new File("product.png");
    Tika tika = new Tika();
    String mimeType = tika.detect(file);
 
    assertEquals(mimeType, "image/png");
}

The library relies on magic markers in the stream prefix, for type resolution.

7. Using Spring’s MediaTypeFactory

MediaTypeFactory is a part of Spring’s web module which provides methods to handle media types. We’ll use its getMediaType() method to get the media type of a file based on its name.

Let’s begin by configuring the Maven dependency:

<dependency>
    <groupId>org.springframework</groupId>
    <artifactId>spring-web</artifactId>
    <version>6.1.6</version>
</dependency>

Now, let’s see how we can use MediaTypeFactory to retrieve the file MIME type:

@Test
public void whenUsingSpringMediaTypeFactory_thenSuccess() {
    final File file = new File("product.png");
    Optional<MediaType> mimeTypeOptional = MediaTypeFactory.getMediaType(file.getName());
    assertTrue(mimeTypeOptional.isPresent());
    assertEquals(mimeTypeOptional.get().toString(), PNG_EXT);
}

The method getMediaType() returns an optional value which may contain a non-null value. We’re using the isPresent() method to prevent the potential errors caused by accessing a null value. This ensures the value exists before we try to access it and avoids unexpected errors.

8. Conclusion

In this article, we’ve looked at the various strategies for obtaining the MIME type of file. Furthermore, we have also analyzed the tradeoffs of the approaches. We have also pointed out the scenarios where we should favor one strategy over the other.

As always, the code used in the examples is available over on GitHub.