Summary of the method of obtaining file suffix name in Java

Preface

In Java development, especially in web applications or file processing scenarios, obtaining file suffix names is a high-frequency requirement. Whether it is file upload verification, type filtering, format verification, or logging, the correct extraction of the suffix name is the core basis.

1. Basic method: lastIndexOf + substring

definition: By positioning the last one.position, intercept the suffix name.

Enter the ginseng：MultipartFile file

rely:none

Code Example：

public static String getExtension(MultipartFile file) {
    if (file == null) {
        return "";
    }
    String originalFilename = ();
    if (originalFilename == null || ()) {
        return "";
    }
    int dotIndex = ('.');
    if (dotIndex == -1 || dotIndex == () - 1) {
        return "";
    }
    return (dotIndex + 1).toLowerCase();
}

Execution effect：

• file namereport_v2.→ Returnxlsx
• file nameimage.→ Return empty string
• file name.gitignore→ Returngitignore

2. String segmentation: split method

definition:pass.Split the file name and take the last element.

Enter the ginseng：MultipartFile file

rely:none

Code Example：

public static String getExtension(MultipartFile file) {
    if (file == null) {
        return "";
    }
    String originalFilename = ();
    if (originalFilename == null) {
        return "";
    }
    String[] parts = ("\\.");
    return  > 1 ? parts[ - 1].toLowerCase() : "";
}

Execution effect：

• file namearchive.v1.7z→ Return7z
• file namefile→ Return empty string

3. Regular expressions

definition: match the last one by regular.The string after that.

Enter the ginseng：MultipartFile file

​​​​​​​rely:none

Code Example：

public static String getExtension(MultipartFile file) {
    if (file == null) {
        return "";
    }
    String originalFilename = ();
    if (originalFilename == null) {
        return "";
    }
    Pattern pattern = ("\\.(\\w+)$");
    Matcher matcher = (originalFilename);
    return () ? (1).toLowerCase() : "";
}

Execution effect：

• file name→ Returnpdf
• file name→ Returngz

4. FilenameUtils of Apache Commons IO

definition: Tool class that uses Apache Commons IO.

Enter the ginseng：MultipartFile file

​​​​​​​rely：

<dependency>
    <groupId></groupId>
    <artifactId>commons-io</artifactId>
    <version>9.0.0</version>
</dependency>

Code Example：

import ;

public static String getExtension(MultipartFile file) {
    if (file == null) {
        return "";
    }
    String originalFilename = ();
    return (originalFilename).toLowerCase();
}

Execution effect：

• file name→ Returntxt
• file namefile.→ Return empty string

5. StringUtils of Spring Framework

definition: Use Spring's tool class.

Enter the ginseng：MultipartFile file

​​​​​​​rely：

<dependency>
    <groupId></groupId>
    <artifactId>spring-core</artifactId>
    <version>6.0.10</version>
</dependency>

Code Example：

import ;

public static String getExtension(MultipartFile file) {
    if (file == null) {
        return "";
    }
    String originalFilename = ();
    return (originalFilename);
}

Execution effect：

• file name→ Returnjpg
• The return value isnullPay attention to null pointers (such asfileNo extension).

6. Use the File class

definition: Through structureFileObject extract file name.

Enter the ginseng：MultipartFile file

​​​​​​​rely:none

Code Example：

import ;

public static String getExtension(MultipartFile file) {
    if (file == null) {
        return "";
    }
    String originalFilename = ();
    if (originalFilename == null) {
        return "";
    }
    File tempFile = new File(originalFilename);
    String name = ();
    int dotIndex = ('.');
    return (dotIndex > 0 && dotIndex < () - 1) 
        ? (dotIndex + 1).toLowerCase() 
        : "";
}

Execution effect：

• file name→ Returncsv
• file nameconfig.→ Return empty string

7. Java NIO's Paths class

definition:passPacket processing path.

Enter the ginseng：MultipartFile file

​​​​​​​rely:none

Code Example：

import ;
import ;

public static String getExtension(MultipartFile file) {
    if (file == null) {
        return "";
    }
    String originalFilename = ();
    if (originalFilename == null) {
        return "";
    }
    Path path = (originalFilename);
    String name = ().toString();
    return (('.') + 1).toLowerCase();
}

Execution effect：

• file name→ Returnpdf
• Good cross-platform compatibility (such as path/home/→ Extracttxt）。

8. Combined with MIME type verification

definition:pass()Verify the suffix.

Enter the ginseng：MultipartFile file

​​​​​​​rely:none

Code Example：

public static boolean validateFile(MultipartFile file) {
    String ext = getExtension(file); // Use any of the above methods    String contentType = ();
    return ("jpg") && ("image/jpeg");
}

Execution effect：

• file nameAnd MIME isimage/jpeg→ Returntrue
• file nameBut MIME isimage/jpeg→ Returnfalse

9. Custom enumeration filtering

definition: The allowed suffix is ​​limited by enumeration.

Enter the ginseng：MultipartFile file

​​​​​​​Code Example：

enum FileType {
    PNG, JPG, JPEG, GIF;

    public static boolean isValid(String ext) {
        for (FileType type : ()) {
            if (().equalsIgnoreCase(ext)) {
                return true;
            }
        }
        return false;
    }
}

public static String getValidExtension(MultipartFile file) {
    String ext = getExtension(file);
    return (ext) ? ext : "";
}

Execution effect：

• file name→ Returnjpg
• file name→ Return empty string

10. Use Lombok's @Cleanup to simplify the code

definition: Reduce resource management code with Lombok.

Enter the ginseng：MultipartFile file

​​​​​​​rely：

<dependency>
    <groupId></groupId>
    <artifactId>lombok</artifactId>
    <version>1.18.28</version>
    <scope>provided</scope>
</dependency>

Code Example：

import ;

public static String getExtension(MultipartFile file) {
    if (file == null) {
        return "";
    }
    @Cleanup
    InputStream inputStream = ();
    // Other logic (such as reading file content)    return "ext"; // Example return value}

Execution effect：

• Automatically closeinputStream, no need to manually manage resources.

Method comparison table

Best Practice Recommendations

1. Dependency selection：

   • Zero dependency scenario: PrioritylastIndexOforsplit。
   • Complex requirements: Using Apache Commons IO or SpringStringUtils。

2. Boundary situation handling：

   • File name with.The beginning (such as.gitignore): Return the suffixgitignore。
   • File name has no extension (such asfile): Returns an empty string.
   • File name with.Ending (such asfile.): Returns an empty string.

3. Suffix is ​​uniform lowercase：

   • usetoLowerCase()Avoid case sensitive issues (such asJPG → jpg）。

4. Security Verification：

   • Combining MIME type and suffix double verification (such asimage/jpegandjpg）。

Summarize

ForMultipartFileGet the suffix name of  , and select it in combination with the project requirements:

• Basic scene:recommendlastIndexOfor Apache Commons IO.
• Spring Ecosystem: Use directly。
• Safety-sensitive scenarios: Combined with MIME type verification.

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