How to Test if a File is PDF (Java)

Here is a simple java function to check if some data represents a PDF file. It is adapted from a C# function posted on Stack Overflow by NinjaCross.

    /**
     * Test if the data in the given byte array represents a PDF file.
     */
    public static boolean is_pdf(byte[] data) {
        if (data != null && data.length > 4 &&
                data[0] == 0x25 && // %
                data[1] == 0x50 && // P
                data[2] == 0x44 && // D
                data[3] == 0x46 && // F
                data[4] == 0x2D) { // -

            // version 1.3 file terminator
            if (data[5] == 0x31 && data[6] == 0x2E && data[7] == 0x33 &&
                    data[data.length - 7] == 0x25 && // %
                    data[data.length - 6] == 0x25 && // %
                    data[data.length - 5] == 0x45 && // E
                    data[data.length - 4] == 0x4F && // O
                    data[data.length - 3] == 0x46 && // F
                    data[data.length - 2] == 0x20 && // SPACE
                    data[data.length - 1] == 0x0A) { // EOL
                return true;
            }

            // version 1.3 file terminator
            if (data[5] == 0x31 && data[6] == 0x2E && data[7] == 0x34 &&
                    data[data.length - 6] == 0x25 && // %
                    data[data.length - 5] == 0x25 && // %
                    data[data.length - 4] == 0x45 && // E
                    data[data.length - 3] == 0x4F && // O
                    data[data.length - 2] == 0x46 && // F
                    data[data.length - 1] == 0x0A) { // EOL
                return true;
            }
        }
        return false;
    }

The function takes a byte[] to make it possible to test data in streams and files. To read a file into a byte array you can use java.nio.Files.readAllBytes, e.g:

    assertTrue(is_pdf(Files.readAllBytes(Paths.get("output.pdf"));

You'll have to handle the IOExceptions from those methods too. To test streamed output, just render to a ByteArrayOutputStream and call the getBytes() method after the output has rendered.

The unit tests for this function are below. Please feel free to contribute test cases, especially ones that fail.

    @Test
    public void test_valid_pdf_1_3_data_is_pdf() {
        assertTrue(is_pdf("%PDF-1.3 CONTENT %%EOF \n".getBytes()));
    }

    @Test
    public void test_valid_pdf_1_4_data_is_pdf() {
        assertTrue(is_pdf("%PDF-1.4 CONTENT %%EOF\n".getBytes()));
    }

    @Test
    public void test_invalid_data_is_not_pdf() {
        assertFalse(is_pdf("Hello World".getBytes()));
    }

About Roger Keays

Roger Keays is an artist, an engineer, and a student of life. He has no fixed address and has left footprints on 40-something different countries around the world. Roger is addicted to surfing. His other interests are music, psychology, languages, the proper use of semicolons, and finding good food.