Cara Membaca File di Java

1. Ikhtisar

Dalam tutorial ini, kita akan menjelajahi berbagai cara untuk membaca dari File di Java .

Pertama, kita akan melihat bagaimana memuat file dari classpath, URL atau dari file JAR, menggunakan kelas Java standar.

Kedua, kita akan melihat cara membaca konten dengan BufferedReader , Scanner , StreamTokenizer , DataInputStream , SequenceInputStream, dan FileChannel . Selain itu, kami akan membahas cara membaca file yang dikodekan UTF-8.

Terakhir, kita akan mempelajari teknik baru untuk memuat dan membaca file di Java 7 dan Java 8.

Artikel ini adalah bagian dari seri “Java - Kembali ke Dasar” di Baeldung.

2. Penyiapan

2.1 Memasukkan File

Dalam kebanyakan contoh dalam artikel ini, kita akan membaca file teks dengan nama file fileTest.txt yang berisi satu baris:

Hello, world!

Dalam beberapa contoh, kami akan menggunakan file yang berbeda. Dalam kasus ini, kami akan menyebutkan file dan isinya secara eksplisit.

2.2 Metode Pembantu

Kami akan menggunakan sekumpulan contoh pengujian hanya menggunakan kelas inti Java, dan dalam pengujian, kami akan menggunakan pernyataan menggunakan Hamcrest matcher.

Pengujian akan membagikan metode readFromInputStream umum yang mengubah InputStream menjadi String agar lebih mudah menegaskan hasil:

private String readFromInputStream(InputStream inputStream) throws IOException { StringBuilder resultStringBuilder = new StringBuilder(); try (BufferedReader br = new BufferedReader(new InputStreamReader(inputStream))) { String line; while ((line = br.readLine()) != null) { resultStringBuilder.append(line).append("\n"); } } return resultStringBuilder.toString(); }

Perhatikan bahwa ada cara lain untuk mencapai hasil yang sama. Anda dapat membaca artikel ini untuk mengetahui beberapa alternatif.

3. Membaca File dari Classpath

3.1. Menggunakan Java Standar

Bagian ini menjelaskan cara membaca file yang tersedia di classpath. Kami akan membaca " fileTest.txt " yang tersedia di src / main / resources :

@Test public void givenFileNameAsAbsolutePath_whenUsingClasspath_thenFileData() { String expectedData = "Hello, world!"; Class clazz = FileOperationsTest.class; InputStream inputStream = clazz.getResourceAsStream("/fileTest.txt"); String data = readFromInputStream(inputStream); Assert.assertThat(data, containsString(expectedData)); }

Dalam cuplikan kode di atas, kami menggunakan kelas saat ini untuk memuat file menggunakan metode getResourceAsStream dan meneruskan jalur absolut file untuk dimuat.

Metode yang sama juga tersedia pada instance ClassLoader :

ClassLoader classLoader = getClass().getClassLoader(); InputStream inputStream = classLoader.getResourceAsStream("fileTest.txt"); String data = readFromInputStream(inputStream);

Kami mendapatkan classLoader dari kelas saat ini menggunakan getClass (). GetClassLoader () .

Perbedaan utamanya adalah saat menggunakan getResourceAsStream pada instance ClassLoader , jalur diperlakukan sebagai absolut mulai dari root classpath.

Saat digunakan pada instance Kelas , jalurnya bisa relatif ke paket, atau jalur absolut, yang diisyaratkan oleh garis miring di depan.

Tentu saja, perhatikan bahwa dalam praktiknya, aliran terbuka harus selalu ditutup , seperti InputStream dalam contoh kami:

InputStream inputStream = null; try { File file = new File(classLoader.getResource("fileTest.txt").getFile()); inputStream = new FileInputStream(file); //... } finally { if (inputStream != null) { try { inputStream.close(); } catch (IOException e) { e.printStackTrace(); } } }

3.2. Menggunakan Perpustakaan commons-io

Opsi umum lainnya adalah menggunakan kelas FileUtils dari paket commons-io :

@Test public void givenFileName_whenUsingFileUtils_thenFileData() { String expectedData = "Hello, world!"; ClassLoader classLoader = getClass().getClassLoader(); File file = new File(classLoader.getResource("fileTest.txt").getFile()); String data = FileUtils.readFileToString(file, "UTF-8"); assertEquals(expectedData, data.trim()); }

Di sini kita meneruskan objek File ke metode readFileToString () dari kelas FileUtils . Kelas utilitas ini berhasil memuat konten tanpa perlu menulis kode boilerplate apa pun untuk membuat instance InputStream dan membaca data.

Perpustakaan yang sama juga menawarkan IOUtilskelas:

@Test public void givenFileName_whenUsingIOUtils_thenFileData() { String expectedData = "Hello, world!"; FileInputStream fis = new FileInputStream("src/test/resources/fileTest.txt"); String data = IOUtils.toString(fis, "UTF-8"); assertEquals(expectedData, data.trim()); }

Di sini kita meneruskan objek FileInputStream ke metode toString () dari kelas IOUtils . Kelas utilitas ini berhasil memuat konten tanpa perlu menulis kode boilerplate apa pun untuk membuat instance InputStream dan membaca data.

4. Membaca dengan BufferedReader

Sekarang mari kita fokus pada berbagai cara untuk mengurai konten file.

Kami akan mulai dengan cara sederhana untuk membaca dari file menggunakan BufferedReader:

@Test public void whenReadWithBufferedReader_thenCorrect() throws IOException { String expected_value = "Hello, world!"; String file; BufferedReader reader = new BufferedReader(new FileReader(file)); String currentLine = reader.readLine(); reader.close(); assertEquals(expected_value, currentLine); }

Perhatikan bahwa readLine () akan mengembalikan null saat akhir file tercapai.

5. Membaca dari File Menggunakan Java NIO

Di JDK7 paket NIO diperbarui secara signifikan.

Mari kita lihat contoh menggunakan kelas Files dan metode readAllLines . The ReadAllLines Metode menerima Path.

Kelas path dapat dianggap sebagai upgrade java.io.File dengan beberapa operasi tambahan.

5.1. Membaca File Kecil

Kode berikut menunjukkan cara membaca file kecil menggunakan kelas File baru :

@Test public void whenReadSmallFileJava7_thenCorrect() throws IOException { String expected_value = "Hello, world!"; Path path = Paths.get("src/test/resources/fileTest.txt"); String read = Files.readAllLines(path).get(0); assertEquals(expected_value, read); }

Note that you can use the readAllBytes() method as well if you need binary data.

5.2. Reading a Large File

If we want to read a large file with Files class, we can use the BufferedReader:

The following code reads the file using the new Files class and BufferedReader:

@Test public void whenReadLargeFileJava7_thenCorrect() throws IOException { String expected_value = "Hello, world!"; Path path = Paths.get("src/test/resources/fileTest.txt"); BufferedReader reader = Files.newBufferedReader(path); String line = reader.readLine(); assertEquals(expected_value, line); }

5.3. Reading a File Using Files.lines()

JDK8 offers the lines() method inside the Files class. It returns a Stream of String elements.

Let’s look at an example of how to read data into bytes and decode using UTF-8 charset.

The following code reads the file using the new Files.lines():

@Test public void givenFilePath_whenUsingFilesLines_thenFileData() { String expectedData = "Hello, world!"; Path path = Paths.get(getClass().getClassLoader() .getResource("fileTest.txt").toURI()); Stream lines = Files.lines(path); String data = lines.collect(Collectors.joining("\n")); lines.close(); Assert.assertEquals(expectedData, data.trim()); }

Using Stream with IO channels like file operations, we need to close the stream explicitly using the close() method.

As we can see, the Files API offers another easy way to read the file contents into a String.

In the next sections, let's have a look at other, less common methods of reading a file, that may be appropriate in some situations.

6. Reading with Scanner

Next, let's use a Scanner to read from the File. Here, we'll use whitespace as the delimiter:

@Test public void whenReadWithScanner_thenCorrect() throws IOException { String file = "src/test/resources/fileTest.txt"; Scanner scanner = new Scanner(new File(file)); scanner.useDelimiter(" "); assertTrue(scanner.hasNext()); assertEquals("Hello,", scanner.next()); assertEquals("world!", scanner.next()); scanner.close(); }

Note that the default delimiter is the whitespace, but multiple delimiters can be used with a Scanner.

The Scanner class is useful when reading content from the console, or when the content contains primitive values, with a known delimiter (eg: a list of integers separated by space).

7. Reading with StreamTokenizer

Next, let's read a text file into tokens using a StreamTokenizer.

The way the tokenizer works is – first, we need to figure out what the next token is – String or number; we do that by looking at the tokenizer.ttype field.

Then, we'll read the actual token based on this type:

  • tokenizer.nval – if the type was a number
  • tokenizer.sval – if the type was a String

In this example we'll use a different input file which simply contains:

Hello 1

The following code reads from the file both the String and the number:

@Test public void whenReadWithStreamTokenizer_thenCorrectTokens() throws IOException { String file = "src/test/resources/fileTestTokenizer.txt"; FileReader reader = new FileReader(file); StreamTokenizer tokenizer = new StreamTokenizer(reader); // token 1 tokenizer.nextToken(); assertEquals(StreamTokenizer.TT_WORD, tokenizer.ttype); assertEquals("Hello", tokenizer.sval); // token 2 tokenizer.nextToken(); assertEquals(StreamTokenizer.TT_NUMBER, tokenizer.ttype); assertEquals(1, tokenizer.nval, 0.0000001); // token 3 tokenizer.nextToken(); assertEquals(StreamTokenizer.TT_EOF, tokenizer.ttype); reader.close(); }

Note how the end of file token is used at the end.

This approach is useful for parsing an input stream into tokens.

8. Reading with DataInputStream

We can use DataInputStream to read binary or primitive data type from a file.

The following test reads the file using a DataInputStream:

@Test public void whenReadWithDataInputStream_thenCorrect() throws IOException { String expectedValue = "Hello, world!"; String file; String result = null; DataInputStream reader = new DataInputStream(new FileInputStream(file)); int nBytesToRead = reader.available(); if(nBytesToRead > 0) { byte[] bytes = new byte[nBytesToRead]; reader.read(bytes); result = new String(bytes); } assertEquals(expectedValue, result); }

9. Reading with FileChannel

If we are reading a large file, FileChannel can be faster than standard IO.

The following code reads data bytes from the file using FileChannel and RandomAccessFile:

@Test public void whenReadWithFileChannel_thenCorrect() throws IOException { String expected_value = "Hello, world!"; String file = "src/test/resources/fileTest.txt"; RandomAccessFile reader = new RandomAccessFile(file, "r"); FileChannel channel = reader.getChannel(); int bufferSize = 1024; if (bufferSize > channel.size()) { bufferSize = (int) channel.size(); } ByteBuffer buff = ByteBuffer.allocate(bufferSize); channel.read(buff); buff.flip(); assertEquals(expected_value, new String(buff.array())); channel.close(); reader.close(); }

10. Reading a UTF-8 Encoded File

Now, let's see how to read a UTF-8 encoded file using BufferedReader. In this example, we'll read a file that contains Chinese characters:

@Test public void whenReadUTFEncodedFile_thenCorrect() throws IOException { String expected_value = "青空"; String file = "src/test/resources/fileTestUtf8.txt"; BufferedReader reader = new BufferedReader (new InputStreamReader(new FileInputStream(file), "UTF-8")); String currentLine = reader.readLine(); reader.close(); assertEquals(expected_value, currentLine); }

11. Reading Content from URL

To read content from a URL, we will use “/” URL in our example as:

@Test public void givenURLName_whenUsingURL_thenFileData() { String expectedData = "Baeldung"; URL urlObject = new URL("/"); URLConnection urlConnection = urlObject.openConnection(); InputStream inputStream = urlConnection.getInputStream(); String data = readFromInputStream(inputStream); Assert.assertThat(data, containsString(expectedData)); }

There are also alternative ways of connecting to a URL. Here we used the URL and URLConnection class available in the standard SDK.

12. Reading a File from a JAR

To read a file which is located inside a JAR file, we will need a JAR with a file inside it. For our example we will read “LICENSE.txt” from the “hamcrest-library-1.3.jar” file:

@Test public void givenFileName_whenUsingJarFile_thenFileData() { String expectedData = "BSD License"; Class clazz = Matchers.class; InputStream inputStream = clazz.getResourceAsStream("/LICENSE.txt"); String data = readFromInputStream(inputStream); Assert.assertThat(data, containsString(expectedData)); }

Here we want to load LICENSE.txt that resides in Hamcrest library, so we will use the Matcher's class that helps to get a resource. The same file can be loaded using the classloader too.

13. Conclusion

As you can see, there are many possibilities for loading a file and reading data from it using plain Java.

You can load a file from various locations like classpath, URL or jar files.

Kemudian Anda dapat menggunakan BufferedReader untuk membaca baris demi baris, Scanner untuk membaca menggunakan pembatas yang berbeda, StreamTokenizer untuk membaca file menjadi token, DataInputStream untuk membaca data biner dan tipe data primitif, SequenceInput Stream untuk menghubungkan beberapa file ke dalam satu aliran, FileChannel untuk membaca lebih cepat dari file besar, dll.

Anda dapat menemukan kode sumber di repo GitHub berikut.