Hur man läser en fil i Java

1. Översikt

I den här handledningen utforskar vi olika sätt att läsa från en fil i Java .

Först får vi se hur man laddar en fil från klassvägen, en URL eller från en JAR-fil med standard Java-klasser.

För det andra får vi se hur man läser innehållet med BufferedReader , Scanner , StreamTokenizer , DataInputStream , SequenceInputStream och FileChannel . Vi kommer också att diskutera hur man läser en UTF-8-kodad fil.

Slutligen utforskar vi de nya teknikerna för att ladda och läsa en fil i Java 7 och Java 8.

Den här artikeln är en del av "Java - Back to Basic" -serien här på Baeldung.

2. Inställning

2.1 Inmatningsfil

I de flesta exempel i den här artikeln läser vi en textfil med filnamn fileTest.txt som innehåller en rad:

Hello, world!

I några exempel använder vi en annan fil. I dessa fall nämner vi filen och dess innehåll uttryckligen.

2.2 Hjälparmetod

Vi använder en uppsättning testexempel med endast Java-klasser och i testerna använder vi påståenden med Hamcrest-matchare.

Tester delar en gemensam readFromInputStream- metod som förvandlar en InputStream till String för att underlätta resultat:

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(); }

Observera att det finns andra sätt att uppnå samma resultat. Du kan läsa den här artikeln för några alternativ.

3. Läsa en fil från klassvägen

3.1. Använda Standard Java

Detta avsnitt förklarar hur man läser en fil som är tillgänglig på en klassväg. Vi läser " fileTest.txt " tillgänglig under 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)); }

I kodavsnittet ovan använde vi den nuvarande klassen för att ladda en fil med getResourceAsStream- metoden och skickade den absoluta sökvägen för filen att ladda.

Samma metod finns också i en ClassLoader- instans:

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

Vi får klassLoader för den aktuella klassen med getClass (). GetClassLoader () .

Huvudskillnaden är att när du använder getResourceAsStream på en ClassLoader- instans, behandlas sökvägen som absolut från början av klassvägen.

När den används mot en klass instans , kan banan vara relativt förpackningen, eller en absolut sökväg, vilket antyds genom den ledande snedstreck.

Observera naturligtvis att öppna strömmar i praktiken alltid ska stängas , till exempel InputStream i vårt exempel:

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. Använda commons-io Library

Ett annat vanligt alternativ är att använda FileUtils- klassen i commons-io- paketet:

@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()); }

Här skickar vi File- objektet till metoden readFileToString () i FileUtils- klassen. Denna verktygsklass lyckas ladda innehållet utan att behöva skriva någon pannkod för att skapa en InputStream- instans och läsa data.

Samma bibliotek erbjuder också IOUtilsklass:

@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()); }

Här skickar vi FileInputStream- objektet till metoden toString () för IOUtils- klassen. Denna verktygsklass lyckas ladda innehållet utan att behöva skriva någon pannkod för att skapa en InputStream- instans och läsa data.

4. Läsning med BufferedReader

Låt oss nu fokusera på olika sätt att analysera innehållet i en fil.

We'll start with a simple way to read from a file using 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); }

Note that readLine() will return null when the end of the file is reached.

5. Reading from a File Using Java NIO

In JDK7 the NIO package was significantly updated.

Let’s look at an example using the Files class and the readAllLines method. The readAllLines method accepts a Path.

Path class can be considered as an upgrade of the java.io.File with some additional operations in place.

5.1. Reading a Small File

The following code shows how to read a small file using the new Files class:

@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.

Sedan kan du använda BufferedReader för att läsa rad för rad, Scanner för att läsa med olika avgränsare, StreamTokenizer för att läsa en fil i tokens, DataInputStream för att läsa binära data och primitiva datatyper, SequenceInput Stream för att länka flera filer till en ström, FileChannel för att läsa snabbare från stora filer etc.

Du hittar källkoden i följande GitHub-repo.