Add constructors for FileDescriptors

src/main/java/com/mpatric/mp3agic/FileWrapper.java

@@ -1,55 +1,87 @@
 package com.mpatric.mp3agic;
 
-import java.io.File;
-import java.io.FileNotFoundException;
-import java.io.IOException;
+import java.io.*;
+import java.nio.channels.FileChannel;
+import java.nio.channels.SeekableByteChannel;
 import java.nio.file.Files;
 import java.nio.file.Path;
 import java.nio.file.Paths;
+import java.nio.file.StandardOpenOption;
 import java.util.concurrent.TimeUnit;
 
 public class FileWrapper {
 
-	protected Path path;
-	protected long length;
-	protected long lastModified;
+	private interface SeekableByteChannelFactory {
+		SeekableByteChannel createByteChannel() throws IOException;
+	}
+
+	private SeekableByteChannelFactory byteChannelFactory;
+	private String filename;
+	private Path absolutePath;
+	private long length;
+	private long lastModified;
 
 	protected FileWrapper() {
 	}
 
+	public FileWrapper(FileDescriptor fileDescriptor) throws IOException {
+		if (fileDescriptor == null) throw new NullPointerException();
+		if (!fileDescriptor.valid()) throw new IOException("FileDescriptor not valid");
+		length = -1;
+		lastModified = -1;
+		byteChannelFactory = () -> {
+			FileChannel channel = new FileInputStream(fileDescriptor).getChannel();
+			length = channel.size();
+			return channel;
+		};
+	}
+
 	public FileWrapper(String filename) throws IOException {
-		this.path = Paths.get(filename);
-		init();
+		this(Paths.get(filename));
 	}
 
 	public FileWrapper(File file) throws IOException {
-		if (file == null) throw new NullPointerException();
-		this.path = Paths.get(file.getPath());
-		init();
+		this(Paths.get(file.getPath()));
 	}
 
 	public FileWrapper(Path path) throws IOException {
 		if (path == null) throw new NullPointerException();
-		this.path = path;
-		init();
+		init(path);
 	}
 
-	private void init() throws IOException {
+	private void init(Path path) throws IOException {
 		if (!Files.exists(path)) throw new FileNotFoundException("File not found " + path);
 		if (!Files.isReadable(path)) throw new IOException("File not readable");
+		absolutePath = path.toAbsolutePath();
+		byteChannelFactory = () -> Files.newByteChannel(path, StandardOpenOption.READ);
+		filename = path.toString();
 		length = Files.size(path);
 		lastModified = Files.getLastModifiedTime(path).to(TimeUnit.MILLISECONDS);
 	}
 
 	public String getFilename() {
-		return path.toString();
+		if (filename == null) {
+			throw new IllegalArgumentException("Cannot get filename from a FileDescriptor");
+		}
+		return filename;
 	}
 
 	public long getLength() {
 		return length;
 	}
 
 	public long getLastModified() {
+		if (lastModified < 0) {
+			throw new IllegalArgumentException("Cannot get last modified from a FileDescriptor");
+		}
 		return lastModified;
 	}
+
+	protected SeekableByteChannel getByteChannel() throws IOException {
+		return byteChannelFactory.createByteChannel();
+	}
+
+	protected Path getAbsolutePath() {
+		return absolutePath;
+	}
 }

src/main/java/com/mpatric/mp3agic/Mp3File.java

@@ -1,13 +1,15 @@
 package com.mpatric.mp3agic;
 
 import java.io.File;
+import java.io.FileDescriptor;
 import java.io.IOException;
 import java.nio.ByteBuffer;
 import java.nio.channels.SeekableByteChannel;
 import java.nio.file.*;
 import java.util.HashMap;
 import java.util.Map;
 import java.util.EnumSet;
+import java.util.Objects;
 
 public class Mp3File extends FileWrapper {
 
@@ -71,6 +73,19 @@ public Mp3File(File file, int bufferLength, boolean scanFile) throws IOException
 		init(bufferLength, scanFile);
 	}
 
+	public Mp3File(FileDescriptor fileDescriptor) throws IOException, UnsupportedTagException, InvalidDataException {
+		this(fileDescriptor, DEFAULT_BUFFER_LENGTH, true);
+	}
+
+	public Mp3File(FileDescriptor fileDescriptor, int bufferLength) throws IOException, UnsupportedTagException, InvalidDataException {
+		this(fileDescriptor, bufferLength, true);
+	}
+
+	public Mp3File(FileDescriptor fileDescriptor, int bufferLength, boolean scanFile) throws IOException, UnsupportedTagException, InvalidDataException {
+		super(fileDescriptor);
+		init(bufferLength, scanFile);
+	}
+
 	public Mp3File(Path path) throws IOException, UnsupportedTagException, InvalidDataException {
 		this(path, DEFAULT_BUFFER_LENGTH, true);
 	}
@@ -90,7 +105,7 @@ private void init(int bufferLength, boolean scanFile) throws IOException, Unsupp
 		this.bufferLength = bufferLength;
 		this.scanFile = scanFile;
 
-		try (SeekableByteChannel seekableByteChannel = Files.newByteChannel(path, StandardOpenOption.READ)) {
+		try (SeekableByteChannel seekableByteChannel = getByteChannel()) {
 			initId3v1Tag(seekableByteChannel);
 			scanFile(seekableByteChannel);
 			if (startOffset < 0) {
@@ -440,7 +455,10 @@ public void removeCustomTag() {
 	}
 
 	public void save(String newFilename) throws IOException, NotSupportedException {
-		if (path.toAbsolutePath().compareTo(Paths.get(newFilename).toAbsolutePath()) == 0) {
+		if (getAbsolutePath() == null) {
+			throw new IllegalArgumentException("Unable to save files based on FileDescriptors");
+		}
+		if (Objects.equals(getAbsolutePath(), Paths.get(newFilename).toAbsolutePath())) {
 			throw new IllegalArgumentException("Save filename same as source filename");
 		}
 		try (SeekableByteChannel saveFile = Files.newByteChannel(Paths.get(newFilename), EnumSet.of(StandardOpenOption.CREATE, StandardOpenOption.TRUNCATE_EXISTING, StandardOpenOption.WRITE))) {
@@ -470,7 +488,7 @@ private void saveMpegFrames(SeekableByteChannel saveFile) throws IOException {
 		if (filePos < 0) return;
 		if (endOffset < filePos) return;
 		ByteBuffer byteBuffer = ByteBuffer.allocate(bufferLength);
-		try (SeekableByteChannel seekableByteChannel = Files.newByteChannel(path, StandardOpenOption.READ)) {
+		try (SeekableByteChannel seekableByteChannel = getByteChannel()) {
 			seekableByteChannel.position(filePos);
 			while (true) {
 				byteBuffer.clear();

src/test/java/com/mpatric/mp3agic/FileWrapperTest.java

@@ -5,9 +5,11 @@
 import java.io.File;
 import java.io.FileNotFoundException;
 import java.io.IOException;
+import java.nio.channels.SeekableByteChannel;
 import java.nio.file.InvalidPathException;
 import java.nio.file.Paths;
 
+import static com.mpatric.mp3agic.TestHelper.toFileDescriptor;
 import static org.junit.Assert.*;
 
 public class FileWrapperTest {
@@ -17,6 +19,15 @@ public class FileWrapperTest {
 	private static final String NON_EXISTENT_FILENAME = "just-not.there";
 	private static final String MALFORMED_FILENAME = "malformed.\0";
 
+	@Test
+	public void shouldReadValidFileDescriptor() throws IOException {
+		FileWrapper fileWrapper = new FileWrapper(toFileDescriptor(new File(VALID_FILENAME)));
+		assertEquals(fileWrapper.getLength(), -1); // length not valid until channel is opened
+		try (SeekableByteChannel channel = fileWrapper.getByteChannel()) {
+			assertEquals(fileWrapper.getLength(), VALID_FILE_LENGTH);
+		}
+	}
+
 	@Test
 	public void shouldReadValidFilename() throws IOException {
 		FileWrapper fileWrapper = new FileWrapper(VALID_FILENAME);
@@ -67,6 +78,11 @@ public void shouldFailForNullFilenameFile() throws IOException {
 		new FileWrapper((java.io.File) null);
 	}
 
+	@Test(expected = NullPointerException.class)
+	public void shouldFailForNullFileDescriptor() throws IOException {
+		new FileWrapper((java.io.FileDescriptor) null);
+	}
+
 	@Test(expected = NullPointerException.class)
 	public void shouldFailForNullPath() throws IOException {
 		new FileWrapper((java.nio.file.Path) null);