使用Path與Files操作文件

Path

Path用於表示目錄名，也可以是一個文件。路徑以根目錄開始的爲據對路徑，否則就是相對路徑。例如假設使用Linux系統：

//絕對路徑：/home/temp
Path absolute = Paths.get("/home", "temp");
Path absolute1 = Paths.get("/home/temp");
//相對路徑：document/work/test.txt
Path relative = Paths.get("document", "work", "test.txt");
Path relative1 = Paths.get("document/work/test.txt");

Paths.get方法接收一個或多個字符串，並使用系統默認的路徑分隔符（Linux爲/，Windows爲\）。Path類提供了一系列方法構建目錄結構。

• resolve(Path other), resolve(String other)：基於this與other構建新的Path，如果other爲絕對路徑，則返回other，否則返回this+other的拼接的路徑：

//path：/home
Path path = Paths.get("/home");
//other：/home/work
Path other = path.resolve("work");
//other1：/home/document
Path other1 = path.resolve("/home/document");
//other2：/home/work
Path other2 = path.resolve(Paths.get("work"));

• resolveSibling(Path other)，resolveSibling(String other)：基於this構建相對同級路徑。如果other是絕對路徑，返回other，否則返回this上級路徑+other。
• relativize(Path other)：返回other相對於this的相對路徑
• toAbsolutePath()：轉換爲絕對路徑。

//path：/home
Path path = Paths.get("/home/work");
//other：/home/document
Path other = path.resolveSibling("document");
//other1：/user/document
Path other1 = path.resolveSibling("/user/document");
//other2：/home/document/user
Path other2 = path.resolveSibling(Paths.get("document/user"));
//relative: ../document/user
Path relative = path.relativize(other2);
//absolute：/Volumns/ServerDevelop/home/work，其中/Volumns/ServerDevelop爲工作目錄
Path absolute = path.toAbsolutePath();

• getParent()：返回上級路徑，若沒有上級路徑則返回null。
• getFileName()：獲取路徑的最好一個元素，有可能是目錄或文件。
• getRoot()：獲取根路徑。
• toFile()：從該路徑創建一個File對象，注意，不是創建一個文件。

Files

Files類使得普通文件操作變得快捷。通過Path提供的文件路徑，可以直接從文件讀寫行、字節、字符串內容，也可以使用流的方式處理文件讀寫。

• Files.readAllLines(Path path)：從path中讀取全部行，返回List<String>；
• Files.readAllLines(Path path)：從path中讀取全部字節，返回byte數組；
• Files.newInputStream(Path path)：以path構建輸入流；
• Files.newOutputStream(Path path, OpenOption options)：以path構建輸出流，其中輸出的方式通過options配置。
• Files.createDirectory(Path path)：創建path目錄，注意指揮創建當級目錄，若創建失敗（如目錄已存在或上級路徑不存在）則會拋出異常。
• Files.createDirectories(Path path)：如果路徑中間目錄不存在這會將中間路徑也創建。
• Files.exists(Path path)：判斷路徑是否存在。
• Files.copy(Path fromPath, Path toPath, CopyOption options)：將fromPath複製到toPath且保留源文件，根據options決定toPath的行爲是覆蓋，複製文件屬性還是原子性移動文件（ATOMIC_MOVE）。
• Files.move(Path fromPath, Path toPath, CopyOption options)：將fromPath複製到toPath，不保留源文件，根據options決定toPath的行爲是覆蓋，複製文件屬性還是原子性移動文件（ATOMIC_MOVE）。
• Files.delete(Path path)：刪除path文件目錄或文件。文件不存在的情況會拋異常。
• Files.deleteIfExists(Path path)：刪除path文件目錄或文件，文件不存在不做任何操作。

public class FilesBasic {
    public static void main(String[] args) throws IOException {
        Path path = Paths.get("./DesignPattern/src/practise/lios/demo/alice.txt");
        //按行讀取全部內容
        List<String> lines = Files.readAllLines(path);
        lines.stream().limit(10).forEach(System.out::println);

        //按字節讀取全部內容
        byte[] bytes = Files.readAllBytes(path);
        String bytesToString = new String(bytes, 0 , 100);
        System.out.println("bytes:\n" + bytesToString);

        //通過InputStream讀取內容
        InputStream inputStream = Files.newInputStream(path);
        Scanner inputScanner = new Scanner(inputStream, StandardCharsets.UTF_8);
        final int maxLine = 10;
        int line = 0;
        System.out.println("Input Stream:");
        while(inputScanner.hasNextLine() && line < maxLine) {
            String contents = inputScanner.nextLine();
            line ++;
            System.out.println(contents);
        }

        //通過OutputStream寫內容到文件
        Path outputDirectory = Paths.get("output");
        //目錄不存在的話創建目錄
        if (! Files.exists(outputDirectory)) {
            Files.createDirectory(outputDirectory);
        }
        //將輸出文件放入到新加的目錄下
        Path outPath = outputDirectory.resolve("output.txt");
        boolean fileExists = Files.exists(outPath);
        OutputStream outputStream;
        if (fileExists) {
            outputStream = Files.newOutputStream(outPath, StandardOpenOption.APPEND);
        } else {
            outputStream = Files.newOutputStream(outPath, StandardOpenOption.CREATE);
        }
        outputStream.write(bytes, 0, 100);
        
        //文件移動、複製及刪除
        Path parentPath = outputDirectory.toAbsolutePath().getParent();
        if (parentPath != null) {
            Path destinationFilePath = parentPath.resolve("output.txt");
            //如果文件存在則替換掉
            Path copyPath = Files.copy(outPath, destinationFilePath, StandardCopyOption.REPLACE_EXISTING);

            Path movedFilePath = parentPath.resolve("output1.txt");
            Path movedPath = Files.move(outPath, movedFilePath, StandardCopyOption.REPLACE_EXISTING);

            Files.delete(outputDirectory);
        }
    }
}

Files獲取文件的基本信息

可以通過Files的方法獲取文件的如下屬性：

• 是否存在、是否是常規文件、是否是鏈接、是否是目錄；
• 是否可讀，是否可寫，是否可執行；
• 文件大小
• 文件的創建、最近修改和最近訪問時間

public static void showFileInfo(Path path) throws IOException {
    boolean fileExists = Files.exists(path);
    System.out.println("File exists: " + fileExists);
    if (fileExists) {
        boolean isHidden = Files.isHidden(path);
        System.out.println("File is hidden: " + isHidden);

        boolean isReadable = Files.isReadable(path);
        System.out.println("File is readable: " + isReadable);

        boolean isWritable = Files.isWritable(path);
        System.out.println("File is writable: " + isWritable);

        boolean isExecutable = Files.isExecutable(path);
        System.out.println("File is executable: " + isExecutable);

        boolean isDirectory = Files.isDirectory(path);
        System.out.println("File is directory: " + isDirectory);

        boolean isSymbolicLink = Files.isSymbolicLink(path);
        System.out.println("File is symbolic link: " + isSymbolicLink);

        boolean isRegularFile = Files.isRegularFile(path);
        if (isRegularFile) {
            long fileSize = Files.size(path);
            System.out.printf("File size: %.1fkB \n",(float)fileSize / 1024);
            //BasicFileAttributes包含上述的全部屬性封裝
            BasicFileAttributes attributes = Files.readAttributes(path, BasicFileAttributes.class);
            System.out.println("File size: " + attributes.size() + "bytes");
            System.out.println("File created at: " + attributes.creationTime());
            System.out.println("File last modified at: " + attributes.lastModifiedTime());
            System.out.println("File last accessed at: " + attributes.lastAccessTime());
        }
    }
}

目錄遍歷

Files.list(Path path)方法將遍歷path下的目錄（不包含下下級目錄），返回Stream<Path>對象；Files.walk(Path path)返回path下的全部目錄（包含全部子孫目錄），返回Stream<Path>對象。

try (Stream<Path> entries = Files.list(path)) {
    entries.forEach(p -> {
        if (Files.isDirectory(p)) {
            System.out.println("Directory: " + p);
        } else {
            System.out.println("File: " + p);
        }
    });
}

try (Stream<Path> entries = Files.walk(path)) {
    entries.forEach(p -> {
        if (Files.isDirectory(p)) {
            System.out.println("Directory: " + p);
        } else {
            System.out.println("File: " + p);
        }
    });
}

文件訪問效率

可以通過使用BufferedInputStream或FileChannel的map方法將文件映射到內存中，從而提高訪問效率。其中FileChannel的map方法支持隨機訪問文件內容。

public class MemoryMapFile {
    public static long checkSumWithInputStream(Path filename) throws IOException {
        try (InputStream inputStream = Files.newInputStream(filename)) {
            CRC32 crc32 = new CRC32();

            int c;
            while((c = inputStream.read()) != -1) {
                crc32.update(c);
            }

            return crc32.getValue();
        }
    }

    public static long checkSumWithBufferedInputStream(Path filename) throws IOException {
        try (BufferedInputStream inputStream = new BufferedInputStream(Files.newInputStream(filename))) {
            CRC32 crc32 = new CRC32();

            int c;
            while((c = inputStream.read()) != -1) {
                crc32.update(c);
            }

            return crc32.getValue();
        }
    }

    public static long checkSumWithRandomAccessFile(Path filename) throws IOException {
        try (RandomAccessFile file = new RandomAccessFile(filename.toFile(), "r")) {
            long length = file.length();

            CRC32 crc32 = new CRC32();
            for (long i = 0; i < length; ++i) {
                file.seek(i);
                int c = file.readByte();
                crc32.update(c);
            }

            return crc32.getValue();
        }
    }

    public static long checkSumWithMappedFile(Path filename) throws IOException {
        try (FileChannel fileChannel = FileChannel.open(filename)) {
            CRC32 crc32 = new CRC32();
            int length = (int)fileChannel.size();
            MappedByteBuffer buffer = fileChannel.map(FileChannel.MapMode.READ_ONLY, 0, length);

            for (int i = 0; i < length; ++i) {
                int c = buffer.get(i);
                crc32.update(c);
            }

            return crc32.getValue();
        }
    }

    public static void main(String[] args) throws IOException {
        Path path = Paths.get("./DesignPattern/src/practise/lios/demo/alice.txt");

        long start = System.currentTimeMillis();
        long crcValue = checkSumWithInputStream(path);
        long end = System.currentTimeMillis();
        System.out.println("CRC value: " + Long.toHexString(crcValue));
        System.out.println("InputStream Running time: " + (end - start) + " milliseconds");

        start = System.currentTimeMillis();
        crcValue = checkSumWithBufferedInputStream(path);
        end = System.currentTimeMillis();
        System.out.println("CRC value: " + Long.toHexString(crcValue));
        System.out.println("BufferedInputStream Running time: " + (end - start) + " milliseconds");

        start = System.currentTimeMillis();
        crcValue = checkSumWithRandomAccessFile(path);
        end = System.currentTimeMillis();
        System.out.println("CRC value: " + Long.toHexString(crcValue));
        System.out.println("RandomAccessFile Running time: " + (end - start) + " milliseconds");

        start = System.currentTimeMillis();
        crcValue = checkSumWithMappedFile(path);
        end = System.currentTimeMillis();
        System.out.println("CRC value: " + Long.toHexString(crcValue));
        System.out.println("MappedFile Running time: " + (end - start) + " milliseconds");
    }
}

運行結果如下：

CRC value: 11891d02
InputStream Running time: 185 milliseconds
CRC value: 11891d02
BufferedInputStream Running time: 9 milliseconds
CRC value: 11891d02
RandomAccessFile Running time: 221 milliseconds
CRC value: 11891d02
MappedFile Running time: 5 milliseconds

可以看到文件訪問的效率爲：MappedFile > BufferInputStream > InputStream > RandomAccessFile