15.10 Java7的NIO.2 15.10.1 Path Paths和Files核心API

发表于 更新于 分类于 ， ， Waline： 本文字数： 12k 阅读时长 ≈ 11 分钟

15.10 Java7的NIO.2

Java7对原有的NIO进行了重大改进,改进主要包括如下两方面的内容。

  • 提供了全面的文件IO和文件系统访问支持。
  • 基于异步Channel的IO。

第一个改进表现为Java7新增的java.nio.file包及各个子包;
第二个改进表现为Java7在java.nio.channels包下增加了多个以Asynchronous开头的Channel接口和类。Java7把这种改进称为NIO.2,本章先详细介绍NIO的第二个改进。

15.1.0.1 Path Paths和Files核心API

早期的Java只提供了一个File类来访问文件系统,但File类的功能比较有限,它不能利用特定文件系统的特性,File所提供的方法的性能也不高。而且,其大多数方法在出错时仅返回失败,并不会提供异常信息

Path接口

NIO.2为了弥补这种不足,引入了一个Path接口,Path接口代表一个平台无关的平台路径。

Path接口方法

方法 描述
Path getRoot() Returns the root component of this path as a Path object, or null if this path does not have a root component.
Path toAbsolutePath() Returns a Path object representing the absolute path of this path.
Path getName(int index) Returns a name element of this path as a Path object.
int getNameCount() 返回Path路径所包含的路径名的数量,例如g:\publishl\codes调用该方法就会返回3。

工具类

除此之外,NIO.2还提供了Files、Paths两个工具类,其中

  • Files包含了大量静态的工具方法来操作文件;
  • Paths则包含了两个返回Path的静态工厂方法。

工具类命名规则

Files和Paths两个工具类非常符合Java一贯的命名风格,比如前面介绍的操作数组的工具类为Arrays,操作集合的工具类为Collections,这种一致的命名风格可以让读者快速了解这些工具类的用途。

Paths工具类方法

Paths接口方法 描述
static Path get(String first, String... more) Converts a path string, or a sequence of strings that when joined form a path string, to a Path.
static Path get(URI uri) Converts the given URI to a Path object.

第一个get方法会将给定的多个字符串连缀成路径,比如Paths.get("g:","publish","codes")就返回g:\publish\codes路径。

程序 Path接口Paths工具类示例

下面程序简单示范了Path接口的功能和用法

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
import java.nio.file.Path;
import java.nio.file.Paths;

public class PathTest {
    public static void main(String[] args) throws Exception {
        // 以当前路径来创建Path对象
        Path path = Paths.get(".");
        System.out.println("path里包含的路径数量：" + path.getNameCount());
        System.out.println("path的根路径：" + path.getRoot());
        // 获取path对应的绝对路径。
        Path absolutePath = path.toAbsolutePath();
        System.out.println(absolutePath);
        // 获取绝对路径的根路径
        System.out.println("absolutePath的根路径：" + absolutePath.getRoot());
        // 获取绝对路径所包含的路径数量
        System.out.println("absolutePath里包含的路径数量：" + absolutePath.getNameCount());
        System.out.println(absolutePath.getName(3));
        // 以多个String来构建Path对象
        Path path2 = Paths.get("g:", "publish", "codes");
        System.out.println(path2);
    }
}

Files工具类

Files类是一个高度封装的工具类,它提供了大量的工具方法来完成文件复制、读取文件内容、写入文件内容等功能,这些原本需要程序员通过IO操作才能完成的功能,现在Files类只要一个工具方法即可。读者应该熟练掌握Files工具类的用法,它所包含的工具方法可以大大地简化文件IO。

流操作

Java8进一步增强了Files工具类的功能,允许开发者使用Stream API来操作文件目录和文件内容。

copy方法

方法 描述
static long copy(InputStream in, Path target, CopyOption... options) Copies all bytes from an input stream to a file.
static long copy(Path source, OutputStream out) Copies all bytes from a file to an output stream.
static Path copy(Path source, Path target, CopyOption... options) Copy a file to a target file.

判断方法

是否可读可写可执行

方法 描述
static boolean isReadable(Path path) Tests whether a file is readable.
static boolean isWritable(Path path) Tests whether a file is writable.
static boolean isExecutable(Path path) Tests whether a file is executable.

是否隐藏 是否符号链接

方法 描述
static boolean isHidden(Path path) Tells whether or not a file is considered hidden.
static boolean isSymbolicLink(Path path) Tests whether a file is a symbolic link.

是否文件 是否目录 是否同一个文件

方法 描述
static boolean isDirectory(Path path, LinkOption... options) Tests whether a file is a directory.
static boolean isRegularFile(Path path, LinkOption... options) Tests whether a file is a regular file with opaque content.
static boolean isSameFile(Path path, Path path2) Tests if two paths locate the same file.

是否存在

方法 描述
static boolean exists(Path path, LinkOption... options) Tests whether a file exists.
static boolean notExists(Path path, LinkOption... options) Tests whether the file located by this path does not exist.

read方法

得到字节数组

方法 描述
static byte[] readAllBytes(Path path) Reads all the bytes from a file.

得到字符串

方法 描述
static String readString(Path path) Reads all content from a file into a string, decoding from bytes to characters using the UTF-8 charset.
static String readString(Path path, Charset cs) Reads all characters from a file into a string, decoding from bytes to characters using the specified charset.

得到所有行

方法 描述
static List<String> readAllLines(Path path) Read all lines from a file.
static List<String> readAllLines(Path path, Charset cs) Read all lines from a file.

得到其他

方法 描述
static <A extends BasicFileAttributes> A readAttributes(Path path, Class<A> type, LinkOption... options) Reads a file’s attributes as a bulk operation.
static Map<String,​Object> readAttributes(Path path, String attributes, LinkOption... options) Reads a set of file attributes as a bulk operation.
static Path readSymbolicLink(Path link) Reads the target of a symbolic link (optional operation).

create方法

创建目录

方法 描述
static Path createDirectories(Path dir, FileAttribute<?>... attrs) Creates a directory by creating all nonexistent parent directories first.
static Path createDirectory(Path dir, FileAttribute<?>... attrs) Creates a new directory.

创建文件

方法 描述
static Path createFile(Path path, FileAttribute<?>... attrs) Creates a new and empty file, failing if the file already exists.

创建链接

方法 描述
static Path createLink(Path link, Path existing) Creates a new link (directory entry) for an existing file (optional operation).
static Path createSymbolicLink(Path link, Path target, FileAttribute<?>... attrs) Creates a symbolic link to a target (optional operation).

创建临时目录

方法 描述
static Path createTempDirectory(String prefix, FileAttribute<?>... attrs) Creates a new directory in the default temporary-file directory, using the given prefix to generate its name.
static Path createTempDirectory(Path dir, String prefix, FileAttribute<?>... attrs) Creates a new directory in the specified directory, using the given prefix to generate its name.

创建临时文件

方法 描述
static Path createTempFile(String prefix, String suffix, FileAttribute<?>... attrs) Creates an empty file in the default temporary-file directory, using the given prefix and suffix to generate its name.
static Path createTempFile(Path dir, String prefix, String suffix, FileAttribute<?>... attrs) Creates a new empty file in the specified directory, using the given prefix and suffix strings to generate its name.

delete方法

方法 描述
static void delete(Path path) Deletes a file.
static boolean deleteIfExists(Path path) Deletes a file if it exists.

set方法

方法 描述
static Path setAttribute(Path path, String attribute, Object value, LinkOption... options) Sets the value of a file attribute.
static Path setLastModifiedTime(Path path, FileTime time) Updates a file’s last modified time attribute.
static Path setOwner(Path path, UserPrincipal owner) Updates the file owner.
static Path setPosixFilePermissions(Path path, Set<PosixFilePermission> perms) Sets a file’s POSIX permissions.

get方法

方法 描述
static Object getAttribute(Path path, String attribute, LinkOption... options) Reads the value of a file attribute.
static FileTime getLastModifiedTime(Path path, LinkOption... options) Returns a file’s last modified time.
static UserPrincipal getOwner(Path path, LinkOption... options) Returns the owner of a file.
static Set<PosixFilePermission> getPosixFilePermissions(Path path, LinkOption... options) Returns a file’s POSIX file permissions.
static <V extends FileAttributeView> V getFileAttributeView(Path path, Class<V> type, LinkOption... options) Returns a file attribute view of a given type.
static FileStore getFileStore(Path path) Returns the FileStore representing the file store where a file is located.

new方法

newBufferedReader

方法 描述
static BufferedReader newBufferedReader(Path path) Opens a file for reading, returning a BufferedReader to read text from the file in an efficient manner.
static BufferedReader newBufferedReader(Path path, Charset cs) Opens a file for reading, returning a BufferedReader that may be used to read text from the file in an efficient manner.

newBufferedWriter

方法 描述
static BufferedWriter newBufferedWriter(Path path, Charset cs, OpenOption... options) Opens or creates a file for writing, returning a BufferedWriter that may be used to write text to the file in an efficient manner.
static BufferedWriter newBufferedWriter(Path path, OpenOption... options) Opens or creates a file for writing, returning a BufferedWriter to write text to the file in an efficient manner.

newByteChannel

方法 描述
static SeekableByteChannel newByteChannel(Path path, OpenOption... options) Opens or creates a file, returning a seekable byte channel to access the file.
static SeekableByteChannel newByteChannel(Path path, Set<? extends OpenOption> options, FileAttribute<?>... attrs) Opens or creates a file, returning a seekable byte channel to access the file.

newDirectoryStream

方法 描述
static DirectoryStream<Path> newDirectoryStream(Path dir) Opens a directory, returning a DirectoryStream to iterate over all entries in the directory.
static DirectoryStream<Path> newDirectoryStream(Path dir, String glob) Opens a directory, returning a DirectoryStream to iterate over the entries in the directory.
static DirectoryStream<Path> newDirectoryStream(Path dir, DirectoryStream.Filter<? super Path> filter) Opens a directory, returning a DirectoryStream to iterate over the entries in the directory.

newInputStream

方法 描述
static InputStream newInputStream(Path path, OpenOption... options) Opens a file, returning an input stream to read from the file.
static OutputStream newOutputStream(Path path, OpenOption... options) Opens or creates a file, returning an output stream that may be used to write bytes to the file.

write方法

write

方法 描述
static Path write(Path path, byte[] bytes, OpenOption... options) Writes bytes to a file.
static Path write(Path path, Iterable<? extends CharSequence> lines, Charset cs, OpenOption... options) Write lines of text to a file.
static Path write(Path path, Iterable<? extends CharSequence> lines, OpenOption... options) Write lines of text to a file.

writeString

方法 描述
static Path writeString(Path path, CharSequence csq, Charset cs, OpenOption... options) Write a CharSequence to a file.
static Path writeString(Path path, CharSequence csq, OpenOption... options) Write a CharSequence to a file.

walk方法

walk

方法 描述
static Stream<Path> walk(Path start, int maxDepth, FileVisitOption... options) Return a Stream that is lazily populated with Path by walking the file tree rooted at a given starting file.
static Stream<Path> walk(Path start, FileVisitOption... options) Return a Stream that is lazily populated with Path by walking the file tree rooted at a given starting file.

walkFileTree

方法 描述
static Path walkFileTree(Path start, FileVisitor<? super Path> visitor) Walks a file tree.
static Path walkFileTree(Path start, Set<FileVisitOption> options, int maxDepth, FileVisitor<? super Path> visitor) Walks a file tree.

返回流的方法

walk

方法 描述
static Stream<Path> walk(Path start, int maxDepth, FileVisitOption... options) Return a Stream that is lazily populated with Path by walking the file tree rooted at a given starting file.
static Stream<Path> walk(Path start, FileVisitOption... options) Return a Stream that is lazily populated with Path by walking the file tree rooted at a given starting file.

find

方法 描述
static Stream<Path> find(Path start, int maxDepth, BiPredicate<Path,​BasicFileAttributes> matcher, FileVisitOption... options) Return a Stream that is lazily populated with Path by searching for files in a file tree rooted at a given starting file.

lines

方法 描述
static Stream<String> lines(Path path) Read all lines from a file as a Stream.
static Stream<String> lines(Path path, Charset cs) Read all lines from a file as a Stream.

list方法

方法 描述
static Stream<Path> list(Path dir) Return a lazily populated Stream, the elements of which are the entries in the directory.

其他方法

方法 描述
static Path move(Path source, Path target, CopyOption... options) Move or rename a file to a target file.
static String probeContentType(Path path) Probes the content type of a file.
static long size(Path path) Returns the size of a file (in bytes).

程序 Files工具类示例

Files是一个操作文件的工具类,它提供了大量便捷的工具方法,下面程序简单示范了Files类的用法。

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
import java.nio.file.*;
import java.nio.charset.*;
import java.io.*;
import java.util.*;

public class FilesTest {
    public static void main(String[] args) throws Exception {
        // 复制文件
        Files.copy(Paths.get("FilesTest.java"), new FileOutputStream("a.txt"));
        // 判断FilesTest.java文件是否为隐藏文件
        System.out.println("FilesTest.java是否为隐藏文件：" 
            + Files.isHidden(Paths.get("FilesTest.java")));
        // 一次性读取FilesTest.java文件的所有行
        List<String> lines = Files.readAllLines(Paths.get("FilesTest.java"), Charset.forName("gbk"));
        System.out.println(lines);
        // 判断指定文件的大小
        System.out.println("FilesTest.java的大小为：" + Files.size(Paths.get("FilesTest.java")));
        List<String> poem = new ArrayList<>();
        poem.add("水晶潭底银鱼跃");
        poem.add("清徐风中碧竿横");
        // 直接将多个字符串内容写入指定文件中
        Files.write(Paths.get("pome.txt"), poem, Charset.forName("gbk"));
        // 使用Java 8新增的Stream API列出当前目录下所有文件和子目录
        Files.list(Paths.get(".")).forEach(path -> System.out.println(path));
        // 使用Java 8新增的Stream API读取文件内容
        Files.lines(Paths.get("FilesTest.java"), Charset.forName("gbk"))
            .forEach(line -> System.out.println(line));
        FileStore cStore = Files.getFileStore(Paths.get("C:"));
        // 判断C盘的总空间，可用空间
        System.out.println("C:共有空间：" + cStore.getTotalSpace());
        System.out.println("C:可用空间：" + cStore.getUsableSpace());
    }
}

上面程序简单示范了Files工具类的用法。

上面示例程序中①号代码使用Stream API列出了指定路径下的所有文件和目录;②号代码则使用了Stream API读取文件内容。