核心技术（卷二）01、第1章-流与文件

流与文件

1. 流

   在Java API中，可以从其中读入一个字节序列的对象称为输入流，可以向其中写入一个字节序列的对象称为输出流。抽象类InputStream和OutputStream构成了I/O类层次机构的基础。

   因为面向字节的流不便于处理以Unicode形式存储的信息（Unicode中每个字符都是由多个字节构成的），所以从抽象类Reader和Writer中继承出了一个专门处理Unicode字符的单独的类层次结构。

2. InputStream、OutputStream、Reader和Writer

   • InputStream:
     

     

     InputStream的所有方法
     
     InputStream有一个abstract int read()抽象方法，该方法读入一个字节并返回这个字节，或者在遇到输入源结尾时返回-1。在设计具体的输入流类时，必须覆盖这个方法以提供实用的功能。InputStream还有两个非抽象的read方法，这两个方法都会调用抽象的read方法。

   • 继承结构：
     

     

     InputStream继承结构
     
     除了在java.io包中有FilterInputStream子类外，其它包中也有，FilterInputStream的所有直接子类有：
     

     image

   • OutputStream:
     

     

     OutputStream的所有方法
     
     与InputStream类似，OutputStream定义了abstract int write()抽象方法，用于向该输出流写出一个字节。

   • 继承结构：
     

     

     `OutputStream`继承结构
     
     FilterOutputStream的直接子类:
     
     

     image

   • Reader:
     

     

     Reader的所有方法

   • 继承结构：

     

     image

   • Writer:
     

     

     Writer的所有方法

   • 继承结构：

     
     
     

     image

   当完成对流的读写后，都应该调用close方法关闭流。否则系统资源的不到释放，如果使用了缓冲区，对于缓冲区最后一个包的读写也有影响。

   4个抽象类都实现了Closeable接口，该接口扩展了java.lang.AutoCloseable接口，所以4个抽象类都可以使用try-with-resource语句。

3. 以文本方式输出

   我们可以使用PrintWriter输出文本，BufferedReader读入文本。

   try(PrintWriter printWriter = new PrintWriter("test2.txt");
       BufferedReader bufferedReader = new BufferedReader(new FileReader("test2.txt"))){
           printWriter.println("this is ");
           printWriter.println("文本输出");
           printWriter.flush();
           while (bufferedReader.ready())
           log.info(bufferedReader.readLine());
       }catch (Exception e){
           e.printStackTrace();
       }
   

4. 字符集

   在JavaSE 1.4中引入的Charset类统一提供了对字符集的转换。

   它有一下几个主要方法：

    //返回一个指定字符集类型对象
    static Charset  forName(String charsetName);
    //返回该字符集的所有别名
    Set<String> aliases();
    //获取当前支持的所有字符集
    public static SortedMap<String,Charset> availableCharsets();
    //将字符序列编码为指定编码的字节序列
    ByteBuffer  encode(CharBuffer cb);
    //将字符串编码为指定编码的字节序列
    ByteBuffer  encode(String str);
    //将指定的字节序列解码，唯一的解码方法
    CharBuffer  decode(ByteBuffer bb)
   

   解码指定字节序列时，需要有字节缓冲区，使用ByteBuffer的wrap方法将一个字节数组转换为一个字节缓冲区。

    static ByteBuffer   wrap(byte[] array, int offset, int length);
    static ByteBuffer   wrap(byte[] array);
    //返回这个缓冲区所管理的字节数组
    byte[]  array();
   

   java.nio.CharBuffer：

    //返回这个字符缓冲区管理的字符数组
    char[]  array();
    //返回给定索引出的码元
    char    charAt(int index);
    //返回由这个缓冲区管理的码元所构成的字符串
    String  toString();
   

   一个完整的编码解码过程：

   
    Charset charSet = Charset.forName("utf-8");
    String text = "this is a 文本";
    //编码
    ByteBuffer byteBuffer = charSet.encode(text);
    byte[] encodeResult = byteBuffer.array();
    //解码
    //解码时，需要将字节缓冲区
    byteBuffer = ByteBuffer.wrap(encodeResult);
    CharBuffer charBuffer = charset.decode(byteBuffer);
    String decodeResult = charBuffer.toString();
   

5. 读写二进制数据

   DataOutput接口的方法来实现以二进制的形式写Java的基本类型数据。
   

   

   DataOutput定义的方法

   如writeInt总是将一个整数写出为4个字节长度的二进制数量值，WriteDouble总是将一个double值写出为8字节的二进制文件。这样产生的结果就是：总是对于给定类型的每一个值，所需的空间都是相同的。并且将其读回也比解析更快。
   DataOutputStream实现了DataOutput接口

   DataInput接口定义了读入二进制数据的方法:
   

   

   DataInput定义的方法
   
   DataInputStream实现了该接口。

6. 随机读写文件

   RandomAccessFile类提供的方法可以帮助我们随机读写文件内容。

   这个类的构造器的第二个参数用于指定访问模式,r/rw（只读/读写）

    RandomAccessFile in = new RandomAccessFile("data.dat","r");//只读
    RandomAccessFile out = new RandomAccessFile("data.dat","rw");//可读写
   

   RandomAccessFile实现了DataInput和DataOutput两个接口，所以这类可以用来读写二进制数据。另外他还有关于文件指针方法:

    //获取文件指针的位置，指针从0开始，最大为文件所包含的字节总数
    long    getFilePointer();
    //将指针置于文件指定的位置
    void    seek(long pos);
   

   因为DataOutput写出的每个类型数据长度固定，在加上随机访问的特性，使用RandomAccessFile类来帮助我们将信息存储为二进制文件是一个很好的选择。

7. ZIP文档

   ZIP文档一压缩格式存储了一个或多个文件，每个ZIP文档都有一个头，包含诸如每个文件名字和所使用的 压缩方法等信息。

   在Java中，使用ZipOutputStream来读入ZIP文档:

   
    //创建文件输出流
    FileOutputStream fileOutputStream = new FileOutputStream("zipfile.zip");
    ZipOutputStream zipOutputStream = new ZipOutputStream(fileOutputStream);
   
    //创建压缩文件项
    ZipEntry zipEntry = new ZipEntry("text.txt");
    ZipEntry zipEntry1 = new ZipEntry("text2.txt");
   
    //写入第一个文件项
    zipOutputStream.putNextEntry(zipEntry);
    //向文件项中写入内容
    String context = "压缩文件text1的内容111";
    zipOutputStream.write(Util.encode(context,"utf-8"));
    //非必须调用
    //zipOutputStream.closeEntry();
    //写入下一个文件项,必须调用这个方法，不然无法写入第二个文件项中
    zipOutputStream.putNextEntry(zipEntry1);
    context = "压缩文件text2的内容222";
    zipOutputStream.write(Util.encode(context,"utf-8"));
   
    //关闭流
    zipOutputStream.close();
    fileOutputStream.close();
   
   

   使用ZipInputStream读入Zip文档：

   //创建输入流
    FileInputStream fileInputStream = new FileInputStream(("zipfile.zip"));
    ZipInputStream zipInputStream = new ZipInputStream(fileInputStream);
    ZipEntry zipEntry;
    while((zipEntry = zipInputStream.getNextEntry())!= null){
        //检查文件项信息
       Util.getLogger().info("file name : " + zipEntry.getName());
       //读文件内容
       byte[] content = new byte[100];
       zipInputStream.read(content);
       Util.getLogger().info(Util.decode(content,"utf-8"));
       //非必须调用
       //zipInputStream.closeEntry();
   
    }
   

8. Path接口

   An object that may be used to locate a file in a file system. It will typically represent a system dependent file path.

   Path表示的是一个目录名序列，其后还还可以跟着一个文件名。

    /**
     * 通过Pahts类的get方法来得到一个Path的实例
     * Paths.get方法可以接受多个字符串，并将它们用默认的文件系统路径分隔符链接起来。
     * 类Unix文件系统是'/'，windows是'\'
     */
    Path path = Paths.get("/home","lrz");
   

   Path有许多操作文件路径的方法：API

9. Files类

   This class consists exclusively of static methods that operate on files, directories, or other types of files.

   Files类包含专门用于操作文件、目录或其他类型文件的静态方法。这些方法使普通文件操作变得简单。

   9.1 读写文件

    static byte[]   readAllBytes(Path path);
    static List<String> readAllLines(Path path);
   
    static Path write(Path path, byte[] bytes, OpenOption... options);
    static Path write(Path path, Iterable<? extends CharSequence> lines, Charset cs, OpenOption... options);
    static Path write(Path path, Iterable<? extends CharSequence> lines, OpenOption... options);
   

   实现OpenOption接口的StandardOpenOption：
   

   

   StandardOpenOption枚举常量解释

   Files类提供的文件读写方法适合中等长度的文本文件，如果是大型文件，或者是二进制文件，还是应该使用流或者读入/读出器

    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.
   
    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.
    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.
   

   9.2 复制、移动、删除文件

    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.
   
    static Path move(Path source, Path target, CopyOption... options)
    //Move or rename a file to a target file.
   
    static void delete(Path path)
    //Deletes a file.
    static boolean  deleteIfExists(Path path)
    //Deletes a file if it exists.
   

   实现CopyOption接口的StandardCopyOption：
   

   

   StandardCopyOption枚举常量解释

   9.3 创建文件和目录

    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.
    static Path createFile(Path path, FileAttribute<?>... attrs)
    //Creates a new and empty file, failing if the file already exists.
    static Path createLink(Path link, Path existing) //硬链接
    //Creates a new link (directory entry) for an existing file (optional operation).
    static Path createSymbolicLink(Path link, Path target, FileAttribute<?>... attrs)//符号链接
    //Creates a symbolic link to a target (optional operation).
    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.
    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 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.
    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.
   

   9.4 获取文件信息

    static boolean  exists(Path path, LinkOption... options)
    //Tests whether a file exists.
    static boolean  isDirectory(Path path, LinkOption... options)
    //Tests whether a file is a directory.
    static boolean  isExecutable(Path path)
    //Tests whether a file is executable.
    static boolean  isHidden(Path path)
    //Tells whether or not a file is considered hidden.
    static boolean  isReadable(Path path)
    //Tests whether a file is readable.
    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.
    static boolean  isSymbolicLink(Path path)
    //Tests whether a file is a symbolic link.
    static boolean  isWritable(Path path)
    //Tests whether a file is writable.
    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 long size(Path path)
    //Returns the size of a file (in bytes).
   

   BasicFileAttributes接口：
   

   

   BasicFileAttributes
   
   LinkOption：
   
   

   LinkOption枚举常量解释

   9.5 迭代目录中的文件

   如果要获取一个目录中所有文件：

    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, DirectoryStream.Filter<? super Path> filter)
    //Opens a directory, returning a DirectoryStream to iterate over the 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.
   

   如果想要访问某个目录的所有子孙成员:

    tatic 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.
   

   

   FileVisitor接口定义的方法：