Java經常使用文件目錄處理代碼集

轉自：http://junglesong.ycool.com/post.1068267.html

創建文件路徑（Constructing a Filename Path）
    String path = File.separator + "a" + File.separator + "b";

在文件路徑和Url之間進行轉換（Converting Between a Filename Path and a URL）
    // Create a file object
    File file = new File("filename");
    
    // Convert the file object to a URL
    URL url = null;
    try {
        // The file need not exist. It is made into an absolute path
        // by prefixing the current working directory
        url = file.toURL();          // file:/d:/almanac1.4/java.io/filename
    } catch (MalformedURLException e) {
    }
    
    // Convert the URL to a file object
    file = new File(url.getFile());  // d:/almanac1.4/java.io/filename
    
    // Read the file contents using the URL
    try {
        // Open an input stream
        InputStream is = url.openStream();
    
        // Read from is
    
        is.close();
    } catch (IOException e) {
        // Could not open the file
    }

從相對文件路徑取得絕對文件路徑（Getting an Absolute Filename Path from a Relative Filename Path）
    File file = new File("filename.txt");
    file = file.getAbsoluteFile();  // c:\temp\filename.txt
    
    file = new File("dir"+File.separatorChar+"filename.txt");
    file = file.getAbsoluteFile();  // c:\temp\dir\filename.txt
    
    file = new File(".."+File.separatorChar+"filename.txt");
    file = file.getAbsoluteFile();  // c:\temp\..\filename.txt
    
    // Note that filename.txt does not need to exist

判斷是否兩個文件路徑是否指向同一個文件（Determining If Two Filename Paths Refer to the Same File）

    File file1 = new File("./filename");
    File file2 = new File("filename");
    
    // Filename paths are not equal
    boolean b = file1.equals(file2);      // false
    
    // Normalize the paths
    try {
        file1 = file1.getCanonicalFile(); // c:\almanac1.4\filename
        file2 = file2.getCanonicalFile(); // c:\almanac1.4\filename
    } catch (IOException e) {
    }
    
    // Filename paths are now equal
    b = file1.equals(file2);              // true

獲得文件所在的文件夾名（Getting the Parents of a Filename Path）

    // Get the parent of a relative filename path
    File file = new File("Ex1.java");
    String parentPath = file.getParent();      // null
    File parentDir = file.getParentFile();     // null
    
    // Get the parents of an absolute filename path
    file = new File("D:\almanac\Ex1.java");
    parentPath = file.getParent();             // D:\almanac
    parentDir = file.getParentFile();          // D:\almanac
    
    parentPath = parentDir.getParent();        // D:\
    parentDir = parentDir.getParentFile();     // D:\
    
    parentPath = parentDir.getParent();        // null
    parentDir = parentDir.getParentFile();     // null

判斷路徑指向的是文件仍是目錄（Determining If a Filename Path Is a File or a Directory）
    File dir = new File("directoryName");
    
    boolean isDir = dir.isDirectory();
    if (isDir) {
        // dir is a directory
    } else {
        // dir is a file
    }

判斷文件或者路徑是否存在（Determining If a File or Directory Exists）
    boolean exists = (new File("filename")).exists();
    if (exists) {
        // File or directory exists
    } else {
        // File or directory does not exist
    }

建立文件（Creating a File）
    try {
        File file = new File("filename");
    
        // Create file if it does not exist
        boolean success = file.createNewFile();
        if (success) {
            // File did not exist and was created
        } else {
            // File already exists
        }
    } catch (IOException e) {
    }

獲得文件的大小（Getting the Size of a File）
    File file = new File("infilename");
    
    // Get the number of bytes in the file
    long length = file.length();

刪除文件（Deleting a File）
    boolean success = (new File("filename")).delete();
    if (!success) {
        // Deletion failed
    }

建立臨時文件（Creating a Temporary File）
    try {
        // Create temp file.
        File temp = File.createTempFile("pattern", ".suffix");
    
        // Delete temp file when program exits.
        temp.deleteOnExit();
    
        // Write to temp file
        BufferedWriter out = new BufferedWriter(new FileWriter(temp));
        out.write("aString");
        out.close();
    } catch (IOException e) {
    }

重命名一個文件或目錄（Renaming a File or Directory）
    // File (or directory) with old name
    File file = new File("oldname");
    
    // File (or directory) with new name
    File file2 = new File("newname");
    
    // Rename file (or directory)
    boolean success = file.renameTo(file2);
    if (!success) {
        // File was not successfully renamed
    }

移動文件或者目錄（Moving a File or Directory to Another Directory）
    // File (or directory) to be moved
    File file = new File("filename");
    
    // Destination directory
    File dir = new File("directoryname");
    
    // Move file to new directory
    boolean success = file.renameTo(new File(dir, file.getName()));
    if (!success) {
        // File was not successfully moved
    }

取得和修改文件或目錄的修改時間（Getting and Setting the Modification Time of a File or Directory）
This example gets the last modified time of a file or directory and then sets it to the current time. 
    File file = new File("filename");
    
    // Get the last modified time
    long modifiedTime = file.lastModified();
    // 0L is returned if the file does not exist
    
    // Set the last modified time
    long newModifiedTime = System.currentTimeMillis();
    boolean success = file.setLastModified(newModifiedTime);
    if (!success) {
        // operation failed.
    }

 

強制更新硬盤上的文件（Forcing Updates to a File to the Disk）
In some applications, such as transaction processing, it is necessary to ensure that an update has been made to the disk. FileDescriptor.sync() blocks until all changes to a file are written to disk. 
    try {
        // Open or create the output file
        FileOutputStream os = new FileOutputStream("outfilename");
        FileDescriptor fd = os.getFD();
    
        // Write some data to the stream
        byte[] data = new byte[]{(byte)0xCA, (byte)0xFE, (byte)0xBA, (byte)0xBE};
        os.write(data);
    
        // Flush the data from the streams and writers into system buffers.
        // The data may or may not be written to disk.
        os.flush();
    
        // Block until the system buffers have been written to disk.
        // After this method returns, the data is guaranteed to have
        // been written to disk.
        fd.sync();
    } catch (IOException e) {
    }

獲得當前的工做目錄（Getting the Current Working Directory）
The working directory is the location in the file system from where the java command was invoked. 
    String curDir = System.getProperty("user.dir");

建立目錄（Creating a Directory）
    // Create a directory; all ancestor directories must exist
    boolean success = (new File("directoryName")).mkdir();
    if (!success) {
        // Directory creation failed
    }
    
    // Create a directory; all non-existent ancestor directories are
    // automatically created
    success = (new File("directoryName")).mkdirs();
    if (!success) {
        // Directory creation failed
    }

刪除目錄（Deleting a Directory）
    // Delete an empty directory
    boolean success = (new File("directoryName")).delete();
    if (!success) {
        // Deletion failed
    }

If the directory is not empty, it is necessary to first recursively delete all files and subdirectories in the directory. Here is a method that will delete a non-empty directory. 
    // Deletes all files and subdirectories under dir.
    // Returns true if all deletions were successful.
    // If a deletion fails, the method stops attempting to delete and returns false.
    public static boolean deleteDir(File dir) {
        if (dir.isDirectory()) {
            String[] children = dir.list();
            for (int i=0; i<children.length; i++) {
                boolean success = deleteDir(new File(dir, children[i]));
                if (!success) {
                    return false;
                }
            }
        }
    
        // The directory is now empty so delete it
        return dir.delete();
    }

列舉文件夾中的文件和子目錄（Listing the Files or Subdirectories in a Directory）
This example lists the files and subdirectories in a directory. To list all descendant files and subdirectories under a directory, see e33 Traversing the Files and Directories Under a Directory. 
    File dir = new File("directoryName");
    
    String[] children = dir.list();
    if (children == null) {
        // Either dir does not exist or is not a directory
    } else {
        for (int i=0; i<children.length; i++) {
            // Get filename of file or directory
            String filename = children[i];
        }
    }
    
    // It is also possible to filter the list of returned files.
    // This example does not return any files that start with `.'.
    FilenameFilter filter = new FilenameFilter() {
        public boolean accept(File dir, String name) {
            return !name.startsWith(".");
        }
    };
    children = dir.list(filter);
    
    
    // The list of files can also be retrieved as File objects
    File[] files = dir.listFiles();
    
    // This filter only returns directories
    FileFilter fileFilter = new FileFilter() {
        public boolean accept(File file) {
            return file.isDirectory();
        }
    };
    files = dir.listFiles(fileFilter);

列舉系統根目錄下的文件（Listing the File System Roots）
UNIX file systems have a single root, `/'. On Windows, each drive is a root. For example the C drive is represented by the root C:\. 
    File[] roots = File.listRoots();
    for (int i=0; i<roots.length; i++) {
        process(roots[i]);
    }

遍歷目錄（Traversing the Files and Directories Under a Directory）
This example implements methods that recursively visits all files and directories under a directory. 
    // Process all files and directories under dir
    public static void visitAllDirsAndFiles(File dir) {
        process(dir);
    
        if (dir.isDirectory()) {
            String[] children = dir.list();
            for (int i=0; i<children.length; i++) {
                visitAllDirsAndFiles(new File(dir, children[i]));
            }
        }
    }
    
    // Process only directories under dir
    public static void visitAllDirs(File dir) {
        if (dir.isDirectory()) {
            process(dir);
    
            String[] children = dir.list();
            for (int i=0; i<children.length; i++) {
                visitAllDirs(new File(dir, children[i]));
            }
        }
    }
    
    // Process only files under dir
    public static void visitAllFiles(File dir) {
        if (dir.isDirectory()) {
            String[] children = dir.list();
            for (int i=0; i<children.length; i++) {
                visitAllFiles(new File(dir, children[i]));
            }
        } else {
            process(dir);
        }
    }

從控制檯讀入文本（Reading Text from Standard Input）
    try {
        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
        String str = "";
        while (str != null) {
            System.out.print("> prompt ");
            str = in.readLine();
            process(str);
        }
    } catch (IOException e) {
    }

從文件中讀入文本（Reading Text from a File）
    try {
        BufferedReader in = new BufferedReader(new FileReader("infilename"));
        String str;
        while ((str = in.readLine()) != null) {
            process(str);
        }
        in.close();
    } catch (IOException e) {
    }

將文件內容讀入一個Byte型數組（Reading a File into a Byte Array）
This example implements a method that reads the entire contents of a file into a byte array. 
See also e35 Reading Text from a File.

    // Returns the contents of the file in a byte array.
    public static byte[] getBytesFromFile(File file) throws IOException {
        InputStream is = new FileInputStream(file);
    
        // Get the size of the file
        long length = file.length();
    
        // You cannot create an array using a long type.
        // It needs to be an int type.
        // Before converting to an int type, check
        // to ensure that file is not larger than Integer.MAX_VALUE.
        if (length > Integer.MAX_VALUE) {
            // File is too large
        }
    
        // Create the byte array to hold the data
        byte[] bytes = new byte[(int)length];
    
        // Read in the bytes
        int offset = 0;
        int numRead = 0;
        while (offset < bytes.length
               && (numRead=is.read(bytes, offset, bytes.length-offset)) >= 0) {
            offset += numRead;
        }
    
        // Ensure all the bytes have been read in
        if (offset < bytes.length) {
            throw new IOException("Could not completely read file "+file.getName());
        }
    
        // Close the input stream and return bytes
        is.close();
        return bytes;
    }

寫文件（Writing to a File）
If the file does not already exist, it is automatically created. 
    try {
        BufferedWriter out = new BufferedWriter(new FileWriter("outfilename"));
        out.write("aString");
        out.close();
    } catch (IOException e) {
    }

向文件中添加內容（Appending to a File）
    try {
        BufferedWriter out = new BufferedWriter(new FileWriter("filename", true));
        out.write("aString");
        out.close();
    } catch (IOException e) {
    }

使用隨即存取文件（Using a Random Access File）
    try {
        File f = new File("filename");
        RandomAccessFile raf = new RandomAccessFile(f, "rw");
    
        // Read a character
        char ch = raf.readChar();
    
        // Seek to end of file
        raf.seek(f.length());
    
        // Append to the end
        raf.writeChars("aString");
        raf.close();
    } catch (IOException e) {
    }

從文件中閱讀UTF-8格式的數據（Reading UTF-8 Encoded Data）
    try {
        BufferedReader in = new BufferedReader(
            new InputStreamReader(new FileInputStream("infilename"), "UTF8"));
        String str = in.readLine();
    } catch (UnsupportedEncodingException e) {
    } catch (IOException e) {
    }

向文件中寫入UTF-8格式的數據（Writing UTF-8 Encoded Data）
    try {
        Writer out = new BufferedWriter(new OutputStreamWriter(
            new FileOutputStream("outfilename"), "UTF8"));
        out.write(aString);
        out.close();
    } catch (UnsupportedEncodingException e) {
    } catch (IOException e) {
    }

從文件中閱讀ISO Latin-1格式的數據（Reading ISO Latin-1 Encoded Data）
    try {
        BufferedReader in = new BufferedReader(
            new InputStreamReader(new FileInputStream("infilename"), "8859_1"));
        String str = in.readLine();
    } catch (UnsupportedEncodingException e) {
    } catch (IOException e) {
    }

向文件中寫入ISO Latin-1 格式的數據（Writing ISO Latin-1 Encoded Data）
    try {
        Writer out = new BufferedWriter(
            new OutputStreamWriter(new FileOutputStream("outfilename"), "8859_1"));
        out.write(aString);
        out.close();
    } catch (UnsupportedEncodingException e) {
    } catch (IOException e) {
    }

序列化實體（Serializing an Object）
The object to be serialized must implement java.io.Serializable. This example serializes a javax.swing.JButton object. 
See also e45 Deserializing an Object.

    Object object = new javax.swing.JButton("push me");
    
    try {
        // Serialize to a file
        ObjectOutput out = new ObjectOutputStream(new FileOutputStream("filename.ser"));
        out.writeObject(object);
        out.close();
    
        // Serialize to a byte array
        ByteArrayOutputStream bos = new ByteArrayOutputStream() ;
        out = new ObjectOutputStream(bos) ;
        out.writeObject(object);
        out.close();
    
        // Get the bytes of the serialized object
        byte[] buf = bos.toByteArray();
    } catch (IOException e) {
    }

反序列化實體（Deserializing an Object）
This example deserializes a javax.swing.JButton object. 
See also e44 Serializing an Object.

    try {
        // Deserialize from a file
        File file = new File("filename.ser");
        ObjectInputStream in = new ObjectInputStream(new FileInputStream(file));
        // Deserialize the object
        javax.swing.JButton button = (javax.swing.JButton) in.readObject();
        in.close();
    
        // Get some byte array data
        byte[] bytes = getBytesFromFile(file);
        // see e36 Reading a File into a Byte Array for the implementation of this method
    
        // Deserialize from a byte array
        in = new ObjectInputStream(new ByteArrayInputStream(bytes));
        button = (javax.swing.JButton) in.readObject();
        in.close();
    } catch (ClassNotFoundException e) {
    } catch (IOException e) {
    }

Implementing a Serializable Singleton
By default, the deserialization process creates new instances of classes. This example demonstrates how to customize the deserialization process of a singleton to avoid creating new instances of the singleton. 
    public class MySingleton implements Serializable {
        static MySingleton singleton = new MySingleton();
    
        private MySingleton() {
        }
    
        // This method is called immediately after an object of this class is deserialized.
        // This method returns the singleton instance.
        protected Object readResolve() {
            return singleton;
        }
    }

Tokenizing Java Source Code The StreamTokenizer can be used for simple parsing of a Java source file into tokens. The tokenizer can be aware of Java-style comments and ignore them. It is also aware of Java quoting and escaping rules.      try {         // Create the tokenizer to read from a file         FileReader rd = new FileReader("filename.java");         StreamTokenizer st = new StreamTokenizer(rd);              // Prepare the tokenizer for Java-style tokenizing rules         st.parseNumbers();         st.wordChars('_', '_');         st.eolIsSignificant(true);              // If whitespace is not to be discarded, make this call         st.ordinaryChars(0, ' ');              // These calls caused comments to be discarded         st.slashSlashComments(true);         st.slashStarComments(true);              // Parse the file         int token = st.nextToken();         while (token != StreamTokenizer.TT_EOF) {             token = st.nextToken();             switch (token) {             case StreamTokenizer.TT_NUMBER:                 // A number was found; the value is in nval                 double num = st.nval;                 break;             case StreamTokenizer.TT_WORD:                 // A word was found; the value is in sval                 String word = st.sval;                 break;             case '"':                 // A double-quoted string was found; sval contains the contents                 String dquoteVal = st.sval;                 break;             case '\'':                 // A single-quoted string was found; sval contains the contents                 String squoteVal = st.sval;                 break;             case StreamTokenizer.TT_EOL:                 // End of line character found                 break;             case StreamTokenizer.TT_EOF:                 // End of file has been reached                 break;             default:                 // A regular character was found; the value is the token itself                 char ch = (char)st.ttype;                 break;             }         }         rd.close();     } catch (IOException e) {     }