Java File类总结和FileUtils类

文件存在和类型判断

　　创建出File类的对象并不代表该路径下有此文件或目录。

　　用public boolean exists()可以判断文件是否存在。

　　File类的对象可以是目录或者文件。

　　如果是目录，public boolean isDirectory()返回true；

　　如果是文件(非目录则是文件)，public boolean isFile()返回true；

　　但是注意需要先判断文件是否存在，如果文件不存在，上面两个方法都返回false,即不存在的File类对象既不是文件也不是目录。

创建文件

　　public boolean createNewFile()会创建一个新的空文件，只有该文件不存在的时候会创建，如果文件已经存在的话则返回false。

创建文件夹

　　public boolean mkdir()

　　创建目录，成功返回true。只能创建一个文件夹，要求所有的父目录都存在，否则创建失败。

　　public boolean mkdirs()

　　创建目录，成功返回true，会创建所有不存在的父目录。(注意即便最后创建失败，但是也可能创建了一些中间目录)。

　　上面两个方法如果要创建的目录已经存在，不再重新创建，都返回false，只有新建目录返回true。

目录操作

　　列出目录中的文件有以下方法可选：

　　String[] list()

　　String[] list(FilenameFilter filter)

　　返回文件名数组。

　　File[] listFiles()

　　File[] listFiles(FileFilter filter)

　　File[] listFiles(FilenameFilter filter)

　　返回File数组。

　　参数是文件或者文件名过滤器。

　　注意返回为空和返回为null的意义是不同的。

　　若不包含(符合条件的)文件，返回为空。

　　但是如果返回为null，则表明调用方法的File对象可能不是一个目录，或者发生了IO错误。

删除文件

　　boolean delete()方法会删除文件，如果File对象是文件则直接删除，对于目录来说，如果是空目录则直接删除，非空目录则无法删除，返回false。

　　如果要删除的文件不能被删除则会抛出IOException。

　　注意：不论是创建文件、创建目录还是删除文件，只有在动作真正发生的时候会返回true。

FileUtils类

　　在项目中写一些工具类包装通用操作是很有必要的，看了一下apache的FileUtils类，copy了一些方法出来：

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */
package com.mengdd.file;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.FileOutputStream;
import java.io.IOException;

/*
 * FileUtils copied from org.apache.commons.io.FileUtils
 */
public class FileUtils {
    /**
     * Construct a file from the set of name elements.
     *
     * @param directory
     *            the parent directory
     * @param names
     *            the name elements
     * @return the file
     */
    public static File getFile(File directory, String... names) {
        if (directory == null) {
            throw new NullPointerException(
                    "directorydirectory must not be null");
        }
        if (names == null) {
            throw new NullPointerException("names must not be null");
        }
        File file = directory;
        for (String name : names) {
            file = new File(file, name);
        }
        return file;
    }

    /**
     * Construct a file from the set of name elements.
     *
     * @param names
     *            the name elements
     * @return the file
     */
    public static File getFile(String... names) {
        if (names == null) {
            throw new NullPointerException("names must not be null");
        }
        File file = null;
        for (String name : names) {
            if (file == null) {
                file = new File(name);
            }
            else {
                file = new File(file, name);
            }
        }
        return file;
    }

    /**
     * Opens a {@link FileInputStream} for the specified file, providing better
     * error messages than simply calling new FileInputStream(file)
     * .
     * 

     * At the end of the method either the stream will be successfully opened,
     * or an exception will have been thrown.
     * 

     * An exception is thrown if the file does not exist. An exception is thrown
     * if the file object exists but is a directory. An exception is thrown if
     * the file exists but cannot be read.
     *
     * @param file
     *            the file to open for input, must not be {@code null}
     * @return a new {@link FileInputStream} for the specified file
     * @throws FileNotFoundException
     *             if the file does not exist
     * @throws IOException
     *             if the file object is a directory
     * @throws IOException
     *             if the file cannot be read
     */
    public static FileInputStream openInputStream(File file) throws IOException {
        if (file.exists()) {
            if (file.isDirectory()) {
                throw new IOException("File '" + file
                        + "' exists but is a directory");
            }
            if (file.canRead() == false) {
                throw new IOException("File '" + file + "' cannot be read");
            }
        }
        else {
            throw new FileNotFoundException("File '" + file
                    + "' does not exist");
        }
        return new FileInputStream(file);
    }

    /**
     * Opens a {@link FileOutputStream} for the specified file, checking and
     * creating the parent directory if it does not exist.
     * 

     * At the end of the method either the stream will be successfully opened,
     * or an exception will have been thrown.
     * 

     * The parent directory will be created if it does not exist. The file will
     * be created if it does not exist. An exception is thrown if the file
     * object exists but is a directory. An exception is thrown if the file
     * exists but cannot be written to. An exception is thrown if the parent
     * directory cannot be created.
     *
     * @param file
     *            the file to open for output, must not be {@code null}
     * @param append
     *            if {@code true}, then bytes will be added to the
     *            end of the file rather than overwriting
     * @return a new {@link FileOutputStream} for the specified file
     * @throws IOException
     *             if the file object is a directory
     * @throws IOException
     *             if the file cannot be written to
     * @throws IOException
     *             if a parent directory needs creating but that fails
     */
    public static FileOutputStream openOutputStream(File file, boolean append)
            throws IOException {
        if (file.exists()) {
            if (file.isDirectory()) {
                throw new IOException("File '" + file
                        + "' exists but is a directory");
            }
            if (file.canWrite() == false) {
                throw new IOException("File '" + file
                        + "' cannot be written to");
            }
        }
        else {
            File parent = file.getParentFile();
            if (parent != null) {
                if (!parent.mkdirs() && !parent.isDirectory()) {
                    throw new IOException("Directory '" + parent
                            + "' could not be created");
                }
            }
        }
        return new FileOutputStream(file, append);
    }

    public static FileOutputStream openOutputStream(File file)
            throws IOException {
        return openOutputStream(file, false);
    }

    /**
     * Cleans a directory without deleting it.
     *
     * @param directory
     *            directory to clean
     * @throws IOException
     *             in case cleaning is unsuccessful
     */
    public static void cleanDirectory(File directory) throws IOException {
        if (!directory.exists()) {
            String message = directory + " does not exist";
            throw new IllegalArgumentException(message);
        }

        if (!directory.isDirectory()) {
            String message = directory + " is not a directory";
            throw new IllegalArgumentException(message);
        }

        File[] files = directory.listFiles();
        if (files == null) { // null if security restricted
            throw new IOException("Failed to list contents of " + directory);
        }

        IOException exception = null;
        for (File file : files) {
            try {
                forceDelete(file);
            }
            catch (IOException ioe) {
                exception = ioe;
            }
        }

        if (null != exception) {
            throw exception;
        }
    }

    // -----------------------------------------------------------------------
    /**
     * Deletes a directory recursively.
     *
     * @param directory
     *            directory to delete
     * @throws IOException
     *             in case deletion is unsuccessful
     */
    public static void deleteDirectory(File directory) throws IOException {
        if (!directory.exists()) {
            return;
        }

        cleanDirectory(directory);

        if (!directory.delete()) {
            String message = "Unable to delete directory " + directory + ".";
            throw new IOException(message);
        }
    }

    /**
     * Deletes a file. If file is a directory, delete it and all
     * sub-directories.
     * 

     * The difference between File.delete() and this method are:
     * 

     * 
A directory to be deleted does not have to be empty.
     * 
You get exceptions when a file or directory cannot be deleted.
     * (java.io.File methods returns a boolean)
     * 
     *
     * @param file
     *            file or directory to delete, must not be {@code null}
     * @throws NullPointerException
     *             if the directory is {@code null}
     * @throws FileNotFoundException
     *             if the file was not found
     * @throws IOException
     *             in case deletion is unsuccessful
     */
    public static void forceDelete(File file) throws IOException {
        if (file.isDirectory()) {
            deleteDirectory(file);
        }
        else {
            boolean filePresent = file.exists();
            if (!file.delete()) {
                if (!filePresent) {
                    throw new FileNotFoundException("File does not exist: "
                            + file);
                }
                String message = "Unable to delete file: " + file;
                throw new IOException(message);
            }
        }
    }

    /**
     * Deletes a file, never throwing an exception. If file is a directory,
     * delete it and all sub-directories.
     * 

     * The difference between File.delete() and this method are:
     * 

     * 
A directory to be deleted does not have to be empty.
     * 
No exceptions are thrown when a file or directory cannot be deleted.
     * 
     *
     * @param file
     *            file or directory to delete, can be {@code null}
     * @return {@code true} if the file or directory was deleted, otherwise
     *         {@code false}
     *
     */
    public static boolean deleteQuietly(File file) {
        if (file == null) {
            return false;
        }
        try {
            if (file.isDirectory()) {
                cleanDirectory(file);
            }
        }
        catch (Exception ignored) {
        }

        try {
            return file.delete();
        }
        catch (Exception ignored) {
            return false;
        }
    }

    /**
     * Makes a directory, including any necessary but nonexistent parent
     * directories. If a file already exists with specified name but it is
     * not a directory then an IOException is thrown.
     * If the directory cannot be created (or does not already exist)
     * then an IOException is thrown.
     *
     * @param directory
     *            directory to create, must not be {@code null}
     * @throws NullPointerException
     *             if the directory is {@code null}
     * @throws IOException
     *             if the directory cannot be created or the file already exists
     *             but is not a directory
     */
    public static void forceMkdir(File directory) throws IOException {
        if (directory.exists()) {
            if (!directory.isDirectory()) {
                String message = "File " + directory + " exists and is "
                        + "not a directory. Unable to create directory.";
                throw new IOException(message);
            }
        }
        else {
            if (!directory.mkdirs()) {
                // Double-check that some other thread or process hasn't made
                // the directory in the background
                if (!directory.isDirectory()) {
                    String message = "Unable to create directory " + directory;
                    throw new IOException(message);
                }
            }
        }
    }

    /**
     * Returns the size of the specified file or directory. If the provided
     * {@link File} is a regular file, then the file's length is returned.
     * If the argument is a directory, then the size of the directory is
     * calculated recursively. If a directory or subdirectory is security
     * restricted, its size will not be included.
     *
     * @param file
     *            the regular file or directory to return the size
     *            of (must not be {@code null}).
     *
     * @return the length of the file, or recursive size of the directory,
     *         provided (in bytes).
     *
     * @throws NullPointerException
     *             if the file is {@code null}
     * @throws IllegalArgumentException
     *             if the file does not exist.
     *
     */
    public static long sizeOf(File file) {

        if (!file.exists()) {
            String message = file + " does not exist";
            throw new IllegalArgumentException(message);
        }

        if (file.isDirectory()) {
            return sizeOfDirectory(file);
        }
        else {
            return file.length();
        }

    }

    /**
     * Counts the size of a directory recursively (sum of the length of all
     * files).
     *
     * @param directory
     *            directory to inspect, must not be {@code null}
     * @return size of directory in bytes, 0 if directory is security
     *         restricted, a negative number when the real total
     *         is greater than {@link Long#MAX_VALUE}.
     * @throws NullPointerException
     *             if the directory is {@code null}
     */
    public static long sizeOfDirectory(File directory) {
        checkDirectory(directory);

        final File[] files = directory.listFiles();
        if (files == null) { // null if security restricted
            return 0L;
        }
        long size = 0;

        for (final File file : files) {

            size += sizeOf(file);
            if (size < 0) {
                break;

            }

        }

        return size;
    }

    /**
     * Checks that the given {@code File} exists and is a directory.
     *
     * @param directory
     *            The {@code File} to check.
     * @throws IllegalArgumentException
     *             if the given {@code File} does not exist or is not a
     *             directory.
     */
    private static void checkDirectory(File directory) {
        if (!directory.exists()) {
            throw new IllegalArgumentException(directory + " does not exist");
        }
        if (!directory.isDirectory()) {
            throw new IllegalArgumentException(directory
                    + " is not a directory");
        }
    }

}

参考资料

　　File类官方文档：

　　http://docs.oracle.com/javase/7/docs/api/java/io/File.html

　　org.apache.commons.io.FileUtils源码：

　　http://grepcode.com/file/repo1.maven.org/maven2/commons-io/commons-io/2.4/org/apache/commons/io/FileUtils.java

　　本博客旧博文：

　　Java IO File类

　　Java IO 用递归实现目录删除和树形目录展示 Java实现