SimpleFileVisitor.java

/*
 * Copyright (c) 2007, 2011, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
 *
 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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package java.nio.file;

import java.io.IOException;
import java.nio.file.attribute.BasicFileAttributes;
import java.util.Objects;

/**
 * A simple visitor of files with default behavior to visit all files and to
 * re-throw I/O errors.
 *
 * <p> Methods in this class may be overridden subject to their general contract.
 *
 * @param <T> The type of reference to the files
 *
 * @since 1.7
 */
// 文件树遍历器的简单实现：除非遇到异常，否则访问所有遇到的子项；可以通过自定义子类来继续扩展该遍历器
public class SimpleFileVisitor<T> implements FileVisitor<T> {
    
    /**
     * Initializes a new instance of this class.
     */
    protected SimpleFileVisitor() {
    }
    
    /**
     * Invoked for a directory before entries in the directory are visited.
     *
     * Unless overridden, this method returns {@link FileVisitResult#CONTINUE CONTINUE}.
     */
    // 【进入目录】遍历文件树过程中遇到了目录，attrs是目录的属性
    @Override
    public FileVisitResult preVisitDirectory(T dir, BasicFileAttributes attrs) throws IOException {
        Objects.requireNonNull(dir);
        Objects.requireNonNull(attrs);
        return FileVisitResult.CONTINUE;
    }
    
    /**
     * Invoked for a file in a directory.
     *
     * Unless overridden, this method returns {@link FileVisitResult#CONTINUE CONTINUE}.
     */
    // 【遇到文件】遍历文件树过程中遇到了文件(而不是目录)，或者递归层次达到了上限，attrs是文件/目录的属性
    @Override
    public FileVisitResult visitFile(T file, BasicFileAttributes attrs) throws IOException {
        Objects.requireNonNull(file);
        Objects.requireNonNull(attrs);
        return FileVisitResult.CONTINUE;
    }
    
    /**
     * Invoked for a file that could not be visited.
     *
     * Unless overridden, this method re-throws the I/O exception that prevented the file from being visited.
     */
    // 【遇到异常】遍历文件树过程中遇到了异常，ex是异常信息
    @Override
    public FileVisitResult visitFileFailed(T file, IOException exc) throws IOException {
        Objects.requireNonNull(file);
        throw exc;
    }
    
    /**
     * Invoked for a directory after entries in the directory, and all of their descendants, have been visited.
     *
     * Unless overridden, this method returns {@link FileVisitResult#CONTINUE CONTINUE}
     * if the directory iteration completes without an I/O exception;
     * otherwise this method re-throws the I/O exception that caused the iteration of the directory to terminate prematurely.
     */
    // 【退出目录】结束了对指定目录的遍历，ex是异常信息
    @Override
    public FileVisitResult postVisitDirectory(T dir, IOException exc) throws IOException {
        Objects.requireNonNull(dir);
        if(exc != null) {
            throw exc;
        }
        return FileVisitResult.CONTINUE;
    }
}