java - WatchService 有时会触发 ENTRY_MODIFY 两次，有时会触发一次

Question

我正在使用来自 Oracle 的这个 WatchService 示例:

import java.nio.file.*;
import static java.nio.file.StandardWatchEventKinds.*;
import static java.nio.file.LinkOption.*;
import java.nio.file.attribute.*;
import java.io.*;
import java.util.*;

public class WatchDir {

private final WatchService watcher;
private final Map<WatchKey,Path> keys;
private final boolean recursive;
private boolean trace = false;

@SuppressWarnings("unchecked")
static <T> WatchEvent<T> cast(WatchEvent<?> event) {
    return (WatchEvent<T>)event;
}

/**
 * Register the given directory with the WatchService
 */
private void register(Path dir) throws IOException {
    WatchKey key = dir.register(watcher, ENTRY_CREATE, ENTRY_DELETE, ENTRY_MODIFY);
    if (trace) {
        Path prev = keys.get(key);
        if (prev == null) {
            System.out.format("register: %s\n", dir);
        } else {
            if (!dir.equals(prev)) {
                System.out.format("update: %s -> %s\n", prev, dir);
            }
        }
    }
    keys.put(key, dir);
}

/**
 * Register the given directory, and all its sub-directories, with the
 * WatchService.
 */
private void registerAll(final Path start) throws IOException {
    // register directory and sub-directories
    Files.walkFileTree(start, new SimpleFileVisitor<Path>() {
        @Override
        public FileVisitResult preVisitDirectory(Path dir, BasicFileAttributes attrs)
                throws IOException
        {
            register(dir);
            return FileVisitResult.CONTINUE;
        }
    });
}

/**
 * Creates a WatchService and registers the given directory
 */
WatchDir(Path dir, boolean recursive) throws IOException {
    this.watcher = FileSystems.getDefault().newWatchService();
    this.keys = new HashMap<WatchKey,Path>();
    this.recursive = recursive;

    if (recursive) {
        System.out.format("Scanning %s ...\n", dir);
        registerAll(dir);
        System.out.println("Done.");
    } else {
        register(dir);
    }

    // enable trace after initial registration
    this.trace = true;
}

/**
 * Process all events for keys queued to the watcher
 */
void processEvents() {
    for (;;) {

        // wait for key to be signalled
        WatchKey key;
        try {
            key = watcher.take();
        } catch (InterruptedException x) {
            return;
        }

        Path dir = keys.get(key);
        if (dir == null) {
            System.err.println("WatchKey not recognized!!");
            continue;
        }

        for (WatchEvent<?> event: key.pollEvents()) {
            WatchEvent.Kind kind = event.kind();

            // TBD - provide example of how OVERFLOW event is handled
            if (kind == OVERFLOW) {
                continue;
            }

            // Context for directory entry event is the file name of entry
            WatchEvent<Path> ev = cast(event);
            Path name = ev.context();
            Path child = dir.resolve(name);

            // print out event
            System.out.format("%s: %s\n", event.kind().name(), child);

            // if directory is created, and watching recursively, then
            // register it and its sub-directories
            if (recursive && (kind == ENTRY_CREATE)) {
                try {
                    if (Files.isDirectory(child, NOFOLLOW_LINKS)) {
                        registerAll(child);
                    }
                } catch (IOException x) {
                    // ignore to keep sample readbale
                }
            }
        }

        // reset key and remove from set if directory no longer accessible
        boolean valid = key.reset();
        if (!valid) {
            keys.remove(key);

            // all directories are inaccessible
            if (keys.isEmpty()) {
                break;
            }
        }
    }
}

static void usage() {
    System.err.println("usage: java WatchDir [-r] dir");
    System.exit(-1);
}

public static void main(String[] args) throws IOException {
    // parse arguments
    if (args.length == 0 || args.length > 2)
        usage();
    boolean recursive = false;
    int dirArg = 0;
    if (args[0].equals("-r")) {
        if (args.length < 2)
            usage();
        recursive = true;
        dirArg++;
    }

    // register directory and process its events
    Path dir = Paths.get(args[dirArg]);
    new WatchDir(dir, recursive).processEvents();
}
}

我正在Windows 7开发一个应用程序，部署环境是rhel 7.2。起初，在这两个操作系统中，每当我复制一个文件时，它都会触发一个 ENTRY_CREATED，然后是两个 ENTRY_MODIFY。第一个 ENTRY_MODIFY 在复制开始时，第二个 ENTRY_MODIFY 在复制结束时。所以我能够理解复制过程已经结束。但是，它现在只在 rhel 7.2 中触发一个 ENTRY_MODIFY。不过，它仍然会在 Windows 7 中触发两个 ENTRY_MODIFY 事件。

我找到了 this在 stackoverflow 中。这个问题问为什么两个 ENTRY_MODIFY 被触发。这不完全是我的问题，但其中一个答案与我的要求存在争议。可悲的是，我的问题在那场争端中没有解决方案。

因为没有 ENTRY_MODIFY 在复制结束时触发，而只是在复制开始时触发，所以我无法理解复制何时完成。您认为这可能是什么原因造成的？能修好吗，我怎么知道复制完成了？我无法更改 rhel 7.2，但除此之外我很乐意接受。提前致谢。

Answer 1

我只是检查文件长度是否为零。这里有一个例子

for (WatchEvent<?> event : key.pollEvents()) {
    Path fileName = (Path) event.context();
    if("tomcat-users.xml".equals(fileName.toString())) {
        Path tomcatUsersXml = tomcatConf.resolve(fileName);
        if(tomcatUsersXml.toFile().length() > 0) {
            load(tomcatUsersXml);
        }
    } 
}