blocks|key|3490653|text|从JDK1.7开始，让应用程序收到文件更改通知的规范方法是使用WatchService应用编程接口。WatchService是事件驱动的。official+tutorial提供了一个示例：|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|3490654|/*
+*+Copyright+(c)+2008,+2010,+Oracle+and/or+its+affiliates.+All+rights+reserved.
+*
+*+Redistribution+and+use+in+source+and+binary+forms,+with+or+without
+*+modification,+are+permitted+provided+that+the+following+conditions
+*+are+met:
+*
+*+++-+Redistributions+of+source+code+must+retain+the+above+copyright
+*+++++notice,+this+list+of+conditions+and+the+following+disclaimer.
+*
+*+++-+Redistributions+in+binary+form+must+reproduce+the+above+copyright
+*+++++notice,+this+list+of+conditions+and+the+following+disclaimer+in+the
+*+++++documentation+and/or+other+materials+provided+with+the+distribution.
+*
+*+++-+Neither+the+name+of+Oracle+nor+the+names+of+its
+*+++++contributors+may+be+used+to+endorse+or+promote+products+derived
+*+++++from+this+software+without+specific+prior+written+permission.
+*
+*+THIS+SOFTWARE+IS+PROVIDED+BY+THE+COPYRIGHT+HOLDERS+AND+CONTRIBUTORS+"AS
+*+IS"+AND+ANY+EXPRESS+OR+IMPLIED+WARRANTIES,+INCLUDING,+BUT+NOT+LIMITED+TO,
+*+THE+IMPLIED+WARRANTIES+OF+MERCHANTABILITY+AND+FITNESS+FOR+A+PARTICULAR
+*+PURPOSE+ARE+DISCLAIMED.++IN+NO+EVENT+SHALL+THE+COPYRIGHT+OWNER+OR
+*+CONTRIBUTORS+BE+LIABLE+FOR+ANY+DIRECT,+INDIRECT,+INCIDENTAL,+SPECIAL,
+*+EXEMPLARY,+OR+CONSEQUENTIAL+DAMAGES+(INCLUDING,+BUT+NOT+LIMITED+TO,
+*+PROCUREMENT+OF+SUBSTITUTE+GOODS+OR+SERVICES;+LOSS+OF+USE,+DATA,+OR
+*+PROFITS;+OR+BUSINESS+INTERRUPTION)+HOWEVER+CAUSED+AND+ON+ANY+THEORY+OF
+*+LIABILITY,+WHETHER+IN+CONTRACT,+STRICT+LIABILITY,+OR+TORT+(INCLUDING
+*+NEGLIGENCE+OR+OTHERWISE)+ARISING+IN+ANY+WAY+OUT+OF+THE+USE+OF+THIS
+*+SOFTWARE,+EVEN+IF+ADVISED+OF+THE+POSSIBILITY+OF+SUCH+DAMAGE.
+*/

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.*;

/**
+*+Example+to+watch+a+directory+(or+tree)+for+changes+to+files.
+*/

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:+%25s\n",+dir);
++++++++++++}+else+{
++++++++++++++++if+(!dir.equals(prev))+{
++++++++++++++++++++System.out.format("update:+%25s+->+%25s\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+%25s+...\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("%25s:+%25s\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+%7C%7C+args.length+>+2)
++++++++++++usage();
++++++++boolean+recursive+=+false;
++++++++int+dirArg+=+0;
++++++++if+(args[0].equals("-r"))+{
++++++++++++if+(args.length+<+2)
++++++++++++++++usage();
++++++++++++recursive+=+true;
++++++++++++dirArg%2B%2B;
++++++++}

++++++++//+register+directory+and+process+its+events
++++++++Path+dir+=+Paths.get(args[dirArg]);
++++++++new+WatchDir(dir,+recursive).processEvents();
++++}
}|code-block|syntax|javascript|3490655|对于单个文件，有多种解决方案，例如：|3490656|3490657|https://dzone.com/articles/listening-to-fileevents-with-java-nio|unordered-list-item|3490658|3490659|请注意，Apache+VFS使用轮询算法，尽管它可以提供更强大的功能。还要注意的是，API不提供确定文件是否已关闭的方法。|3490660|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.oracle.com/javase/9/docs/api/java/nio/file/WatchService.html|1|https://docs.oracle.com/javase/tutorial/essential/io/notification.html|2^0|V|C|0|1X|H|1|0|0|0|0|0|1S|2|0|0|0^^$0|@$1|2|3|4|5|6|7|12|8|@]|9|@$A|13|B|14|1|15]|$A|16|B|17|1|18]]|C|$]]|$1|D|3|E|5|F|7|19|8|@]|9|@]|C|$G|H]]|$1|I|3|J|5|6|7|1A|8|@]|9|@]|C|$]]|$1|K|3|-4|5|6|7|1B|8|@]|9|@]|C|$]]|$1|L|3|M|5|N|7|1C|8|@]|9|@$A|1D|B|1E|1|1F]]|C|$]]|$1|O|3|-4|5|6|7|1G|8|@]|9|@]|C|$]]|$1|P|3|Q|5|6|7|1H|8|@]|9|@]|C|$]]|$1|R|3|-4|5|6|7|1I|8|@]|9|@]|C|$]]]|S|$T|$5|U|V|W|C|$X|Y]]|Z|$5|U|V|W|C|$X|10]]|11|$5|U|V|W|C|$X|M]]]]

Since JDK 1.7, the canonical way to have an application be notified of changes to a file is using the <a href="https://docs.oracle.com/javase/9/docs/api/java/nio/file/WatchService.html" rel="nofollow noreferrer">WatchService</a> API. The WatchService is event-driven. The <a href="https://docs.oracle.com/javase/tutorial/essential/io/notification.html" rel="nofollow noreferrer">official tutorial</a> provides an example:
<pre class="lang-java prettyprint-override"><code>/*
 * Copyright (c) 2008, 2010, Oracle and/or its affiliates. All rights reserved.
 *
 * Redistribution and use in source and binary forms, with or without
 * modification, are permitted provided that the following conditions
 * are met:
 *
 * - Redistributions of source code must retain the above copyright
 * notice, this list of conditions and the following disclaimer.
 *
 * - Redistributions in binary form must reproduce the above copyright
 * notice, this list of conditions and the following disclaimer in the
 * documentation and/or other materials provided with the distribution.
 *
 * - Neither the name of Oracle nor the names of its
 * contributors may be used to endorse or promote products derived
 * from this software without specific prior written permission.
 *
 * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS &quot;AS
 * IS&quot; AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
 * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
 * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR
 * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
 * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
 * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
 * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF
 * LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING
 * NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF THIS
 * SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */

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.*;

/**
 * Example to watch a directory (or tree) for changes to files.
 */

public class WatchDir {

 private final WatchService watcher;
 private final Map&lt;WatchKey,Path&gt; keys;
 private final boolean recursive;
 private boolean trace = false;

 @SuppressWarnings(&quot;unchecked&quot;)
 static &lt;T&gt; WatchEvent&lt;T&gt; cast(WatchEvent&lt;?&gt; event) {
 return (WatchEvent&lt;T&gt;)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(&quot;register: %s\n&quot;, dir);
 } else {
 if (!dir.equals(prev)) {
 System.out.format(&quot;update: %s -&gt; %s\n&quot;, 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&lt;Path&gt;() {
 @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&lt;WatchKey,Path&gt;();
 this.recursive = recursive;

 if (recursive) {
 System.out.format(&quot;Scanning %s ...\n&quot;, dir);
 registerAll(dir);
 System.out.println(&quot;Done.&quot;);
 } 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(&quot;WatchKey not recognized!!&quot;);
 continue;
 }

 for (WatchEvent&lt;?&gt; 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&lt;Path&gt; ev = cast(event);
 Path name = ev.context();
 Path child = dir.resolve(name);

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

 // if directory is created, and watching recursively, then
 // register it and its sub-directories
 if (recursive &amp;&amp; (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(&quot;usage: java WatchDir [-r] dir&quot;);
 System.exit(-1);
 }

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

 // register directory and process its events
 Path dir = Paths.get(args[dirArg]);
 new WatchDir(dir, recursive).processEvents();
 }
}
</code></pre>
For individual files, various solutions exist, such as:
<ul>
<li><a href="https://dzone.com/articles/listening-to-fileevents-with-java-nio" rel="nofollow noreferrer">https://dzone.com/articles/listening-to-fileevents-with-java-nio</a></li>
</ul>
Note that Apache VFS uses a polling algorithm, although it may offer greater functionality. Also note that the API does not offer a way to determine whether a file has been closed.

blocks|key|273720|text|如果你愿意花点钱，JNIWrapper是一个很有用的带有Winpack的库，你将能够在某些文件上获得文件系统事件。不幸的是，只能在windows上运行。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|273721|参见https://www.teamdev.com/jniwrapper。|offset|length|273722|否则，求助于本机代码并不总是一件坏事，特别是当提供的最好的是轮询机制而不是本机事件时。|273723|我注意到，在某些计算机上，Java文件系统操作可能很慢，如果处理不当，很容易影响应用程序的性能。|273724|entityMap|0|LINK|mutability|MUTABLE|url|https://www.teamdev.com/jniwrapper^0|0|2|Y|0|0|0|0^^$0|@$1|2|3|4|5|6|7|R|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|S|8|@]|9|@$D|T|E|U|1|V]]|A|$]]|$1|F|3|G|5|6|7|W|8|@]|9|@]|A|$]]|$1|H|3|I|5|6|7|X|8|@]|9|@]|A|$]]|$1|J|3|-4|5|6|7|Y|8|@]|9|@]|A|$]]]|K|$L|$5|M|N|O|A|$P|Q]]]]

If you are willing to part with some money, JNIWrapper is a useful library with a Winpack, you will be able to get file system events on certain files. Unfortunately windows only.

See <a href="https://www.teamdev.com/jniwrapper" rel="nofollow noreferrer">https://www.teamdev.com/jniwrapper</a>.

Otherwise, resorting to native code is not always a bad thing especially when the best on offer is a polling mechanism as against a native event.

I've noticed that Java file system operations can be slow on some computers and can easily affect the application's performance if not handled well.

blocks|key|273754|text|有一个名为jnotify的库，它包装了linux上的inotify，也支持windows。我从来没有用过它，我不知道它有多好，但我想说的是，它值得一试。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|273755|entityMap|0|LINK|mutability|MUTABLE|url|http://jnotify.sourceforge.net/|1|http://en.wikipedia.org/wiki/Inotify^0|5|7|0|Q|7|1|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@$A|O|B|P|1|Q]|$A|R|B|S|1|T]]|C|$]]|$1|D|3|-4|5|6|7|U|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]|L|$5|G|H|I|C|$J|M]]]]

There is a lib called <a href="http://jnotify.sourceforge.net/" rel="noreferrer">jnotify</a> that wraps <a href="http://en.wikipedia.org/wiki/Inotify" rel="noreferrer">inotify</a> on linux and has also support for windows. Never used it and I don't know how good it is, but it's worth a try I'd say.

blocks|key|3490773|text|我以前写过一个日志文件监视器，我发现轮询单个文件的属性对系统性能的影响实际上非常小，每秒几次。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3490774|作为NIO.2的一部分，Java7添加了WatchService+API|offset|length|3490775|3490776|+WatchService应用编程接口专为需要通知文件更改事件的应用程序而设计。|blockquote|3490777|3490778|entityMap|0|LINK|mutability|MUTABLE|url|http://download.oracle.com/javase/tutorial/essential/io/notification.html^0|0|K|G|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|T|8|@]|9|@$D|U|E|V|1|W]]|A|$]]|$1|F|3|-4|5|6|7|X|8|@]|9|@]|A|$]]|$1|G|3|H|5|I|7|Y|8|@]|9|@]|A|$]]|$1|J|3|-4|5|6|7|Z|8|@]|9|@]|A|$]]|$1|K|3|-4|5|6|7|10|8|@]|9|@]|A|$]]]|L|$M|$5|N|O|P|A|$Q|R]]]]

I've written a log file monitor before, and I found that the impact on system performance of polling the attributes of a single file, a few times a second, is actually very small.

Java 7, as part of NIO.2 has added the <a href="http://download.oracle.com/javase/tutorial/essential/io/notification.html" rel="noreferrer">WatchService API</a>

<blockquote>
 The WatchService API is designed for applications that need to be notified about file change events.
</blockquote>

blocks|key|999431|text|“更多NIO功能”具有文件监视功能，具体实现取决于底层操作系统。应该在JDK7中。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|999432|更新:添加到JavaSE7中。Chris+Janicki提供了一个link+to+the+relevant+Java+tutorial。|offset|length|999433|entityMap|0|LINK|mutability|MUTABLE|url|https://docs.oracle.com/javase/tutorial/essential/io/notification.html^0|0|X|Y|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|O|8|@]|9|@$D|P|E|Q|1|R]]|A|$]]|$1|F|3|-4|5|6|7|S|8|@]|9|@]|A|$]]]|G|$H|$5|I|J|K|A|$L|M]]]]

&quot;More NIO features&quot; has file watch functionality, with implementation dependent upon the underlying OS. Should be in JDK7.
Update: Was added to Java SE 7. Chris Janicki offers a <a href="https://docs.oracle.com/javase/tutorial/essential/io/notification.html" rel="nofollow noreferrer">link to the relevant Java tutorial</a>.

blocks|key|4338374|text|我使用的是Apache+Commons的VFS+API，下面是一个如何在不影响性能的情况下监控文件的示例：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4338375|DefaultFileMonitor|offset|length|4338376|entityMap|0|LINK|mutability|MUTABLE|url|http://commons.apache.org/proper/commons-vfs/commons-vfs2/apidocs/org/apache/commons/vfs2/impl/DefaultFileMonitor.html^0|0|0|I|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|O|8|@]|9|@$D|P|E|Q|1|R]]|A|$]]|$1|F|3|-4|5|6|7|S|8|@]|9|@]|A|$]]]|G|$H|$5|I|J|K|A|$L|M]]]]

I use the VFS API from Apache Commons, here is an example of how to monitor a file without much impact in performance:

<a href="http://commons.apache.org/proper/commons-vfs/commons-vfs2/apidocs/org/apache/commons/vfs2/impl/DefaultFileMonitor.html" rel="nofollow noreferrer">DefaultFileMonitor</a>

blocks|key|273883|text|有一个用于文件和文件夹监视的商业跨桌面库，名为JxFileWatcher。可从此处下载：http://www.teamdev.com/jxfilewatcher/|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|273884|你也可以在网上看到它的实际效果：http://www.teamdev.com/jxfilewatcher/onlinedemo/|273885|entityMap|0|LINK|mutability|MUTABLE|url|http://www.teamdev.com/jxfilewatcher/|1|http://www.teamdev.com/jxfilewatcher/onlinedemo/^0|18|11|0|0|G|1C|1|0^^$0|@$1|2|3|4|5|6|7|P|8|@]|9|@$A|Q|B|R|1|S]]|C|$]]|$1|D|3|E|5|6|7|T|8|@]|9|@$A|U|B|V|1|W]]|C|$]]|$1|F|3|-4|5|6|7|X|8|@]|9|@]|C|$]]]|G|$H|$5|I|J|K|C|$L|M]]|N|$5|I|J|K|C|$L|O]]]]

There is a commercial cross-desktop library for files and folders watching called JxFileWatcher.
It can be downloaded from here:
<a href="http://www.teamdev.com/jxfilewatcher/" rel="nofollow noreferrer">http://www.teamdev.com/jxfilewatcher/</a>

Also you can see it in action online:
 <a href="http://www.teamdev.com/jxfilewatcher/onlinedemo/" rel="nofollow noreferrer">http://www.teamdev.com/jxfilewatcher/onlinedemo/</a>

blocks|key|3490889|text|您还可以考虑Apache+Commons+JCI+(Java编译器接口)。尽管此API似乎侧重于类的动态编译，但它在监视文件更改的API中也包含了类。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|3490890|示例：http://commons.apache.org/jci/usage.html|offset|length|3490891|entityMap|0|LINK|mutability|MUTABLE|url|http://commons.apache.org/jci/usage.html^0|0|3|14|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|O|8|@]|9|@$D|P|E|Q|1|R]]|A|$]]|$1|F|3|-4|5|6|7|S|8|@]|9|@]|A|$]]]|G|$H|$5|I|J|K|A|$L|M]]]]

You may also consider the Apache Commons JCI (Java Compiler Interface). Although this API seems to be focused on dynamic compilation of classes, it also includes classes in its API that monitors file changes.

Example:
<a href="http://commons.apache.org/jci/usage.html" rel="nofollow noreferrer">http://commons.apache.org/jci/usage.html</a>

blocks|key|1004172|text|Spring+Integration提供了一种很好的监视目录和文件的机制：http://static.springsource.org/spring-integration/reference/htmlsingle/#files。我非常确定它是跨平台的(我在mac、linux和windows上都用过它)。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|1004173|entityMap|0|LINK|mutability|MUTABLE|url|http://static.springsource.org/spring-integration/reference/htmlsingle/#files^0|11|25|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@$A|M|B|N|1|O]]|C|$]]|$1|D|3|-4|5|6|7|P|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]]]

Spring Integration provides a nice mechanism for watching Directories and files: <a href="http://static.springsource.org/spring-integration/reference/htmlsingle/#files" rel="nofollow">http://static.springsource.org/spring-integration/reference/htmlsingle/#files</a>. Pretty sure it's cross platform (I've used it on mac, linux, and windows).

blocks|key|4338475|text|每次读取属性文件时，我都会运行这段代码，只有在自上次读取后修改过的情况下才实际读取该文件。希望这对某些人有帮助。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|4338476|private+long+timeStamp;
private+File+file;

private+boolean+isFileUpdated(+File+file+)+{
++this.file+=+file;
++this.timeStamp+=+file.lastModified();

++if(+this.timeStamp+!=+timeStamp+)+{
++++this.timeStamp+=+timeStamp;
++++//Yes,+file+is+updated
++++return+true;
++}
++//No,+file+is+not+updated
++return+false;
}|code-block|syntax|javascript|4338477|在Log4J+FileWatchdog中也使用了类似的方法。|offset|length|style|CODE|4338478|entityMap|0|LINK|mutability|MUTABLE|url|http://grepcode.com/file/repo1.maven.org/maven2/log4j/log4j/1.2.17/org/apache/log4j/helpers/FileWatchdog.java#74^0|0|0|7|C|7|C|0|0^^$0|@$1|2|3|4|5|6|7|U|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|V|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|W|8|@$I|X|J|Y|K|L]]|9|@$I|Z|J|10|1|11]]|A|$]]|$1|M|3|-4|5|6|7|12|8|@]|9|@]|A|$]]]|N|$O|$5|P|Q|R|A|$S|T]]]]

I run this snippet of code every time I go to read the properties file, only actually reading the file if it has been modified since the last time I read it. Hope this helps someone.

<pre><code>private long timeStamp;
private File file;

private boolean isFileUpdated( File file ) {
 this.file = file;
 this.timeStamp = file.lastModified();

 if( this.timeStamp != timeStamp ) {
 this.timeStamp = timeStamp;
 //Yes, file is updated
 return true;
 }
 //No, file is not updated
 return false;
}
</code></pre>

Similar approach is used in Log4J <a href="http://grepcode.com/file/repo1.maven.org/maven2/log4j/log4j/1.2.17/org/apache/log4j/helpers/FileWatchdog.java#74" rel="nofollow"><code>FileWatchdog</code></a>.

blocks|key|274018|text|Java+commons-io有一个FileAlterationObserver。它结合FileAlterationMonitor进行轮询。类似于commons+VFS。优点是它的依赖性要小得多。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|274019|编辑:更少的依赖项不是真的，它们对于VFS是可选的。但它使用的是java+File而不是VFS抽象层。|274020|entityMap|0|LINK|mutability|MUTABLE|url|http://commons.apache.org/proper/commons-io//javadocs/api-release/org/apache/commons/io/monitor/FileAlterationObserver.html^0|I|M|0|0|0^^$0|@$1|2|3|4|5|6|7|N|8|@]|9|@$A|O|B|P|1|Q]]|C|$]]|$1|D|3|E|5|6|7|R|8|@]|9|@]|C|$]]|$1|F|3|-4|5|6|7|S|8|@]|9|@]|C|$]]]|G|$H|$5|I|J|K|C|$L|M]]]]

Java commons-io has a <a href="http://commons.apache.org/proper/commons-io//javadocs/api-release/org/apache/commons/io/monitor/FileAlterationObserver.html">FileAlterationObserver</a>. it does polling in combination with a FileAlterationMonitor. Similar to commons VFS. The advantag is that it has much less dependencies.

edit: Less dependencies is not true, they are optional for VFS. But it uses java File instead of the VFS abstraction layer.

blocks|key|1004212|text|您可以使用FileReader监听文件更改。请看下面的例子|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1004213|//+File+content+change+listener+
private+String+fname;
private+Object+lck+=+new+Object();
...+
public+void+run()
{
++++try
++++{
++++++++BufferedReader+br+=+new+BufferedReader(+new+FileReader(+fname+)+);
++++++++String+s;
++++++++StringBuilder+buf+=+new+StringBuilder();
++++++++while(+true+)
++++++++{
++++++++++++s+=+br.readLine();
++++++++++++if(+s+==+null+)
++++++++++++{
++++++++++++++++synchronized(+lck+)
++++++++++++++++{
++++++++++++++++++++lck.wait(+500+);
++++++++++++++++}
++++++++++++}
++++++++++++else
++++++++++++{
+++++++++++++++System.out.println(+"s+=+"+%2B+s+);
++++++++++++}

++++++++}
++++}
++++catch(+Exception+e+)
++++{
++++++++e.printStackTrace();
++++}
}|code-block|syntax|javascript|1004214|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

You can listen file changes using a FileReader. Plz see the example below

<pre><code>// File content change listener 
private String fname;
private Object lck = new Object();
... 
public void run()
{
 try
 {
 BufferedReader br = new BufferedReader( new FileReader( fname ) );
 String s;
 StringBuilder buf = new StringBuilder();
 while( true )
 {
 s = br.readLine();
 if( s == null )
 {
 synchronized( lck )
 {
 lck.wait( 500 );
 }
 }
 else
 {
 System.out.println( "s = " + s );
 }

 }
 }
 catch( Exception e )
 {
 e.printStackTrace();
 }
}
</code></pre>

blocks|key|4338581|text|不过，轮询最后修改的文件属性是一个简单而有效的解决方案。只需定义一个类来扩展我的FileChangedWatcher并实现onModified()方法：|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|4338582|import+java.io.File;

public+abstract+class+FileChangedWatcher
{
++++private+File+file;

++++public+FileChangedWatcher(String+filePath)
++++{
++++++++file+=+new+File(filePath);
++++}

++++public+void+watch()+throws+InterruptedException
++++{
++++++++long+currentModifiedDate+=+file.lastModified();

++++++++while+(true)
++++++++{
++++++++++++long+newModifiedDate+=+file.lastModified();

++++++++++++if+(newModifiedDate+!=+currentModifiedDate)
++++++++++++{
++++++++++++++++currentModifiedDate+=+newModifiedDate;
++++++++++++++++onModified();
++++++++++++}

++++++++++++Thread.sleep(100);
++++++++}
++++}

++++public+String+getFilePath()
++++{
++++++++return+file.getAbsolutePath();
++++}

++++protected+abstract+void+onModified();
}|code-block|syntax|javascript|4338583|entityMap^0|14|I|1P|C|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@$9|N|A|O|B|C]|$9|P|A|Q|B|C]]|D|@]|E|$]]|$1|F|3|G|5|H|7|R|8|@]|D|@]|E|$I|J]]|$1|K|3|-4|5|6|7|S|8|@]|D|@]|E|$]]]|L|$]]

Polling the last modified file property is a simple yet effective solution though. Just define a class extending my <code>FileChangedWatcher</code> and implement the <code>onModified()</code> method:

<pre><code>import java.io.File;

public abstract class FileChangedWatcher
{
 private File file;

 public FileChangedWatcher(String filePath)
 {
 file = new File(filePath);
 }

 public void watch() throws InterruptedException
 {
 long currentModifiedDate = file.lastModified();

 while (true)
 {
 long newModifiedDate = file.lastModified();

 if (newModifiedDate != currentModifiedDate)
 {
 currentModifiedDate = newModifiedDate;
 onModified();
 }

 Thread.sleep(100);
 }
 }

 public String getFilePath()
 {
 return file.getAbsolutePath();
 }

 protected abstract void onModified();
}
</code></pre>

blocks|key|274082|text|与其他答案类似，下面是我如何使用文件、计时器和TimerTask将其作为后台线程以设定的时间间隔轮询来运行。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|274083|import+java.io.File;
import+java.util.Timer;
import+java.util.TimerTask;

public+class+FileModifiedWatcher
{
++private+static+File+file;
++private+static+int+pollingInterval;
++private+static+Timer+fileWatcher;
++private+static+long+lastReadTimeStamp+=+0L;

++public+static+boolean+init(String+_file,+int+_pollingInterval)
++{
++++file+=++new+File(_file);
++++pollingInterval+=+_pollingInterval;+//+In+seconds

++++watchFile();

++++return+true;
++}

++private+static+void+watchFile()
++{
++++if+(+null+==+fileWatcher+)
++++{
++++++System.out.println("START");

++++++fileWatcher+=+new+Timer();

++++++fileWatcher.scheduleAtFixedRate(new+TimerTask()
++++++{
++++++++@Override
++++++++public+void+run()
++++++++{

++++++++++if+(+file.lastModified()+>+lastReadTimeStamp+)
++++++++++{
++++++++++++System.out.println("File+Modified");
++++++++++}

++++++++++lastReadTimeStamp+=+System.currentTimeMillis();
++++++++}
++++++},+0,+1000+*+pollingInterval);
++++}

++}
}|code-block|syntax|javascript|274084|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|I|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|J|8|@]|9|@]|A|$E|F]]|$1|G|3|-4|5|6|7|K|8|@]|9|@]|A|$]]]|H|$]]

Similar to the other answers, here's how I did it using File, Timer, and TimerTask to let this run as a background thread polling at set intervals.

<pre><code>import java.io.File;
import java.util.Timer;
import java.util.TimerTask;

public class FileModifiedWatcher
{
 private static File file;
 private static int pollingInterval;
 private static Timer fileWatcher;
 private static long lastReadTimeStamp = 0L;

 public static boolean init(String _file, int _pollingInterval)
 {
 file = new File(_file);
 pollingInterval = _pollingInterval; // In seconds

 watchFile();

 return true;
 }

 private static void watchFile()
 {
 if ( null == fileWatcher )
 {
 System.out.println("START");

 fileWatcher = new Timer();

 fileWatcher.scheduleAtFixedRate(new TimerTask()
 {
 @Override
 public void run()
 {

 if ( file.lastModified() &gt; lastReadTimeStamp )
 {
 System.out.println("File Modified");
 }

 lastReadTimeStamp = System.currentTimeMillis();
 }
 }, 0, 1000 * pollingInterval);
 }

 }
}
</code></pre>

I'd like to be notified when a file has been changed in the file system. I have found nothing but a thread that polls the lastModified File property and clearly this solution is not optimal.

File changed listener in Java

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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当文件系统中的文件发生更改时，我希望收到通知。我只发现了一个轮询lastModified文件属性的线程，显然这个解决方案并不是最优的。

问Java中的文件更改监听器
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