我希望在文件系统中的文件被更改时收到通知。 我只找到了一个轮询lastModified文件属性的线程,显然这个解决方案并不是最优的。
我希望在文件系统中的文件被更改时收到通知。 我只找到了一个轮询lastModified文件属性的线程,显然这个解决方案并不是最优的。
我曾经编写过一个日志文件监视器,并且发现每秒轮询单个文件的属性几次对系统性能的影响实际上非常小。
Java 7作为NIO.2的一部分,新增了WatchService API
WatchService API是专门为需要通知文件更改事件的应用程序设计的。
自JDK 1.7以来,让应用程序收到文件变化通知的规范方式是使用WatchService API。 WatchService是事件驱动的。 官方教程提供了一个示例:
/*
* 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: %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();
}
}
对于单个文件,有多种解决方案,例如:
请注意,Apache VFS使用轮询算法,虽然可能提供更多功能。此外,请注意该API不提供确定文件是否已关闭的方法。
更多的NIO功能
拥有文件监控功能,其实现取决于底层操作系统。应该在JDK7中提供。
更新:已添加到Java SE 7。Chris Janicki提供了一个相关Java教程的链接:https://docs.oracle.com/javase/tutorial/essential/io/notification.html。
"每次读取属性文件时,我都会运行这段代码,只有在自上次读取以来文件已被修改时才实际读取文件。希望对某些人有所帮助。
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;
}
在Log4J中也使用了类似的方法FileWatchdog
。
// 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();
}
}
FileChangedWatcher
类并实现onModified()
方法即可: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();
}