应用场景：
代码可以实现文件变化后的监听，如文件变化，自动重新加载文件内容，实现配置文件的热部署。
代码：
package com.yx.demo.filemonitor;
import java.io.File;
import java.lang.ref.WeakReference;
import java.util.ArrayList;
import java.util.Collection;
import java.util.HashMap;
import java.util.Iterator;
import java.util.List;
import java.util.Timer;
import java.util.TimerTask;
/**
* FileMonitor
* 文件监控器
*
* @author yx
* @date 2019/12/21 0:59
*/
public class FileMonitor {
/**
* 每2秒更新的默认监控器
*/
private static FileMonitor defaultFileMonitor = new FileMonitor(2 * 1000);
private Timer timer_;
private HashMap<File, FileEntry> files_; // File -> Long
private List<FileEntry> fileEntrys = new java.util.concurrent.CopyOnWriteArrayList<FileEntry>();
private Collection<WeakReference<FileListener>> listeners_; // of WeakReference(FileListener)
private long pollingInterval = 10000;
public static FileMonitor getDefaultFileMonitor() {
return defaultFileMonitor;
}
/**
* Create a file monitor instance with specified polling interval.
*
* @param pollingInterval Polling interval in milli seconds.
*/
public FileMonitor(long pollingInterval) {
this.pollingInterval = pollingInterval;
files_ = new HashMap<File, FileEntry>();
listeners_ = new ArrayList<WeakReference<FileListener>>();
timer_ = new Timer("FileMonitor", true);
timer_.schedule(new FileMonitorNotifier(), 0, pollingInterval);
}
/**
* Stop the file monitor polling.
*/
public void stop() {
timer_.cancel();
timer_ = null;
}
public void start() {
if (timer_ == null) {
timer_ = new Timer(true);
timer_.schedule(new FileMonitorNotifier(), 0, pollingInterval);
}
}
/**
* Add file to listen for. File may be any java.io.File (including a
* directory) and may well be a non-existing file in the case where the
* creating of the file is to be trepped.
* <p>
* More than one file can be listened for. When the specified file is
* created, modified or deleted, listeners are notified.
*
* @param file File to listen for.
*/
public void addFile(String id, File file) {
if (!files_.containsKey(file)) {
FileEntry entry = new FileEntry(id, file, file.exists() ? file.lastModified() : -1);
files_.put(file, entry);
}
}
/**
* 添加监控文件实体。
*
* @param fileEntry
*/
public void addFileEntry(FileEntry fileEntry) {
if (!fileEntrys.contains(fileEntry)) {
fileEntrys.add(fileEntry);
}
}
/**
* 通过文件实体的标识判断监控文件实体是否存在。
*
* @param id
* @return
*/
public boolean fileEntryExists(String id) {
if (id == null) {
return false;
}
for (int i = 0; i < fileEntrys.size(); i++) {
if (id.equals(fileEntrys.get(i).getId())) {
return true;
}
}
return false;
}
/**
* 通过文件实体标识删除一个监控文件实体。
*
* @param id
*/
public void removeFileEntry(String id) {
if (id == null) {
return;
}
for (int i = 0; i < fileEntrys.size(); i++) {
if (id.equals(fileEntrys.get(i).getId())) {
fileEntrys.remove(i);
return;
}
}
}
/**
* Remove specified file for listening.
*
* @param file File to remove.
*/
public void removeFile(File file) {
files_.remove(file);
}
/**
* Add listener to this file monitor.
*
* @param fileListener Listener to add.
*/
public void addListener(FileListener fileListener) {
// Don't add if its already there
for (Iterator<WeakReference<FileListener>> i = listeners_.iterator(); i.hasNext(); ) {
WeakReference<FileListener> reference = i.next();
FileListener listener = (FileListener) reference.get();
if (listener == fileListener) {
return;
}
}
// Use WeakReference to avoid memory leak if this becomes the
// sole reference to the object.
listeners_.add(new WeakReference<FileListener>(fileListener));
}
/**
* Remove listener from this file monitor.
*
* @param fileListener Listener to remove.
*/
public void removeListener(FileListener fileListener) {
for (Iterator<WeakReference<FileListener>> i = listeners_.iterator(); i.hasNext(); ) {
WeakReference<FileListener> reference = (WeakReference<FileListener>) i.next();
FileListener listener = (FileListener) reference.get();
if (listener == fileListener) {
i.remove();
break;
}
}
}
/**
* This is the timer thread which is executed every n milliseconds according
* to the setting of the file monitor. It investigates the file in question
* and notify listeners if changed.
*/
private class FileMonitorNotifier extends TimerTask {
@Override
public void run() {
try {
for (Iterator<FileEntry> i = fileEntrys.iterator(); i.hasNext(); ) {
try {
FileEntry entry = i.next();
if (entry == null || !entry.check()) {
i.remove();
}
} catch (Throwable t) {
t.printStackTrace();
System.out.println("执行文件监控发生错误:" + t.getMessage());
}
}
// Loop over the registered files and see which have changed.
// Use a copy of the list in case listener wants to alter the
// list within its fileChanged method.
Collection<File> files = new ArrayList<File>(files_.keySet());
for (Iterator<File> i = files.iterator(); i.hasNext(); ) {
File file = i.next();
try {
FileEntry fileEntry = files_.get(file);
long lastModifiedTime = fileEntry.getLastModified();
long newModifiedTime = file.exists() ? file.lastModified() : -1;
//logger.debug(file.getAbsolutePath());
//logger.debug(" {}=>{}", lastModifiedTime, newModifiedTime);
// Chek if file has changed
if (newModifiedTime != lastModifiedTime) {
//logger.debug("file changed {})", file.getAbsolutePath());
fileEntry.setLastModified(newModifiedTime);
// Register new modified time
files_.put(file, fileEntry);
if (fileEntry.getFileListener() != null) {
fileEntry.getFileListener().fileChanged(fileEntry);
} else {
// Notify listeners
for (Iterator<WeakReference<FileListener>> j =
listeners_.iterator(); j.hasNext(); ) {
WeakReference<FileListener> reference =
(WeakReference<FileListener>) j.next();
FileListener listener = (FileListener) reference.get();
// Remove from list if the back-end object has been GC'd
if (listener == null) {
j.remove();
} else {
listener.fileChanged(fileEntry);
}
}
}
}
} catch (Throwable t) {
if (file != null) {
t.printStackTrace();
System.out.println(
"file monitor execute error, file=" + file.getAbsolutePath() +
t.getMessage());
} else {
System.out.println(
"file monitor execute error, file=null" + t.getMessage());
}
}
}
} catch (Throwable t) {
System.out.println("执行文件监控发生错误" + t.getMessage());
}
}
}
}
package com.yx.demo.filemonitor;
/**
* FileListener
*
* @author yx
* @date 2019/12/21 0:55
*/
public interface FileListener {
/**
*
* @param fileEntry
*/
public void fileChanged(FileEntry fileEntry);
}
package com.yx.demo.filemonitor;
import java.io.File;
import java.lang.ref.WeakReference;
/**
* FileEntry
* 文件Entry，如果FileEntry指定了FileListener，那么当文件发生变动时只触发指定的FileListener
*
* @author yx
* @date 2019/12/21 0:56
*/
public class FileEntry {
String id;
File file;
long lastModified;
FileListener fileListener = null;
Object userData;
WeakReference<Object> reference = null;
/**
* 构造函数。
*
* @param id
* @param file
*/
public FileEntry(String id, File file) {
this(id, file, file.exists() ? file.lastModified() : -1);
}
public FileEntry(Object reference, String id, File file) {
this(id, file, file.exists() ? file.lastModified() : -1);
reference = new WeakReference<Object>(reference);
}
/**
* 构造函数。
*
* @param id 标识
* @param file 要监控的文件
* @param lastmodified 最后修改日期
*/
public FileEntry(String id, File file, long lastmodified) {
super();
this.id = id;
this.file = file;
this.lastModified = lastmodified;
}
public boolean check() {
if (reference != null && reference.get() == null) {
//监控对象已经不存在，请求FileMonitor删除自己
return false;
}
long newModifiedTime = file.exists() ? file.lastModified() : -1;
if (lastModified != newModifiedTime) {
this.lastModified = newModifiedTime;
FileListener ls = this.getFileListener();
if (ls == null) {
return false;
} else {
try {
ls.fileChanged(this);
} catch (Exception e) {
e.printStackTrace();
System.err.println("执行文件监控事件监听" + e.getMessage());
}
return true;
}
} else {
return true;
}
}
public String getId() {
return id;
}
public void setId(String id) {
this.id = id;
}
public File getFile() {
return file;
}
public void setFile(File file) {
this.file = file;
}
public long getLastModified() {
return lastModified;
}
public void setLastModified(long lastModified) {
this.lastModified = lastModified;
}
public FileListener getFileListener() {
return fileListener;
}
public void setFileListener(FileListener fileListener) {
this.fileListener = fileListener;
}
public Object getUserData() {
return userData;
}
public void setUserData(Object userData) {
this.userData = userData;
}
}
使用demo:
// 文件路径
String fileName = "conf/database.xml";
// 文件监控
FileListener fileListener = new FileListener() {
@Override
public void fileChanged(FileEntry fileEntry) {
// TODO 文件变化后的业务处理
}
};
File file = new File(fileName);
FileEntry fileEntry = new FileEntry("database", file);
// 设置文件监控
fileEntry.setFileListener(fileListener);
FileMonitor.getDefaultFileMonitor().addFileEntry(fileEntry);