我有這段代碼。// On a threadtry { WatchService watcher = FileSystems.getDefault().newWatchService(); Path directory = Paths.get("properties"); WatchKey watchKey = directory.register(watcher, StandardWatchEventKinds.ENTRY_MODIFY); while (true) { for (WatchEvent<?> event : watchKey.pollEvents()) { Path changed = (Path) event.context(); if (changed.toString().equals("radar.properties")) { System.out.println("read call:"); readProperties(); } } if (!watchKey.reset()) { break; } }} catch (IOException e) { FCSLogger.LOGGER.log(Level.SEVERE, "Exception while setting up WatchService", e);}// Method called by the above codeprivate void readProperties() { try { InputStream input = new FileInputStream(Paths.get("properties", "radar.properties").toString()); Properties prop = new Properties(); prop.load(input); updateRate = Integer.parseInt(prop.getProperty("updateRate")); System.out.println(updateRate); input.close(); } catch (IOException e) { e.printStackTrace(); }}它在第一次調用時返回正確的結果，然后阻塞整個線程。我已經隔離了這個方法中的錯誤，因為當沒有調用這個方法時，其他一切都完美無缺。我想知道我在這里做錯了什么。控制臺輸出快照：// First change of file:read call:10read call:10// Second change of file:read call:// I keep changing but nothing happens:
1 回答
眼眸繁星
TA貢獻1873條經驗 獲得超9個贊
它可能是readPropertiesthrows NumberFormatException，并導致您的觀察線程退出。
您可以包裝調用readProperties并捕獲異常；這樣至少觀察者會在出現異常情況時繼續觀察。
你應該使用take, 以便觀察線程阻塞。您當前的解決方案導致 100% 的 CPU 使用率。
請參閱下面修改后的代碼。我添加了一個編寫器線程來更新文件，并且（不出所料？）readProperties可能會失敗，因為我們在寫入文件時正在訪問該文件。一個可能的輸出是：
....
property=5000
property=null
java.lang.NumberFormatException: null
at java.base/java.lang.Integer.parseInt(Integer.java:614)
at java.base/java.lang.Integer.parseInt(Integer.java:770)
at cl.ClientPoll.readProperties(ClientPoll.java:26)
at cl.ClientPoll.run(ClientPoll.java:46)
property=6000
property=7000
....
因此在觀看時，您可以： 跳過錯誤并繼續；或使用其他API，使正在寫入的文件在寫入時被鎖定。
示例代碼
package cl;
import java.io.File;
import java.io.FileInputStream;
import java.io.FileWriter;
import java.io.IOException;
import java.io.InputStream;
import java.nio.file.FileSystems;
import java.nio.file.Path;
import java.nio.file.Paths;
import java.nio.file.StandardWatchEventKinds;
import java.nio.file.WatchEvent;
import java.nio.file.WatchKey;
import java.nio.file.WatchService;
import java.util.Properties;
public class ClientPoll extends Thread {
private void readProperties() {
try {
Path path = Paths.get("radar.properties");
InputStream input =new FileInputStream(path.toString());
Properties prop = new Properties();
prop.load(input);
String property = prop.getProperty("updateRate");
System.out.println("property="+property);
int updateRate = Integer.parseInt(property);
// System.out.println(updateRate);
input.close();
} catch (IOException e) {
e.printStackTrace(System.out);
}
}
@Override
public void run() {
try {
WatchService watcher = FileSystems.getDefault().newWatchService();
Path directory = Paths.get(".");
WatchKey watchKey = directory.register(watcher, StandardWatchEventKinds.ENTRY_MODIFY);
while (true) {
WatchKey wk = watcher.take();
for (WatchEvent<?> event : wk.pollEvents()) {
Path changed = (Path) event.context();
if (changed.toString().equals("radar.properties")) {
try {
readProperties();
} catch (Exception e) {
e.printStackTrace(System.out);
}
}
}
if (!watchKey.reset()) {
break;
}
}
} catch (IOException e) {
e.printStackTrace(System.out);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
public static void main(String[] args) {
new ClientPoll().start();
new Writer().start();
}
}
class Writer extends Thread {
@Override
public void run() {
try {
for (int i=0;i<10;i++) {
File f = new File("radar.properties");
FileWriter fw = new FileWriter(f);
fw.write("updateRate="+i*1000);
fw.close();
sleep(1000L);
}
} catch (Exception e) {
e.printStackTrace(System.out);
}
System.out.println("exit");
System.exit(0);
}
}
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