Question

我有这段代码。

// On a thread
try {
    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 code

private 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();
    }
}

它在第一次调用时返回正确的结果，然后阻塞整个线程。我隔离了此方法中的错误，因为在没有调用此方法的情况下，其他所有东西都可以正常工作。我想知道我在做什么错。控制台输出快照：

// First change of file:
read call:
10
read call:
10

// Second change of file:
read call:

// I keep changing but nothing happens:

Answer 1

可能是readProperties抛出了NumberFormatException，这导致您的观察者线程退出。

您可以包装对readProperties的调用并捕获异常；这样至少在异常情况下观察者会继续观察。

并且您应该使用take，以便监视程序线程阻塞。您当前的解决方案导致100％的CPU使用率。

请参阅下面的修改后的代码。我添加了一个写程序线程来更新文件，并且（毫无疑问？）readProperties可能会失败，因为我们正在写文件时正在访问文件。可能的输出是：

....
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，观看时，您可以：跳过错误并继续；或使用其他API，以便在写入时锁定正在写入的文件。

示例代码

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);
  }
}

Java属性类错误

1 个答案: