In Java, shutdown hooks provide a way for applications to perform cleanup tasks upon termination. This can be useful for ensuring the program shuts down gracefully in the event of an unexpected interruption.
Consider the following application that writes numbers to a file in batches:
<code class="java">import java.io.File; import java.io.FileNotFoundException; import java.io.FileOutputStream; import java.io.PrintWriter; public class GracefulShutdownTest1 { private final File f; private final int N; public GracefulShutdownTest1(File f, int N) { this.f = f; this.N = N; } public void run() { try (PrintWriter pw = new PrintWriter(new FileOutputStream(f))) { for (int i = 0; i < N; ++i) { writeBatch(pw, i); } } catch (FileNotFoundException e) { e.printStackTrace(); } } private void writeBatch(PrintWriter pw, int i) { for (int j = 0; j < 100; ++j) { pw.write(Integer.toString(i * 100 + j)); if ((j + 1) % 10 == 0) { pw.write('\n'); } else { pw.write(' '); } } } public static void main(String[] args) { if (args.length < 2) { System.out.println("args = [file] [N] " + "where file = output filename, N=batch count"); } else { new GracefulShutdownTest1(new File(args[0]), Integer.parseInt(args[1])).run(); } } }</code>
To integrate a shutdown hook into this application, follow these steps:
<code class="java">private static volatile boolean keepRunning = true; // ... public void run() { try (PrintWriter pw = new PrintWriter(new FileOutputStream(f))) { for (int i = 0; i < N && keepRunning; ++i) { writeBatch(pw, i); } } catch (FileNotFoundException e) { e.printStackTrace(); } } // ... public static void main(String[] args) { final Thread mainThread = Thread.currentThread(); Runtime.getRuntime().addShutdownHook(new Thread() { public void run() { keepRunning = false; mainThread.join(); } }); new GracefulShutdownTest1(new File(args[0]), Integer.parseInt(args[1])).run(); }</code>
By implementing a shutdown hook, you can provide a more resilient application that performs necessary cleanup tasks before terminating.
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