如何解决Java ShutdownHook无法正常运行
我想要做的是在while(true)循环中运行一些代码,然后当我按下IntelliJ或控件c中的终止按钮时,第二段代码运行干净地终止并保存了我所有的进度到一个文件。目前,我的程序正在使用在我的主要方法中运行的以下代码运行:
File terminate = new File(terminatePath);
while(!terminate.canRead()) {
// process
}
// exit code
但是,为了终止代码,我必须在“ terminatePath”目录中创建一个文件,并且当我想再次开始运行时,必须删除该文件。这是很草率和烦人的事情,所以我想学习正确的方法来做这样的事情。我在网上找到的大多数情况都说使用关闭钩子,并在下面提供此代码:
Runtime.getRuntime().addShutdownHook(new Thread() {
public void run() {
// exit code
}
});
然后我将while循环直接放在主要方法中的该钩子下面:
public static void main(String[] args) {
Runtime.getRuntime().addShutdownHook(new Thread() {
public void run() {
// exit code
}
});
while (true) {
// process
}
}
但是在这段代码中,关闭钩子似乎并不是最后运行的东西。终止后,退出代码将立即运行,然后还会执行while循环的更多迭代。
我假设我错误地应用了退出挂钩,但是我似乎无法在线找到正确的方法。我可以更改此代码以使while循环在运行出口挂钩之前可靠地停止吗?谢谢。
解决方法
Windows用户的序言:通常在Windows 10上,我从 IntelliJ IDEA , Eclipse 或 Git Bash运行Java程序。它们都不会触发Ctrl-C
上的任何JVM关闭钩子,可能是因为它们以比常规Windows终端 cmd.exe 更少协作的方式杀死进程。因此,为了测试整个场景,我确实必须从 cmd.exe 或 PowerShell 运行Java。
更新:在 IntelliJ IDEA 中,您可以单击“退出”按钮,就像从左到右的箭头指向一个空的正方形-而不是“停止”按钮,就像一个实心的正方形一样在典型的音频/视频播放器上。另请参阅here和here。
请查看Javadoc for Runtime.addShutdownHook(Thread)
。它说明了shutdown钩子只是一个初始化但未启动的线程,它将在JVM关闭时启动。它还指出,您应该以线程安全的方式进行防御性编码,因为不能保证所有其他线程都已被中止。
让我向您展示这种效果。不幸的是,由于不幸的是您没有提供MCVE,因为什么代码片段都不起作用来重现您的问题并不是特别有帮助,所以我创建了一个代码片段来解释您的情况似乎正在发生什么:
public class Result {
private long value = 0;
public long getValue() {
return value;
}
public void setValue(long value) {
this.value = value;
}
@Override
public String toString() {
return "Result{value=" + value + '}';
}
}
import java.io.*;
public class ResultShutdownHookDemo {
private static final File resultFile = new File("result.txt");
private static final Result result = new Result();
private static final Result oldResult = new Result();
public static void main(String[] args) throws InterruptedException {
loadPreviousResult();
saveResultOnExit();
calculateResult();
}
private static void loadPreviousResult() {
try (BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(new FileInputStream(resultFile)))) {
result.setValue(Long.parseLong(bufferedReader.readLine()));
oldResult.setValue(result.getValue());
System.out.println("Starting with intermediate result " + result);
}
catch (IOException e) {
System.err.println("Cannot read result,starting from scratch");
}
}
private static void saveResultOnExit() {
Runtime.getRuntime().addShutdownHook(new Thread(() -> {
System.out.println("Shutting down after progress from " + oldResult + " to " + result);
try { Thread.sleep(500); }
catch (InterruptedException ignored) {}
try (PrintStream out = new PrintStream(new FileOutputStream(resultFile))) {
out.println(result.getValue());
}
catch (IOException e) {
System.err.println("Cannot write result");
}
}));
}
private static void calculateResult() throws InterruptedException {
while (true) {
result.setValue(result.getValue() + 1);
System.out.println("Running,current result value is " + result);
Thread.sleep(100);
}
}
}
此代码的作用是简单地增加一个包装在Result
类中的数字,以便具有一个可变的对象,该对象可以声明为final并在关闭钩子线程中使用。它确实是由
- 从上次运行保存的文件中加载中间结果(如果可能的话,否则从0开始计数),
- 每100毫秒增加一次值,
- 在JVM关闭期间将当前的中间结果写入文件中(为了演示您的问题,人为地将关闭挂钩的速度降低了500毫秒)。
现在,如果我们这样运行程序3次,总是在大约一秒钟后按Ctrl-C
,输出将是这样的:
my-path> del result.txt
my-path> java -cp bin ResultShutdownHookDemo
Cannot read result,starting from scratch
Running,current result value is Result{value=1}
Running,current result value is Result{value=2}
Running,current result value is Result{value=3}
Running,current result value is Result{value=4}
Running,current result value is Result{value=5}
Running,current result value is Result{value=6}
Running,current result value is Result{value=7}
Shutting down after progress from Result{value=0} to Result{value=7}
Running,current result value is Result{value=8}
Running,current result value is Result{value=9}
Running,current result value is Result{value=10}
Running,current result value is Result{value=11}
Running,current result value is Result{value=12}
my-path> java -cp bin ResultShutdownHookDemo
Starting with intermediate result Result{value=12}
Running,current result value is Result{value=13}
Running,current result value is Result{value=14}
Running,current result value is Result{value=15}
Running,current result value is Result{value=16}
Running,current result value is Result{value=17}
Shutting down after progress from Result{value=12} to Result{value=17}
Running,current result value is Result{value=18}
Running,current result value is Result{value=19}
Running,current result value is Result{value=20}
Running,current result value is Result{value=21}
Running,current result value is Result{value=22}
my-path> java -cp bin ResultShutdownHookDemo
Starting with intermediate result Result{value=22}
Running,current result value is Result{value=23}
Running,current result value is Result{value=24}
Running,current result value is Result{value=25}
Running,current result value is Result{value=26}
Running,current result value is Result{value=27}
Running,current result value is Result{value=28}
Running,current result value is Result{value=29}
Running,current result value is Result{value=30}
Shutting down after progress from Result{value=22} to Result{value=30}
Running,current result value is Result{value=31}
Running,current result value is Result{value=32}
Running,current result value is Result{value=33}
Running,current result value is Result{value=34}
Running,current result value is Result{value=35}
我们看到以下效果:
- 实际上,在启动关闭挂钩之后,主线程会继续运行一段时间。
- 在第2次和第3次运行中,程序将继续运行,并且主线程最后一次将其值打印到控制台,而不是等待500 ms之前使用shutdown钩子线程打印的值。
经验教训:
- 不要相信正常的线程在关闭挂钩运行时已经全部关闭。比赛条件可能会发生。
- 如果您要确保首先打印的内容也是写入结果文件的内容,请在
Result
实例上进行同步,例如由synchronized(result)
。 - 了解关闭钩子的目的是关闭资源,而不是关闭线程。因此,您确实需要使其成为线程安全的。
如您所见,在此示例中,即使没有线程安全,也没有发生任何不好的情况,因为Result
实例是一个非常简单的对象,我们将其保存为一致状态。即使我们保存了一个中间结果,也不会造成任何危害,并且此后该计算将继续进行。在下一次运行时,该程序将从保存的位置重新开始其工作。
唯一丢失的工作是关闭挂钩保存结果后所做的工作,只要不影响文件或数据库之类的其他外部资源,这就不成问题。
如果是后者,则需要确保在保存中间结果之前,将通过shutdown挂钩关闭这些资源。这可能会在主应用程序线程中导致错误,但要避免不一致。您可以通过向close()
添加Result
方法并在关闭后调用getter或setter时引发错误来模拟此情况。因此,shutdown hook不会终止其他线程,也不会依赖于它们被终止,它只是在必要时照顾(同步并)关闭资源以提供一致性。
更新:这是其中Result
类具有close
方法并且已调整saveResultOnExit
方法以使用它的变体。方法loadPreviousResult
和calculateResult
保持不变。请注意,关闭挂钩在复制要写入另一个变量的中间结果之后如何使用synchronized
并关闭资源。如果要保持Result
上的同步保持打开状态直到将其写入文件后,复制就不是必须的。但是,在那种情况下,您需要确保内部结果状态不能被另一个线程以任何方式更改,即资源封装很重要。
public class Result {
private long value = 0;
private boolean closed = false;
public long getValue() {
if (closed)
throw new RuntimeException("resource closed");
return value;
}
public void setValue(long value) {
if (closed)
throw new RuntimeException("resource closed");
this.value = value;
}
public void close() {
closed = true;
}
@Override
public String toString() {
return "Result{value=" + value + '}';
}
}
import java.io.*;
public class ResultShutdownHookDemo {
private static final File resultFile = new File("result.txt");
private static final Result result = new Result();
private static final Result oldResult = new Result();
public static void main(String[] args) throws InterruptedException {
loadPreviousResult();
saveResultOnExit();
calculateResult();
}
private static void loadPreviousResult() {
try (BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(new FileInputStream(resultFile)))) {
result.setValue(Long.parseLong(bufferedReader.readLine()));
oldResult.setValue(result.getValue());
System.out.println("Starting with intermediate result " + result);
}
catch (IOException e) {
System.err.println("Cannot read result,starting from scratch");
}
}
private static void saveResultOnExit() {
Runtime.getRuntime().addShutdownHook(new Thread(() -> {
long resultToBeSaved;
synchronized (result) {
System.out.println("Shutting down after progress from " + oldResult + " to " + result);
resultToBeSaved = result.getValue();
result.close();
}
try { Thread.sleep(500); }
catch (InterruptedException ignored) {}
try (PrintStream out = new PrintStream(new FileOutputStream(resultFile))) {
out.println(resultToBeSaved);
}
catch (IOException e) {
System.err.println("Cannot write result");
}
}));
}
private static void calculateResult() throws InterruptedException {
while (true) {
result.setValue(result.getValue() + 1);
System.out.println("Running,current result value is " + result);
Thread.sleep(100);
}
}
}
现在,您会在控制台上看到来自主线程的异常,因为它在关闭挂钩已经关闭资源之后尝试继续工作。但这在关闭期间无关紧要,并且可以确保我们确切地知道关闭期间写入输出文件的内容,并且在此期间没有其他线程修改要写入的对象。
my-path> del result.txt
my-path> java -cp bin ResultShutdownHookDemo
Cannot read result,current result value is Result{value=7}
Shutting down after progress from Result{value=0} to Result{value=7}
Exception in thread "main" java.lang.RuntimeException: resource closed
at Result.getValue(Result.java:7)
at ResultShutdownHookDemo.calculateResult(ResultShutdownHookDemo.java:51)
at ResultShutdownHookDemo.main(ResultShutdownHookDemo.java:11)
my-path> java -cp bin ResultShutdownHookDemo
Starting with intermediate result Result{value=7}
Running,current result value is Result{value=12}
Shutting down after progress from Result{value=7} to Result{value=12}
Exception in thread "main" java.lang.RuntimeException: resource closed
at Result.getValue(Result.java:7)
at ResultShutdownHookDemo.calculateResult(ResultShutdownHookDemo.java:51)
at ResultShutdownHookDemo.main(ResultShutdownHookDemo.java:11)
my-path> java -cp bin ResultShutdownHookDemo
Starting with intermediate result Result{value=12}
Running,current result value is Result{value=17}
Shutting down after progress from Result{value=12} to Result{value=17}
Exception in thread "main" java.lang.RuntimeException: resource closed
at Result.getValue(Result.java:7)
at ResultShutdownHookDemo.calculateResult(ResultShutdownHookDemo.java:51)
at ResultShutdownHookDemo.main(ResultShutdownHookDemo.java:11)
版权声明:本文内容由互联网用户自发贡献,该文观点与技术仅代表作者本人。本站仅提供信息存储空间服务,不拥有所有权,不承担相关法律责任。如发现本站有涉嫌侵权/违法违规的内容, 请发送邮件至 dio@foxmail.com 举报,一经查实,本站将立刻删除。