Interview shock 26: how to stop threads correctly?

stay Java The implementation methods of stopping threads in are as follows 3 Kind of ：

1. Custom interrupt identifier , Stop thread .
2. Use thread interrupt method interrupt Stop thread .
3. Use stop Stop thread .

among stop Method is @Deprecated Expiration method of decoration , That is, the deprecated expiration method , because stop Method will directly stop the thread , In this way, the thread is not given enough time to process the saved work before stopping , It will cause the problem of incomplete data , Therefore, it is not recommended to use . and There are also some problems with custom interrupt identification , So in general ,interrupt Method is the most ideal way to stop threads , Next, let's look at their specific differences .

1. Custom interrupt identifier

Custom interrupt identifier is to define a variable in the program to determine whether the thread should interrupt execution , The specific implementation code is as follows ：

class FlagThread extends Thread {
    //  Custom interrupt identifier 
    public volatile boolean isInterrupt = false;
 @Override
    public void run() {
        //  If  true ->  Interrupt execution 
        while (!isInterrupt) {
            //  Business logic processing 
        }
    }
}

but The problem with customizing interrupt identifiers is ： Thread interruption is not timely enough . Because the thread is executing , Unable to call while(!isInterrupt) To determine whether the thread is terminated , It can only determine whether to terminate the current thread at the next run , So it doesn't interrupt the thread in time , For example, the following code ：

class InterruptFlag {
    //  Custom interrupt identifier 
    private static volatile boolean isInterrupt = false;

    public static void main(String[] args) throws InterruptedException {
        //  Create interruptible thread instances 
        Thread thread = new Thread(() -> {
            while (!isInterrupt) { //  If  isInterrupt=true  Then stop the thread 
                System.out.println("thread  Execution steps 1： The thread is about to go to sleep ");
                try {
                    //  Sleep  1s
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
                System.out.println("thread  Execution steps 2： The thread executes the task ");
            }
        });
        thread.start(); //  Start thread 

        //  Sleep  100ms, wait for  thread  The thread runs 
        Thread.sleep(100);
        System.out.println(" The main thread ： An attempt was made to terminate the thread  thread");
        //  Modify interrupt identifier , Interrupt threads 
        isInterrupt = true;
    }
}

The execution result of the above code is shown in the figure below ： What we expect is ： The thread executes step 1 after , Received instruction to interrupt thread , Then don't perform any more steps 2 了 , However, it can be seen from the above implementation results , Using a custom interrupt identifier is not possible to achieve our expected results , This is the custom interrupt identifier , The problem of not responding in time .

2.interrupt Interrupt threads

Use interrupt Method can be given to the thread executing the task , Send an instruction to interrupt the thread , It does not interrupt the thread directly , Instead, it sends a signal to interrupt the thread , Leave the initiative of whether the thread is being interrupted to the coder . Compared with the custom interrupt identifier , It can receive interrupt instruction in time , As shown in the following code ：

public static void main(String[] args) throws InterruptedException {
    //  Create interruptible thread instances 
    Thread thread = new Thread(() -> {
        while (!Thread.currentThread().isInterrupted()) {
            System.out.println("thread  Execution steps 1： The thread is about to go to sleep ");
            try {
                //  Sleep  1s
                Thread.sleep(1000);
            } catch (InterruptedException e) {
                System.out.println("thread  Thread received interrupt instruction , Execute interrupt operation ");
                //  Interrupt the task execution of the current thread 
                break;
            }
            System.out.println("thread  Execution steps 2： The thread executes the task ");
        }
    });
    thread.start(); //  Start thread 

    //  Sleep  100ms, wait for  thread  The thread runs 
    Thread.sleep(100);
    System.out.println(" The main thread ： An attempt was made to terminate the thread  thread");
    //  Modify interrupt identifier , Interrupt threads 
    thread.interrupt();
}

The execution result of the above code is shown in the figure below ： As can be seen from the above results , After the thread receives the interrupt instruction , Immediately interrupted the thread , Compared with the previous method of customizing interrupt identifier , It can respond to interrupt thread instructions in a more timely manner .

3.stop Stop thread

stop Method can stop the thread , But it is already an obsolete method that is not recommended , This can be done through Thread Class ,stop Source code is as follows ： As can be seen from the picture above ,stop The way is to be @Deprecated Modified deprecated expiration method , And the first sentence of the note shows stop The method is not safe . In the latest version Java in , This method has been removed directly , Therefore, it is strongly not recommended to use .

summary

This article introduces the method of stopping threads 3 Methods ：

1. Stop method of custom interrupt identifier , The disadvantage of this method is that it can not respond to the interrupt request in time ;
2. Use interrupt Interrupt thread method , This method sends an interrupt signal to the thread , It can respond to interruptions in a timely manner , It is also the most recommended method ;
3. And finally stop Method , Although it can also stop threads , But this method is outdated and not recommended , stay Java The latest version has been removed directly , So it's not recommended .

It's up to you to judge right and wrong , Disdain is to listen to people , Gain or loss is more important than number .

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