What is thread interrupt in Java? What problems will Java thread interruption cause?

The content of this article is about what is thread interruption in Java? What problems will Java thread interruption cause? It has certain reference value. Friends in need can refer to it. I hope it will be helpful to you.

What is a thread interrupt?

In fact, there is more than one execution thread in our Java program. Only when all threads have finished running, the Java program will be considered finished. The official description is as follows: This Java program can end when all non-daemon threads finish running, or when one of the threads calls the System.exit() method.

Application scenarios of thread interruption

Let’s give an example first. For example, if we are downloading a blockbuster of more than 500M, we click to start downloading. At this time, it is equivalent to starting a thread. Go download our files. However, our internet speed is not very strong at this time. Dozens of KB files are running here. As a young man, I can’t wait any longer. If I don’t download, our first operation at this time is to end the download. This operation of downloading files is actually closer to the program. At this time, we need to interrupt the thread.

Let’s write the download code next to see how to interrupt a thread. Here I have assumed that you have mastered how to create a thread. In this program we Simulate downloading, first get the system time, and then enter the loop to get the system time each time. If the time exceeds 10 seconds, we will interrupt the thread and not continue downloading. The download speed is 1M per second:

public void run() {

       int number = 0;

       // 记录程序开始的时间
       Long start = System.currentTimeMillis();

       while (true) {

           // 每次执行一次结束的时间
           Long end = System.currentTimeMillis();

           // 获取时间差
           Long interval = end - start;

           // 如果时间超过了10秒，那么我们就结束下载
           if (interval >= 10000) {
               // 中断线程
               interrupted();
               System.err.println("太慢了，我不下了");
               return;
           } else if (number >= 500) {
               System.out.println("文件下载完成");
               // 中断线程
               interrupted();
               return;
           }

           number++;
           System.out.println("已下载" + number + "M");

           try {
               Thread.sleep(2000);
           } catch (InterruptedException e) {
               e.printStackTrace();
           }
       }
   }

How to interrupt the thread

The Thread class provides us with a method to interrupt the thread. Let's first take a look at how this method interrupts the thread:

public static boolean interrupted() { 
      return currentThread().isInterrupted(true);
   }

This The method is to check whether the current thread is interrupted. Interruption returns true, and non-interruption returns false

private native boolean isInterrupted(boolean ClearInterrupted);

By looking at the source code, we can find that interrupting the thread is to call the method to check whether the thread is interrupted, and set the value. is true. At this time, when you call the method to check whether the thread is interrupted, it will return true.

Everyone needs to pay attention to a problem here: the Thread.interrupted() method only modifies the status of the current thread to tell it to be interrupted, but for non-blocked threads, it only changes the interruption status, that is, Thread.isInterrupted () returns true. For threads in a cancelable blocking state, such as threads waiting on these functions, Thread.sleep(), this thread will throw an InterruptedException after receiving the interrupt signal, and the interrupt status will be set at the same time. is true.

The reason why InterruptedException is caused by thread sleep

In fact, everyone has a little understanding of this, so I will write an incorrect example. Let's take a look and solve this problem thoroughly. Figure it out:

public void run() {

       int number = 0;

       while (true) {
           // 检查线程是否被中断，中断就停止下载
           if (isInterrupted()) {

               System.err.println("太慢了，我不下了");
               return;
           } else if (number >= 500) {
               System.out.println("下载完成");
               return;
           }

           number++;
           System.out.println("已下载" + number + "M");

           try {
               Thread.sleep(2000);
           } catch (InterruptedException e) {
               e.printStackTrace();
           }
       }
   }

This is our main program, wait for 10 seconds and then interrupt the thread

public static void main(String[] args) throws InterruptedException {

       Thread thread = new PrimeGenerator();

       // 启动线程
       thread.start();

       // 等待10秒后中断线程
       Thread.sleep(1000);

       // 中断线程
       thread.interrupt();
   }

It looks like a very ordinary program, but the fact is not what you see. In fact, this This piece of code will throw InterruptedException, let's analyze the reason.

Here we must first understand that the Thread.interrupt() method will not interrupt a running thread. When the Thread.sleep() method is called, it will no longer be occupied at this time. CPU, let’s analyze our program. We have to wait 10 seconds to download. The speed of each download is 0.5M/S. That is, when we download to 5M, the waiting time has expired. At this time, we call Thread.interrupt( ) method interrupts the thread, but the sleep in the run() method will continue to be executed. It will not give up executing the following code due to the interruption. Then at this time, when it executes Thread.sleep() again, it will An InterruptedException is thrown because the current thread has been interrupted.

Speaking of which, do you already understand the reason for this exception? In addition, there are two other reasons why threads generate InterruptedException exceptions. Improper use of the wait() and join() methods can also cause threads to throw this exception.

Two ways to check whether a thread is interrupted

There is a method interrupted() in the Thread class that can be used to check when the current thread is interrupted, and isInterrupted( ) method can be used to check whether the current thread is interrupted.

The underlying method of interrupting a thread is to set this property to true, and the isInterrupted() method just returns the property value.

One difference between these two methods is that isInterrupted() cannot change the attribute value of interrupted(), but the interrupted() method can change the attribute value of interrupted, so in When determining when a thread is interrupted, we recommend using isInterrupted().

The above is the detailed content of What is thread interrupt in Java? What problems will Java thread interruption cause?. For more information, please follow other related articles on the PHP Chinese website!