Example analysis of daemon threads and non-daemon threads in Java

This article mainly introduces the daemon thread and non-daemon thread of Java. The editor thinks it is quite good. Now I will share it with you and give it as a reference. Let’s follow the editor to take a look.

Recently studied the basic knowledge of Java again, and found that too much knowledge was slightly ignored in the past. Comparing Java’s thread mechanism, there are two types of threads in Java: User Thread( User thread), Daemon Thread (daemon thread), (PS: I ignored it before).

It is estimated that students who have studied Unix development but have not studied Java in detail will be confused. There is no so-called daemon thread concept in the operating system. There is only daemon process, but the Java language mechanism is built on the JVM. Basically, it means that the Java platform shields the bottom layer of the operating system, so it can construct a mechanism that is beneficial to itself in its own virtual platform, and the designer of the language or platform is more or less responsible for the collection. Influenced by Unix thinking, and the daemon thread mechanism is suitable for platforms such as JVM, daemon threads came into being.

The role of Daemon is to provide services for the running of other threads, such as GC threads. In fact, there is essentially no difference between User Thread threads and Daemon Thread daemon threads. The only difference lies in the departure of the virtual machine: if all User Threads evacuate, then Daemon Thread will have no threads to serve, so The virtual machine also exits.

The daemon thread is not provided inside the virtual machine. Users can also set the daemon thread by themselves. Method: public final void setDaemon(boolean on); but there are a few points to note:

1 ), thread.setDaemon(true) must be set before thread.start(), otherwise an IllegalThreadStateException will be thrown. You cannot set a running regular thread to be a daemon thread. (Note: This point is obviously different from the daemon process. After the daemon process is created, it allows the process to get rid of the control of the original session + the process gets rid of the control of the original process group + the process gets rid of the control of the original control terminal; so it relies on virtual The language mechanism of the machine is essentially different from the system-level language)

2). The new threads generated in the Daemon thread are also Daemon's. (This is an essential difference: the child process that comes out of the daemon process fork() is no longer a daemon process. Although it copies the process-related information of the parent process, the parent process of the child process is not the init process. , the so-called daemon process essentially means "the parent process dies, init adopts it, and then files 0, 1, 2 are all /dev/null, and the current directory is /")

3), not all Applications can be assigned to Daemon threads to perform services, such as read and write operations or calculation logic. Because the virtual machine may have exited before Daemon Thread can perform the operation.

Example:

//完成文件输出的守护线程任务

import java.io.*; 

 

class TestRunnable implements Runnable{ 

  public void run(){ 

        try{ 

         Thread.sleep(1000);//守护线程阻塞1秒后运行 

         File f=new File("daemon.txt"); 

         FileOutputStream os=new FileOutputStream(f,true); 

         os.write("daemon".getBytes()); 

      } 

        catch(IOException e1){ 

     e1.printStackTrace(); 

        } 

        catch(InterruptedException e2){ 

         e2.printStackTrace(); 

      } 

  } 

} 

public class TestDemo2{ 

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

  { 

    Runnable tr=new TestRunnable(); 

    Thread thread=new Thread(tr); 

        thread.setDaemon(true); //设置守护线程 

    thread.start(); //开始执行分进程 

  } 

}

Run result: There is no "daemon" string in the file daemon.txt.

But if thread.setDaemon(true); //Set daemon thread is commented out, the file daemon.txt can be written into the daemon string

The standard for JRE to judge whether a program has ended is that all foreground execution threads have finished executing, regardless of the status of background threads. Therefore, you must pay attention to this issue when using background threads.

But where is the actual application of daemon Thread? For example, for a Servlet in a web server, when the container starts, a service thread, that is, a scheduling thread, is initialized in the background, which is responsible for processing http requests. Then for each request, the scheduling thread takes out a worker thread from the thread pool to process the request, so that To achieve the purpose of concurrency control.

A picture taken from the Internet to facilitate everyone’s understanding:

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