jvm - Are Java new objects atomic?

Question

public static void main(Sting args[]){
     Object a=null;
     new Thread(){
              a=new xxx()
     }.start();

     new Thread(){
              a=new xxx()
     }.start();
}

I would like to ask, there is complex object initialization logic in the xxx() method. Is the object created by the new keyword atomic? If not, will there be a problem of object initialization disorder?

Answer 1

I don’t understand what you mean, if my guess is correct:

It all depends on the specific logic in your construction method. After all, the code is written by humans.

public class Test {
    static class A{
        public A(){
            try {
                SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd:hh:mm:ss:SS");
                System.out.println(sdf.format(new Date()) + "--begin --从线程" + Thread.currentThread().getName() + "中创建A");
                Thread.sleep(2000);
                System.out.println(sdf.format(new Date()) + "--end--从线程" + Thread.currentThread().getName() + "中创建A");
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
    
    public static void main(String[] args) {
        new Thread(new Runnable(){

            @Override
            public void run() {
                System.out.println("A is " +new A());
            }
            
        }).start();
        
        new Thread(new Runnable(){

            @Override
            public void run() {
                System.out.println("A is " +new A());
            }
            
        }).start();
    }
}

Output:

2017-06-16:11:46:43:780--begin --从线程Thread-1中创建A
2017-06-16:11:46:43:780--begin --从线程Thread-0中创建A
2017-06-16:11:46:45:786--end--从线程Thread-0中创建A
2017-06-16:11:46:45:786--end--从线程Thread-1中创建A
A is nwe.Test$A@1e6a629c
A is nwe.Test$A@27fcb25d

Another example, the constructor contains a synchronization block, and each thread needs to wait for the execution of the previous thread to complete before it can execute.

import java.text.*;
import java.util.Date;

public class Test {
    static class A{
        public A(){
            try {
                synchronized (Test.class) {
                    SimpleDateFormat sdf = new SimpleDateFormat("yyyy-MM-dd:hh:mm:ss:SS");
                    System.out.println(sdf.format(new Date()) + "--begin --从线程" + Thread.currentThread().getName() + "中创建A");
                    Thread.sleep(2000);
                    System.out.println(sdf.format(new Date()) + "--end--从线程" + Thread.currentThread().getName() + "中创建A");
                }
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
    
    public static void main(String[] args) {
        new Thread(new Runnable(){

            @Override
            public void run() {
                System.out.println("A is " +new A());
            }
            
        }).start();
        
        new Thread(new Runnable(){

            @Override
            public void run() {
                System.out.println("A is " +new A());
            }
            
        }).start();
    }
}

Output:

2017-06-16:11:49:33:548--begin --从线程Thread-0中创建A
2017-06-16:11:49:35:549--end--从线程Thread-0中创建A
A is nwe.Test$A@717c3e10
2017-06-16:11:49:35:550--begin --从线程Thread-1中创建A
2017-06-16:11:49:37:553--end--从线程Thread-1中创建A
A is nwe.Test$A@27280786

Answer 2

It is recommended to refer to the thread-safe singleton mode

Answer 3

No, for example, if multiple pieces of logic are written in the construction method, it can be interrupted when the construction method is executed.

Answer 4

The description of "atomicity" is too abstract. When the poster asks questions, it is best not to think that everyone has exactly the same understanding of a certain word. All I can say is that the constructor is thread-safe. For each object, the constructor will only be executed once and by one thread.