why java thread cannot get the newest value

Question

I have a simple program to test for multi-thread in java

public class Test {
    public static void main(String[] args) {
        final Data data = new Data();

        for (int i = 0; i < 3; i++) {
            new Thread(new Runnable() {
                public void run() {
                    for (int j = 0; j < 5; j++) {
                        data.set(new Random().nextInt(30));
                    }
                }
            }).start();
        }       

        for (int i = 0; i < 3; i++) {
            new Thread(new Runnable() {
                public void run() {
                    for (int j = 0; j < 5; j++) {
                        data.get();
                    }
                }
            }).start();
        }
    }
}

class Data {    
    private int data;   

    public void set(int data) {
        try {
            Thread.sleep(20);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(Thread.currentThread().getName() + " ready to write data");
        this.data = data;
        System.out.println(Thread.currentThread().getName() + " write " + this.data);
    }   

    public void get() {
        try {
            Thread.sleep(20);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println(Thread.currentThread().getName() + " ready to read data");
        System.out.println(Thread.currentThread().getName() + " read " + this.data);
    }
}

The result is random, while in one time, I got the result like that:

Thread-3 ready to read data
Thread-2 ready to write data
Thread-1 ready to write data
Thread-0 ready to write data
Thread-0 write 20
Thread-4 ready to read data
Thread-1 write 10
Thread-5 ready to read data
Thread-2 write 10
Thread-3 read 0
Thread-5 read 20
Thread-4 read 20
etc

I'm puzzled about that, the data variable in Data class is shared by all threads, while the last write operation is Thread-2 write 10, I think the data value should be 10 now, why Thread-3 read 0, Thread-5 and Thread-4 read 20? I think they should read 10

while, you're right, I should add volatile for data variable, but after that, the result becomes:

Thread-0 ready to write data
Thread-2 ready to write data
Thread-3 ready to read data
Thread-1 ready to write data
Thread-5 ready to read data
Thread-5 read 22
Thread-3 read 16
Thread-4 ready to read data
Thread-2 write 16
Thread-0 write 22
etc

why the read operation can be ahead of write operation, 22 and 16 is read before they are write, at least the print value shows that

I know I have not added synchronized, I'm puzzled about the reason for it, multi threads go into the set and get method, but why the thread cannot get the newest value — gesanri, Jul 05 '18 at 03:24
@gesanri variable values are cached thread-locally in java. If you want to access the latest value from another thread you need to mark it `volatile` which will force it to be read from main memory. — gregwhitaker, Jul 05 '18 at 03:25
That is an essential part of multi threading: without precautions there are no guarantees when/if thread A sees values written by thread B. — GhostCat, Jul 05 '18 at 03:26

why java thread cannot get the newest value

0 Answers0