如何在线程中同步使用

Question

Without synchronized i ran this below code, and i am getting in sequential order as below.

Thread-1 got 1
Thread-0 got 2
Thread-1 got 3
Thread-0 got 4
Thread-1 got 5
Thread-0 got 6
Thread-1 got 7
Thread-0 got 8
Thread-1 got 9
Thread-0 got 10
Thread-1 got 11
Thread-0 got 12
Thread-1 got 13
Thread-0 got 14
Thread-1 got 15
Thread-0 got 16
Thread-1 got 17
Thread-0 got 18
Thread-1 got 19
Thread-0 got 20
Thread-1 got 21
Thread-0 got 22
Thread-1 got 23
Thread-0 got 24
Thread-1 got 25
Thread-0 got 26
Thread-1 got 27
Thread-0 got 28
Thread-1 got 29
Thread-0 got 30
Thread-1 got 31
Thread-0 got 32
Thread-1 got 33
Thread-0 got 34
Thread-1 got 35
Thread-0 got 36
Thread-0 got 38
Thread-1 got 37

shouldn't i get something like repeated sequence as i am not using synchronized? Why am not getting output such as below,

Thread-1 got 1 Thread-0 got 1 Thread-1 got 3 Thread-0 got 3 Thread-1 got 4 Thread-0 got 4

the code i used below and Output is in sequence without using thread safety.

public class T1 {

    public static void main(String[] args) {

        Increment inc = new Increment();

        runI r = new runI(inc);
        r.start();

        runI r1 = new runI(inc);
        r1.start();

    }

}

class runI extends Thread {

    Increment iv = null;

    public runI(Increment iv) {
        this.iv = iv;
    }

    public void run() {

        for (int i = 1; i < 20; i++) {

            System.out.println(Thread.currentThread().getName() + " got " + iv.getNext());
        }

    }

}
public class Increment {

    int value = 0;

    public int getNext() {

        value++;
        return value;

    }

}

Answer 1

Note System.out.println will synchronized on the PrintStream instance. So this behavior makes the multi threads will do sync on printing. In your case, make the concurrent issue at low probability.

Use one of below ways to produce concurrent issue easily:

1. only print result at last

This way will reduce the affect of sync on PrintStream instance.

StringBuilder builder = new StringBuilder();
for (int i = 1; i < 20; i++) {
    builder.append(Thread.currentThread().getName() + " got " + iv.getNext()).append("\n");
}
System.out.println(builder.toString());

instead of printing it each time:

for (int i = 1; i < 20; i++) {
    System.out.println(Thread.currentThread().getName() + " got " + iv.getNext());
}

2. Use more threads

On my own pc(cpu with 6 cores), use 4 or more threads will easy produce the concurrent issue. Below is the result under 4 threads:

Thread-1 got 1
Thread-2 got 2
Thread-3 got 3
Thread-0 got 1
Thread-3 got 6
Thread-2 got 5
Thread-1 got 4
Thread-2 got 9
Thread-3 got 8
Thread-3 got 12
Thread-3 got 13
Thread-0 got 7
Thread-3 got 14
Thread-3 got 16
Thread-3 got 17
Thread-3 got 18
Thread-2 got 11
Thread-1 got 10
Thread-2 got 20
Thread-3 got 19
Thread-3 got 23
Thread-0 got 15
Thread-3 got 24
Thread-2 got 22
Thread-1 got 21
Thread-1 got 28
Thread-2 got 27
Thread-3 got 26
Thread-3 got 31
Thread-3 got 32
Thread-3 got 33
Thread-0 got 25
Thread-3 got 34
Thread-2 got 30
Thread-1 got 29
Thread-2 got 37
Thread-2 got 39
Thread-3 got 36
Thread-0 got 35
Thread-3 got 41
Thread-2 got 40
Thread-1 got 38
Thread-2 got 43
Thread-0 got 42
Thread-2 got 45
Thread-1 got 44
Thread-1 got 48
Thread-2 got 47
Thread-0 got 46
Thread-2 got 50
Thread-1 got 49
Thread-1 got 53
Thread-1 got 54
Thread-2 got 52
Thread-0 got 51
Thread-2 got 56
Thread-1 got 55
Thread-2 got 58
Thread-2 got 60
Thread-0 got 57
Thread-1 got 59
Thread-0 got 61
Thread-1 got 62
Thread-1 got 64
Thread-0 got 63
Thread-1 got 65
Thread-0 got 66
Thread-0 got 68
Thread-0 got 69
Thread-0 got 70
Thread-0 got 71
Thread-0 got 72
Thread-0 got 73
Thread-0 got 74
Thread-1 got 67
Thread-1 got 75

Here we can see the number 1 repeat in more than one threads.

Answer 2

Well, I guess it's just a coincidence that in your case you had a right sequence. Without synchronization there is no guarantee to get it, eg for me your code has returned this:

Thread-0 got 1
Thread-0 got 3
Thread-0 got 4
Thread-1 got 2
Thread-0 got 5
Thread-1 got 6
Thread-0 got 7
Thread-1 got 8
Thread-0 got 9
Thread-1 got 10
Thread-0 got 11
Thread-1 got 12
Thread-0 got 13
Thread-1 got 14
Thread-0 got 15
Thread-0 got 17
Thread-0 got 18
Thread-1 got 16
Thread-0 got 19
Thread-1 got 20
Thread-0 got 21
Thread-1 got 22
Thread-0 got 23
Thread-1 got 24
Thread-0 got 25
Thread-1 got 26
Thread-0 got 27
Thread-1 got 28
Thread-0 got 29
Thread-1 got 30
Thread-0 got 31
Thread-1 got 32
Thread-0 got 33
Thread-1 got 34
Thread-1 got 35
Thread-1 got 36
Thread-1 got 37
Thread-1 got 38

In general it mostly depends on a CPU type and a bunch of another factors. Take a look at a nice tutorial for more information about multithreading and concurrent programming