public class Main {
    public static void main(String[] args) throws InterruptedException {
        threadSafeTest();
    }

    public static void threadSafeTest() throws InterruptedException {
        MyThread1 thread1 = new MyThread1();
        MyThread1 thread2 = new MyThread1();
        thread1.start();
        Thread.currentThread().sleep(250);
        thread2.start();
    }
}

class ThreadSafeSingletone{
    private static final ThreadSafeSingletone instance = new ThreadSafeSingletone();
    private Integer ratio = new Integer(0);
    private ThreadSafeSingletone(){

    }
    public static ThreadSafeSingletone getInstance(){
        return instance;
    }

    public Integer getRatio() {
        return ratio;
    }

    public void setRatio(Integer ratio) {
        this.ratio = ratio;
    }

    public synchronized void printRatio() {
        Integer ratio = getRatio();
        try {
            Thread.currentThread().sleep(500);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        System.out.println("Thread " + Thread.currentThread().getId() + ": " + ratio + "+1=" + (getRatio() + 1));
        setRatio(getRatio() + 1);
    }
}

class MyThread1 extends Thread{
    private ThreadSafeSingletone singletone = ThreadSafeSingletone.getInstance();
    @Override
    public void run(){
        for(int i = 0; i < 5; i++) {
            //System.out.println("!" + Thread.currentThread().getId());
            singletone.printRatio();
        }
    }
}

Thread 10: 0+1=1
Thread 10: 1+1=2
Thread 10: 2+1=3
Thread 10: 3+1=4
Thread 10: 4+1=5
Thread 11: 5+1=6
Thread 11: 6+1=7
Thread 11: 7+1=8
Thread 11: 8+1=9
Thread 11: 9+1=10

Thread 10: 0+1=1
Thread 11: 1+1=2
Thread 10: 2+1=3
Thread 11: 3+1=4
Thread 10: 4+1=5
Thread 11: 5+1=6
Thread 10: 6+1=7
Thread 11: 7+1=8
Thread 10: 8+1=9
Thread 11: 9+1=10

Thread 10: 0+1=1
Thread 11: 1+1=2
Thread 10: 2+1=3
Thread 11: 3+1=4
Thread 10: 4+1=5
Thread 11: 5+1=6
Thread 10: 6+1=7
Thread 11: 7+1=8
Thread 10: 8+1=9
Thread 11: 9+1=10

Thread 10: 0+1=1
Thread 11: 1+1=2
Thread 10: 2+1=3
Thread 11: 3+1=4
Thread 10: 4+1=5
Thread 11: 5+1=6
Thread 10: 6+1=7
Thread 11: 7+1=8
Thread 10: 8+1=9
Thread 11: 9+1=10