java 线程加synchronized和不加synchronized有什么区别？

红太狼 2024-03-26 03:57 23阅读 0赞

/**
     * 加synchronized
     */
    public class SyncThread {
     
        private int count;
     
        public synchronized void increase() {
            count++;
        }
     
        public static void main(String[] args) {
            SyncThread syncThread = new SyncThread();
            for (int i = 0; i < 10; i++) {
                new Thread(() -> {
                    for (int j = 0; j < 1000; j++)
                        syncThread.increase();
                }).start();
            }
            while (Thread.activeCount() > 2) {
                Thread.yield();
            }
            System.out.println(syncThread.count);  // 输出 10000
        }
    }

/**
     * 不加synchronized
     */
    public class UnsyncThread {
     
        private int count;
     
        public void increase() {
            count++;
        }
     
        public static void main(String[] args) {
            UnsyncThread unsyncThread = new UnsyncThread();
            for (int i = 0; i < 10; i++) {
                new Thread(() -> {
                    for (int j = 0; j < 1000; j++)
                        unsyncThread.increase();
                }).start();
            }
            while (Thread.activeCount() > 2) {
                Thread.yield();
            }
            System.out.println(unsyncThread.count);  // 输出小于 10000
        }
    }

> 加synchronized：保证了多线程并发时count的正确性，最后输出结果为10000；  
> 不加synchronized：因为多线程并发，可能会出现count++操作被打断的情况，最后输出结果小于10000。