synchronized 不是万能，错误加锁

Java支持多个线程同时访问一个对象或者对象的成员变量，synchronized 关键字可以修饰方法或者同步代码块的方法，确保多个线程在同一时刻，只能有一个线程处于方法或者同步代码块中，保证了线程对变量访问的可见性和排他性

 

public class Demo052501 {

    private static class ThreadDemo implements Runnable{
        private Integer i;

        public  ThreadDemo(Integer i){
            this.i = i;
        }
        @Override
        public void run() {
            synchronized (i){
                Thread thread = Thread.currentThread();
                System.out.println(thread.getName() + "-------"+ i+ "@----" + System.identityHashCode(this.i));
                i++;
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }

    public static void main(String[] args) {
        ThreadDemo worker = new ThreadDemo(1);

        for(int i =0;i<6;i++){
            new Thread(worker).start();
        }
    }
}

预期输出：

i : 1-2-3-4-5-6

实际输出：

 

错误加锁原因：

.class 文件反编译，i++ 的代码为：

Integer.valueOf()

synchronized 内置锁，锁的是对象，保证对象唯一！！！

 

解决方法

public class Demo052501 {

    private static class ThreadDemo implements Runnable{
        private Integer i;
        private Object o = new Object();

        public  ThreadDemo(Integer i){
            this.i = i;
        }
        @Override
        public void run() {
            synchronized (o){
                Thread thread = Thread.currentThread();
                System.out.println(thread.getName() + "-------"+ i+ "@----" + System.identityHashCode(this.i));
                i++;
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        }
    }

    public static void main(String[] args) {
        ThreadDemo worker = new ThreadDemo(1);

        for(int i =0;i<6;i++){
            new Thread(worker).start();
        }
    }
}