Java 实例级别的锁和类级别的锁

实例级别的锁

实例代码

@Slf4j
public class AddCompareDemo {
    private int a, b;

    public void add() {
        for (int i = 0; i < 10000; i++) {
            a++;
            b++;
        }
    }

    public void compare() {
        for (int i = 0; i < 10000; i++) {
            if (a < b) {
                log.info("a:{},b:{},{}", a, b, a > b);
            }
        }
    }

}

执行

@SpringBootApplication
public class NewdemoApplication {

    public static void main(String[] args) throws Exception {
        SpringApplication.run(NewdemoApplication.class, args);
        AddCompareDemo demo = new AddCompareDemo();
        new Thread(() -> {
            demo.add();
        }).start();
        new Thread(() -> {
            demo.compare();
        }).start();
    }

}

执行结果

 INFO 39336 --- [      Thread-30] com.example.newdemo.AddCompareDemo       : a:5262,b:5894,true

解决

这是同一个实例之间的行程安全问题。在方法上增加synchronize字段

@Slf4j
public class AddCompareDemo {
    private int a, b;

    public synchronized void add() {
        for (int i = 0; i < 10000; i++) {
            a++;
            b++;
        }
    }

    public synchronized void compare() {
        for (int i = 0; i < 10000; i++) {
            if (a < b) {
                log.info("a:{},b:{},{}", a, b, a > b);
            }
        }
    }
}

即可解决问题

类级别

代码

public class ClassLevelSyncDemo {

    private static int data;//类基本静态字段

    public synchronized void count() {//此处默认是实例级别的锁
        data++;
    }

    public static int getData() {
        return data;
    }
}

执行

@SpringBootApplication
@Slf4j
public class NewdemoApplication {

    public static void main(String[] args) throws Exception {
        SpringApplication.run(NewdemoApplication.class, args);
        IntStream.rangeClosed(1, 100000).parallel().forEach(i ->
        {
            new ClassLevelSyncDemo().count();
        });
        log.info("" + ClassLevelSyncDemo.getData());

    }

}

执行结果

28988 不是100000.

解决方法

public class ClassLevelSyncDemo {

    private static int data;//类基本静态字段
    private static Object object = new Object();

    public void count() {
        synchronized (object) {
            data++;
        }
    }

    public static int getData() {
        return data;
    }
}