synchronized实战：synchronized 锁升级过程

news2024/11/19 23:36:17

在这里插入图片描述

下面程序通过对加锁前后Object对象字节码的打印验证了对象由无锁到偏向锁的过程。

public class T01 {
    public static void main(String[] args) {
        Object o = new Object();
        System.out.println(ClassLayout.parseInstance(o).toPrintable());
        o.hashCode();
        System.out.println(ClassLayout.parseInstance(o).toPrintable());
        synchronized (o){
            System.out.println(ClassLayout.parseInstance(o).toPrintable());
        }
    }
}

运行上面程序，输出结果如下
在这里插入图片描述

Java 中的synchronized锁升级过程实际上是锁的粗化（Coarsening）和锁的细化（Fine-grained Locking）之间的权衡过程。锁升级的过程会根据执行情况来动态调整锁的粒度，以优化性能。下面是一个简单的程序示例，演示了锁的升级过程：

public class LockUpgradeDemo {

    private static final Object lock = new Object();

    public static void main(String[] args) {
        // 启动两个线程
        Thread thread1 = new Thread(new CoarseLockTask());
        Thread thread2 = new Thread(new FineGrainedLockTask());

        thread1.start();
        thread2.start();
    }

    // 粗粒度锁任务
    static class CoarseLockTask implements Runnable {
        @Override
        public void run() {
            synchronized (lock) {
                System.out.println("Coarse-grained Lock: Start");

                try {
                    Thread.sleep(1000); // 模拟耗时操作
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }

                System.out.println("Coarse-grained Lock: End");
            }
        }
    }

    // 细粒度锁任务
    static class FineGrainedLockTask implements Runnable {
        @Override
        public void run() {
            System.out.println("Fine-grained Lock: Start");

            synchronized (lock) {
                try {
                    Thread.sleep(500); // 模拟耗时操作
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }

                System.out.println("Fine-grained Lock: Inside synchronized block");
            }

            System.out.println("Fine-grained Lock: End");
        }
    }
}