在非静态方法中使用同步块锁定实例

按照下面的代码,我有两个类A的实例-a1和a2。并分别对两个实例调用foo()方法。

foo()方法中有一个同步块,它被锁定在调用对象上。因为这是一个实例级锁定,所以这两个方法应该同时开始执行,因为它们是从两个不同的实例调用的。但是,他们是按顺序被处决的。

我的理解是“this”指的是调用Runnable实例,它是不同的,甚至线程也是不同的。因此,Thread.sleep不应该阻塞另一个线程。那么,为什么run的两个调用没有并行发生呢?

main <time> Inside A.run
Thread-0 <time> Inside A.run
Thread-0 <time+4s> Exiting A.run
main <time+5s> Exiting A.run

实际输出(按顺序执行)

main <time> Inside A.run
main <time+5s> Exiting A.run
Thread-0 <time+5s> Inside A.run
Thread-0 <time+9s> Exiting A.run 

import java.time.*;
import java.time.format.DateTimeFormatter;

public class Test {

    public static void main(String[] args) {
        /*A a1 = new A(5000); A a2 = new A(4000);
        a1.foo(); a2.foo();*/
        A a1 = new A(5000); A a2 = new A(4000);
        Thread t = new Thread(a2);
        /*a1.run(); t.start();*/
        t.start(); a1.run(); // <-- putting t.start() before a1.run() solves the issue
    }

}

class A implements Runnable {
    public long waitTime;
    public A() {}
    public A(long timeInMs) {
        waitTime = timeInMs;
    }
    public void run() {
        synchronized(this) {
            try {
                DateTimeFormatter formatter = DateTimeFormatter.ofPattern("yyyy/MM/dd HH:mm:ss");
                LocalDateTime time = LocalDateTime.now();
                System.out.println(Thread.currentThread().getName() + " " + formatter.format(time) + " Inside A.run");
                Thread.sleep(waitTime);
                time = LocalDateTime.now();
                System.out.println(Thread.currentThread().getName() + " " + formatter.format(time) + " Exiting A.run");
            } catch (InterruptedException e) {}
        }
    }

}