一个关于死锁的问题

class A
{
    public synchronized void foo(B b)
    {
        System.out.println("当前线程名："+Thread.currentThread().getName()
                        +"进入了A的实例发foo方法");
        try
        {
            Thread.sleep(200);
        }
        catch(InterruptedException ie)
        {
            ie.printStackTrace();
        }
        System.out.println("当前线程名："+Thread.currentThread().getName()
                        +"尝试调用B实例的last()方法");
        b.last();
    }
    public synchronized void last()
    {
        System.out.println("进入了A类的last()方法");
    }
}
class B
{
    public synchronized void bar(A a)
    {
        System.out.println("当前线程名："+Thread.currentThread().getName()
                        +"进入了B的实例发foo方法");
        try
        {
            Thread.sleep(200);
        }
        catch(InterruptedException ie)
        {
            ie.printStackTrace();
        }
        System.out.println("当前线程名："+Thread.currentThread().getName()
                        +"尝试调用A实例的last()方法");
        a.last();
    }
    public synchronized void last()
    {
        System.out.println("进入了B类的last()方法");
    }
}
public class DeadLockDemo implements Runnable
{
    A a = new A();
    B b = new B();
    public void info()
    {
        Thread.currentThread().setName("主线程");
        a.foo(b);
        System.out.println("进入主线程之后");
    }
    @Override
    public void run()
    {
        Thread.currentThread().setName("副线程");
        b.bar(a);
        System.out.println("进入副线程之后");
    }
    public static void main(String[] args)
    {
        DeadLockDemo d = new DeadLockDemo();
        d.info();
        Thread t1 = new Thread(d);
        t1.start();
        /**
         *把d.info()放在前面就能正常运行
         *DeadLockDemo d = new DeadLockDemo();
         *d.info();
         *Thread t1 = new Thread(d);
         *t1.start();
         */
    }
}