目的：用两个线程来共同打印a - z，而且没有线程安全问题

运行下面代码后，每次只有一个线程启动，这个问题在哪里？
package java16;

public class PrintL implements Runnable{
    private char c = 'a';
    @Override
    public synchronized void run() {
        // TODO Auto-generated method stub
        while (c <= 'z') {
            System.out.println(Thread.currentThread().getName() + ": " + c);

            try {
                Thread.currentThread().sleep(10);
            } catch (InterruptedException e) {
                e.printStackTrace();
            }

            c++;
        }
    }
    public static void main(String[] args) {
        PrintL printL = new PrintL();
        Thread thread1 = new Thread(printL);
        Thread thread2 = new Thread(printL);
        thread1.start();
        thread2.start();
    }
}