在方法中定义一个类的写法

在《JAVA并发编程实战》这本书里，看到一种以前没有见过的写法，就是在方法里面定义一个类，这种写法太少见了，而且搜索的话也没有什么博客介绍过。在此记录一下书中给出的代码，是符合java的语法的

public class TimedRun2 {
    private static final ScheduledExecutorService cancelExec = newScheduledThreadPool(1);

    public static void timedRun(final Runnable r,
                                long timeout, TimeUnit unit)
            throws InterruptedException {
        class RethrowableTask implements Runnable {
            private volatile Throwable t;

            public void run() {
                try {
                    r.run();
                } catch (Throwable t) {
                    this.t = t;
                }
            }

            void rethrow() {
                if (t != null)
                    throw launderThrowable(t);
            }
        }

        RethrowableTask task = new RethrowableTask();
        final Thread taskThread = new Thread(task);
        taskThread.start();
        cancelExec.schedule(new Runnable() {
            public void run() {
                taskThread.interrupt();
            }
        }, timeout, unit);
        taskThread.join(unit.toMillis(timeout));
        task.rethrow();
    }
}