内部类无法访问从内部调用构造函数的方法初始化的外部类成员变量

下面的代码由于未初始化外部成员变量而引发空指针异常。 它正在为可运行变量抛出NPE in run方法。我已经在newFixedThreadPool方法中初始化了ConcurrentLinkedQueue类型的本地valiable,并调用参数化构造函数来初始化runnable成员变量。

顺便说一下,当我直接初始化runnable变量而不创建任何局部变量时,代码将成功运行。

有人能解释一下吗

import java.util.concurrent.ConcurrentLinkedQueue;

public class MyExecuterService  {

    private  ConcurrentLinkedQueue<Runnable> runnables;
    //private AtomicBoolean execute=AtomicBoolean.true;
    private PoolWorkerThread poolWorkerThreads[];

    public MyExecuterService()
    {}

    public MyExecuterService(ConcurrentLinkedQueue<Runnable> runnables,PoolWorkerThread poolWorkerThreads1[])
    {
        this.runnables=runnables;
        poolWorkerThreads=poolWorkerThreads1;
    }

    private class PoolWorkerThread extends Thread{

        @Override
        public void run() {
            while(true) {
                System.out.println(runnables.size()+"runnable"+runnables);
                synchronized (runnables) {

                    if(runnables.size()==0)
                    {
                        try {
                        runnables.wait();
                        } catch (InterruptedException e) {
                            e.printStackTrace();
                        }
                    }
                    else
                    {
                        Runnable task=runnables.poll();
                        task.run();
                        //notify();
                    }
                }   
            }
        }
    }

    public  MyExecuterService newFixedThreadPool(int i)
    {
        ConcurrentLinkedQueue<Runnable> runnableQueue= new ConcurrentLinkedQueue<Runnable>();

        PoolWorkerThread [] threads= new PoolWorkerThread[i];
        for (int j = 0; j < i; j++) {
            threads[j]= new PoolWorkerThread();
            threads[j].start();
        }
        return new MyExecuterService(runnableQueue, threads);
    }

    public void execute(Runnable runnable) {
        System.out.println(runnables.size());
        synchronized(runnables)
        {
            runnables.add(runnable);
            runnables.notify();
        }
    }
    public ConcurrentLinkedQueue<Runnable> getRunnables() {
        return runnables;
    }

    public void setRunnables(ConcurrentLinkedQueue<Runnable> runnables) {
        this.runnables = runnables;        
    }
}