Java中在子线程中获取固定大小的连接池出现错误，如果解释？报错

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问题1. 不太好描述，直接看代码吧

public class QueueConsumer extends EndPoint implements Runnable {

// 线程池
private ExecutorService executor = null;

public QueueConsumer(String endPointName, String hosts, String ports,
        String username, String password) throws Exception {
    super(endPointName, hosts, ports, username, password);
    // 初始化线程池
    executor = Executors.newCachedThreadPool();
}

@Override
public void run() {
    try {
        QueueingConsumer consumer = new QueueingConsumer(channel);
        channel.basicConsume(endPointName, false, consumer);
        while (true) {

            // 信息到达时
            Delivery delivery = consumer.nextDelivery();
            
            // 初始化业务线程对象
            BusinessClient businessClient = new BusinessClient(channel, delivery);
            // 执行任务
            executor.execute(businessClient);
        }
    } catch (Exception e) {
        LogUtil.printException("消息队列异常", e, LogUtil.LOG_LEVEL_ERROR);
    } finally {
        try {
            close();
        } catch (Exception e) {
            LogUtil.printException("关闭消息队列异常", e, LogUtil.LOG_LEVEL_ERROR);
        }
    }
}

}

public class BusinessClient implements Runnable { ... 省略

// 主要代码
List<Callable<Object>> taskList = new ArrayList<Callable<Object>>();
for (BaseRequestData<BaseResponseData> queryData : queryDataLst) {
    QueryThread queryThread = new QueryThread(queryData);
    taskList.add(queryThread);
}

// 问题点：此处使用Executors.newFixedThreadPool(i)会抛出IllegalArgumentException
          但是使用Executors.newCachedThreadPool()则不会有问题.不是很理解，为什么此处不可以用newFixedThreadPool(i)?
          
ExecutorService executor = Executors.newCachedThreadPool();
List<Future<Object>> results = executor.invokeAll(taskList);
executor.shutdown();

... 省略

}

问题2. 我如果定义一个线程池工厂，将线程池定义为static final，每个需要并发的地方都使用工厂来获得线程池，

   那么问题来了，我这个线程池会自动扩展或者释放线程吗？
   
   

纯粹自学的Java, 对于一些基础知识不是很理解，望不吝赐教！谢谢