使用 singleThreadExecutor 和 executorServices 执行时出现 NullpointerException答案

【问题标题】：Getting NullpointerException while executing with singleThreadExecutor and executorServices使用 singleThreadExecutor 和 executorServices 执行时出现 NullpointerException
【发布时间】：2019-06-21 06:35:17
【问题描述】：

我创建了一个程序，使用线程从文件中读取数据并显示。我得到空指针异常，第一次线程 1 打印来自文本文件的第一条记录和它从文件中删除的第二个数据，它抛出空指针

public static void main(String[] args) throws InterruptedException, ExecutionException {
        ExecutorService es = Executors.newSingleThreadExecutor();
        List<String> tasks = getUsersFromFile("new_users.txt");
        DoctorDao dao = new DoctorDao();
        for (String data : tasks) {
            Future<Boolean> result = es.submit(new DoctorTask(data, dao));
            while (result.get()) {
            System.out.println("data stored!!!");
            }
        }
        es.shutdown();
        System.out.println("task done!!!");
    }

    public static List<String> getUsersFromFile(String fileName) {
        List<String> users = new ArrayList<>();
        try (BufferedReader reader = new BufferedReader(new FileReader(new File(fileName)))) {
            String line = null;
            while ((line = reader.readLine()) != null) {
                users.add(line);
            }
        } catch (FileNotFoundException ex) {
            // Logger.getLogger(TestExecutors.class.getName()).log(Level.SEVERE, null, ex);
        } catch (IOException ex) {
            // Logger.getLogger(TestExecutors.class.getName()).log(Level.SEVERE, null, ex);
        }
        return users;

在Callable实现类中

    public Boolean call() throws Exception {
        Boolean status = false;
        System.out.println(Thread.currentThread().getName() + " processing record for : " + doctorRecord);
        StringTokenizer tokenizer = new StringTokenizer(this.doctorRecord, ",");
        Doctor doc = null;
        while (tokenizer.hasMoreTokens()) {
            doc = new Doctor();
            doc.setEmailAddress(tokenizer.nextToken());
            doc.setName(tokenizer.nextToken());
            doc.setId(Integer.valueOf(tokenizer.nextToken()));
            status = dao.saveUser(doc);
        }

        return status;
    }

Exception in thread "main" java.util.concurrent.ExecutionException: java.lang.NullPointerException
    at java.util.concurrent.FutureTask.report(FutureTask.java:122)
    at java.util.concurrent.FutureTask.get(FutureTask.java:192)
    at com.executor.demo.readfile.TestDotorExecutor.main(TestDotorExecutor.java:23)
Caused by: java.lang.NullPointerException
    at com.executor.demo.readfile.DoctorDao.saveUser(DoctorDao.java:5)
    at com.executor.demo.readfile.DoctorTask.call(DoctorTask.java:28)
    at com.executor.demo.readfile.DoctorTask.call(DoctorTask.java:1)
    at java.util.concurrent.FutureTask.run(FutureTask.java:266)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
    at java.lang.Thread.run(Thread.java:748)

【问题讨论】：

标签： executorservice executor

【解决方案1】：

我在statiDB类返回空值的代码中出错了，我没有初始化List Object，所以我修改了代码如下所示

上述问题中缺少以下完整代码

在 dao 类中：

public class DoctorDao {
    public boolean saveUser(Doctor doc) {
        boolean storedStatus = StaticDB.getDoctorList().add(doc);
        return storedStatus;
    }
}

在数据库类中

class StaticDB {
    static List<Doctor> docList=null;
        
    public static List<Doctor> getDoctorList() {
        Doctor doc1 = new Doctor(111,"Shalu","ss@gmail.com");
        Doctor doc2 = new Doctor(112,"luke","luke@gmail.com");
        docList = new ArrayList<Doctor>(); //This was missing 
        docList.add(doc1);
        docList.add(doc2);

        return docList;
    }
}

【讨论】：