如何修复FileReader错误，其中getCause在读取csv文件时指向null？

Question

我正在测试csv文件读取，我一直得到同样的错误。显示的错误是“无法读取请求的文件。原因：null”。这是我的代码的一个愚蠢的版本，仍然会产生相同的错误。

public class PersonReader {
    public static List<Person> readPersons(String path) throws IOException{
        ArrayList<Person> persons = new ArrayList<>();
        BufferedReader reader = null;

        try {
            reader = new BufferedReader(new FileReader(path));
            String line = null; // read first line

            while((line = reader.readLine()) != null) {
                persons.add(new Person(line));
            }
        } finally {
            if(reader != null) {
                reader.close();
            }
        }

        return persons;
}

public class Person {
    private String name;

    public Person(String name) {
        this.name = name;
    }

    @Override
    public String toString() {
        return name;
    }
}

public class Main{
    public static void main(String[] args) {
        List<Person> persons = null;

        try {
            persons = PersonReader.readPersons("sample/Data.csv");
        } catch (IOException e) {
            System.err.println("Could not read the requested file. Cause: " + e.getCause());
        }

        if(persons == null) { // some error has occurred
            System.exit(1);
        }

        System.out.println(persons);
    }
}

我试过的PersonReader的替代版本会产生相同的错误

public class PersonReader {
    public static List<Person> readPersons(String path) throws IOException{
        ArrayList<Person> persons = new ArrayList<>();
        BufferedReader reader = null;

        try {
            reader = Files.newBufferedReader(Paths.get(path));
            String line = null;

            while((line=reader.readLine()) != null) {
                persons.add(new Person(line));
            }
        } finally {
            if(reader != null) {
                reader.close();
            }
        }

        return persons;
    }

StackTrace出品

java.io.FileNotFoundException:   (The system cannot find the path specified)
    at java.base/java.io.FileInputStream.open0(Native Method)
    at java.base/java.io.FileInputStream.open(FileInputStream.java:219)
    at java.base/java.io.FileInputStream.<init>(FileInputStream.java:157)
    at java.base/java.io.FileInputStream.<init>(FileInputStream.java:112)
    at java.base/java.io.FileReader.<init>(FileReader.java:60)
    at sample.PersonReader.readPersons(PersonReader.java:17)
    at sample.Main.main(Main.java:14)

Answer 1

我用System.err.println("Could not read the requested file. Cause: " + e.getCause());取代e.printStackTrace()得到这个：

java.io.FileNotFoundException: sample/Data.csv (No such file or directory)
    at java.io.FileInputStream.open0(Native Method)
    at java.io.FileInputStream.open(FileInputStream.java:195)
    at java.io.FileInputStream.<init>(FileInputStream.java:138)
    at java.io.FileInputStream.<init>(FileInputStream.java:93)
    at java.io.FileReader.<init>(FileReader.java:58)
    at xxx.yyy.zzz.PersonReader.readPersons(A.java:37)
    at xxx.yyy.zzz.A.main(A.java:73)

问题是FileReader无法使用其相对路径找到文件 - 相对于当前工作目录没有文件“sample / Data.csv”。您可以提供绝对路径，或者如果您可以在项目资源中添加文件，则可以执行以下操作：

  InputStream inputStream = PersonReader.class.getClassLoader().getResourceAsStream("sample/Data.csv");
  InputStreamReader streamReader = new InputStreamReader(inputStream, StandardCharsets.UTF_8);
  BufferedReader reader = new BufferedReader(streamReader);

Answer 2

在Java中，Exception是Throwable的子类。 Throwable.getCause()方法返回“导致抛出此抛出物的throwable”（根据Javadoc）。这通常可以是null。

相反，调用Throwable.getMessage()，这个“[r] eturns这个throwable的详细消息字符串”。