java - 如何测试IO异常？

Question

我的代码中有一个 WriterReader 对象，它使用 FileOutputStream 和 ObjectOutputStream。因此，将对象写入文件会引发 IO 异常。我正在尝试一种方法来处理 FileNotFound 异常并抛出其他 IO 异常。此外，在处理 FileNotFound 异常时，我想使用 JUnit 5 对其进行测试。

我尝试了以下测试，使用日期作为唯一文件名，断言该文件 DNE，然后将对象写入该 DNE 的文件，这应该触发 FileNotFound 异常。在处理该异常时，我基本上创建了一个同名的新文件，并在此之前创建了目录。

代码:

@Test
void testWriteReadObjectFileNotFound() {
    String timeStamp = new SimpleDateFormat("yyyyMMddHHmmss").format(new Date());
    File file = new File("data\\test\\" + timeStamp );
    try {
        assertFalse(file.exists());
        System.out.println(file.exists());
        String s1 = "testObj";
        wr.writeObject(s1, file.getName());
        String s2 = (String) wr.readObject(file.getName());
        assertEquals("testObj", s2);
        assertTrue(file.exists());
    } catch (IOException e) {
        fail();
    } catch (ClassNotFoundException e) {
        fail();
    }
    file.delete();
}

//MODIFIES: file at filepath
//EFFECTS: Writes given object to file
public void writeObject(Object serObj, String fileName) throws IOException {
    FileOutputStream fileOut;
    ObjectOutputStream objectOut;
    try {
        new FileOutputStream(filepath + fileName);
    } catch (FileNotFoundException e) {
        File file = new File(filepath + fileName);
        file.getParentFile().mkdirs();
        file.createNewFile();
    } finally {
        fileOut = new FileOutputStream(filepath + fileName);
        objectOut = new ObjectOutputStream(fileOut);
        objectOut.writeObject(serObj);
        objectOut.close();
    }
}

但在我的代码覆盖范围内，它显示了以下行:

catch (FileNotFoundException e) {
        File file = new File(filepath + fileName);
        file.getParentFile().mkdirs();
        file.createNewFile();

不包括在内。有人能解释一下这种情况吗？

编辑:

在我用建议更新我的代码后，现在 intelliJ 显示我的测试涵盖了 write 方法，而在线 Jacoco Code Coverage 服务显示它根本没有涵盖。

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Answer 1

首先你错误地使用了try/catch结构。您滥用异常工具作为控制结构。应该是这样的:

public void writeObject(Object serObj, String fileName) throws 
    IOException {
        File theFile = new File(filepath + fileName);
        if(!theFile.exists()) {
            file.getParentFile().mkdirs();
            file.createNewFile();
        }

        try (FileOutputStream fileOut = new FileOutputStream(filepath + fileName);
             ObjectOutputStream objectOut = new ObjectOutputStream(fileOut)) {
             objectOut.writeObject(serObj);
        }
    }

有了这个，您的问题就变成了:我如何测试 if block ？

您可以通过将方法的第二个参数从 String 更改为 File 来实现:

public void writeObject(Object serObj, File theFile) throws IOException {
        if(!theFile.exists()) {
            file.getParentFile().mkdirs();
            file.createNewFile();
        }

        try (FileOutputStream fileOut = new FileOutputStream(filepath + fileName);
             ObjectOutputStream objectOut = new ObjectOutputStream(fileOut)) {
             objectOut.writeObject(serObj);
        }
    }

让此方法的调用者负责创建 File 对象。

然后在您的测试中，您可以传递一个mock 作为第二个参数。使用 Mockito 作为模拟框架，测试将如下所示:

class Test { 
  private String timeStamp = new SimpleDateFormat("yyyyMMddHHmmss").format(new Date());
  @Rule
  public final MethodRule mockito = MockitoJunit.rule(); 

  @Spy
  private File theFile = new File("data\\test\\" + timeStamp );

  @Mock
  private File theParent;

  @Before
  public void setup(){
     doReturn(theParent).when(theFile).getParentFile();
  }
  @Test
  void testWriteReadObjectFileNotFound() {
    // arrange
    doReturn(Boolean.FALSE).when(theFile).exists();
    String s1 = "testObj";
    // act
    wr.writeObject(s1, theFile);  
    // assert
    verify(theParent).mkdirs();
    verify.(theFile).createNewFile();
 }
}