我正在对下面的代码段进行测试，我很难让这个方法抛出ioexception，这样我就可以获得100%的覆盖率。我曾试图嘲笑chararrayreader和stringwriter类，但没有成功。非常感谢您的帮助！
要测试的类

public static final String getValue(String content) {
    if (content == null) return null;
    CharArrayReader reader = new CharArrayReader(content.toCharArray());
    StringWriter writer = new StringWriter();
    try {
        int c;
        while ((c = reader.read()) != -1) {
            if (isChinese((char)c)) {
                writer.write(c);
            } else {
                if ( (char)c > 0x20 && (char)c < 0x7f ) {
                    writer.write(c);
                } else {
                    writer.write(' ');
                }
            }
        }
    } catch (IOException e) {
        return null;
    } finally {
        reader.close();
    }
    return writer.toString();
}

我的尝试

@Test
public void getValue_Exception() throws IOException {
    String content = "asd";
    char[] chara = null;
    CharArrayReader reader = mock(CharArrayReader.class);
    when(content.toCharArray()).thenReturn(chara);
    when(reader.read()).thenThrow(IOException.class);
    StringWriter writer = mock(StringWriter.class);
    doThrow(IOException.class).when(writer).write(anyInt());
    spyController.getValue(content);
}