JDBC间接调用时返回空ResultSet（通过不同的Java方法）

Question

我运行了JUnit测试，在名为delete的持久化类中调用了以下PStudent方法：

@Override
public boolean delete(int studentID) {
    System.out.println("delete - studentID = " + studentID);
    if (studentID < 1)
        throw new IllegalArgumentException("Parameter \"studentID\" "
            + "must be greater than 0!");

    Student student = get(studentID);
    if (student == null)
        return false;

    // I would do important things now if only the ResultSet in get()
    // weren't always empty

    return true;
}

这是从get()内部调用时似乎无法正常工作的delete()方法（相同的子类）：

@Override
public Student get(int studentID) {
    if (studentID < 1)
        throw new IllegalArgumentException("Parameter \"studentID\" "
            + "muss größer als 0 sein!");

    Student student = null;

    // get student data
    try {
        String sql = "SELECT * FROM student WHERE studentID = ?";
        PreparedStatement ps = conn.prepareStatement(sql);
        System.out.println("get - studentID = " + studentID);
        // SQuirreL showed that this ID did exist in "student"
        ps.setInt(1, studentID);
        ResultSet rs = ps.executeQuery();

        if (rs.next()) {
            // This is what normally happens when this method is called
            // directly (some 'student.something = rs.getInt("something")'
            // commands). Works all fine.
            student = getStudentFromResults(rs);
        } else {
            // This is what happens when called from within delete().
            errorLog.log(Level.WARNING, "Empty ResultSet!");
            return null;
        }

    } catch (SQLException e) {
        errorLog.log(Level.WARNING, "Error during query!", e);
        return null;
    }

    return student;
}

正如评论所示，get方法返回一个空的ResultSet，尽管查询是正确的（get方法在JUnit测试期间的任何时候直接调用时工作正常相同参数），参数studentID本身是正确的（通过不同点的几个System.out.println命令检查）。这是相应的控制台输出（ID“2”是正确的）：

delete - studentID = 2
get - studentID = 2
mar 02, 2016 12:17:32 EM some.package.SomeClass get
WARNING: Empty ResultSet!

连接conn的检索方式如下：

String URL = "jdbc:derby://localhost:1527/my_db";
Connection conn = DriverManager.getConnection(URL, "user", "password");

编辑：这些是JUnit测试。所有以P开头的类名都是表示逻辑和数据库之间接口的持久化类。对于每个这些对象，在实例化期间打开一个连接（如上所示）：

public class LoggingTest {

private PActionLogElement pLog;

@Before
public void setUp() {
    MyDataSource.setTestMode(true);
    try {
        pLog = new PActionLogElement(1);
    } catch (SQLException e) {
        fail("PActionLogElement(int): Verbindung zur DB konnte nicht "
            + "aufgebaut werden!");
    }
    // empty log in order to make sure that the assertions work properly
    pLog.delete(null);
    pLog.close();
    pLog = null;
}

@After
public void tearDown() {
    MyDataSource.setTestMode(false);
}


@Test
public void testPStudentLogging() {
    PStudent ps = null;
    try {
        ps = new PStudent(1);
    } catch (SQLException e) {
        fail("testPStudentLogging(): PStudent(int) -- Verbindung zur "
            + "DB konnte nicht hergestellt werden!");
    }
    try {
        pLog = new PActionLogElement(1);
    } catch (SQLException e) {
        fail("testPStudentLogging(): PActionLogElement(int) -- "
            + "Verbindung zur DB konnte nicht hergestellt werden!");
    }
    // Set up new model object and fill it with (correct) values
    Student s = new Student();
    s.setFirstName("Test");
    s.setLastName("Person");
    s.setEmail("test@person.de");
    s.setPassword("test");
    Set<GlobalRoleEnum> globalRoles = new HashSet<>();
    globalRoles.add(GlobalRoleEnum.OTHER);
    s.setGlobalRoles(globalRoles);
    s.setMatrNum("123");
    s.setDegreeCourse("Katastrophenmanagement");

    /******************
     * PStudent.add() *
     ******************/
    int studentID = ps.add(s);
    if (studentID == BusinessObjectPersistence.FAIL)
        fail("testPStudentLogging(): PStudent.add() gibt FAIL zurück.");
    s.setStudentID(studentID);
    // System.out.println("studentID = " + studentID);

    List<ActionLogElement> log = pLog.get(null);
    assertEquals(2, log.size());
    assertTrue(log.get(1).getContent().contains(s.getFirstName()));
    assertTrue(log.get(1).getContent().contains(s.getLastName()));
    assertTrue(log.get(1).getContent().contains(
        "(studentID " + studentID + ")"));
    // clear log
    pLog.delete(null);

    /*********************
     * PStudent.update() *
     *********************/
    s.setDegreeCourse("Skandinavistik");
    if (!ps.update(s))
        fail("testPStudentLogging(): PStudent.update() stinkt.");

    log = pLog.get(null);
    assertEquals(2, log.size());
    assertTrue(log.get(1).getContent().contains(s.getFirstName()));
    assertTrue(log.get(1).getContent().contains(s.getLastName()));
    assertTrue(log.get(1).getContent().contains(
        "(studentID " + studentID + ")"));
    // clear log
    pLog.delete(null);

    /*********************
     * PStudent.delete() *
     *********************/
    if (!ps.delete(studentID))
        fail("testPStudentLogging(): PStudent.delete() hat "
            + "Stimmungsschwankungen.");
    // Executes "SELECT * FROM log" and returns all log elements
    log = pLog.get(null);
    assertEquals(2, log.size());
    assertTrue(log.get(1).getContent().contains(s.getFirstName()));
    assertTrue(log.get(1).getContent().contains(s.getLastName()));
    assertTrue(log.get(1).getContent().contains(
        "(studentID " + studentID + ")"));

    // close connections
    pLog.close();
    ps.close();

}
}