我是否必须为双向关系设置双方？

Question

@Entity
public class A {
@GeneratedValue
@Id
private long id;

    public long getId() {
        return id;
    }

    public void setId(final long id) {
        this.id = id;
    }

    @OneToMany(mappedBy = "a")
    List<B> bs;

    public List<B> getBs() {
        return bs;
    }

    public void setBs(final List<B> bs) {
        this.bs = bs;
    }
}
@Entity
public class B {
    @GeneratedValue
    @Id
    private long id;

    public long getId() {
        return id;
    }

    public void setId(final long id) {
        this.id = id;
    }

    @ManyToOne
    @JoinTable
    A a;

    public A getA() {
        return a;
    }

    public void setA(final A a) {
        this.a = a;
    }
}

建立关系，我必须打电话

b.setA(a);
a.getBs().add(b);

为什么两者都是必要的，为什么仅仅做是不够的

b.setA(a);

要么

a.getBs().add(b);

？

该关系存储在联接表中，b.setA(a)并将更新该联接表。

但是当我之后进行查询时，它a.getBs()为空。这是为什么？

这是一个测试案例，说明了这个问题。请注意，最后一个断言失败。

public class QuickTestAB2 {

    private static String dbUrlBase = "jdbc:derby:testData/db/test.db";

    private static String dbUrlCreate = dbUrlBase + ";create=true";

    private static String dbUrlDrop = dbUrlBase + ";drop=true";

    private EntityManagerFactory factory;

    private EntityManager em;

    public Map<String, String> createPersistenceMap(final String dbUrl) {
        final Map<String, String> persistenceMap = new HashMap<>();
        persistenceMap.put("javax.persistence.jdbc.url", dbUrl);
        return persistenceMap;
    }

    public void dropDatabase() throws Exception {
        if (em != null && em.isOpen()) {
            em.close();
        }
        if (factory != null && factory.isOpen()) {
            factory.close();
        }
        try (Connection conn = DriverManager.getConnection(dbUrlDrop)) {

        } catch (final SQLException e) {
            // always

        }
    }

    public void deleteDatabase() throws Exception {
        dropDatabase();
        final File file = new File("testData/db/test.db");
        if (file.exists()) {
            FileUtils.forceDelete(file);
        }
    }

    public void createNewDatabase() throws SQLException, IOException {

        FileUtils.forceMkdir(new File("testData/db"));
        try (Connection conn = DriverManager.getConnection(dbUrlCreate)) {

        }
    }

    @BeforeClass
    public static void setUpBeforeClass01() throws Exception {
        Tests.enableLog4J();
        JPATests.enableJPA();

    }

    @AfterClass
    public static void tearDownAfterClass01() throws Exception {

    }

    @Before
    public void setUp01() throws Exception {

        deleteDatabase();
        createNewDatabase();
        final Map<String, String> map = createPersistenceMap(dbUrlCreate);
        factory = Persistence.createEntityManagerFactory("pu", map);

    }

    @After
    public void tearDown01() throws Exception {
        if (em != null && em.isOpen()) {
            em.close();
        }
        em = null;
        if (factory != null && factory.isOpen()) {
            factory.close();
        }
        factory = null;
    }

    @Test
    public void test01() throws Exception {
        em = factory.createEntityManager();
        final A a = new A();
        final B b = new B();
        b.setA(a);
        try {
            em.getTransaction().begin();
            em.persist(a);
            em.persist(b);
            em.getTransaction().commit();
        } finally {
            em.close();
        }
        em = factory.createEntityManager();
        B b2;
        A a2;
        try {
            em.getTransaction().begin();
            Query q = em.createQuery("SELECT b FROM B b");
            b2 = (B) q.getSingleResult();
            q = em.createQuery("SELECT a FROM A a");
            a2 = (A) q.getSingleResult();
            em.getTransaction().commit();
        } finally {
            em.close();
        }
        assertThat(a2, is(not(nullValue())));
        assertThat(b2, is(not(nullValue())));

        assertThat(b2.getA(), is(not(nullValue())));
        assertThat(a2.getBs().isEmpty(), is(false));

    }

}

动机： 当数量增加时，通过更改“仅一侧”来更改双向关系会很有用a.Bs。在这种情况下，UPDATE SELECT拥有方的查询比调用a.getBs().remove(b) See

Answer 1

如下面NikosParaskevopoulos的回答所述，JPA和Java对象要求您设置关系的双方。只要设置了拥有方，数据库就会随着关系的变化而更新，但是非拥有方仅在手动设置数据库或强制刷新或重新加载数据库时才会反映数据库中的内容。从单独的上下文中读取实体不会强制重新加载，因为您的JPA提供程序可以使用二级缓存。这是EclipseLink中的默认设置。您的其他读取是从共享缓存中返回A，与原始对象一样，它没有将B添加到其B列表中。

最简单的解决方案是将B预先设置为A的列表。但是，这里的其他选项是使用em.refresh（a）或查询提示来强制A的刷新，或者禁用共享的缓存。