Question

我正在尝试从一个相当简单的查询中获取结果，并在jsp页面上写出这些结果。使用Netbeans运行Glassfish 3.1。当我运行项目时，我从例程返回一个空列表。

当我右键单击“服务”中的表并点击“查看数据”时，我可以看到该表已填充，当我将查询复制并通过SQL命令窗口并运行它时，它会给出长度为2的预期列表。

有一些类似的问题（如this one），其中没有一个似乎有太多帮助。我是新手，所以我可能没有理解其他问题的解决方案。

这里有很多因素在起作用，我不确定要包括什么。我已经包含了执行查询的例程，调用该例程的JSP代码，我正在使用的实体类，以及我在项目运行时获得的服务器日志的输出。如果需要更多信息，请告诉我，我会把它提出来。

我真的很感激任何帮助。

格里夫

常规执行查询：

public static LinkedList<String> getCategories(EntityManager entityManager) {
    try {
        Query query = entityManager.createQuery(
                "SELECT DISTINCT i.category from ItemEntity i");
        List resultList = query.getResultList();
        if (!resultList.isEmpty()) {
            return new LinkedList<String>(resultList);
        }
    } catch(Exception e) {
        System.out.println(e);
    } finally {
        return new LinkedList<String>();
    }
}

调用例程的JSP代码：

<body>
    <h1>Store</h1>
    <h2>Categories</h2>
    <%
        Context environmentContext 
                = (Context) new InitialContext().lookup("java:comp/env");
        EntityManager entityManager 
                = (EntityManager) environmentContext.lookup("persistence/dbunit");

        LinkedList<String> categories = DataBase.getCategories(entityManager);

        ListIterator<String> categoryIterator = categories.listIterator();
        String category = "";
    %>

    <form action="category.jsp">
        <%  while (categoryIterator.hasNext()) { %>
                <%  category = categoryIterator.next(); %>
                <input type="submit" class="submitButtonAsLink" value="<%= category %>" name="<%= category %>" /><br />
        <% } %>
    </form> 
</body>

ItemEntity.java：

@Entity
public class ItemEntity implements Serializable {
    private static final long serialVersionUID = 1L;

@Id @GeneratedValue(strategy = GenerationType.AUTO)
    private String id;

    private String title;
    private String longDescription;
    private double cost;
    private String category;

    public String getId() {
        return id;
    }

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

    @Override
    public int hashCode() {
        int hash = 0;
        hash += (id != null ? id.hashCode() : 0);
        return hash;
    }

    @Override
    public boolean equals(Object object) {
        // TODO: Warning - this method won't work in the case the id fields are not set  
        if (!(object instanceof ItemEntity)) {
            return false;
        }
        ItemEntity other = (ItemEntity) object;
        if ((this.id == null && other.id != null) || (this.id != null && !this.id.equals(other.id))) {
            return false;
        }
        return true;
    }

    @Override
    public String toString() {
        return "store.model.entities.ItemTable[ id=" + id + " ]";
    }

    //getters and setters omitted. 
    //There are getters and setters for every field except id.
}

服务器日志：

INFO: file:/Users/griffgeorge/Dropbox/school/design-arch/labs/Lab4Exercise/build/web/WEB-INF/classes/_Lab4ExercisePU logout successful
INFO: Instantiated an instance of org.hibernate.validator.engine.resolver.JPATraversableResolver.
INFO: EclipseLink, version: Eclipse Persistence Services - 2.3.0.v20110604-r9504
INFO: file:/Users/griffgeorge/Dropbox/school/design-arch/labs/Lab4Exercise/build/web/WEB-INF/classes/_Lab4ExercisePU login successful
WARNING: Multiple [2] JMX MBeanServer instances exist, we will use the server at index [0] : [com.sun.enterprise.v3.admin.DynamicInterceptor@1ed957d].
WARNING: JMX MBeanServer in use: [com.sun.enterprise.v3.admin.DynamicInterceptor@1ed957d] from index [0] 
WARNING: JMX MBeanServer in use: [com.sun.jmx.mbeanserver.JmxMBeanServer@1a84f3c] from index [1] 
WARNING: PER01000: Got SQLException executing statement "CREATE TABLE ITEMENTITY (ID VARCHAR(255) NOT NULL, CATEGORY VARCHAR(255), COST FLOAT, LONGDESCRIPTION VARCHAR(255), TITLE VARCHAR(255), PRIMARY KEY (ID))": java.sql.SQLException: Table/View 'ITEMENTITY' already exists in Schema 'APP'.
WARNING: PER01000: Got SQLException executing statement "CREATE TABLE SEQUENCE (SEQ_NAME VARCHAR(50) NOT NULL, SEQ_COUNT DECIMAL(15), PRIMARY KEY (SEQ_NAME))": java.sql.SQLException: Table/View 'SEQUENCE' already exists in Schema 'APP'.
WARNING: PER01000: Got SQLException executing statement "INSERT INTO SEQUENCE(SEQ_NAME, SEQ_COUNT) values ('SEQ_GEN', 0)": java.sql.SQLIntegrityConstraintViolationException: The statement was aborted because it would have caused a duplicate key value in a unique or primary key constraint or unique index identified by 'SQL111012114449870' defined on 'SEQUENCE'.
INFO: WEB0671: Loading application [Lab4Exercise] at [/Lab4Exercise]

Answer 1

您的getCategories()方法始终会返回一个空列表，因为finally中的返回始终会运行（即使在第一个return之后）。

根本不需要finally条款。你可以简化到这个

public static LinkedList<String> getCategories(EntityManager entityManager) {
    try {
        Query query = entityManager.createQuery(
                "SELECT DISTINCT i.category from ItemEntity i");
        return new LinkedList<String>(query.getResultList());
    } catch(Exception e) {
        System.out.println(e);
        return new LinkedList<String>();
    }
}

如果query.getResultList()不为空，则返回非空字符串链接列表。

如果query.getResultList()为空，则返回一个空的字符串链表。

如果发生异常，则返回空列表。正如您之前所做的那样，即使您的查询返回数据，也始终返回一个空列表。

要证明finally中的回报是通过的，请看一下

public class TestFinally {
    public static void main(String[] args) {
        System.out.println(TestFinally.test());
    }

    public static int test() {
        try {
            return 1;
        } finally {
            return 0;
        }  
    }
}

返回

Answer 2

你的persistence.xml怎么样？是指向您期望的数据源吗？然后是指向您期望的连接池的数据源吗？

为什么我不能从这个相当简单的查询中得到任何结果？

2 个答案: