java - Java 中的 ArrayList 打印最后插入的值？

Question

我有以下java类

package com.picvik.model;

import java.util.Date;

public class ViewAlbum {

private Integer albumid;
private String albumname;
private String description;
private String location;
private Date date;
private Integer uid;

public Integer getAlbumid() {
    return albumid;
}
public void setAlbumid(Integer albumid) {
    this.albumid = albumid;
}
public String getAlbumname() {
    return albumname;
}
public void setAlbumname(String albumname) {
    this.albumname = albumname;
}
public String getDescription() {
    return description;
}
public void setDescription(String description) {
    this.description = description;
}
public String getLocation() {
    return location;
}
public void setLocation(String location) {
    this.location = location;
}
public Date getDate() {
    return date;
}
public void setDate(Date date) {
    this.date = date;
}
public Integer getUid() {
    return uid;
}
public void setUid(Integer uid) {
    this.uid = uid;
}

}

我正在从数据库中检索数据并将其添加到我的数组列表中，就像这样

public ArrayList getAllAlbums(Integer uid) {
    ViewAlbum album = new  ViewAlbum();
    ArrayList<ViewAlbum>allAlbums = new ArrayList<ViewAlbum>();
    try {
        String qstring = "SELECT albumid, albumname, description, location," +
                " date, uid FROM picvik_picture_album WHERE " +
                "uid = '" + uid + "';";

        System.out.println(qstring);
        connection = com.picvik.util.MySqlConnection.getInstance().getConnection();
        ptmt = connection.prepareStatement(qstring);
        resultSet = ptmt.executeQuery();
        while(resultSet.next()) {
            //System.out.println(resultSet.getString("albumname"));
            album.setAlbumid(resultSet.getInt("albumid"));
            album.setAlbumname(resultSet.getString("albumname"));
            album.setDescription(resultSet.getString("description"));
            album.setLocation(resultSet.getString("location"));
            album.setDate(resultSet.getDate("date"));
            album.setUid(resultSet.getInt("uid"));
            allAlbums.add(album);
        }

        resultSet.close();
        ptmt.close();
        connection.close();


    } catch (Exception e) {
        e.printStackTrace();
    }   
    return allAlbums;
}

但是当我尝试打印存储在数组列表中的值时。它总是给我最后插入的记录。

<div class="row">
                <div class="span10">
                    <s:iterator value="allAlbums">
                        <s:property value="albumname"/>
                    </s:iterator>   
                </div>
            </div>

Answer 1

在这里，

ViewAlbum album = new ViewAlbum();
// ...

while (resultSet.next()) {
    album.setAlbumid(resultSet.getInt("albumid"));
    // ...
    allAlbums.add(album);
}

您正在为所有记录重复使用完全相同的 album 实例。实例的数据在循环中每次都会被覆盖。该列表不包含实例的副本，但包含对单个实例的引用 的副本。您知道，Java 是面向对象的。

您应该为每条记录创建一个新的 album 实例。将实例化移到循环内。

// ...

while (resultSet.next()) {
    ViewAlbum album = new ViewAlbum();
    album.setAlbumid(resultSet.getInt("albumid"));
    // ...
    allAlbums.add(album);
}

另见:

• How can I pass an Integer class correctly by reference?

---

与具体问题无关，您应该在 finally block 中关闭 JDBC 资源，或者在 try() 中打开它们try-with-resources 语句，否则在执行查询或处理结果集时出现异常时它们仍然会泄漏。您还应该将 JDBC 资源的声明移到方法 block 内部，否则您也会遇到线程安全问题。最后但同样重要的是，您应该使用 PreparedStatement 的 setter 方法在 SQL 字符串中设置用户控制的变量。如果它们是字符串，您就会遇到 SQL 注入(inject)攻击漏洞。

另见:

• Java Iterator backed by a ResultSet
• JDBC MySql connection pooling practices to avoid exhausted connection pool