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在我的項目中插入技術jpa之前，我的數據庫中有一個數據對象數組。它正常工作。jpa中的收集對象
但現在II使用JPA，他告訴我： 「空指針異常」
這是我的第一選擇功能
public Nodes select(HttpServletRequest request) throws Dao_exception
{
try
{
Class.forName("com.mysql.jdbc.Driver");
} catch (ClassNotFoundException e)
{
}
String url = "jdbc:mysql://localhost/akinator";
String utilisateur = "root";
String motDePasse = "root";
Connection connexion = null;
Statement statement = null;
ResultSet resultset = null;
try
{
connexion = (Connection) DriverManager.getConnection(url, utilisateur, motDePasse);
statement = (Statement) connexion.createStatement();
resultset = (ResultSet) statement.executeQuery(SQL_SELECT);
List<Nodes> nodeList = new ArrayList<Nodes>();
while (resultset.next())
{
Nodes node = new Nodes(resultset.getLong("id"),
resultset.getString("question"),
resultset.getString("result"),
resultset.getLong("id_left_node_children"),
resultset.getLong("id_right_node_children"));
nodeList.add(node);
}
nodes = nodeList.toArray(new Nodes[nodeList.size()]);
}
catch (SQLException e)
{
e.printStackTrace();
}
return nodes[0];
}
這裏同樣的功能在JPA
public List<Nodes> select (HttpServletRequest request) throws Dao_exception
{
List <Nodes> nodeList = new ArrayList<Nodes>();
nodeList = em.createQuery("SELECT n FROM Nodes n").getResultList();
return nodeList;
}
這裏是我的servlet
@EJB
private Node_dao nodes_dao;
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException
{
Node_dao nodess = new Node_dao();
Nodes cursor = new Nodes();
cursor = nodess.select(request).get(1);
request.setAttribute("Nodes", cursor);
this.getServletContext().getRequestDispatcher(VUE).forward(request, response);
}
這裏我的課節點
@Entity
@Table (name = "node")
public class Nodes
{
@Id
@GeneratedValue (strategy = GenerationType.IDENTITY)
@Column(name="id")
private Long id_node;
@Column (name = "question")
private String question;
@Column (name = "result")
private String result;
@Column (name = "id_left_node_children")
private Long id_left_node_children;
@Column (name = "id_right_node_children")
private Long id_right_node_children;
Nodes left;
Nodes right;
private final static Map<Long, Nodes> nodes=new HashMap<Long,Nodes>();
public Nodes(Long id, String question, String result, Long id_left_node_children, Long id_right_node_children)
{
this.id_node=id;
this.question=question;
this.result=result;
this.id_left_node_children=id_left_node_children;
this.id_right_node_children=id_right_node_children;
nodes.put(id, this);
}
和我的錯誤
java.lang.NullPointerException
at dao.Node_dao.select(Node_dao.java:32)
at servlet.Game.doGet(Game.java:45)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:687)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1682)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:318)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:160)
at org.apache.catalina.core.StandardPipeline.doInvoke(StandardPipeline.java:734)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:673)
at com.sun.enterprise.web.WebPipeline.invoke(WebPipeline.java:99)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:174)
at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:357)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:260)
at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:188)
at org.glassfish.grizzly.http.server.HttpHandler.runService(HttpHandler.java:191)
at org.glassfish.grizzly.http.server.HttpHandler.doHandle(HttpHandler.java:168)
at org.glassfish.grizzly.http.server.HttpServerFilter.handleRead(HttpServerFilter.java:189)
at org.glassfish.grizzly.filterchain.ExecutorResolver$9.execute(ExecutorResolver.java:119)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeFilter(DefaultFilterChain.java:288)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeChainPart(DefaultFilterChain.java:206)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.execute(DefaultFilterChain.java:136)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.process(DefaultFilterChain.java:114)
at org.glassfish.grizzly.ProcessorExecutor.execute(ProcessorExecutor.java:77)
at org.glassfish.grizzly.nio.transport.TCPNIOTransport.fireIOEvent(TCPNIOTransport.java:838)
at org.glassfish.grizzly.strategies.AbstractIOStrategy.fireIOEvent(AbstractIOStrategy.java:113)
at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.run0(WorkerThreadIOStrategy.java:115)
at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.access$100(WorkerThreadIOStrategy.java:55)
at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy$WorkerThreadRunnable.run(WorkerThreadIOStrategy.java:135)
at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:564)
at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.run(AbstractThreadPool.java:544)
at java.lang.Thread.run(Thread.java:744)
是什麼創造了我的名單？ 感謝
完美！
但現在我有一個新的錯誤。
這裏我新的錯誤
java.lang.Error: Unresolved compilation problem:
Type mismatch: cannot convert from List<Nodes> to Nodes
at servlet.Game.doGet(Game.java:40)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:687)
at javax.servlet.http.HttpServlet.service(HttpServlet.java:790)
at org.apache.catalina.core.StandardWrapper.service(StandardWrapper.java:1682)
at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:318)
at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:160)
at org.apache.catalina.core.StandardPipeline.doInvoke(StandardPipeline.java:734)
at org.apache.catalina.core.StandardPipeline.invoke(StandardPipeline.java:673)
at com.sun.enterprise.web.WebPipeline.invoke(WebPipeline.java:99)
at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:174)
at org.apache.catalina.connector.CoyoteAdapter.doService(CoyoteAdapter.java:357)
at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:260)
at com.sun.enterprise.v3.services.impl.ContainerMapper.service(ContainerMapper.java:188)
at org.glassfish.grizzly.http.server.HttpHandler.runService(HttpHandler.java:191)
at org.glassfish.grizzly.http.server.HttpHandler.doHandle(HttpHandler.java:168)
at org.glassfish.grizzly.http.server.HttpServerFilter.handleRead(HttpServerFilter.java:189)
at org.glassfish.grizzly.filterchain.ExecutorResolver$9.execute(ExecutorResolver.java:119)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeFilter(DefaultFilterChain.java:288)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.executeChainPart(DefaultFilterChain.java:206)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.execute(DefaultFilterChain.java:136)
at org.glassfish.grizzly.filterchain.DefaultFilterChain.process(DefaultFilterChain.java:114)
at org.glassfish.grizzly.ProcessorExecutor.execute(ProcessorExecutor.java:77)
at org.glassfish.grizzly.nio.transport.TCPNIOTransport.fireIOEvent(TCPNIOTransport.java:838)
at org.glassfish.grizzly.strategies.AbstractIOStrategy.fireIOEvent(AbstractIOStrategy.java:113)
at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.run0(WorkerThreadIOStrategy.java:115)
at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy.access$100(WorkerThreadIOStrategy.java:55)
at org.glassfish.grizzly.strategies.WorkerThreadIOStrategy$WorkerThreadRunnable.run(WorkerThreadIOStrategy.java:135)
at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.doWork(AbstractThreadPool.java:564)
at org.glassfish.grizzly.threadpool.AbstractThreadPool$Worker.run(AbstractThreadPool.java:544)
at java.lang.Thread.run(Thread.java:744)
可以粘貼錯誤堆棧跟蹤器 – Kick
你應該告訴我們你在哪裏得到你的NullPointerException。 –
對不起，我的空指針異常是「nodeList = em.createQuery（」SELECT n FROM Nodes n「）。getResultList（）;」 – Biohazard401