知識點：

1，hql是沒有直接insert的語句

2，使用hibernate對資料庫（postgresql）進行插入資料操作（法一：原生SQL的insert；法二：hibernate自帶save方法）

3，設定主鍵自增

4，mapping報錯Unknown Entity(情況一:設定@Entity對映是導錯jar；情況二：未向配置檔案cfg.xml加入對映檔案hbm.xml;情況三：一個project裡生成兩個配置檔案啊！！太愚蠢了我就是因為這個找了一天的bug最後在老師提點下才知道只需要建一個cfg.xml檔案然後每次只需要新增對映的mapping語句即可)

以上具體解決法請看程式碼                                   ps:有彩蛋！

在次解釋一下，通常情況下是利用hibernate自帶的save()方法提交事務後即可向資料庫中插入資料，但此處製作留言板需要主鍵id每插入一條就自增一次。如果每次使用save方法會導致主鍵值重複的問題，原因：

hibernate的save方法：當主鍵一樣的時候，第二次呼叫save方法就是執行更新操作

彩蛋有該問題解決方法，對比起來使用原生SQL更方便

正式開始                                                                                                                     

資料庫設計（表名：messa）

主鍵（id）自增：id資料型別設為serial

配置檔案cfg.xml

<hibernate-configuration>
 <session-factory>
 
  <property name="hibernate.connection.driver_class">org.postgresql.Driver</property>
  <property name="hibernate.connection.password">password</property>
  <property name="hibernate.connection.url">jdbc:postgresql://localhost:5432/jspdb?useUnicode=true&characterEncoding=UTF-8</property>
  <property name="hibernate.connection.username">username</property>
  <property name="hibernate.dialect">org.hibernate.dialect.PostgreSQLDialect</property>
<!--重點-->
 <property name="hibernate.temp.use_jdbc_metadata_defaults">false</property><!--用於解決報錯Disabling contextual LOB creation as createClob() method threw err-->
 <property name="show_sql">true</property><!--顯示SQL語句-->
<property name="connection.autocommit">true </property> 
<!--同一個專案不需要多次新建配置檔案，在hibernate.cfg.xml裡新增對映檔案路徑即可，否則mapping報錯Unknown Entity-->
  <mapping class="LoginUsers.beans.Users" resource="loginusers/beans/Users.hbm.xml"/>
  <mapping class="LoginUsers.beans.Messas" resource="loginusers/beans/Messas.hbm.xml"/>
  
 </session-factory>

 
</hibernate-configuration>

bean:

import javax.persistence.Entity; //一定注意不要導錯這個包，如果匯入的是org.hibernate帶的會導致mapping報錯Unknown Entity
import javax.persistence.Id;
import javax.persistence.Table;

@Entity
@Table(name = "messa")

public class Messas {
	
	
	@Id      //id對映位置不要寫錯。對應private int id
	private int id;
	private String mname;
	private String subject;
	private String memo;

	public int getId() {
		return id;
	}

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

	public String getMname() {
		return mname;
	}

	public void setMname(String mname) {
		this.mname = mname;
	}

	public String getSubject() {
		return subject;
	}

	public void setSubject(String subject) {
		this.subject = subject;
	}

	public String getMemo() {
		return memo;
	}

	public void setMemo(String memo) {
		this.memo = memo;
	}

	
}

Messas類 對應生成的對映檔案hbm.xml

<hibernate-mapping>
    <class name="loginusers.beans.Messas" table="MESSAS">
        <id name="id" type="java.lang.String">
            <column name="ID" />
            <generator class="assigned" />
        </id>
        <property name="mname" type="java.lang.String">
            <column name="MNAME" />
        </property>
        <property name="subject" type="java.lang.String">
            <column name="SUBJECT" />
        </property>
        <property name="memo" type="java.lang.String">
            <column name="MEMO" />
        </property>
    </class>
</hibernate-mapping>

Dao:

import java.sql.PreparedStatement;

import org.hibernate.SQLQuery;
import org.hibernate.Session;
import org.hibernate.SessionFactory;
import org.hibernate.boot.MetadataSources;
import org.hibernate.boot.registry.StandardServiceRegistry;
import org.hibernate.boot.registry.StandardServiceRegistryBuilder;

import loginusers.beans.Messas;


public class MessasDao {
	public void insertMsg(String mname,String subject,String memo) {
	
//	 	StandardServiceRegistry registry = new StandardServiceRegistryBuilder().configure().build();
//	 	SessionFactory sf = new MetadataSources( registry ).buildMetadata().buildSessionFactory();
//		Session session = sf.openSession();
//		
	Session session =getSession();
	session.beginTransaction();
//在hibernate中執行原生SQL的insert語句完成插入操作
 String sql="insert into messa(mname,subject,memo) values(?,?,?)";
	  SQLQuery query = session.createSQLQuery(sql);
	  query.addEntity(Messas.class);
	 query.setString(0,mname);
     query.setString(1,subject);
     query.setString(2,memo);
    query.executeUpdate();//提交至資料庫
//		Messas msg = new Messas();
//		msg.setMname(mname);
//		msg.setSubject(subject);
//		msg.setMemo(memo);
	//System.out.println(mname+""+subject+""+memo);
//		session.save(msg);
	//session.getTransaction().commit();//提交事務至資料庫
  	session.close();
	}
	private Session getSession() {
 	StandardServiceRegistry registry = new StandardServiceRegistryBuilder().configure().build();
	 	SessionFactory sf = new MetadataSources( registry ).buildMetadata().buildSessionFactory();
	Session session = sf.openSession();
		return session;
	}
}

servlet：

public class savemsg extends HttpServlet {
	private static final long serialVersionUID = 1L;
   
    public savemsg() {
        super();
     
    }

	
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		request.setCharacterEncoding("utf-8");
		response.setContentType("text/html; charset=UTF-8");
		
		PrintWriter out=response.getWriter();
		String name=request.getParameter("name");
		String subject=request.getParameter("subject");
		String memo=request.getParameter("memo");
			
		MessasDao mdao=new MessasDao();
		mdao.insertMsg(name, subject,memo);
		
	}


	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
	
		doGet(request, response);
	}

}

jsp:

<html xmlns="http://www.w3.org/1999/xhtml">
 <script src="https://cdn.bootcss.com/jquery/3.3.1/jquery.js"></script>

<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
<title>留言板</title>
<script type="text/javascript">  
$(function(){  
       $("#tijiao").click(function(){ 
           var name=$("#name").val();  
           var subject = $("#subject").val();
           var memo=$("#memo").val();  
      $.post("savemsg", {"name" : name,"subject":subject,"memo":memo}, function(msg) {
				alert(msg);
				});
      alert("aa");});
     
		 $(".cz").click(function (){   //設定重置按鈕，點選清空留言板
          $("#memo").val('');
		   
            })
      
       })
     
     </script>
</head>
<body>
<h2><!-- 【${user.username}】 -->請發表你的留言</h2>
<div id="menu">  
    <p>暱稱：<input type="text" id="name"></p> 
    <p>主題：<input type="text" id="subject"></p>
    <p>內容：</p>
    <p><textarea value="在這留言" style="width: 600px; height: 150px"id="memo">在這留言</textarea></p>  
    <p><input type="button" value="提交" id="tijiao">  <input type="button" value="重置" class="cz" ></p> 
  
    <p><h1>所有留言</h1></p>  
    <ul id="ul1">  
    </ul>  
    </div>  
</body>
</html>

本地伺服器執行jsp：

點選提交後資料庫：

彩蛋：其實用save方法也是可以完成主鍵自增的插入，改兩處即可，這裡不贅述，上圖。。。