即使关闭连接后，数据仍然存在于h2数据库表中

Posted 2023-04-09

【中文标题】即使关闭连接后，数据仍然存在于h2数据库表中

【英文标题】：Even after closing the connection, data still exists in h2 database tables

【发布时间】：2017-11-27 10:09:54

【问题描述】：

当我浏览许多 H2 数据库教程时，默认情况下，关闭与数据库的最后一个连接会关闭数据库。对于内存数据库，这意味着内容会丢失。

我已经创建了 Employee1 表，插入了记录并关闭了连接。但是，如果我在某个时间后重新连接同一个数据库，我仍然可以检索 Employee1 数据。为什么数据仍然存在？

package connection;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;

public class Connector 

static Connection conn = null;
static Statement stmt = null;

public static void main(String[] args) 

    System.out.println("Welcome!");
    Connector connector = new Connector();
    connector.createConnection();
    connector.createTable("Employee2");
    connector.insertRecord("Employee2");
    connector.readRecord("Employee2");
    connector.readRecord("Employee1"); //Employee1 Table which is created in previous execution but still it reads the data
    connector.closeConnection();


public void createConnection() 
    try 
        System.out.println("Creating connection");
        // STEP 1: Register JDBC driver
        Class.forName("org.h2.Driver");
        // STEP 2: Open a connection
        System.out.println("Connecting to database...");
        conn = DriverManager.getConnection("jdbc:h2:mem/db1", "sa", "");
     catch (SQLException se) 
        // Handle errors for JDBC
        se.printStackTrace();
     catch (Exception e) 
        // Handle errors for Class.forName
        e.printStackTrace();
    



public void createTable(String tableName) 
    try 
        // STEP 3: Execute a query
        System.out.println("Creating table in given database with the name of ..." + tableName);
        stmt = conn.createStatement();
        String sql = "CREATE TABLE " + tableName + "(id INTEGER not NULL, " + " first VARCHAR(255), "
                + " last VARCHAR(255), " + " age INTEGER, " + " PRIMARY KEY ( id ))";
        stmt.executeUpdate(sql);
        System.out.println("Created table in given database...");

     catch (SQLException se) 
        // Handle errors for JDBC
        se.printStackTrace();
     catch (Exception e) 
        // Handle errors for Class.forName
        e.printStackTrace();
    


public void insertRecord(String tableName) 
    try 
        // STEP 3: Execute a query
        stmt = conn.createStatement();
        String sql = "INSERT INTO " + tableName + " VALUES (500, 'Zara', 'Ali', 18)";

        stmt.executeUpdate(sql);
        sql = "INSERT INTO " + tableName + " VALUES (501, 'Mahnaz', 'Fatma', 25)";

        stmt.executeUpdate(sql);
        sql = "INSERT INTO " + tableName + " VALUES (502, 'Zaid', 'Khan', 30)";

        stmt.executeUpdate(sql);
        sql = "INSERT INTO " + tableName + " VALUES(503, 'Sumit', 'Mittal', 28)";

        stmt.executeUpdate(sql);
        System.out.println("Inserted records into the table...");
     catch (SQLException se) 
        // Handle errors for JDBC
        se.printStackTrace();
     catch (Exception e) 
        // Handle errors for Class.forName
        e.printStackTrace();
    


public void readRecord(String tableName) 
    try 
        System.out.println("Reading data from "+tableName);
        stmt = conn.createStatement();
        String sql = "SELECT id, first, last, age FROM " + tableName;
        ResultSet rs = stmt.executeQuery(sql);

        // STEP 4: Extract data from result set
        while (rs.next()) 
            // Retrieve by column name
            int id = rs.getInt("id");
            int age = rs.getInt("age");
            String first = rs.getString("first");
            String last = rs.getString("last");

            // Display values
            System.out.print("ID: " + id);
            System.out.print(", Age: " + age);
            System.out.print(", First: " + first);
            System.out.println(", Last: " + last);
        
        // STEP 5: Clean-up environment
        rs.close();
     catch (SQLException se) 
        // Handle errors for JDBC
        se.printStackTrace();
     catch (Exception e) 
        // Handle errors for Class.forName
        e.printStackTrace();
     finally 
        // finally block used to close resources
        try 
            if (stmt != null)
                stmt.close();
         catch (SQLException se2) 
         // nothing we can do
     // end try


public void closeConnection() 
    try 
        if (conn != null) 
            conn.close();
            System.out.println("Connection Closed..");
        
     catch (SQLException se) 
        se.printStackTrace();
     // end finally try

上述程序的输出：

Welcome!
Creating connection
Connecting to database...
Creating table in given database with the name of ...Employee2
Created table in given database...
Inserted records into the table...
Reading data from Employee2
ID: 500, Age: 18, First: Zara, Last: Ali
ID: 501, Age: 25, First: Mahnaz, Last: Fatma
ID: 502, Age: 30, First: Zaid, Last: Khan
ID: 503, Age: 28, First: Sumit, Last: Mittal
Reading data from Employee1
ID: 400, Age: 18, First: freeze, Last: Ali
ID: 401, Age: 25, First: dora, Last: Fatma
ID: 402, Age: 30, First: xer, Last: Khan
ID: 403, Age: 28, First: kilo, Last: Mittal
Connection Closed..

【问题讨论】：

查看是否设置了其中之一：***.com/questions/27057900/…

实际上我的问题是即使关闭连接后，内容并没有丢失它仍然存在。即使我尝试过 conn = DriverManager.getConnection("jdbc:h2:mem/db1;DB_CLOSE_DELAY=0", "sa", "");但它不起作用。

我了解您的问题。我没有解释。

【参考方案1】：

如你所见here

jdbc:h2:mem/db1

使用mem/db1 的相对路径连接到本地数据库文件，因此数据仍然存在，因为它以文件的形式保存。

对于内存数据库，连接字符串应该是：

jdbc:h2:mem:db1

注意/ 和: 之间的区别

【讨论】：

非常感谢。我经常混淆和编辑导致错误的代码。