Java連接MySQL數據庫是常見的開發場景之一，比如查詢數據庫中存儲了多少行數據等。下面是簡單的Java代碼示例，演示了如何使用Java語言來查詢MySQL數據庫中存儲了多少行數據。

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
public class CheckRowCount {
public static void main(String[] args) {
Connection connection = null;
Statement statement = null;
ResultSet resultSet = null;
try {
Class.forName("com.mysql.jdbc.Driver");
connection = DriverManager.getConnection("jdbc:mysql://localhost:3306/test_db","root","password");
statement = connection.createStatement();
String selectQuery = "SELECT COUNT(*) FROM student";
resultSet = statement.executeQuery(selectQuery);
while(resultSet.next()) {
int rowCount = resultSet.getInt(1);
System.out.println("Number of rows in table: " + rowCount);
}
} catch (ClassNotFoundException | SQLException ex) {
ex.getMessage();
}
finally {
try {
if(statement!=null) {
statement.close();
}
if(connection!=null) {
connection.close();
}
}
catch(Exception ex) {
ex.getMessage();
}
}
}
}

以上是一個簡單的Java應用程序，它連接本地MySQL數據庫，并查詢Test_DB數據庫中student表存儲了多少行數據。