我有一个 Java 方法，它应该从一个 MySQL 行中获取列值并使用这些值创建一个字符串。运行时，它会生成 SQL 错误 1078“在结果集开始之前”。
这是发生错误的类（问题在 listPosesInSection 方法中：
/** Class used to access the database */
import java.sql.*;
import java.util.ArrayList;
import javax.swing.JOptionPane;
public class YogaDatabaseAccess {
String dbUrl = "jdbc:mysql://localhost/yoga";
private Connection connection;
private ResultSet rset;
private ResultSetMetaData rsMetaData;
private Statement statement;
private PreparedStatement pStatementAll = null;
private PreparedStatement pStatementPartial = null;
// Strings for queries and updates
String strListPosesNotPrimary;
String strInsertNewClass;
String strInsertNewSection;
String strInsertNewPose;
String strUpdateClass;
String strUpdateSection;
String strUpdatePose;
String strArrangePoseOrder;
private String[] poseArray;
// Constructor
YogaDatabaseAccess() {
connectToDatabase();
}
// Method that connects to database
private void connectToDatabase() {
try {
connection = DriverManager.getConnection(dbUrl, "Kyle", "Kullerstrand@2");
System.out.println("Database connected");
}
catch(SQLException e) {
System.out.println(e.getMessage());
}
}
// Query that returns lists to be used with combo boxes
public String listForBoxes(String listName) {
// List to be returned
String strList = "";
// Determine name of the database table for this list
String listTableName;
if (listName == "pose")
listTableName = listName + "s";
else if (listName == "class")
listTableName = listName + "es";
else
listTableName = listName;
// Determine the database column name for this list
String listColumnName = listName + "_name";
// Run the query
try {
statement = connection.createStatement();
rset = statement.executeQuery("SELECT DISTINCT " + listColumnName + " FROM " + listTableName +
" ORDER BY " + listColumnName);
while (rset.next()){
strList = strList + rset.getString(listColumnName) + ", ";
}
} catch (SQLException e) {
e.printStackTrace();
}
return strList;
}
// Query that returns list of primary poses for a section
public String listPrimaryPoses(String sectionName) {
// List to be returned
String strList = "";
// Run the query
try {
statement = connection.createStatement();
rset = statement.executeQuery("SELECT DISTINCT pose_name FROM poses WHERE primarily_suitable_for = '" + sectionName +
"' OR primarily_suitable_for = 'Anything' ORDER BY pose_name");
while (rset.next()){
strList = strList + rset.getString("pose_name") + ", ";
}
} catch (SQLException e) {
e.printStackTrace();
}
return strList;
}
// Query that returns list of secondary poses for a section
public String listSecondaryPoses(String sectionName) {
// List to be returned
String strList = "";
// Run the query
try {
statement = connection.createStatement();
rset = statement.executeQuery("SELECT DISTINCT pose_name FROM poses WHERE sometimes_suitable_for = '" + sectionName + "' ORDER BY pose_name");
while (rset.next()){
strList = strList + rset.getString("pose_name") + ", ";
}
} catch (SQLException e) {
e.printStackTrace();
}
return strList;
}
// Query that returns the poses within a specific section
public String listPosesInSection(String tableName, String sectionName) {
String strList;
StringBuilder strBuilderList = new StringBuilder("");
// Run the query
try {
statement = connection.createStatement();
// Query will collect all columns from one specific row
rset = statement.executeQuery("SELECT * FROM " + tableName + " WHERE " + tableName + "_name = '" + sectionName + "'");
while (rset.next()) {
for (int i = 2; i <= countColumnsInTable(tableName); i++) // First value (0) is always null, skip section name (1)
if (rset.getString(i) != null) // If column has a value
strBuilderList.append(rset.getString(i) + "\n");
}
} catch (SQLException e) {
e.printStackTrace();
}
strList = strBuilderList.toString();
return strList.replaceAll(", $",""); // Strips off the trailing comma
}
// Insert statement that inserts a new class into the classes table
public void insertNewClass(String className) {
/** String insert = "INSERT INTO poses (pose_name, primarily_suitable_for, sometimes_suitable_for) values(?, ?, ?)";
System.out.println("About to create the prepared statement");
// Run the insert
try {
pStatement = connection.prepareStatement(insert);
// statement.execute("INSERT IGNORE INTO poses VALUES ('" + poseName + "', '" + suitableFor + "', '" + suitableForSometimes + "')");
pStatement.setString(1, poseName);
pStatement.setString(2, suitableFor);
pStatement.setString(3, suitableForSometimes);
System.out.println("Created the prepared statement");
// execute query, and return number of rows created
pStatement.executeUpdate();
} catch (SQLException e) {
e.printStackTrace();
} */
}
// Insert statement that inserts a new pose into poses table
public void insertNewPose(String poseName, String suitableFor, String suitableForSometimes) {
String insertAll = "INSERT INTO poses (pose_name, primarily_suitable_for, sometimes_suitable_for) values(?, ?, ?)";
String insertPartial = "INSERT INTO poses (pose_name, primarily_suitable_for) values(?, ?)";
// Run the insert
try {
if (suitableForSometimes == "NULL") { // Insert statement contains a null value for sometimes suitable column
pStatementPartial = connection.prepareStatement(insertPartial);
pStatementPartial.setString(1, poseName);
pStatementPartial.setString(2, suitableFor);
pStatementPartial.executeUpdate();
} else { // Insert statement contains values for all three columns
pStatementAll = connection.prepareStatement(insertAll);
pStatementAll.setString(1, poseName);
pStatementAll.setString(2, suitableFor);
pStatementAll.setString(3, suitableForSometimes);
pStatementAll.executeUpdate();
}
} catch (SQLException e) {
System.err.println("SQLException: " + e.getMessage() + ":" + e.getSQLState());
JOptionPane.showMessageDialog(null, "This pose already exists.");
} finally {
SQLWarning w;
try {
for (w = connection.getWarnings(); w != null; w = w.getNextWarning())
System.err.println("WARNING: " + w.getMessage() + ":" + w.getSQLState());
} catch (SQLException e) {
e.printStackTrace();
JOptionPane.showMessageDialog(null, "An unknown error in the yoga design program has occurred.");
}
}
}
// Insert statement that inserts a new section into warmup, work or restore sections
public void insertNewSection(String sectionType, String sectionName, ArrayList<String> poses) {
System.out.println("insertNewSection method was called");
int maxColumns = countColumnsInTable(sectionType);
poseArray = new String[poses.size()];
poseArray = poses.toArray(poseArray);
if (poseArray.length == 0)
JOptionPane.showMessageDialog(null, "There are no poses in this section. Please add poses.");
// Create a list of columns of the table for the INSERT statement
StringBuilder columns = new StringBuilder(sectionType + "_name");
for (int c = 1; c < maxColumns; c++)
columns.append(", pose_" + c);
// Create a string list of poses, separated by commas, from the array
StringBuilder values = new StringBuilder();
values.append("'" + poseArray[0] + "'");
for (int v = 1; v < poseArray.length - 1; v++)
values.append(", '" + poseArray[v] + "'");
// make sure query uses correct number of columns by padding the query with NULL
for (int i = poseArray.length; i < maxColumns; i++)
values.append(", NULL");
String posesToAddToSection = values.toString();
// The string containing the entire insert statement
String insert = "INSERT INTO " + sectionType + " (" + columns + ") VALUES ('" + sectionName + "', " + posesToAddToSection + ")";
// Run the insert
try {
statement = connection.createStatement();
statement.executeUpdate(insert);
} catch (SQLException e) {
System.err.println("SQLException: " + e.getMessage() + ":" + e.getSQLState());
JOptionPane.showMessageDialog(null, "An error in the yoga design program has occurred. SQLException: " +
e.getMessage() + ":" + e.getSQLState());
} finally {
SQLWarning w;
try {
for (w = connection.getWarnings(); w != null; w = w.getNextWarning())
System.err.println("WARNING: " + w.getMessage() + ":" + w.getSQLState());
} catch (SQLException e) {
e.printStackTrace();
JOptionPane.showMessageDialog(null, "An unknown error in the yoga design program has occurred.");
}
}
}
// Statement that deletes rows from tables
public void deleteRow(String tableName, String columnName, String rowName) {
String delete = "DELETE FROM " + tableName + " WHERE " + columnName + " = '" + rowName + "'";
// Run the insert
try {
statement = connection.createStatement();
statement.executeUpdate(delete);
System.out.println("Delete statement was run on Java's end.");
} catch (SQLException e) {
System.err.println("SQLException: " + e.getMessage() + ":" + e.getSQLState());
JOptionPane.showMessageDialog(null, "Sorry, something went wrong: SQLException: " +
e.getMessage() + ":" + e.getSQLState());
} finally {
SQLWarning w;
try {
for (w = connection.getWarnings(); w != null; w = w.getNextWarning())
System.err.println("WARNING: " + w.getMessage() + ":" + w.getSQLState());
} catch (SQLException e) {
e.printStackTrace();
}
}
}
// Method for getting the number of columns in a table using metadata
public int countColumnsInTable(String sectionType) {
int count = 16;
try {
// System.out.println(sectionType);
statement = connection.createStatement();
rset = statement.executeQuery("SELECT * FROM " + sectionType);
rsMetaData = rset.getMetaData();
count = rsMetaData.getColumnCount();
// System.out.println("Column count is " + count);
} catch (SQLException e) {
e.printStackTrace();
}
return count;
}
// Close the database and release resources
public void closeDatabase() {
try {
connection.close();
} catch (SQLException e) {
e.printStackTrace();
}
}
}
这是错误列表的开头：
java.sql.SQLException: Before start of result set
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:1078)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:989)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:975)
at com.mysql.jdbc.SQLError.createSQLException(SQLError.java:920)
at com.mysql.jdbc.ResultSetImpl.checkRowPos(ResultSetImpl.java:855)
at com.mysql.jdbc.ResultSetImpl.getStringInternal(ResultSetImpl.java:5773)
at com.mysql.jdbc.ResultSetImpl.getString(ResultSetImpl.java:5693)
at YogaDatabaseAccess.listPosesInSection(YogaDatabaseAccess.java:125)
at YogaSectionDesigner$5.actionPerformed(YogaSectionDesigner.java:229)