是否可以在不为每一列调用 getInt(..) getString(..) 的情况下检索整行？
我有多个线程，每个线程都需要将结果写入某个线程安全集合。
我希望能够将行直接写入此集合，然后解析此集合的成员并根据列类型检索值。
【问题讨论】：
Objects 并将其存储在Collection 中。考虑在线程中以块的形式查询数据。
这是一个使用查询来计算查询大小然后将处理拆分到多个线程的示例。
我使用 MySQL，所以查询是为此编写的。您需要更改数据库引擎的查询。
public static void main(String[] args) throws Exception {
final int count;
try (final Connection conn = DATA_SOURCE.getConnection()) {
final String countQuery = "SELECT COUNT(*) FROM my_table";
try (final PreparedStatement ps = conn.prepareStatement(countQuery);
final ResultSet resultSet = ps.executeQuery()) {
resultSet.next();
count = resultSet.getInt(1);
}
}
final int chunksize = 1000;
final Queue<SqlResult> results = new ConcurrentLinkedQueue<>();
final ExecutorService es = Executors.newFixedThreadPool(10);
for (int end = 0; end < count; end += chunksize) {
es.execute(new ResultReader(count, end, DATA_SOURCE, results));
}
}
private static class ResultReader implements Runnable {
private final int start;
private final int size;
private final DataSource dataSource;
private final Queue<SqlResult> results;
public ResultReader(int start, int size, DataSource dataSource, Queue<SqlResult> results) {
this.start = start;
this.size = size;
this.dataSource = dataSource;
this.results = results;
}
@Override
public void run() {
try (final Connection connection = dataSource.getConnection()) {
final String query = "SELECT id, something, somethingElse FROM my_table LIMIT ?, ?";
try (final PreparedStatement ps = connection.prepareStatement(query)) {
ps.setInt(1, start);
ps.setInt(2, size);
try (final ResultSet rs = ps.executeQuery()) {
while (rs.next()) {
final SqlResult sqlResult = new SqlResult();
sqlResult.setId(rs.getInt("id"));
sqlResult.setSomething(rs.getString("something"));
sqlResult.setSomethingElse(rs.getString("somethingElse"));
results.add(sqlResult);
}
}
}
} catch (SQLException ex) {
Logger.getLogger(App.class.getName()).log(Level.SEVERE, null, ex);
}
}
}
private static class SqlResult {
private int id;
private String something;
private String somethingElse;
public int getId() {
return id;
}
public void setId(int id) {
this.id = id;
}
public String getSomething() {
return something;
}
public void setSomething(String something) {
this.something = something;
}
public String getSomethingElse() {
return somethingElse;
}
public void setSomethingElse(String somethingElse) {
this.somethingElse = somethingElse;
}
}
这假设您已经使用DataSource 进行连接池。
每个工作人员使用LIMIT 运行查询并将结果处理为SqlResult 对象并将它们添加到并发Queue。
【讨论】：
LIMIT ?, ? 不是 MySQL 保留的密钥吗？ AFAIK 它不适用于 Oracle 或 SQL Server。
你可以建立一个像这样的类，它将sql数据类型映射到java数据类型：
class Row
{
public Map <Object,Class> row;
public static Map <String, Class> TYPE;
static
{
TYPE = new HashMap<String, Class>();
TYPE.put("INTEGER", Integer.class);
TYPE.put("TINYINT", Byte.class);
TYPE.put("SMALLINT", Short.class);
TYPE.put("BIGINT", Long.class);
TYPE.put("REAL", Float.class);
TYPE.put("FLOAT", Double.class);
TYPE.put("DOUBLE", Double.class);
TYPE.put("DECIMAL", BigDecimal.class);
TYPE.put("NUMERIC", BigDecimal.class);
TYPE.put("BOOLEAN", Boolean.class);
TYPE.put("CHAR", String.class);
TYPE.put("VARCHAR", String.class);
TYPE.put("LONGVARCHAR", String.class);
TYPE.put("DATE", Date.class);
TYPE.put("TIME", Time.class);
TYPE.put("TIMESTAMP", Timestamp.class);
// ...
}
public Row ()
{
row = new HashMap<Object,Class>();
}
public <T> void add (T data)
{
row.put(data, data.getClass());
}
public void add (Object data, String sqlType)
{
add((Row.TYPE.get(sqlType)) data);
}
public static void formTable (ResultSet rs, ArrayList<Row> table)
{
if (rs == null) return;
ResultSetMetaData rsmd = rs.getMetaData();
int NumOfCol = rsmd.getColumnCount();
while (rs.next())
{
row = new Row ();
for(int i = 1; i <= NumOfCol; i++)
{
row.add(rs.getObject(i), rsmd.getColumnTypeName(i));
}
table.add(row);
}
}
}
你可以这样使用它：
List<Row> table = new ArrayList<Row>();
Row row = null;
ResultSet rs = st.executeQuery("SELECT * FROM table_name");
Row.formTable(rs, table);
然后您可以检索字段并将它们转换为各自的数据类型：
for (Row row : table)
{
for (Object data : row.row.getKeySet())
{
System.out.print(" > " + ((row.row.get(data) data));
}
System.out.println();
}
【讨论】：
行表示为列表：
import java.math.BigDecimal;
import java.sql.Date;
import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.sql.SQLException;
import java.sql.Time;
import java.sql.Timestamp;
import java.util.AbstractMap;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;
import java.util.logging.Level;
import java.util.logging.Logger;
/**
* @author Adam Dziedzic
*
*/
public class Row {
public List<Entry<Object, Class>> row;
public static Map<String, Class> TYPE;
static {
TYPE = new HashMap<String, Class>();
TYPE.put("INTEGER", Integer.class);
TYPE.put("TINYINT", Byte.class);
TYPE.put("SMALLINT", Short.class);
TYPE.put("BIGINT", Long.class);
TYPE.put("REAL", Float.class);
TYPE.put("FLOAT", Double.class);
TYPE.put("DOUBLE", Double.class);
TYPE.put("DECIMAL", BigDecimal.class);
TYPE.put("NUMERIC", BigDecimal.class);
TYPE.put("BOOLEAN", Boolean.class);
TYPE.put("CHAR", String.class);
TYPE.put("VARCHAR", String.class);
TYPE.put("LONGVARCHAR", String.class);
TYPE.put("DATE", Date.class);
TYPE.put("TIME", Time.class);
TYPE.put("TIMESTAMP", Timestamp.class);
TYPE.put("SERIAL",Integer.class);
// ...
}
public Row() {
row = new ArrayList<Entry<Object, Class>>();
}
public <T> void add(T data) {
row.add(new AbstractMap.SimpleImmutableEntry<Object,Class>(data, data.getClass()));
}
public void add(Object data, String sqlType) {
Class castType = Row.TYPE.get(sqlType.toUpperCase());
try {
this.add(castType.cast(data));
} catch (NullPointerException e) {
e.printStackTrace();
Logger lgr = Logger.getLogger(Row.class.getName());
lgr.log(Level.SEVERE, e.getMessage()+" Add the type "+sqlType+" to the TYPE hash map in the Row class.", e);
throw e;
}
}
public static void formTable(ResultSet rs, List<Row> table)
throws SQLException {
if (rs == null)
return;
ResultSetMetaData rsmd;
try {
rsmd = rs.getMetaData();
int NumOfCol = rsmd.getColumnCount();
while (rs.next()) {
Row current_row = new Row();
for (int i = 1; i <= NumOfCol; i++) {
current_row.add(rs.getObject(i), rsmd.getColumnTypeName(i));
}
table.add(current_row);
}
} catch (SQLException e) {
throw e;
}
}
}
用法：
List<Row> table = new ArrayList<Row>();
ResultSet rs = st.executeQuery("SELECT * FROM table_name");
Row.formTable(rs, table);
for (Row row : table)
{
for (Entry<Object, Class> col: row.row)
{
System.out.print(" > " + ((col.getValue()).cast(col.getKey())));
}
System.out.println();
}
【讨论】：
我使用了 adam.cajf 答案，非常有用，但是我需要添加一行代码来添加函数，否则它会在我的特定数据集中引发错误。
if (data != null) {
【讨论】：
没有。
根据 Java 文档，没有一种方法可以让您在不指定列标签或索引的情况下检索结果。
https://docs.oracle.com/javase/7/docs/api/java/sql/ResultSet.html
【讨论】：