Try-with-resources關閉以前打開的連接？

Question

在這段代碼中，連接是否會關閉？

Connection con = DriverManager.getConnection(...); 
try (Statement st = con.createStatement()) { 
    ResultSet rs = st.executeQuery(...); 
    ... 
} 

Answer 1

不，調用close on語句不會關閉連接。這將阻止我們在已經打開的連接上創建下一個語句，這意味着我們必須爲每個語句創建新的連接（這可能是昂貴的操作）。

你可以很容易地測試它。試着在上一個關閉後創建另一個語句。

Connection con = DriverManager.getConnection(dbUrl,user,pass); 
try (Statement st = con.createStatement()) { 
    ResultSet rs = st.executeQuery(sqlQuery1); 
    simplePrint(rs); 
} catch (SQLException e) { 
    e.printStackTrace(); 
}//here `st` will be closed automatically by try-with-resources 

try (Statement st = con.createStatement()) {//lets try to create next statement 
    ResultSet rs = st.executeQuery(sqlQuery2); 
    simplePrint(rs); 
} catch (SQLException e) { 
    e.printStackTrace(); 
} 

simplePrint方法可能如下：

public static void simplePrint(ResultSet rs) throws SQLException { 
    ResultSetMetaData meta = rs.getMetaData(); 
    while (rs.next()) { 
     for (int i = 1; i <= meta.getColumnCount(); i++) { 
      System.out.print(rs.getObject(i)+"\t"); 
     } 
     System.out.println(); 
    } 
}