Try-with-resources关闭以前打开的连接？

Question

在这段代码中，连接是否会关闭？

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

Answer 1

不，调用close on语句不会关闭连接。这将阻止我们在已经打开的连接上创建下一个语句，这意味着我们必须为每个语句创建新的连接（这可能是昂贵的操作）。

你可以很容易地测试它。试着在上一个关闭后创建另一个语句。

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(); 
    } 
}