我应该如何在JDBC中使用try-with-resources?

问题

我有一种方法可以使用JDBC从数据库中获取用户:

public List<User> getUser(int userId) {
    String sql = "SELECT id, name FROM users WHERE id = ?";
    List<User> users = new ArrayList<User>();
    try {
        Connection con = DriverManager.getConnection(myConnectionURL);
        PreparedStatement ps = con.prepareStatement(sql); 
        ps.setInt(1, userId);
        ResultSet rs = ps.executeQuery();
        while(rs.next()) {
            users.add(new User(rs.getInt("id"), rs.getString("name")));
        }
        rs.close();
        ps.close();
        con.close();
    } catch (SQLException e) {
        e.printStackTrace();
    }
    return users;
}

我应该如何使用Java 7try-with-resources来改进这段代码?

我已尝试使用下面的代码,但它使用了许多try,并没有提高的可读性much。我应该用另一种方式使用try-with-resources吗?

public List<User> getUser(int userId) {
    String sql = "SELECT id, name FROM users WHERE id = ?";
    List<User> users = new ArrayList<>();
    try {
        try (Connection con = DriverManager.getConnection(myConnectionURL);
             PreparedStatement ps = con.prepareStatement(sql);) {
            ps.setInt(1, userId);
            try (ResultSet rs = ps.executeQuery();) {
                while(rs.next()) {
                    users.add(new User(rs.getInt("id"), rs.getString("name")));
                }
            }
        }
    } catch (SQLException e) {
        e.printStackTrace();
    }
    return users;
}

#1 热门回答(145 赞)

我意识到这很久以前就已经回答了,但是想要建议一种额外的方法来避免嵌套的try-with-resources双块。

public List<User> getUser(int userId) {
    try (Connection con = DriverManager.getConnection(myConnectionURL);
         PreparedStatement ps = createPreparedStatement(con, userId); 
         ResultSet rs = ps.executeQuery()) {

         // process the resultset here, all resources will be cleaned up

    } catch (SQLException e) {
        e.printStackTrace();
    }
}

private PreparedStatement createPreparedStatement(Connection con, int userId) throws SQLException {
    String sql = "SELECT id, username FROM users WHERE id = ?";
    PreparedStatement ps = con.prepareStatement(sql);
    ps.setInt(1, userId);
    return ps;
}

#2 热门回答(59 赞)

在你的示例中不需要外部尝试,因此你至少可以从3下降到2,并且你也不需要在资源列表的末尾关闭;。使用两个try块的优点是你的所有代码都是预先存在的,因此你不必引用单独的方法:

public List<User> getUser(int userId) {
    String sql = "SELECT id, username FROM users WHERE id = ?";
    List<User> users = new ArrayList<>();
    try (Connection con = DriverManager.getConnection(myConnectionURL);
         PreparedStatement ps = con.prepareStatement(sql)) {
        ps.setInt(1, userId);
        try (ResultSet rs = ps.executeQuery()) {
            while(rs.next()) {
                users.add(new User(rs.getInt("id"), rs.getString("name")));
            }
        }
    } catch (SQLException e) {
        e.printStackTrace();
    }
    return users;
}

#3 热门回答(2 赞)

以下是使用lambdas和JDK 8 Supplier的简洁方法,以适应外部尝试中的所有内容:

try (Connection con = DriverManager.getConnection(JDBC_URL, prop);
            PreparedStatement stmt = ((Supplier<PreparedStatement>)() -> {
                try {
                  PreparedStatement s = con.prepareStatement(
                        "SELECT userid, name, features FROM users WHERE userid = ?");
                  s.setInt(1, userid);
                  return s;
                } catch (SQLException e) { throw new RuntimeException(e); }
            }).get();
          ResultSet resultSet = stmt.executeQuery()) {
    }