从多个线程设置和更新并发哈希映射

Question

I am implementing a Client Lookup Map where i am using ConcurrentHashamp.

    private ConcurrentHashMap<String,SignupDTO> map = new <String,SignupDTO> ConcurrentHashMap();*
    public  SignupDTO get(String opcode) 
    {
      return this.map.get(opcode);
    }
    public  void set(String opcode,SignupDTO dto) 
    {
      this.map.put(opcode, dto);
    }

Here get and set function called from multiple thread to update and get client information. I have another function update() which is called from a specific thread after certain time and update the map with updated client information.

public  void update() 
    {
        Connection connection = null;
        Statement stmt = null;
        ResultSet rs = null;

        try 
        {
            connection = DatabaseManager.getInstance().getConnection();
            stmt = connection.createStatement();    
            String sql= "select * from client";
            rs = stmt.executeQuery(sql);
            while (rs.next()) 
            {
                SignupDTO row = new SignupDTO();
                ......
                            ..........
                this.map.put(key, row);         
            }

        } 
    }

get and set operation on map will be thread safe or not? please help me.

Answer 1

As @nosid pointed out, your update method will not be thread safe (= it can update only half of map, then someone can read mixture of old/new values, and the your update method will finish updating the other half).

I recommend you put your query result into a Map (not nesseserly concurrent one) and the update with putAll method from your ConcurrentHashMap, which is thread-safe (or at least official documentation caims so).