Java中的DAO类花费太多时间来获取数据

Question

I have the following Data in my DAO class

public List<Environment> fetchMiddlewareVersions() throws SQLException{
      System.out.println("reached version");
      Environment environment;
      List<Environment> environments=new ArrayList<Environment>();
 try{
         connection=DBConnectionUtil.getConnection();
         statement=connection.createStatement();
         statement.setFetchSize(100);
         preparedStatement=connection.prepareStatement("select * from     middleware_version_details order by application,environment");
         resultSet=preparedStatement.executeQuery();
            while(resultSet.next())
            {
                environment = new Environment();
                environment.setAppName(resultSet.getString("APPLICATION"));
                environment.setHostName(resultSet.getString("HOSTNAME"));
                environment.setSoftwareComponent(resultSet.getString("SOFTWARE_COMPONENT"));
                environment.setVersion(resultSet.getString("VERSION"));
                environment.setInstallPath(resultSet.getString("INSTALL_PATH"));
                environment.setRemarks(resultSet.getString("REMARKS"));
                environment.setEnvironmental(resultSet.getString("ENVIRONMENT"));
                environments.add(environment);  

            }
    }

By the time I get the entire data into JSP page, it has consumed 20-30 seconds already. How do I Increase speed of the Fetch. I tried DynaCache and it hasn't helped.

Answer 1

So barring any sort of connectivity issues, it almost always comes down to the number of records you're fetching. If you're fetching A TON of records, the method will not return until it has gone through each item and created an array object.

I would try adding a LIMIT and OFFSET clause to your SQL Statement to only retrieve records, say, 25 at a time. setFetchSize( int ) does not affect the number of overall records, only the number of records the underlying transport will fetch at a time from your sever. (also, move your SQL query into a static final variable:

 private static final String SQL_FETCH_MIDDLEWARE_VERSION = 
      "SELECT * FROM middleware_version_details order by application, environment " +
      "LIMIT = ? OFFSET = ?";

then set the limit and the offset in your prepared statement like so:

 preparedStatement.setInt( 1, <RECORD COUNT> );
 preparedStatement.setInt( 2, <RECORD START> );

Third, do you have an index on application and environment ? If you do not and you will be constantly ordering, filtering and joining on those columns, you should add an index.

Fourth, and it's a minor point but one that I adhere to, is that doing resultSet.getString( "<COLUMN NAME>" ) will cause another call to go look up the column index. It's not usually a huge deal, but if you're trying to be as performant as possible, you should use the numeric index. You can do this by creating private static variables holding the index:

 private static int INDEX_ENVIRONMENT = 6;

or you can use a counter and just insure that the columns are in the correct order, something like this:

while(resultSet.next())
{
     int iC = 0;

     environment = new Environment();
     environment.setAppName(resultSet.getString( iC++ ));
     environment.setHostName(resultSet.getString( iC++ ));
     environment.setSoftwareComponent(resultSet.getString( iC++ ));
     environment.setVersion(resultSet.getString( iC++ ));
     environment.setInstallPath(resultSet.getString( iC++ ));
     environment.setRemarks(resultSet.getString( iC++ ));
     environment.setEnvironmental(resultSet.getString( iC++ ));
     environments.add(environment);  
 }

Just insure that you're setting the variables in the correct order and it will be slightly more performant. I like this counter approach as well because it allows me to easily adapt to changing schemas.