Java：线程安全数据集实现

Question

I have created a custom SQLDataset implementation where it takes a SQL query and returns a List of LinkedHashmap back to the requestcontroller to be displayed in JSP or download in Excel format.

Could you please let me know if the approach is thread safe?

SqlDataset.java

package com.sqle.core;

import com.util.QueryProcessor;

import java.io.Serializable;
import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;

public class SqlDataset implements Serializable {
    private String query;
    private QueryProcessor qp;
    private ResultSet rsSet;
    private List<LinkedHashMap<String, Object>> rsList = new ArrayList<>();
    private ArrayList<String> dataHeader = new ArrayList<>();

    public SqlDataset() {}

    public SqlDataset(String uquery) {
        this.query = uquery;
    }

    private ResultSet getQueryResult() throws Exception {
        qp = new QueryProcessor(query);
        this.rsSet = qp.getQueryResultSet();
        return this.rsSet;
    }

    public List<LinkedHashMap<String, Object>> getResult() throws Exception {
        return this.getValues(this.getQueryResult());
    }

    public List<LinkedHashMap<String, Object>> getResult(String query) throws Exception {
        this.query = query;
        return this.getValues(this.getQueryResult());
    }

    public int getRowCount() {
        return this.rsList.size();
    }

    public ArrayList getHeaders() {
        for (LinkedHashMap<String, Object> aRsList : this.rsList) {
            for (Map.Entry<String, Object> dh : aRsList.entrySet()) {
                if (!this.dataHeader.contains(dh.getKey()))
                    this.dataHeader.add(dh.getKey());
            }
        }
        return this.dataHeader;
    }

    private List<LinkedHashMap<String, Object>> getValues(ResultSet rs) throws SQLException {
        ResultSetMetaData rmd = rs.getMetaData();
        int columns = rmd.getColumnCount();

        while (rs.next()) {
            LinkedHashMap<String, Object> row = new LinkedHashMap<>(columns);
            for (int i = 1; i <= columns; ++i) {
                row.put(rmd.getColumnName(i), rs.getObject(i));
            }
            this.rsList.add(row);
        }
        return this.rsList;
    }
}

Below is the code written in request controller:

protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
    String act = request.getParameter("act").toLowerCase();
    RequestDispatcher rd = request.getRequestDispatcher("sqleditor.jsp");

    try {
        if (act.equalsIgnoreCase("exec")) {
            String uqry = request.getParameter("isql");
            if (!uqry.isEmpty()) {
                SqlDataset sd = new SqlDataset(uqry);
                rslist = sd.getResult();
                if (sd.getRowCount() > 0) {
                    headRow = sd.getHeaders();
                    request.setAttribute("resHead", headRow);
                    request.setAttribute("result", rslist);
                } else {
                    throw new NoDataException("No data found to display");
                }
            } else {
                throw new NoDataException("Please enter a query");
            }
            rd.forward(request, response);
        } else if (act.equalsIgnoreCase("excel")) {
            String uqry = request.getParameter("isql");
            if (!uqry.isEmpty()) {
                try {
                    SqlDataset sd = new SqlDataset();
                    rslist = sd.getResult(uqry);
                    if (sd.getRowCount() > 0) {
                        headRow = sd.getHeaders();
                        response.reset();
                        response.setContentType("application/vnd.ms-excel");
                        response.setHeader("Content-Disposition", "attachment; filename=\"" + FILENAME + "\"");
                        ExcelWriter ew = new ExcelWriter();
                        ew.initExcelfile(rslist, headRow, response.getOutputStream());
                    } else {
                        throw new NoDataException("No data found to download");
                    }
                } catch (Exception evar1) {
                    throw new AppException(evar1.getMessage());
                }
            } else {
                throw new NoDataException("Please enter a query");
            }
        }
    } catch (SQLException evar2) {
        request.setAttribute("errormsg", evar2.getMessage());
        rd.forward(request, response);
    } catch (NullPointerException evar3) {
        request.setAttribute("errormsg", evar3.getMessage());
        rd.forward(request, response);
    } catch (Exception evar4) {
        request.setAttribute("errormsg", evar4.getMessage());
        rd.forward(request, response);
    }
}

Will this code work is multiple users use this application and running different queries successively?

Modified SQLdataset class:

package com.sqle.core;

import com.util.QueryProcessor;

import java.io.Serializable;
import java.sql.ResultSet;
import java.sql.ResultSetMetaData;
import java.sql.SQLException;
import java.util.ArrayList;
import java.util.LinkedHashMap;
import java.util.List;
import java.util.Map;

public class SqlDataset implements Serializable {

    private List<LinkedHashMap<String, Object>> rsList = new ArrayList<>();

    public SqlDataset() {}

    private ResultSet getQueryResult(String query) throws Exception {
        QueryProcessor qp = new QueryProcessor(query);
        ResultSet rsSet = qp.getQueryResultSet();
        return rsSet;
    }

    public List<LinkedHashMap<String, Object>> getResult(String query) throws Exception {
        return this.getValues(this.getQueryResult(query));
    }

    public ArrayList getHeaders() {
        ArrayList<String> dataHeader = new ArrayList<>();

        for (LinkedHashMap<String, Object> aRsList : this.rsList) {
            for (Map.Entry<String, Object> dh : aRsList.entrySet()) {
                if (!dataHeader.contains(dh.getKey()))
                    dataHeader.add(dh.getKey());
            }
        }
        return dataHeader;
    }

    private List<LinkedHashMap<String, Object>> getValues(ResultSet rs) throws SQLException {
        ResultSetMetaData rmd = rs.getMetaData();
        int columns = rmd.getColumnCount();

        while (rs.next()) {
            LinkedHashMap<String, Object> row = new LinkedHashMap<>(columns);
            for (int i = 1; i <= columns; ++i) {
                row.put(rmd.getColumnName(i), rs.getObject(i));
            }
            this.rsList.add(row);
        }
        return this.rsList;
    }

    public int getRowCount() {
        return this.rsList.size();
    }
}

Answer 1

It depends on how you use this class.

With your doPost method in controller it is thread safe for multiple users because you create new SqlDataset object every time.

It means it will be used only by thread which processes a single request.

Your Controller code is re-entrant and thread safe.

BTW in case if you plan to use your SqlDataset as singleton (eg Spring bean or such) - it is not thread safe. It has instance variables used in process - it means SqlDataset methods are not re-entrant.

just think about them...

• do you really need private QueryProcessor qp; while you create new instance every time in the getQueryResult() method?

• do you really need private ArrayList<String> dataHeader = new ArrayList<>(); while you just return it from getHeaders() - why do not just create new ArrayList before for loops inside the method. ...and so on...

If you make everything passed to methods as parameters and return everything created inside methods it will be fully thread safe.

Singletons may have only immutable instance variables (logically almost constants) to keep some settings or properties applicable to any threads which use it.

Answer 2

The SqlDataset.java itself is not thread safe as you have instance variables in it.

However if you only use it in some of your request controller 's methods then there will be no problems. This is because a Servlet is not thread safe but the Servlets methods are that.