java - 使 Java Servlet 线程安全

Question

我在网页上执行 API 时收到的响应很困惑。

稍微回溯一下，发现 Angular 是以正确的方式触发它的，但是 API 对并发请求的响应是混合的。

在网上搜索发现Java servlet中的全局变量可能会触发异常行为，因为一个线程可能会访问当前正在由另一个变量处理的变量。

因此，我意识到 PrintWriter 在这里造成了一个问题。但我遇到的困难是如何摆脱这个问题。

而且也可能当前的问题与我的结论根本无关。如有错误之处，请指正。

@WebServlet(name = "WorkSpaceDetails", urlPatterns = {"/WorkSpaceDetails"})
public class WorkSpaceDetails extends HttpServlet {

    PrintWriter out;
    private static final long serialVersionUID = 1L;


    private void processRequest(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException, Exception {
        String wid = request.getParameter("id");

        try {

            id = Integer.parseInt(wid);
            response.setContentType("application/json");
            response.setCharacterEncoding("utf-8");
            out = response.getWriter();
            Connection conn = null;
            PreparedStatement prx = null;
            PreparedStatement pry = null;
            ResultSet set = null;

            ResultSetMetaData metaData;
            String query;
            JSONObject obj = new JSONObject();
            String WorkSpaceType;
            List<HashMap> completeData = new ArrayList<HashMap>();
            List<String> ColoumnNames = new ArrayList<String>();
            List<String> Data = new ArrayList<String>();
            try {
                query = "-";
                conn = boneCPConnectionPool.getConnection();
                prx = conn.prepareStatement(query);
                prx.setInt(1, id);
                set = prx.executeQuery();
                while (set.next()) {
                    WorkSpaceType = set.getString("WorkSpaceType");
                    System.out.println("WorkSpaceType:" + WorkSpaceType);
                    if (WorkSpaceType.equalsIgnoreCase("1")) {
                        System.out.println("" + 1);
                        query = "-";
                    } else {
                        System.out.println("" + 0);
                        query = "-";
                    }
                    conn = boneCPConnectionPool.getConnection();
                    pry = conn.prepareStatement(query);
                    pry.setInt(1, id);
                    set = pry.executeQuery();
                    metaData = set.getMetaData();
                    int count = metaData.getColumnCount();
                    for (int i = 1; i <= count; i++) {
                        System.out.println("" + metaData.getColumnName(i));
                        ColoumnNames.add(metaData.getColumnName(i));
                    }
                    while (set.next()) {
                        HashMap tmp = new HashMap();
                        for (int i = 0; i < ColoumnNames.size(); i++) {

                            String cname = ColoumnNames.get(i);

                            tmp.put(cname, set.getString(cname));
                            tmp.put("WorkSpaceType", WorkSpaceType);
                        }
                        System.out.println("" + tmp);
                        completeData.add(tmp);
                        generateJson(completeData);
                    }


                }

            } catch (Exception e) {
                e.printStackTrace();
                out.println(e);
            } finally {

                if (conn != null) {
                    try {
                        conn.close();
                    } catch (Exception e) {
                    }
                }
                if (set != null) {
                    try {
                        set.close();
                    } catch (SQLException ignore) {
                    }
                }
                if (prx != null) {
                    try {
                        prx.close();
                    } catch (SQLException ignore) {
                    }
                }
                if (pry != null) {
                    try {
                        pry.close();
                    } catch (SQLException ignore) {
                    }
                }

            }
        } catch (Exception e) {

            JSONObject obj = new JSONObject();
            obj.put("Error", "BLANK ID");
            System.out.println(obj.toString());
            e.printStackTrace();
        }
    }

    private void generateJson(List<HashMap> data) {
        JSONObject obj = new JSONObject();
        try {
            for (HashMap map : data) {

                Iterator it = map.entrySet().iterator();

                while (it.hasNext()) {

                    Map.Entry pair = (Map.Entry) it.next();
                    String key = (String) pair.getKey();

                    String value = (String) pair.getValue();
                    if (value == null) {
                        value = "blank";
                    }
                    obj.put(key, value);
                }
            }

            out.println(obj.toString());

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

    }

    @Override
    protected void doGet(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        out = response.getWriter();
        JSONObject obj = new JSONObject();
        try {
            obj.put("Error", "API Protected Get Request");
        } catch (JSONException ex) {
            Logger.getLogger(WorkSpaceDetails.class.getName()).log(Level.SEVERE, null, ex);
        }
        out.println(obj.toString());
        out.close();
        System.out.println("API Protected:Get Request");
    }

    @Override
    protected void doPost(HttpServletRequest request, HttpServletResponse response)
            throws ServletException, IOException {
        out = response.getWriter();
        JSONObject obj = new JSONObject();
        try {
            processRequest(request, response);
        } catch (NumberFormatException ex) {
            try {
                obj.put("Error", "ID NULL");
            } catch (JSONException ex1) {
                Logger.getLogger(WorkSpaceDetails.class.getName()).log(Level.SEVERE, null, ex1);
            }
            out.println(obj.toString());
        } catch (Exception ex) {

            try {
                obj.put("Error", "TOKEN NULL");
            } catch (JSONException ex1) {
                Logger.getLogger(WorkSpaceDetails.class.getName()).log(Level.SEVERE, null, ex1);
            }
            out.println(obj.toString());
            out.close();

        }
    }

    @Override
    public String getServletInfo() {
        return "Servlet to get info of a Workspace";
    }

}

Answer 1

而不是声明PrintWriter out;作为类成员，将其作为 processRequest() 内的局部变量，就像您已经在 doGet() 中所做的那样和doPost() 。这样，可能并发访问 servlet 的多个线程就不会互相干扰。

更新

看来processRequest()仅从 doPost() 调用，所以我建议您:

• 摆脱PrintWriter out类(class)成员
• 声明本地 PrintWriter out在doGet()
• 声明本地 PrintWriter out在doPost()
• 密码 out作为 processRequest() 的参数，其签名可能是:

private void processRequest(
    HttpServletRequest request, HttpServletResponse response, PrintWriter out);

更新

• 密码 out作为 generateJson() 的参数
• 密码 out作为调用“generateJson”方法的参数 processRequest()

private void generateJson(List<HashMap> data, PrintWriter out);

generateJson(completeData, out);