org.postgresql.util.PSQLException：ResultSet没有正确定位，也许你需要调用next

Question

String id = request.getParameter("id");

    try {
            Class.forName("org.postgresql.Driver");
            Connection con = DriverManager.getConnection("jdbc:postgresql://172.10.10.3:5432/stepup380qty","stepup380qty","stepup380qty");
            String query = "select binarydata from ad_image where ad_image_id =" + id;
            Statement stm = con.createStatement();
            ResultSet rs = stm.executeQuery(query);
            rs.next();
            String img = rs.getString("binarydata");
            byte[] img2 = new byte[img.length()];
            InputStream imgstream = rs.getBinaryStream("binarydata");
            int index = imgstream.read(img2, 0, img.length());
            stm.close();
            response.reset();
            response.setContentType("image/jpg");
            response.getOutputStream().write(img2,0,img2.length);
            response.getOutputStream().flush();
            imgstream.close();
    }
    catch (SQLException e) { 
        e.printStackTrace(); 
        }
    catch (Exception e) { 
        e.printStackTrace(); 
        }
}

这是我的代码，我收到此错误

org.postgresql.util.PSQLException: ResultSet not positioned properly, perhaps you need to call next.
    at org.postgresql.jdbc2.AbstractJdbc2ResultSet.checkResultSet(AbstractJdbc2ResultSet.java:2881)
    at org.postgresql.jdbc2.AbstractJdbc2ResultSet.getString(AbstractJdbc2ResultSet.java:1956)
    at org.postgresql.jdbc2.AbstractJdbc2ResultSet.getString(AbstractJdbc2ResultSet.java:2568)
    at ireportImage.IreportIMAGE.doGet(IreportIMAGE.java:48)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:624)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:731)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)
    at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:505)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:169)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:956)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:436)
    at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1078)
    at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:625)
    at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:316)
    at java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
    at java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
    at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)
    at java.lang.Thread.run(Unknown Source)