在java中生成随机否的问题

Question

i have written a code like this but it is giving exceptions.actually i want to generate a random value.save into the database and return the same to the user

import java.math.BigInteger;
import java.security.SecureRandom;

public static String recoverpassword(String uid,String ans) {
    try {
        verifyans.setString(1,uid);
        verifyans.setString(2,ans);
        ResultSet resultSet = verifyans.executeQuery();

        if (resultSet.next()) {
            SecureRandom random = new SecureRandom();
            String newpass = new BigInteger(130, random).toString(32);
            resetpass.setString(1,newpass);
            resetpass.setString(2,uid);
            resetpass.executeUpdate();                          
            return newpass;
        } else {
            return "xxx";
        }

    } catch(Exception e) {
        System.out.println("exception" +e);
        e.printStackTrace();
        return "xxx";
    }
}

im getting null pointer exceptions like:

com.ibm.lims.Users@1d193c9]
exceptionjava.lang.NullPointerException
java.lang.NullPointerException
    at org.tranql.connector.jdbc.ConnectionHandle.connectionError(ConnectionHandle.java:103)
    at org.tranql.connector.jdbc.PreparedStatementHandle.executeUpdate(PreparedStatementHandle.java:105)
    at com.ibm.lims.LimsHandler.recoverpassword(LimsHandler.java:940)
    at org.apache.jsp.recoverpasswordresult_jsp._jspService(recoverpasswordresult_jsp.java:78)
    at org.apache.jasper.runtime.HttpJspBase.service(HttpJspBase.java:70)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:806)
    at org.apache.jasper.servlet.JspServletWrapper.service(JspServletWrapper.java:369)
    at org.apache.jasper.servlet.JspServlet.serviceJspFile(JspServlet.java:342)
    at org.apache.jasper.servlet.JspServlet.service(JspServlet.java:267)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:806)
    at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290)
    at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206)
    at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:233)
    at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:191)
    at org.apache.geronimo.tomcat.valve.DefaultSubjectValve.invoke(DefaultSubjectValve.java:56)
    at org.apache.geronimo.tomcat.GeronimoStandardContext$SystemMethodValve.invoke(GeronimoStandardContext.java:406)
    at org.apache.geronimo.tomcat.valve.GeronimoBeforeAfterValve.invoke(GeronimoBeforeAfterValve.java:47)
    at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:128)
    at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:102)
    at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:109)
    at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:567)
    at org.apache.geronimo.tomcat.valve.ThreadCleanerValve.invoke(ThreadCleanerValve.java:40)
    at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:293)
    at org.apache.coyote.http11.Http11Processor.process(Http11Processor.java:849)
    at org.apache.coyote.http11.Http11Protocol$Http11ConnectionHandler.process(Http11Protocol.java:583)
    at org.apache.tomcat.util.net.JIoEndpoint$Worker.run(JIoEndpoint.java:454)
    at java.lang.Thread.run(Unknown Source)

hey i tested the code removing those securerandom class call.i just passed a string value in double quotes like "sample" and it worked. means the problem is with the securerandom class.i think the value is going as an integer in a string value field so database server is sending exception. plz give me some code to generate random string alternatively –

Answer 1

There are several problems playing a role here.

First the root cause of the NPE:

java.lang.NullPointerException
    at org.tranql.connector.jdbc.ConnectionHandle.connectionError(ConnectionHandle.java:103)

This is clearly a bug in the TranQL JDBC driver. There was some unexpected null pointer during the ConnectionHandle#connectionError() method so that it was unable to handle the actual connection error. To fix this particular problem (so that you don't get a NPE, but a more clear and driver-specific SQLException ), either upgrade the driver or replace it by a more decent one (I've never heard of TranQL before).

As to the real cause of the whole problem; judging from the stacktrace it look like that there's no means of an active connection. Judging from the non-threadsafe code it look like that the preparedStatement was already used before and has somehow implicitly been closed / detached from the connection.

At least it clearly boils down to the non-threadsafe and non-resourceleak-safe JDBC code you have there. The normal JDBC idiom is that you should always acquire and close the Connection , PreparedStatement and ResultSet in the shortest possible scope . Thus, already inside the very same (non-static!) method block. The statement and resultset should never be shared among threads. The connection can be, but you shouldn't take it in your own hands. Use a decent connection pool API for this, eg C3P0.

Here's an example according the ideal JDBC idiom:

public String recoverPassword(Long userId, String answer) throws SQLException {
    Connection connection = null;
    PreparedStatement verifyAnswer = null;
    ResultSet resultSet = null;
    String newPassword = null;
    PreparedStatement resetPassword = null;

    try {
        connection = database.getConnection();
        verifyAnswer = connection.prepareStatement(SQL_VERIFY_ANSWER);
        verifyAnswer.setLong(1, userId);
        verifyAnswer.setString(2, answer);
        resultSet = statement.executeQuery();

        if (resultSet.next()) {
            SecureRandom random = new SecureRandom();
            newPassword = new BigInteger(130, random).toString(32);
            resetPassword = connection.prepareStatement(SQL_RESET_PASSWORD);
            resetPassword.setString(1, newPassword);
            resetPassword.setLong(2, userId);
            resetPassword.executeUpdate();                          
        }
    } finally {
        // Always free resources in reversed order.
        if (resetPassword != null) try { resetPassword.close(); } catch (SQLException logOrIgnore) {}
        if (resultSet != null) try { resultSet.close(); } catch (SQLException logOrIgnore) {}
        if (verifyAnswer != null) try { verifyAnswer.close(); } catch (SQLException logOrIgnore) {}
        if (connection != null) try { connection.close(); } catch (SQLException logOrIgnore) {}
    }

    return newPassword;
}

Note that I hinted the userId to be a Long . You really don't want to have String ID's in both your code and datamodel. You can find more information, hints and other examples about writing solid JDBC code here .

That said, the following part of the stacktrace

at com.ibm.lims.LimsHandler.recoverpassword(LimsHandler.java:940)
at org.apache.jsp.recoverpasswordresult_jsp._jspService(recoverpasswordresult_jsp.java:78)

implies that you're writing raw Java code in a JSP file using the old-fashioned scriptlets. To save future maintenance and debugging problems, I would strongly recommend you not to do so, but just use a Servlet class for that.

Answer 2

Have you tried stepping through this with a debugger?

From the code posted it looks like maybe uid is being passed in as null at the top of the method.

You also shouldn't store passwords as plain text in the database, you should use a salted one-way hash of the password instead.

Answer 3

The problem you have is not related to random gen of number. You might take a look at you DB connection parameters, at the sql request you are trying to execute, ... But please, be more specific "I'm getting NPE" is not a question :)