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Null Pointer when converting to JSON

I am using WDCalendar in Struts2. I converted a Hashmap to a Json String using GSON, but suddenly I get a NULL pointer exception, and a Convert Hashmap for a JSON string

result = new Gson().toJson(ret);

My code is:

 public static String loadAllEvents(String start, String end) throws SQLException { PreparedStatement ps1 = null; ResultSet rs1 = null; Gson gson = new Gson(); String result = ""; Connection con = ULDBConnectionUtility.getDBConnection(); HashMap<String, Object> ret = new HashMap<String, Object>(); try{ String strSql = "SELECT * FROM EVENT_MANAGER";// WHERE START_DATE BETWEEN '"+start+"' AND '"+end+"'"; ps1 = con.prepareStatement(strSql); rs1 = ps1.executeQuery(); SimpleDateFormat fromDB = new SimpleDateFormat("yyyy-MM-dd'T'HH:mm:ss"); java.text.SimpleDateFormat sourceFormat = new SimpleDateFormat("EEE MMM dd yyyy HH:mm:ss ZZZ"); ArrayList<ArrayList> eventArrayList = new ArrayList<ArrayList>(); while(rs1.next()){ ArrayList EvtData = new ArrayList(); EvtData.add(rs1.getInt("EVENT_ID"));//0 EvtData.add(rs1.getString("EVENT_TITLE"));//1 EvtData.add(sourceFormat.format(fromDB.parse(rs1.getString("START_DATE"))));//2 EvtData.add(sourceFormat.format(fromDB.parse(rs1.getString("END_DATE"))));//3 EvtData.add(rs1.getInt("IS_ALL_DAY_EVENT"));//4 EvtData.add(Integer.parseInt("0"));//5 EvtData.add(rs1.getInt("RECURRING_RULE"));//6 EvtData.add(rs1.getString("COLOR"));//7 EvtData.add(Integer.parseInt("1"));//8 EvtData.add(rs1.getString("LOCATION"));//9 EvtData.add("");//10 eventArrayList.add(EvtData); } String error = ""; ret.put("events", eventArrayList); ret.put("issort", true); ret.put("start", sourceFormat.format(fromDB.parse(start))); ret.put("end", sourceFormat.format(fromDB.parse(end))); ret.put("error", error); System.out.println("ret : "+ret); result = new Gson().toJson(ret); } catch (Exception e) { e.printStackTrace(); } finally { try { if (rs1 != null) { rs1.close(); } if (ps1 != null) { ps1.close(); } if (con != null) { con.close(); } } catch (SQLException e) { e.printStackTrace(); } } return result; }

HashMap before moving on to conversion:

 {events=[[98, EvebtTitle, Mon Dec 03 2012 10:45:00 +0530, Wed Oct 17 2012 11:15:00 +0530, 0, 0, 0, 0, 1, NJ, ], [118, testDec, Wed Dec 05 2012 09:00:00 +0530, Wed Dec 05 2012 10:00:00 +0530, 0, 0, 0, null, 1, null, ], [67, Test45, Wed Dec 05 2012 10:30:00 +0530, Tue Oct 16 2012 11:00:00 +0530, 1, 0, 0, 0, 1, NY, ], [119, sd, Tue Dec 04 2012 00:00:00 +0530, Tue Dec 04 2012 00:00:00 +0530, 1, 0, 0, null, 1, null, ], [78, Test, Fri Dec 07 2012 09:30:00 +0530, Mon Oct 15 2012 10:15:00 +0530, 0, 0, 0, 1, 1, LA, ], [66, Event 2, Tue Dec 04 2012 10:00:00 +0530, Tue Oct 16 2012 10:30:00 +0530, 0, 0, 1, 0, 1, CA, ]], start=Thu Jan 01 1970 05:30:00 +0530, issort=true, end=Thu Jan 01 1970 05:30:00 +0530, error=null}

Error:

 20:49:15,597 ERROR [STDERR] java.lang.NullPointerException 20:49:15,599 ERROR [STDERR] at com.google.gson.DefaultTypeAdapters$CollectionTypeAdapter.serialize(DefaultTypeAdapters.java:443) 20:49:15,601 ERROR [STDERR] at com.google.gson.DefaultTypeAdapters$CollectionTypeAdapter.serialize(DefaultTypeAdapters.java:430) 20:49:15,604 ERROR [STDERR] at com.google.gson.JsonSerializationVisitor.visitUsingCustomHandler(JsonSerializationVisitor.java:128) 20:49:15,605 ERROR [STDERR] at com.google.gson.ObjectNavigator.accept(ObjectNavigator.java:96) 20:49:15,606 ERROR [STDERR] at com.google.gson.JsonSerializationContextDefault.serialize(JsonSerializationContextDefault.java:47) 20:49:15,608 ERROR [STDERR] at com.google.gson.DefaultTypeAdapters$CollectionTypeAdapter.serialize(DefaultTypeAdapters.java:445) 20:49:15,609 ERROR [STDERR] at com.google.gson.DefaultTypeAdapters$CollectionTypeAdapter.serialize(DefaultTypeAdapters.java:430) 20:49:15,610 ERROR [STDERR] at com.google.gson.JsonSerializationVisitor.visitUsingCustomHandler(JsonSerializationVisitor.java:128) 20:49:15,611 ERROR [STDERR] at com.google.gson.ObjectNavigator.accept(ObjectNavigator.java:96) 20:49:15,612 ERROR [STDERR] at com.google.gson.JsonSerializationContextDefault.serialize(JsonSerializationContextDefault.java:47) 20:49:15,613 ERROR [STDERR] at com.google.gson.DefaultTypeAdapters$MapTypeAdapter.serialize(DefaultTypeAdapters.java:509) 20:49:15,613 ERROR [STDERR] at com.google.gson.DefaultTypeAdapters$MapTypeAdapter.serialize(DefaultTypeAdapters.java:489) 20:49:15,614 ERROR [STDERR] at com.google.gson.JsonSerializationVisitor.visitUsingCustomHandler(JsonSerializationVisitor.java:128) 20:49:15,615 ERROR [STDERR] at com.google.gson.ObjectNavigator.accept(ObjectNavigator.java:96) 20:49:15,616 ERROR [STDERR] at com.google.gson.JsonSerializationContextDefault.serialize(JsonSerializationContextDefault.java:47) 20:49:15,617 ERROR [STDERR] at com.google.gson.Gson.toJson(Gson.java:269) 20:49:15,618 ERROR [STDERR] at com.google.gson.Gson.toJson(Gson.java:220) 20:49:15,618 ERROR [STDERR] at com.google.gson.Gson.toJson(Gson.java:200) 20:49:15,619 ERROR [STDERR] at dao.UtilityDao.loadAllEvents(UtilityDao.java:174) 20:49:15,620 ERROR [STDERR] at utility.CalendarUtility.loadAllList(CalendarUtility.java:64) 20:49:15,621 ERROR [STDERR] at servlet.DataFeed.doGet(DataFeed.java:56) 20:49:15,622 ERROR [STDERR] at servlet.DataFeed.doPost(DataFeed.java:97) 20:49:15,623 ERROR [STDERR] at javax.servlet.http.HttpServlet.service(HttpServlet.java:637) 20:49:15,624 ERROR [STDERR] at javax.servlet.http.HttpServlet.service(HttpServlet.java:717) 20:49:15,625 ERROR [STDERR] at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:290) 20:49:15,626 ERROR [STDERR] at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:206) 20:49:15,627 ERROR [STDERR] at org.jboss.web.tomcat.filters.ReplyHeaderFilter.doFilter(ReplyHeaderFilter.java:96) 20:49:15,
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2 answers

resolved:

Some event field values ​​were NULL in the database, and that was the reason.

In ret.put("events", eventArrayList);

eventArrayList refers to ArrayList<ArrayList> , and several field values ​​in <ArrayList> are NULL,

Above Ex: (Look at the Null value in the internal arraylist, which is in the second index)

 {events=[[98, EvebtTitle, Mon Dec 03 2012 10:45:00 +0530, Wed Oct 17 2012 11:15:00 +0530, 0, 0, 0, 0, 1, NJ, ], [118, testDec, Wed Dec 05 2012 09:00:00 +0530, Wed Dec 05 2012 10:00:00 +0530, 0, 0, 0, null, 1, null, ]]}

After removing the Null values ​​from the internal ArrayList, everything works fine.

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One thing that comes to my mind about the method

public String toJson (Object src)

GSON documentation that states: “This method should be used when the specified object is not a common type. If the object is of a common type, use toJson (Object, Type) instead.

Another thing is that null pointer exception can be caused by any other object, and not just HashMap. It seems that you took it for granted, but I recommend debugging your code.

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