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how to Fix JAVA error NullPointerException(with jdbc)

I do not speak English well

An error occurred while doing JDBC.

I knew during coding that ' executequery ' should only be done by ' select ' in the database, and ' insert ' should be ' executeupdate '.

However, when I did executeupdate, I had a problem with the variable and changed it to " int "

Then an error occurred.

How can it be carried out as I intended?

I'll show you the code.

import java.io.BufferedReader;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.ResultSet;
import java.sql.SQLException;
import java.sql.Statement;
import java.util.ArrayList;

public class EdgeCloud {
static ArrayList<String> deleteDB = new ArrayList<>();
static ArrayList<String> DBCarIP = new ArrayList<>();
static ArrayList<String> ContainerCarIP = new ArrayList<>();
static String carSSID = "192.0.1";
static String state;
static String upDateInputContainer;
static String upDateInputCar;
static int i = 0;
static String cIP;

//-------------------------------------------( main start )----------------------------------------------------//
public static void main(String args) throws Exception {
System.out.println("main start");
while(true) {
// JDBC(state="check");
if(carSSID!=null) {
makeContainer();
}else {
UPDATE();
}
}
}
//-------------------------------------------( main end )----------------------------------------------------//


//-------------------------------------------( make container start )----------------------------------------------------//
public static void makeContainer() throws Exception {
System.out.println("mc start");

String makeContainer = "/usr/local/bin/docker run --name "+carSSID+" ubuntu:16.04";
Runtime runtime = Runtime.getRuntime();
Process process = runtime.exec(makeContainer);
InputStream is = process.getInputStream();
InputStreamReader isr = new InputStreamReader(is);
BufferedReader br = new BufferedReader(isr);
String line;

while((line = br.readLine()) != null) {

System.out.println(line);
}

String ipload = "/usr/local/bin/docker inspect -f {{.NetworkSettings.IPAddress}} "+carSSID;

IPLoad(ipload);
}

//-------------------------------------------( make container end )----------------------------------------------------//


//-------------------------------------------( get container ip start )----------------------------------------------------//
public static void IPLoad(String ipload) throws Exception {
System.out.println("ipload start");

upDateInputCar = null;
upDateInputContainer = null;
upDateInputCar = carSSID;
carSSID = null;
state = "insert";
Runtime runtime = Runtime.getRuntime();
Process process = runtime.exec(ipload);
InputStream is = process.getInputStream();
InputStreamReader isr = new InputStreamReader(is);
BufferedReader br = new BufferedReader(isr);
while((cIP = br.readLine()) != null) {
upDateInputContainer= cIP;
System.out.println("cip : "+cIP);
}
System.out.println("upd"+upDateInputContainer);
JDBC(state);
}

//-------------------------------------------( get container ip end )----------------------------------------------------//


//-------------------------------------------( update start )----------------------------------------------------//
public static void UPDATE() throws Exception {
state = "check";
JDBC(state);
String getContainerName = "/usr/local/bin/docker ps --format {{.Names}}";
Runtime runtime = Runtime.getRuntime();
Process process = runtime.exec(getContainerName);
InputStream is = process.getInputStream();
InputStreamReader isr = new InputStreamReader(is);
BufferedReader br = new BufferedReader(isr);
String line;

while((line = br.readLine()) != null) {
ContainerCarIP.add(line);
}
deleteDB.addAll(DBCarIP);
deleteDB.removeAll(ContainerCarIP);

if(deleteDB.get(0)!=null) {

state="delete";
while(deleteDB.get(i++)!=null) {
//JDBC(state);
}
i = 0;
}
}

//-------------------------------------------( update end )----------------------------------------------------//


//-------------------------------------------( socket start )----------------------------------------------------//

/*socket place
-
-
-
-
-
-
*/

//-------------------------------------------( soket end )----------------------------------------------------//


//-------------------------------------------( JDBC start )----------------------------------------------------//
public static void JDBC(String state){
System.out.println("JDBC start");
Connection conn = null ;
Statement stmt = null;
String query = null;

if(state=="insert") {
query = "insert into inpo (ip, carid) values ('"+upDateInputContainer+"', '"+upDateInputCar+"')";
System.out.println(query);
}

if(state=="delete") {
query = "delete from inpo where carid = " +deleteDB.get(i);
}

if(state=="check") {
query = "select carid from inpo";
}

try{
Class.forName("com.mysql.cj.jdbc.Driver").newInstance();
}
catch(ClassNotFoundException e ){
System.out.println( "드라이버 연결 에러." ) ;
}
catch(Exception etc) {
System.out.println(etc.getMessage());
}
try{
// String url = "jdbc:mysql://192.168.44.122:3306/ssidIp?serverTimezone=UTC&autoReconnect=true&useSSL=false";
String url = "jdbc:mysql://localhost:3306/ssidIp?serverTimezone=UTC&autoReconnect=true&useSSL=false";
String userPass = "123456789" ;
conn = DriverManager.getConnection(url, userId, userPass);
stmt = conn.createStatement();
ResultSet rs = null;
int rss = 0;
if(state=="check") {
rs = stmt.executeQuery(query);
}else {
rss = stmt.executeUpdate(query);
}//////////////////////////////////////////////////////
if (state=="check") {
rs = stmt.getResultSet();
}
System.out.println("------------------------------------");
System.out.println("debug1 ");

if(state=="check") {
System.out.println("debug 2");
while (rs.next()) {
String str = rs.getNString(1);
DBCarIP.add(str);
}
}else {
System.out.println("debug 3");

while(rs.next()) { // here NullpointerException
System.out.println("debug 4");
String str = rs.getNString(1);
System.out.println(str);
}
}

System.out.println("연결");
stmt.close();
conn.close();
}
catch( SQLException e ){
System.out.println( "SQLException : " + e.getMessage() ) ;
}
}
}
//-------------------------------------------( JDBC end )----------------------------------------------------//


ERROR part

main start
mc start
ipload start
cip : 172.17.0.3
upd172.17.0.3
JDBC start
insert into inpo (ip, carid) values ('172.17.0.3', '192.0.1')
------------------------------------
debug1
debug 3
Exception in thread "main" java.lang.NullPointerException
at edgeCloud.EdgeCloud.JDBC(EdgeCloud.java:195)
at edgeCloud.EdgeCloud.IPLoad(EdgeCloud.java:86)
at edgeCloud.EdgeCloud.makeContainer(EdgeCloud.java:61)
at edgeCloud.EdgeCloud.main(EdgeCloud.java:33)


rs

rss

JDBC(String)

1 Answer
1

You’re reading from rs instead of rss but rs is null for an insert or delete query.

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