Develop Reference

MAC OS com.mysql.cj.exceptions.CJCommunicationsException: Communications link failure

I am trying to figure out why my app has trouble connecting to my database, only on mac OS so I'm using a small program to run on terminal on OS to see what was causing that.
It seems the hostname is causing the issue, but my service provider cannot give me an ip address to try if it fixes my issue. Does anyone have a fix for this or an idea on what I could try? Thanks a lot for your time.
here is the connection program. the database user can select only, the hostname is correct and password as well. This issue only happens on MAC OS and not all the time. My database accepts conneciton from all ip, I used a wildcard. Any idea on what to try next is appreciated.
import java.sql.Connection;
import java.sql.DriverManager;
public class test
{
public static void main(String[] args) {
System.out.println("\nMySQL JDBC Connection To G4THER DBase Tester");
Connection conn = null;
try {
Class.forName("com.mysql.cj.jdbc.Driver").newInstance();
String userName = "app";
String password = "password";
String url = "jdbc:mysql://hostname:3306/dbase";
conn = DriverManager.getConnection(url, userName, password);
System.out.println("\nDatabase Connection Established...");
Thread.sleep(1500L);
}
catch (Exception ex) {
System.err.println("Cannot connect to database server");
ex.printStackTrace();
}
finally {
if (conn != null)
try {
System.out.println("\nNow Lets terminate the Connection...");
Thread.sleep(1500L);
conn.close();
System.out.println("\nDatabase connection terminated...");
Thread.sleep(1500L);
System.out.println("\n***** Everything works fine Ol' man *****");
}
catch (Exception ex) {
System.out.println("Error in connection termination!");
}
}
}
}
here is the error log.
Last login: Thu Dec 12 12:25:13 on ttys000
Michaels-iMac:~ michael$ cd downloads
Michaels-iMac:downloads michael$ java -jar dbasetester.jar
***** MySQL JDBC Connection To G4THER DBase Tester *****
Cannot connect to database server
com.mysql.cj.jdbc.exceptions.CommunicationsException: Communications link failure
The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.
at com.mysql.cj.jdbc.exceptions.SQLError.createCommunicationsException(SQLError.java:174)
at com.mysql.cj.jdbc.exceptions.SQLExceptionsMapping.translateException(SQLExceptionsMapping.java:64)
at com.mysql.cj.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:836)
at com.mysql.cj.jdbc.ConnectionImpl.(ConnectionImpl.java:456)
at com.mysql.cj.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:246)
at com.mysql.cj.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:199)
at java.sql/java.sql.DriverManager.getConnection(DriverManager.java:677)
at java.sql/java.sql.DriverManager.getConnection(DriverManager.java:228)
at test.main(test.java:18)
Caused by: com.mysql.cj.exceptions.CJCommunicationsException: Communications link failure
The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.
at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at java.base/jdk.internal.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:62)
at java.base/jdk.internal.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:45)
at java.base/java.lang.reflect.Constructor.newInstanceWithCaller(Constructor.java:500)
at java.base/java.lang.reflect.Constructor.newInstance(Constructor.java:481)
at com.mysql.cj.exceptions.ExceptionFactory.createException(ExceptionFactory.java:61)
at com.mysql.cj.exceptions.ExceptionFactory.createException(ExceptionFactory.java:105)
at com.mysql.cj.exceptions.ExceptionFactory.createException(ExceptionFactory.java:151)
at com.mysql.cj.exceptions.ExceptionFactory.createCommunicationsException(ExceptionFactory.java:167)
at com.mysql.cj.protocol.a.NativeSocketConnection.connect(NativeSocketConnection.java:91)
at com.mysql.cj.NativeSession.connect(NativeSession.java:144)
at com.mysql.cj.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:956)
at com.mysql.cj.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:826)
... 6 more
Caused by: java.net.UnknownHostException: cpl81.hosting24.com: nodename nor servname provided, or not known
at java.base/java.net.Inet6AddressImpl.lookupAllHostAddr(Native Method)
at java.base/java.net.InetAddress$PlatformNameService.lookupAllHostAddr(InetAddress.java:930)
at java.base/java.net.InetAddress.getAddressesFromNameService(InetAddress.java:1499)
at java.base/java.net.InetAddress$NameServiceAddresses.get(InetAddress.java:849)
at java.base/java.net.InetAddress.getAllByName0(InetAddress.java:1489)
at java.base/java.net.InetAddress.getAllByName(InetAddress.java:1348)
at java.base/java.net.InetAddress.getAllByName(InetAddress.java:1282)
at com.mysql.cj.protocol.StandardSocketFactory.connect(StandardSocketFactory.java:132)
at com.mysql.cj.protocol.a.NativeSocketConnection.connect(NativeSocketConnection.java:65)
... 9 more
Michaels-iMac:downloads michael$
thanks

I was able to fix the issue using an IP address instead of the hostname. I used a site to find the IP with the hostname and changed my code. So far so good.

Catching Java SQLExceptions

I do not have a lot of experience in Java and would appreciate if you bring a light on the question.
I have the following piece of code (method) to establish the JDBC connection with a PostgreSQL database :
public void establishDBConnection() throws SQLException {
try (Connection connection = DriverManager.getConnection(
"jdbc:postgresql://" + dbServer + ":" + dbPort + "/" +
dbDatabase, dbUser, dbPassword))
catch (SQLException e) {
logger.error("Connection to Postgres Database Failed: " +
e.printStackTrace();
}
}
I expect that the SQLException would be caught by the catch block as soon as the method can't establish connection with the database (server is down). The catch block puts the exception on the console but I see that the same exception is displayed earlier right after the getConnection method execution. Thus, I have 2 quite the same exceptions on the console. See below.
The question is what is the reason for this and how to force the application to display only the exception generated by the catch block.
Logs:
Jul 07, 2017 9:11:53 PM org.postgresql.Driver connect
SEVERE: Connection error:
**org.postgresql.util.PSQLException: Connection to localhost:5433 refused. Check that the hostname and port are correct and that the postmaster is accepting TCP/IP connections.**
at org.postgresql.core.v3.ConnectionFactoryImpl.openConnectionImpl(ConnectionFactoryImpl.java:265)
at org.postgresql.core.ConnectionFactory.openConnection(ConnectionFactory.java:49)
at org.postgresql.jdbc.PgConnection.<init>(PgConnection.java:194)
at org.postgresql.Driver.makeConnection(Driver.java:431)
at org.postgresql.Driver.connect(Driver.java:247)
at java.sql.DriverManager.getConnection(DriverManager.java:664)
at java.sql.DriverManager.getConnection(DriverManager.java:247)
at excel2db.service.impl.DBConnectionPostgresImpl.establishDBConnection(DBConnectionPostgresImpl.java:38)
at excel2db.excel2db.main(excel2db.java:86)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:144)
Caused by: java.net.ConnectException: Connection refused: connect
at java.net.DualStackPlainSocketImpl.waitForConnect(Native Method)
at java.net.DualStackPlainSocketImpl.socketConnect(DualStackPlainSocketImpl.java:85)
at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:350)
at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:206)
at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:188)
at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:172)
at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
at java.net.Socket.connect(Socket.java:589)
at org.postgresql.core.PGStream.<init>(PGStream.java:62)
at org.postgresql.core.v3.ConnectionFactoryImpl.openConnectionImpl(ConnectionFactoryImpl.java:144)
... 13 more
21:11:53.465 [main] ERROR e.s.impl.DBConnectionPostgresImpl - **Connection to Postgres Database Failed: Connection to localhost:5433 refused. Check that the hostname and port are correct and that the postmaster is accepting TCP/IP connections.**

There is only one exception being printed in your example: the SQLException being caught. What you think is a second exception is only (as the text shows) the cause of the SQLException: Postgres throws a SQLException, which is itself being thrown because there was a network ConnectException when Portgres tried to connect.
This chain of exceptions is very handy when you need to diagnose a failure. It allows going back to the original, low-level source of a problem. Just like, for example, it can be useful to know that you didn't got your mail (NoMailException) because the post office is on strike (OnStrikeException), because the government decided to lower the wages (WagesTooLowException). If you just had the original NoMailException, you wouldn't be able to know the reason why you got no mail, know that it's probably temporary, etc.
In this particular case, you can deduce that it was impossible to connect to the server, and not for example, that the password was incorrect.
Chaining exceptions is simply achieved by calling one of the exception constructors that take another exception as argument (most exceptions do). See https://docs.oracle.com/javase/8/docs/api/java/lang/Exception.html#Exception-java.lang.String-java.lang.Throwable-.

You need to load the driver's class into memory. Try This code :
try {
Class.forName("org.postgresql.Driver");
Connection connection = DriverManager.getConnection(jdbc:postgresql://server_address:5432/database_name?user=usernam&password=password);
} catch (Exception exception) {
exception.printStackTrace();
}

Connection to Oracle 11g from network machine fails via java

I have orace 11g running on 192.168.1.217 and I am trying to connect it using JDBC driver with java and it gives me following error
IO Error: The Network Adapter could not establish the connection
Library I am using is ojdbc6.jar
Here is my code
public void makeOracleConnection() {
try {
Class.forName("oracle.jdbc.OracleDriver");
oraCon = DriverManager.getConnection("jdbc:oracle:thin:#192.168.1.217:1521:orcl", "hr", "hr");
oraStmt = oraCon.createStatement();
oraRsStmt=oraCon.createStatement(ResultSet.CONCUR_READ_ONLY,ResultSet.TYPE_SCROLL_INSENSITIVE);
} catch (Exception e) {
System.out.println("Error while making connection with Database : " + e.getMessage());
}
}
I have also tried to ping on 192.168.1.217 then pins is successful.
Also TNSLISTENER is running on that machine.
please help.
Please find print stack trace here
run:
java.sql.SQLRecoverableException: IO Error: The Network Adapter could not establish the connection
at oracle.jdbc.driver.T4CConnection.logon(T4CConnection.java:743)
at oracle.jdbc.driver.PhysicalConnection.connect(PhysicalConnection.java:657)
at oracle.jdbc.driver.T4CDriverExtension.getConnection(T4CDriverExtension.java:32)
at oracle.jdbc.driver.OracleDriver.connect(OracleDriver.java:560)
at java.sql.DriverManager.getConnection(DriverManager.java:664)
at java.sql.DriverManager.getConnection(DriverManager.java:247)
at test.oracle.makeOracleConnection(oracle.java:30)
at test.oracle.<init>(oracle.java:21)
at test.oracle.main(oracle.java:69)
Caused by: oracle.net.ns.NetException: The Network Adapter could not establish the connection
at oracle.net.nt.ConnStrategy.execute(ConnStrategy.java:470)
at oracle.net.resolver.AddrResolution.resolveAndExecute(AddrResolution.java:506)
at oracle.net.ns.NSProtocol.establishConnection(NSProtocol.java:595)
at oracle.net.ns.NSProtocol.connect(NSProtocol.java:230)
at oracle.jdbc.driver.T4CConnection.connect(T4CConnection.java:1452)
at oracle.jdbc.driver.T4CConnection.logon(T4CConnection.java:496)
... 8 more
Caused by: java.net.ConnectException: Connection refused: connect
at java.net.DualStackPlainSocketImpl.waitForConnect(Native Method)
at java.net.DualStackPlainSocketImpl.socketConnect(DualStackPlainSocketImpl.java:85)
at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:350)
at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:206)
at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:188)
at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:172)
at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
at java.net.Socket.connect(Socket.java:589)
at oracle.net.nt.TcpNTAdapter.connect(TcpNTAdapter.java:163)
at oracle.net.nt.ConnOption.connect(ConnOption.java:159)
at oracle.net.nt.ConnStrategy.execute(ConnStrategy.java:428)
... 13 more
BUILD SUCCESSFUL (total time: 1 second)

You get the error
java.net.ConnectException: Connection refused: connect
Which means that there is nothing listening on the machine and port you are trying to connect to. Your Java code looks correct so I would continue to investigate that Oracle is actually listening on port 1521 on 192.168.1.217.
If you run run netstat -n on the server you should find a line that looks like
TCP [::]:1521 [::]:0 LISTENING
If something really is listening on that port. If you do not find that line, check your Oracle configuration.

Try to connect with some other tool, ie sqlplus to verify that the issue is not with Oracle. If you cannot connect with sqlplus/sql developer, make sure that your oracle is configured to allow remote connections, and also listens on given addresses/ports

public void makeOracleConnection() {
try {
Class.forName("oracle.jdbc.OracleDriver");
Connection oraCon = DriverManager.getConnection("jdbc:oracle:thin:#192.168.1.217:1521:orcl", "hr", "hr");
Statement oraStmt = oraCon.createStatement();
//oraRsStmt=oraCon.createStatement(ResultSet.CONCUR_READ_ONLY,ResultSet.TYPE_SCROLL_INSENSITIVE);
ResultSet rs = oraStmt.executeQuery("select hello as result from dual");
while(rs.next()) {
System.out.println(rs.getString("result"));
}
}
catch (Exception e)
System.out.println("Error while making connection with Database : " + e.getMessage());
}
}
Try this out. Hope it'll help. I also don't like your connection path. Is it right? I think it should be something like this:
jdbc:oracle:thin:#(DESCRIPTION=(ADDRESS_LIST=(LOAD_BALANCE=YES)(ADDRESS=(PROTOCOL=tcp)(HOST=ip adres)(PORT=port)))(CONNECT_DATA=(SERVICE_NAME = orcl)))","username","password"

How to check the default port number for mysql connection? Connection not working! jdbc connection

I am trying to connect with database using jdbc in java file. It is not connecting at all and giving me the error constantly "Something went wrong"; I guess it is because of the port number because all other data such as username, password and other code seems correct.
I want to check the default port number so that I can try it properly. I did try using all three of these 8080, 80 and 3306 but it shows me error.
Here port 8080 is used for HTTP server, 3306 is supposed to be default from the research and 80 randomly.
try {
Class.forName("com.mysql.jdbc.Driver");
System.out.println("Driver found");
} catch (ClassNotFoundException e) {
System.out.println("Driver not found");
}
String url="jdbc:mysql://localhost:8080 or 80 or 3306 or without port number/test";
String user="user";
String password="";
Connection con=null;
try {
con=DriverManager.getConnection(url, user, password);
System.out.println("Success");
} catch (SQLException e){
e.printStackTrace();
}
}
The error is giving below when used String url="jdbc:mysql://localhost:3306/test";
Driver found
com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure
The last packet sent successfully to the server was 0 milliseconds ago. The driver has not received any packets from the server.
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
at com.mysql.jdbc.Util.handleNewInstance(Util.java:377)
at com.mysql.jdbc.SQLError.createCommunicationsException(SQLError.java:1036)
at com.mysql.jdbc.MysqlIO.<init>(MysqlIO.java:338)
at com.mysql.jdbc.ConnectionImpl.coreConnect(ConnectionImpl.java:2232)
at com.mysql.jdbc.ConnectionImpl.connectOneTryOnly(ConnectionImpl.java:2265)
at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2064)
at com.mysql.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:790)
at com.mysql.jdbc.JDBC4Connection.<init>(JDBC4Connection.java:44)
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(NativeConstructorAccessorImpl.java:39)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(DelegatingConstructorAccessorImpl.java:27)
at java.lang.reflect.Constructor.newInstance(Constructor.java:513)
at com.mysql.jdbc.Util.handleNewInstance(Util.java:377)
at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:395)
at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:325)
at java.sql.DriverManager.getConnection(DriverManager.java:582)
at java.sql.DriverManager.getConnection(DriverManager.java:185)
at com.town.connect.Dbconnection.main(Dbconnection.java:26)
Caused by: java.net.ConnectException: Connection refused
at java.net.PlainSocketImpl.socketConnect(Native Method)
at java.net.PlainSocketImpl.doConnect(PlainSocketImpl.java:382)
at java.net.PlainSocketImpl.connectToAddress(PlainSocketImpl.java:241)
at java.net.PlainSocketImpl.connect(PlainSocketImpl.java:228)
at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:431)
at java.net.Socket.connect(Socket.java:527)
at com.mysql.jdbc.StandardSocketFactory.connect(StandardSocketFactory.java:213)
at com.mysql.jdbc.MysqlIO.<init>(MysqlIO.java:297)
... 15 more

You can access those settings via
mysql> show variables;

DriverManager.getConnection("jdbc:mysql://"+dbaddress+"/"+dbname+"?user=" + username + "&password=" + password)
This statement can help you. No need to define any other port. Just define the following variables as per your mysql configuration.
dbaddress
dbname
username
password
And if this doesn't works can you post the stacktrace or a screenshot for your error ?

If you want to know the port number you can use following command
SHOW VARIABLES WHERE Variable_name = 'port';

java.rmi.ConnectException: Connection refused to host: 127.0.0.1

I've tried to used RMI, here is server side. at first it worked without any exception, but now after three times whenever i try to run the below code, i will get some errors
The code is:
import java.rmi.server.UnicastRemoteObject;
/**
* Created by elyas on 12/11/14 AD.
*/
public class LogicImplement extends UnicastRemoteObject implements Logic
{
public LogicImplement() throws Exception
{
java.rmi.registry.LocateRegistry.createRegistry(6060);
java.rmi.Naming.rebind("Object1",this);
}
#Override
public int sum(int a, int b) throws Exception
{
int result = a + b;
System.out.println("ana sum executed");
return result;
}
public static void main(String[] args)
{
try
{
LogicImplement logicImplement = new LogicImplement();
} catch (Exception e)
{
e.printStackTrace();
}
}
}
The error is like this: i've tried to change the Object1 to for example Object2, but again i will get error, also i change the port number...
what is solution?
java.rmi.ConnectException: Connection refused to host: 127.0.0.1; nested exception is:
java.net.ConnectException: Connection refused
at sun.rmi.transport.tcp.TCPEndpoint.newSocket(TCPEndpoint.java:619)
at sun.rmi.transport.tcp.TCPChannel.createConnection(TCPChannel.java:216)
at sun.rmi.transport.tcp.TCPChannel.newConnection(TCPChannel.java:202)
at sun.rmi.server.UnicastRef.newCall(UnicastRef.java:341)
at sun.rmi.registry.RegistryImpl_Stub.rebind(Unknown Source)
at java.rmi.Naming.rebind(Naming.java:177)
at LogicImplement.<init>(LogicImplement.java:12)
at LogicImplement.main(LogicImplement.java:27)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:57)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:606)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:120)
Caused by: java.net.ConnectException: Connection refused
at java.net.PlainSocketImpl.socketConnect(Native Method)
at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:339)
at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:200)
at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:182)
at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
at java.net.Socket.connect(Socket.java:579)
at java.net.Socket.connect(Socket.java:528)
at java.net.Socket.<init>(Socket.java:425)
at java.net.Socket.<init>(Socket.java:208)
at sun.rmi.transport.proxy.RMIDirectSocketFactory.createSocket(RMIDirectSocketFactory.java:40)
at sun.rmi.transport.proxy.RMIMasterSocketFactory.createSocket(RMIMasterSocketFactory.java:147)
at sun.rmi.transport.tcp.TCPEndpoint.newSocket(TCPEndpoint.java:613)
... 12 more

java.net.ConnectException: Connection refused
There could be couple of reasons for this exception:
You have not started your rmiregistry in background.
You are trying to connect to the wrong port number.
Your firewall maybe blocking the connections.

The Answer was very simple, By default, the registry runs on port 1099. To start the registry on a different port, specify the port number on the command line. Do not forget to unset your CLASSPATH environment variable. for more information check this link: Running the Example Programs
** So for fixing this code i must change the port number form 6060 to 1099
notice that: if 1099 is used by other services you have to test 1100, and if 1100 is used too, you have yo use 1101 and so on. :-)

Since this was a port number issue, there is another way to start the registry on the port number you want:
java.rmi.registry.Registry rmiRegistry = java.rmi.registry.LocateRegistry.
createRegistry(6060); // Creates a registry on 6060
rmiRegistry.rebind("Object1",this); // Binds to registry created on 6060

You haven't started the RMI Registry.
When you get past this, if it still happens when calling the remote method, see item A.1 of the RMI FAQ.