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.

java.net.ConnectException: Connection refused timeout

I saw a lot of "java.net.ConnectException: Connection refused" questions but none referring to timeout of this error. My problem is I have to connect to a server that, in some cases, is blocked (connected by another software to the same port). So, I'm doing a loop with some max retries to try to connect:
My current code (of course, is depending on a lot of configurations for my software, but is working fine):
public TCPConnector(TCPDefinition tcpDefinition) throws IAException {
ivTcpDefinition = tcpDefinition;
// Initialize the socket
boolean retry = false;
int counter = 1;
do {
try {
ivSocket = new Socket();
ivSocket.connect(new InetSocketAddress(tcpDefinition.getHostname(), tcpDefinition.getPort()), tcpDefinition.getConnectTimeOut());
ivSocket.setSoTimeout(tcpDefinition.getAckTimeOut());
retry = false;
}
catch (UnknownHostException uhe) {
throw new IAException(null, new StringBuffer("Can't find host: ").append(tcpDefinition.getHostname()).toString(), uhe);
}
catch (SocketException see) {
StringBuilder sb = new StringBuilder("Connection refused to host ").append(tcpDefinition.getHostname()).
append(" port ").append(tcpDefinition.getPort()).append(". Connection Attempt Nr. ").append(counter);
logger.error(sb.toString(), see);
retry = true;
if (counter++ > tcpDefinition.getConnectRetries())
throw new IAException(null, sb.toString(), see);
else
logger.error("will retry to connect");
}
catch (IOException ioe) {
StringBuilder sb = new StringBuilder("I/O error while connecting to host ").append(tcpDefinition.getHostname()).
append(" port ").append(tcpDefinition.getPort()).append(". Connection Attempt Nr. ").append(counter);
logger.error(sb.toString(), ioe);
retry = true;
if (counter++ > tcpDefinition.getConnectRetries())
throw new IAException(null, sb.toString(), ioe);
else
logger.error("will retry to connect");
}
}
while (retry);
}
Well, the problem is this:
On Windows, every second, the SocketException is thrown, instead the IOException, while I have configured a timeout of 5000 msec to ivSocket.connect
On Linux, this is thrown every millisecond!!
Windows:
2019-12-05 12:40:47,609 ERROR DefaultQuartzScheduler_Worker-1 TCPConnector - Connection refused to host localhost port 13002. Connection Attempt Nr. 1
java.net.ConnectException: Connection refused: connect
at java.net.PlainSocketImpl.socketConnect(Native Method)
2019-12-05 12:40:48,703 ERROR DefaultQuartzScheduler_Worker-1 TCPConnector - Connection refused to host localhost port 13002. Connection Attempt Nr. 2
java.net.ConnectException: Connection refused: connect
Linux:
2019-12-05 12:45:47,609 ERROR DefaultQuartzScheduler_Worker-1 TCPConnector - Connection refused to host localhost port 13002. Connection Attempt Nr. 1
java.net.ConnectException: Connection refused: connect
at java.net.PlainSocketImpl.socketConnect(Native Method)
2019-12-05 12:45:47,610 ERROR DefaultQuartzScheduler_Worker-1 TCPConnector - Connection refused to host localhost port 13002. Connection Attempt Nr. 2
java.net.ConnectException: Connection refused: connect
Why the timeout is not executed? Well this is not exactly right. If I configure a timeout less than 1 second on Windows, then the timeout is executed. 500 msec:
2019-12-05 11:47:07,375 ERROR DefaultQuartzScheduler_Worker-1 TCPConnector - I/O error while connecting to host localhost port 13002. Connection Attempt Nr. 1
java.net.SocketTimeoutException: connect timed out
at java.net.PlainSocketImpl.socketConnect(Native Method)
2019-12-05 11:47:07,875 ERROR DefaultQuartzScheduler_Worker-1 TCPConnector - I/O error while connecting to host localhost port 13002. Connection Attempt Nr. 2
java.net.SocketTimeoutException: connect timed out
at java.net.PlainSocketImpl.socketConnect(Native Method)
It is possible to configure a "connect refuse" timeout?

There is no such thing as a "connection refused timeout".
"Connection refused" happens when the server sees the connection request, but there is no service listening for connections on the IP + port that the request is directed to. The server then "refuses" the connection. This typically happens instantly, so so no timeout is triggered.
"Connection timed out" happens (typically) when something stops the connection request from reaching the server1, 2. So the client-side will wait for the response from the server, and then resend / wait a few times. And eventually the time allotted for establishing a connection will expire ... and the connection times out.
As you can see these are different scenarios. And they are reported back to the Java client-side differently.
So the reason you are not getting timeouts is that the "connection refused" responses are coming back quick enough that your configured timeout is not exceeded.
That might also explain why setting the connect timeout small might have changed the behavior. There may also be issues with the granularity of the timeout that the OS allows Java to set.
To investigate this further, I think we would need a minimal reproducible example. For example, we need to see how you have implemented the code that manages the server-socket and accepts connections on the server side.
1 - The blockage could be on the server's reply packets.
2 - There are various possible causes for this kind of thing. The most likely are a firewall blocking traffic somewhere, a network routing problem, or using a private IP address on the wrong network.

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();
}

Local PostgreSQL refusing connection with IntelliJ IDEA

A week ago I installed PostgreSQL with Homebrew following this tutorial. Everything worked fined and I was perfectly able to connect to the database in my Java Example Project. Two days ago, I've tried to see the code again and Java is throwing the following error:
org.postgresql.util.PSQLException: Connection to localhost:5432 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:239)
at org.postgresql.core.ConnectionFactory.openConnection(ConnectionFactory.java:66)
at org.postgresql.jdbc2.AbstractJdbc2Connection.<init>(AbstractJdbc2Connection.java:127)
at org.postgresql.jdbc3.AbstractJdbc3Connection.<init>(AbstractJdbc3Connection.java:29)
at org.postgresql.jdbc3g.AbstractJdbc3gConnection.<init>(AbstractJdbc3gConnection.java:21)
at org.postgresql.jdbc4.AbstractJdbc4Connection.<init>(AbstractJdbc4Connection.java:41)
at org.postgresql.jdbc4.Jdbc4Connection.<init>(Jdbc4Connection.java:24)
at org.postgresql.Driver.makeConnection(Driver.java:414)
at org.postgresql.Driver.connect(Driver.java:282)
at java.sql.DriverManager.getConnection(DriverManager.java:664)
at java.sql.DriverManager.getConnection(DriverManager.java:247)
at com.company.Main.main(Main.java:16)
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:497)
at com.intellij.rt.execution.application.AppMain.main(AppMain.java:140)
Caused by: java.net.ConnectException: Connection refused
at java.net.PlainSocketImpl.socketConnect(Native Method)
at java.net.AbstractPlainSocketImpl.doConnect(AbstractPlainSocketImpl.java:345)
at java.net.AbstractPlainSocketImpl.connectToAddress(AbstractPlainSocketImpl.java:206)
at java.net.AbstractPlainSocketImpl.connect(AbstractPlainSocketImpl.java:188)
at java.net.SocksSocketImpl.connect(SocksSocketImpl.java:392)
at java.net.Socket.connect(Socket.java:589)
at org.postgresql.core.PGStream.<init>(PGStream.java:61)
at org.postgresql.core.v3.ConnectionFactoryImpl.openConnectionImpl(ConnectionFactoryImpl.java:121)
... 16 more
At the beginning I've thought postgresql just stopped listening the port, but it is listening:
$ lsof -i :5432
COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME
postgres 39225 barbarity 5u IPv6 0x4ef6e84c8de72f23 0t0 TCP localhost:postgresql (LISTEN)
postgres 39225 barbarity 6u IPv4 0x4ef6e84ca3a79493 0t0 TCP localhost:postgresql (LISTEN)
And also I can connect from both PGAdmin and PSQL:
$ psql -d prakt -U barbarity
psql (9.4.2)
Type "help" for help.
prakt=#
Then I tried to uninstall and reinstall from homebrew, but it didn't work neither. I've also tried Postgress.app and nothing also.
Do you have any ideas?
I'm using:
Java SDK 1.8
JDBC 41
PostgreSQL 9.4.2
IntelliJ IDEA 14.1.3
(I put the java code just in case, but it was working a week ago)
import java.sql.*;
public class Main {
public static void main(String[] args) {
try {
Class.forName("org.postgresql.Driver");
} catch (ClassNotFoundException e) {
e.printStackTrace();
}
String query = new StringBuilder()
.append("SELECT * FROM customer WHERE id_customer = ? or f_name = ?").toString();
try (Connection connection = DriverManager.getConnection("jdbc:postgresql:prakt", "barbarity", "");
PreparedStatement preparedStatement = connection.prepareStatement(query);){
preparedStatement.setInt(1, 1);
preparedStatement.setString(2,"Richard");
try (ResultSet resultSet = preparedStatement.executeQuery();){
while (resultSet.next()) {
System.out.println(new StringBuilder()
.append("Name: ")
.append(resultSet.getString(2))
.toString());
}
resultSet.close();
} catch (SQLException e) {
e.printStackTrace();
}
} catch (SQLException e) {
e.printStackTrace();
}
}
}

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';