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java.net.UnknownHostException: <<hostname>> : unknown error

I'm trying to establish Oracle DB connection from the application which runs on an EC2 instance of AWS. Oracle DB is in on-prem server. Firewall has been opened and I'm able to telnet to the SCAN and VIP hosts of that DB from my EC2 instance. But, still I'm getting the below exception:
Caused by: oracle.net.ns.NetException: The Network Adapter could not establish the connection
at oracle.net.nt.ConnStrategy.execute(ConnStrategy.java:392) ~[ojdbc6-11.2.0.3.jar:11.2.0.3.0]
at oracle.net.resolver.AddrResolution.resolveAndExecute(AddrResolution.java:434) ~[ojdbc6-11.2.0.3.jar:11.2.0.3.0]
at oracle.net.ns.NSProtocol.establishConnection(NSProtocol.java:687) ~[ojdbc6-11.2.0.3.jar:11.2.0.3.0]
at oracle.net.ns.NSProtocol.connect(NSProtocol.java:343) ~[ojdbc6-11.2.0.3.jar:11.2.0.3.0]
at oracle.jdbc.driver.T4CConnection.connect(T4CConnection.java:1102) ~[ojdbc6-11.2.0.3.jar:11.2.0.3.0]
at oracle.jdbc.driver.T4CConnection.logon(T4CConnection.java:320) ~[ojdbc6-11.2.0.3.jar:11.2.0.3.0]
... 239 common frames omitted
Caused by: java.net.UnknownHostException: <<hostname>>: unknown error
at java.net.Inet4AddressImpl.lookupAllHostAddr(Native Method) ~[na:1.8.0_40-internal]
at java.net.InetAddress$2.lookupAllHostAddr(InetAddress.java:907) ~[na:1.8.0_40-internal]
at java.net.InetAddress.getAddressesFromNameService(InetAddress.java:1302) ~[na:1.8.0_40-internal]
at java.net.InetAddress.getAllByName0(InetAddress.java:1255) ~[na:1.8.0_40-internal]
at java.net.InetAddress.getAllByName(InetAddress.java:1171) ~[na:1.8.0_40-internal]
at java.net.InetAddress.getAllByName(InetAddress.java:1105) ~[na:1.8.0_40-internal]
at oracle.net.nt.TcpNTAdapter.connect(TcpNTAdapter.java:117) ~[ojdbc6-11.2.0.3.jar:11.2.0.3.0]
at oracle.net.nt.ConnOption.connect(ConnOption.java:133) ~[ojdbc6-11.2.0.3.jar:11.2.0.3.0]
at oracle.net.nt.ConnStrategy.execute(ConnStrategy.java:370) ~[ojdbc6-11.2.0.3.jar:11.2.0.3.0]
JDBC URL:
jdbc:oracle:thin:#(DESCRIPTION =(ADDRESS_LIST =(ADDRESS = (PROTOCOL = TCP)
(HOST = <<IP address of the host>>)(PORT = 1590)))(LOAD_BALANCE = yes)
(CONNECT_DATA =(SERVICE_NAME = <<example.service.com>>)(FAILOVER_MODE =(TYPE = SELECT)
(METHOD = BASIC))))`

Are you using java 8 ? If so then probably it's related to this bug
Java 7 or 9 would probably give you a more useful error message instead of "unknown error" (eg possibly "Name or service not known")
Apart from that, did you try a tnsping from the host you're trying to connect from ?
Also as per the below in the Oracle driver documentation when using a tnsnames entry in the jdbc URL it should be as below, using OCI driver :
Note that you can also specify the database by a TNSNAMES entry. You can find the available TNSNAMES entries listed in the file tnsnames.ora on the client computer from which you are connecting. For example, if you want to connect to the database on host myhost as user scott with password tiger that has a TNSNAMES entry of MyHostString, enter:
Connection conn = DriverManager.getConnection
("jdbc:oracle:oci8:#MyHostString","scott","tiger");

JDBC oracle connection error: ORA-12519, TNS:no appropriate service handler found

In my project I am using jdbc to connect to a oracle 12c instance in a multi-threading environment, earlier we had an oracle 9i instance and we were using ojdbc6 and it was working perfectly but we receltly got this oracle 12c instance which gave following error at JDBC connection point.
java.sql.SQLException: Listener refused the connection with the following error:
ORA-12519, TNS:no appropriate service handler found
at oracle.jdbc.driver.T4CConnection.logon(T4CConnection.java:774)
at oracle.jdbc.driver.PhysicalConnection.connect(PhysicalConnection.java:688)
at oracle.jdbc.driver.T4CDriverExtension.getConnection(T4CDriverExtension.java:39)
at oracle.jdbc.driver.OracleDriver.connect(OracleDriver.java:691)
at java.sql.DriverManager.getConnection(Unknown Source)
at java.sql.DriverManager.getConnection(Unknown Source)
So I thought it may be because of the older driver version that we had, so I incorporated ojdbc8 which I found over the internet to be compatible with 12 but the above error is still there. My JDK version is 1.8.
I'd appreciate any input on resolving this issue. Thanks in advance.

Sid. I think the bug occurred dut to the Oracle initialization parameter setting problem.
Please use the command line as below:
SQL > SHOW the PARAMETER of the SESSION
------------------------------------ ----------- ------------------------------
java_max_sessionspace_size integer 0
java_soft_sessionspace_limit integer 0
license_max_sessions integer 0
license_sessions_warning integer 0
session_cached_cursors integer 50
session_max_open_files integer 10
sessions integer 600
shared_server_sessions integer
the other command line:
SQL> SHOW PARAMETER PROCESS
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
aq_tm_processes integer 0
db_writer_processes integer 2
gcs_server_processes integer 2
global_txn_processes integer 1
job_queue_processes integer 1000
log_archive_max_processes integer 4
processes integer 150
According to the Oracle documentation, SESSIONS and the TRANSACTIONS of the initialization parameters should be derived from the PROCESSES, according to the default setting SESSIONS = 1.1 + 5 by the PROCESSES.
SESSIONS currently set up to 600, and the PROCESSES of setting has not changed, still for 150, led to too much user session to connect to the Oracle, Oracle do not have enough background PROCESSES to support these SESSIONS.
The direct solution is set the appropriate the PROCESSES.

"com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure" to remote database

I tried to connect to my remote MySQL database, but I failed and got this error.
com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure
The confusion is that It was effective when I used MySQL-Front tool to connect the remote database, and I can ping to the IP address successfully. but when I used my code, it wound show the error after about ten seconds.
Also when I used the wrong username or password in my code, it wound show the wrong verification immediately. Did that prove it is no problem to set up conection?
Here is my code(It can work on my localhost database):
public static void main(String[] args) {
String url = "jdbc:mysql://(IP address):3306/";
String dbName = "talk";
String driver = "com.mysql.jdbc.Driver";
String userName = "talkroot";
String password = "123456";
try {
Class.forName(driver).newInstance();
Connection conn = DriverManager.getConnection(url + dbName,
userName, password);
System.out.println("connect");
conn.close();
} catch (Exception e) {
e.printStackTrace();
}
System.out.println("end");
}
Here is the error:
com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure
Last packet sent to the server was 0 ms ago.
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
at java.lang.reflect.Constructor.newInstance(Unknown Source)
at com.mysql.jdbc.Util.handleNewInstance(Util.java:406)
at com.mysql.jdbc.SQLError.createCommunicationsException(SQLError.java:1074)
at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2120)
at com.mysql.jdbc.ConnectionImpl.<init>(ConnectionImpl.java:723)
at com.mysql.jdbc.JDBC4Connection.<init>(JDBC4Connection.java:46)
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
at java.lang.reflect.Constructor.newInstance(Unknown Source)
at com.mysql.jdbc.Util.handleNewInstance(Util.java:406)
at com.mysql.jdbc.ConnectionImpl.getInstance(ConnectionImpl.java:302)
at com.mysql.jdbc.NonRegisteringDriver.connect(NonRegisteringDriver.java:282)
at java.sql.DriverManager.getConnection(Unknown Source)
at java.sql.DriverManager.getConnection(Unknown Source)
at com.test.TestMySQL.main(TestMySQL.java:16)
Caused by: com.mysql.jdbc.exceptions.jdbc4.CommunicationsException: Communications link failure
Last packet sent to the server was 21298 ms ago.
at sun.reflect.NativeConstructorAccessorImpl.newInstance0(Native Method)
at sun.reflect.NativeConstructorAccessorImpl.newInstance(Unknown Source)
at sun.reflect.DelegatingConstructorAccessorImpl.newInstance(Unknown Source)
at java.lang.reflect.Constructor.newInstance(Unknown Source)
at com.mysql.jdbc.Util.handleNewInstance(Util.java:406)
at com.mysql.jdbc.SQLError.createCommunicationsException(SQLError.java:1074)
at com.mysql.jdbc.MysqlIO.reuseAndReadPacket(MysqlIO.java:3009)
at com.mysql.jdbc.MysqlIO.reuseAndReadPacket(MysqlIO.java:2895)
at com.mysql.jdbc.MysqlIO.checkErrorPacket(MysqlIO.java:3438)
at com.mysql.jdbc.MysqlIO.sendCommand(MysqlIO.java:1951)
at com.mysql.jdbc.MysqlIO.sqlQueryDirect(MysqlIO.java:2101)
at com.mysql.jdbc.ConnectionImpl.execSQL(ConnectionImpl.java:2548)
at com.mysql.jdbc.ConnectionImpl.configureClientCharacterSet(ConnectionImpl.java:1802)
at com.mysql.jdbc.ConnectionImpl.initializePropsFromServer(ConnectionImpl.java:3444)
at com.mysql.jdbc.ConnectionImpl.createNewIO(ConnectionImpl.java:2062)
... 12 more
Caused by: java.net.SocketException: Software caused connection abort: recv failed
at java.net.SocketInputStream.socketRead0(Native Method)
at java.net.SocketInputStream.read(Unknown Source)
at java.net.SocketInputStream.read(Unknown Source)
at com.mysql.jdbc.util.ReadAheadInputStream.fill(ReadAheadInputStream.java:113)
at com.mysql.jdbc.util.ReadAheadInputStream.readFromUnderlyingStreamIfNecessary(ReadAheadInputStream.java:160)
at com.mysql.jdbc.util.ReadAheadInputStream.read(ReadAheadInputStream.java:188)
at com.mysql.jdbc.MysqlIO.readFully(MysqlIO.java:2452)
at com.mysql.jdbc.MysqlIO.reuseAndReadPacket(MysqlIO.java:2906)
... 20 more
Anyone can help me please?

This com.mysql.jdbc.exceptions.jdbc4.CommunicationsException exception occurs if your database connection is idle for long time. This idle connection returns true on connection.isClosed(); but if we try to execute statement then it will fire this exception so I will suggest to go with database pooling.

Usually this error occurs when you don't use right port number. May be the machine which working on this database (client'db) has different port number. (Ex: 3307) . Go task manager==> Services ==> services and check is there other version of mysql. if they use 3306 change your port to 3307.

you can try to replace the mysql-connector-java-version.jar jar file. Some people update the jar file from 5.1.10 to 5.1.14, then it works. Wish you are luck..

update your drive version to:
<dependency>
<groupId>mysql</groupId>
<artifactId>mysql-connector-java</artifactId>
<version>5.1.18</version>

Double Check the following things:
Verify whether the IP address/Port Number is correct or not
Ping to the server using ping "IP address" it may tells is the server in startup mode/shutdown mode
If you are getting connection objects from connection pool, verify
once connections availability.
Hope this information helps...

I solved it by removing the port number from the connection URL.
So instead of using connection string as:
jdbc:mysql://12x.xxx.xxx.xxx:1527/yourdbname
use
jdbc:mysql://12x.xxx.xxx.xxx/yourdbname

Also when I used the wrong username or password in my code, it wound show the wrong verification immediately. Did that prove it is no problem to set up conection?
this is probably done locally. so it doesn't relate to why your code isn't working.
There seems to be many ways to go about this, so you can try multiple things; people have solved this link failure in different ways as per their own situations:
Here are some the solutions:
changing "bind-address" attribute
Uncomment "bind-address" attribute or change it to one of the following IPs:
bind-address="127.0.0.1"
or
bind-address="0.0.0.0"
commenting out "skip-networking"
If there is a "skip-networking" line in your MySQL config file, make it comment by adding "#" sign at the beginning of that line.
change "wait_timeout" and "interactive_timeout"
Add these lines to the MySQL config file:
[wait_timeout][1] = number
interactive_timeout = number
connect_timeout = number
check Operating System proxy settings
Make sure the Fire wall, or Anti virus soft wares don't block MySQL service.
change connection string
Check your query string. your connection string should be some thing like this:
dbName = "my_database";
dbUserName = "root";
dbPassword = "";
String connectionString = "jdbc:mysql://localhost/" + dbName + "?user=" + dbUserName + "&password=" + dbPassword + "&useUnicode=true&characterEncoding=UTF-8";
Make sure you don't have spaces in your string. All the connection string should be continues without any space characters.
Try to replace "localhost" with your port, like 127.0.0.1.
Also try to add port number to your connection string, like:
String connectionString = "jdbc:mysql://localhost:3306/my_database?user=root&password=Pass&useUnicode=true&characterEncoding=UTF-8";
Usually default port for MySQL is 3306.
Don't forget to change username and password to the username and password of your MySQL server.
update your JDK driver library file
test different JDK and JREs (like JDK 6 and 7)
don't change max_allowed_packet
"[max_allowed_packet][4]" is a variable in MySQL config file that indicates the maximum packet size, not the maximum number of packets. So it will not help to solve this error.
change tomcat security
change TOMCAT6_SECURITY=yes to TOMCAT6_SECURITY=no
use validationQuery property
use validationQuery="select now()" to make sure each query has responses
AutoReconnect
Add this code to your connection string:
&autoReconnect=true&failOverReadOnly=false&maxReconnects=10

Your MySQL server isn't running. Check the same in the services page of the task manager if you are running Windows.

If you are working with Amazon Ubuntu instance then you have to follow this steps:
Check the amazon instance
Check the security group of instance
Assign the mysql/aura service and add your machine ip here or
service provider ip.

Please update your mysql configuration file in /etc/mysql/my.cnf file
By default, MySQL only allows connections from the localhost address. The configuration file is usually found in /etc/mysql/my.cnf
You will want to comment out the bind-address line by making it look like this:
#bind-address = 127.0.0.1
Next, you will need to restart the database server:
sudo /etc/init.d/mysql restart

I had faced this issue and struggled a lot like a lot. I did everything single thing said in all the portals and not worked for me. The only thing which worked to me was updating my jar file to the current new version . Earlier I was using 5.1.38 and now updated to 8.0.15 and just that my program worked super fine.

Cassandra AssertionError

I received an OOM exception at one point in Cassandra. Mine is a single instance running on a modestly powered server, and i was doing some load testing, so no surprise there.
But, i have subsequently been unable to use the instance. When i list the keyspaces, only "system" is shown. But when i try to recreate the keyspace i was testing with, Hector responds with the dreaded "All host pools marked down. Retry burden pushed out to client." message, and the Cassandra log has the following stack trace:
ERROR [MigrationStage:1] 2012-04-27 20:47:00,863 AbstractCassandraDaemon.java (line 134) Exception in thread Thread[MigrationStage:1,5,main]
java.lang.AssertionError
at org.apache.cassandra.db.DefsTable.updateKeyspace(DefsTable.java:441)
at org.apache.cassandra.db.DefsTable.mergeKeyspaces(DefsTable.java:339)
at org.apache.cassandra.db.DefsTable.mergeSchema(DefsTable.java:269)
at org.apache.cassandra.service.MigrationManager$1.call(MigrationManager.java:214)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
at java.util.concurrent.FutureTask.run(FutureTask.java:138)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:662)
ERROR [Thrift:9] 2012-04-27 20:47:00,864 CustomTThreadPoolServer.java (line 204) Error occurred during processing of message.
java.lang.RuntimeException: java.util.concurrent.ExecutionException: java.lang.AssertionError
at org.apache.cassandra.utils.FBUtilities.waitOnFuture(FBUtilities.java:372)
at org.apache.cassandra.service.MigrationManager.announce(MigrationManager.java:191)
at org.apache.cassandra.service.MigrationManager.announceNewKeyspace(MigrationManager.java:129)
at org.apache.cassandra.thrift.CassandraServer.system_add_keyspace(CassandraServer.java:987)
at org.apache.cassandra.thrift.Cassandra$Processor$system_add_keyspace.getResult(Cassandra.java:3370)
at org.apache.cassandra.thrift.Cassandra$Processor$system_add_keyspace.getResult(Cassandra.java:3358)
at org.apache.thrift.ProcessFunction.process(ProcessFunction.java:32)
at org.apache.thrift.TBaseProcessor.process(TBaseProcessor.java:34)
at org.apache.cassandra.thrift.CustomTThreadPoolServer$WorkerProcess.run(CustomTThreadPoolServer.java:186)
at java.util.concurrent.ThreadPoolExecutor$Worker.runTask(ThreadPoolExecutor.java:886)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:908)
at java.lang.Thread.run(Thread.java:662)
Caused by: java.util.concurrent.ExecutionException: java.lang.AssertionError
at java.util.concurrent.FutureTask$Sync.innerGet(FutureTask.java:222)
at java.util.concurrent.FutureTask.get(FutureTask.java:83)
at org.apache.cassandra.utils.FBUtilities.waitOnFuture(FBUtilities.java:368)
... 11 more
Caused by: java.lang.AssertionError
at org.apache.cassandra.db.DefsTable.updateKeyspace(DefsTable.java:441)
at org.apache.cassandra.db.DefsTable.mergeKeyspaces(DefsTable.java:339)
at org.apache.cassandra.db.DefsTable.mergeSchema(DefsTable.java:269)
at org.apache.cassandra.service.MigrationManager$1.call(MigrationManager.java:214)
at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:303)
at java.util.concurrent.FutureTask.run(FutureTask.java:138)
... 3 more
The old keyspace was still in the data dir, so i moved it, but that didn't help. It seems that the system data still has an invalid reference somewhere. Does anyone know how to fix this?
Edit: from the CLI, a "describe cluster;" only describes the "system" keyspace. But when i "use system;" and then "list schema_keyspaces;" the following is displayed:
Using default limit of 100
-------------------
RowKey: mango
=> (column=durable_writes, value=true, timestamp=29127788177516974)
=> (column=name, value=mango, timestamp=29127788177516974)
=> (column=strategy_class, value=org.apache.cassandra.locator.SimpleStrategy, timestamp=29127788177516974)
=> (column=strategy_options, value={"replication_factor":"1"}, timestamp=29127788177516974)
1 Row Returned.
Elapsed time: 1107 msec(s).
"mango" is the keyspace that i can no longer access, but it is still in there to some degree. Is there any way to fix it?

The problem almost certainly is that the recreated keyspace is inconsistent with the commit log or data stored with the original definition. Shut down the Cassandra server and clear out the commitlog, saved_caches, and data directory corresponding to the keyspace. The locations of these directories are in cassandra.yaml - look for data_file_directories, saved_caches_directory, and commitlog_directory.

This problem is due to inconsistency and you can go for following steps.
1) In your case this is OK to clear "data" , "saved_caches" and "commitlog" directories as you dont have any critical data and other Keyspaces.
2) In the scenarios where you have some critical data and you can not delete above mentioned directories do the following.
Use nodetool drain to Empty the commitlog on all the nodes of the cluster.
Then delete all the "LocationInfo*" files from "/data/system" directories and restart the cluster.

JNDI with Active Directory PartialResultException

I am basically walking the LDAP tree in Active Directory.
At each level I query for "(objectClass=*)". When I do this on the root eg "dc=example,dc=com" I get the exception below. This works fine on our other LDAP instances. For some reason only on our Active Directory server I get this exception. I also get the same exception when using JXplorer on our Active Directory server.
From reading around online I found people saying you should turn on following, not sure what that means... So on my controls object (javax.naming.directory.SearchControls) that I pass with the query I call searchControls.setDerefLinkFlag(true). I also have tried setting it to false with the same result. Any a suggestions on what else could cause this? Maybe how I could fix it?
Note: In this post I changed the baseDn from dc=<my company domain> to example for my companies privacy.
javax.naming.PartialResultException: Unprocessed Continuation Reference(s); remaining name 'dc=example,dc=com'
at com.sun.jndi.ldap.LdapCtx.processReturnCode(LdapCtx.java:2820)
at com.sun.jndi.ldap.LdapCtx.processReturnCode(LdapCtx.java:2794)
at com.sun.jndi.ldap.LdapCtx.searchAux(LdapCtx.java:1826)
at com.sun.jndi.ldap.LdapCtx.c_search(LdapCtx.java:1749)
at com.sun.jndi.toolkit.ctx.ComponentDirContext.p_search(ComponentDirContext.java:368)
at com.sun.jndi.toolkit.ctx.PartialCompositeDirContext.search(PartialCompositeDirContext.java:338)
at com.sun.jndi.toolkit.ctx.PartialCompositeDirContext.search(PartialCompositeDirContext.java:321)
at javax.naming.directory.InitialDirContext.search(InitialDirContext.java:248)
at com.motio.pi.gui.panels.useraccess.ldap.LDAPConnector.query(LDAPConnector.java:262)
at com.motio.pi.gui.selector.directory.CognosDirectoryBrowserController.expandCognosTreeNode(CognosDirectoryBrowserController.java:99)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:597)
at com.motio.pi.utils.PIThreadDelegate$1.run(PIThreadDelegate.java:54)
at java.lang.Thread.run(Thread.java:662)

So when I was creating my naming context with the method:
javax.naming.ldap.InitialLdapContext.InitialLdapContext(
Hashtable<?, ?> environment, Control[] connCtls)
In the argument environment there is a property with the name Context.REFERRAL and its value should be set to: follow. This was the setting that I needed.

If you get an exception while referral usage in follow (for example: connection timed out) you can use referral ignore but you dont want to get partial exception you can use 3268 port number instead of 389 this port is using global catalog for ldap. You can find info from following link;
https://technet.microsoft.com/en-us/library/how-global-catalog-servers-work(v=ws.10).aspx