How to Resolve Java.net.ConnectException: Connection Refused Error

Do you keep facing a similar situation when trying to implement a TCP connection, everything seems fine from the server side, but when running the client program, you are facing an error, “java.net.connectexception: connection refused. Want to fix it? Keep reading!

Java.net.ConnectException: Connection refused is the most frequent kind of networking exception occurring in Java. Typically, Java books don’t tell you how to handle these problems quickly. This is why we have compiled this comprehensive guide to help you with the exception that is helpful for Java professionals during network programming.

In this blog, we will discuss

• The reasons why Java.net.ConnectException: Connection refused error occurs
• How to fix Java.net.ConnectException: Connection refused error
• How online Java training will help you impart in-depth Java knowledge 

Without further delay, let’s solve your Java.net.ConnectException: Connection refused error.

The Java.net.ConnectException: Connection refused error comes when you try to make a TCP connection from the client to the server. A similar error may come during RMI (Remote Method Invocation) because the RMI method is also underneath the TCP_IP protocol. 

Let’s know the reasons why the Java.net.ConnectException: Connection refused error occurs.

Do you know what is an exception in Java? An exception in Java is primarily an unexpected or exceptional event that occurs during program execution, in other words, during run time, and typically results in disruption of the program.

The network error “java.net.ConnectException: Connection refused” clearly states that the client is trying to make a TCP connection with the server and fails. Here are the possible reasons:

• Server or Client is not in a network: Either client or server or both of them are not in the network like LAN or internet; in that case, Java will throw an error - java.net.ConnectException: Connection refused. 
• If the server is down: If the server is down or not working properly or there is a low network connection, in that case also Java will throw an error - java.net.ConnectException: Connection refused, you must be thinking, what do I do?

The most irritating thing to notice here is that the reasons are possibly different. Still, Java throws a single error in all possible scenarios that may be difficult to deal with the Java programmer. If there is any server issue, then you can check the status of the server with the command “ping,” or there are lots of similar options to get the response back from the port. 

• The server is running but not listening to the port: The most typical case is when the server is running well but not listening to the port. This is the most advanced reason that could not be figured out quickly. The developer needs to check out the network configuration properly. In the case of hierarchical configuration, there are chances of overriding where default settings will override your actual settings. 
• Firewall Restriction: If the network is protected by firewalls, then the same network error will be thrown by the Java programming language. Here, the Java developer needs to submit a request to the network administrator for continuous communication between IP addresses and the port. In case permission is not allowed by the firewall, then it will show the same error again and again. 
• Incorrect host port combination: When you are working on networks, the most obvious terms you should know about are our host, port, MAC address, IP address, router, switch, etc. If you are unsure of any of the terms, then networking programming may be challenging for the developers. The next possible situation is the wrong combination of host and port number. 

This is possible that you have provided either the wrong host or port number then the network will throw the same error again. Here the only solution is to check for active configuration settings and make necessary changes as required for successful communication. 

• Server is stopped: Suppose a server administration gave some IP address and PORT to  access the server. But he stopped the server, but if your client programs try to access the server with administrator-provided credentials, the client program will get the exception.
• Protocol mismatch:  HTTP, RMI, Websocket, etc., uses TCP as the underlying protocol. HTTP request starts with http://, and RMI protocol starts with rmi// or WebSocket protocol starts with ws://. If we request an HTTP request with rmi protocol, it will throw the java.net.ConnectException: Connection refused.

During a network connection, you have to make sure that the correct protocol has been passed as expected by the server. For RMI, Client-Server architecture, and HTTP, the most obvious protocol to establish a network connection includes TCP.

Want to brush up your Java knowledgebase, check out this tutorial that speaks at length about Java language, Java operators, and control flow.

In the above section, we have discussed all possible reasons closely. Here is the right time to approach a solution to deal with the “java.net.ConnectException: Connection refused” network error professionally.

1. First of all, use the ping command to check if the server is connected to the port or not. If you are able to ping successfully without any error, then it shows that the client and server both are well connected, and the reason is somewhat different.
2. In the second step, try to connect with the server and port with telnet. The “Telnet” command is an easy way to check connectivity to the server from a client. You would be able to know whether the problem is with the server or client code. Also, you would be able to know if the serving is running well or not.
3. Check the configuration of the client that will tell you about the correct IP address and port number. For highly complex connections, you should try RMI calls so that you can be sure of the right protocol and the connection string. You can use the same process when using MySQL and JDBC.

Check firewall connections, and Windows firewall logs, and make sure that networks are accessible to each other to make successful communication. Also, you have to make sure that the source and destination computer should be on the same network, host, or router.

Let’s know how you can resolve the issue at the network layer. If you are sure about subnet creation, IP addressing, and routing, then this would be easy for you to solve the network issues correctly and quickly too.

To establish communication at the network layer, we need a global addressing scheme that is named as logical addressing in the network world. These logical addresses are referred to as IPv4 and IPv6 addresses. Although the IPv6 address scheme has been deployed into the network world, still many organizations rely on IPv4 addressing only. 

This is the reason why every network engineer should know how to troubleshoot IPv4 logical addressing on devices. Online Java training selfpace will help Java developers master different Java programming skills. Technical issues may be faced by network engineers while troubleshooting IPv4 like bad address, subnet mask or default gateway address, etc.

When IPv4 addresses are deployed on devices, they are assigned dynamically through the DHCP protocol. The next topic of interest is Network Address Translation (NAT). NAT is required to translate IPv4 logical addresses to public addresses that can be further routed on the internet.

This adds another complexity to the environment as you need to know how to troubleshoot NAT so that devices can be accessed quickly, even externally to the organization. This is true that we are using IPv4 as of now, but soon there will come a time when IPv6 address schemes will be deployed in the organization. 

So you should also know how to troubleshoot IPv6 successfully. In future tutorials, we will discuss IPv6 addressing schemes, addressing technologies, and their troubleshooting tricks in deep that can help you in starting a successful Java development career with organizations. As of now, we are limited to troubleshooting tricks for IPv4 logical addressing at the network layer.

Here, we will focus on a deep understanding of IPv4 addressing and how to check whether each of the devices is addressed correctly on the network or not.

Every IPv4 address comprises two major parts – one network/subnet mask and the other is the host part. When two devices want to communicate together, then it is necessary that both of them should share the same network/subnet mask.

In case, the subnet mask for both of the systems is different then it will notify back with addressing issues that need to be troubleshot by a network engineer.

In the given figure, the sample subnet for both PCs is (10.1.1.0/26) and they are sharing default gateway R1. Let us discuss on the network addressing how it works actually. Take an example where PC1 wants to communicate with PC2. In that case, it will start for DNS lookup to know the IP address for the PC2. After DNS lookup, IP address 10.1.1.20 is returned.

Now, PC1 will check whether PC2 is located on the same network or they are sharing the same subnet mask or not. For this purpose, it will convert the Decimal number to Binary to get accurate results.

As you can see both binary bits are exactly the same, so it is clear that PC1 and PC2 share the same network and packets can be exchanged directly without any need of default gateway R1. In technical words, PC1 will create a MAC address of its own system as the source and MAC address For PC2 as the destination. Here’s how you can get MAC address from IP1.

Now let us see what happens when PC1 wants to communicate with a web server (192.0.2.1). It will again start for DNS lookup to know the IP address of the web server. After DNS lookup, IP address 192.0.2.1 is returned. Now, PC1 will check whether PC2 is located on the same network or they are sharing the same subnet mask or not. For this purpose, it will convert the Decimal number to Binary to get accurate results.

As you can see that both binary bits are not exactly the same, so it is clear that PC1 and the web server don’t share the same network, and packets cannot be exchanged directly. Here, you need the default gateway R1 to establish the communication further. In technical words, PC1 will create the MAC address of its own system as the source and the MAC address of the web server as the destination. 

The above discussion makes sure that the correct IP address is paramount for success. Now the question comes as to what will happen if PC1 is configured with the wrong subnet mask by mistake. This is given in the following figure.

The above discussion makes sure that correct IP address is paramount for success. Now the question comes to what will happen if PC1 is configured with the wrong subnet mask by mistake. This is given in figure 9-2.

After comparison PC1 concludes that PC2 is not on the same network and it needs to route the packets to the default gateway R1. SO, it will create a MAC address of its own system as the source and a MAC address of the default gateway R1 as the destination. 

As a result, the packets will be addressed to the wrong device even if both of the systems were connected to the same network only. This is an example of Incorrect IP addressing that needs to troubleshoot further by a network engineer. Not only an improper subnet mask will cause issues, but improper IP addressing along with a right subnet mask will also cause connectivity issues. 

Additionally, if the default gateway is not configured correctly, then it will route the packets to the wrong destination that is not acceptable to any of the organization. Being a troubleshooter or network engineer, you should be aware of all possible connectivity issues, you may face during network configuration.

If you are aware of all possible situations only then you can resolve the issues quickly. The IP information can be verified on Windows PC with "ipconfig" command and you can check the same details on a router or switch with show IP interface inter- face_typeinterface_number command.

Here is a quick tour that will help you determine IP addresses within a subnet. Figure 9-3 “Determining IP Address within a Subnet,” will give you a clear idea of how they are actually assigned.

Take the last octet of the subnet mask and subtract it from 256. In this case, the last octet is 192 and when it is subtracted from 256, the result would be 64. So the subnet for this network can be defined as 10.1.1.0/26 to 10.1.1.63/26. The next subnet would be 10.1.1.64/26 to 10.1.1.127/26. The third subnet would be 10.1.1.128/26 to 10.1.1.191/26 and so on.

Let’s look at some frequently asked questions and answers to solve all your queries related to Java.net.ConnectException: Connection refused error.

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Have you got your solution to Java.net.ConnectException: Connection refused error? Getting such errors is common, but knowing the solution and the main cause can make the process of resolving them easier. Employers want their developers to be efficient and smart enough to have such knowledge.

Save this blog to use all the tricks the next time you get stuck with Java.net.ConnectException: Connection refused error. If you want to learn more about networks and Java to  become an efficient Java developer, you need professional online Java training, and Janbask Training offers exactly that. What are you waiting for? If you have the passion and interest to learn Java and make a career with it, join the Java course now.