How to Use the ASP.NET Core SignalR Java Client for Real-Time Messaging !

If you're looking for a way to connect to an ASP.NET Core SignalR server from Java code, including Android apps, you're in luck. The Java client for SignalR enables you to receive and send messages to a hub in real-time, just like the JavaScript client and the .NET client. In this article, we'll show you how to use the SignalR Java client to connect to a hub and exchange messages.

Installing the SignalR Java client package

To get started, you'll need to install the SignalR Java client package. The signalr-7.0.0 JAR file allows clients to connect to SignalR hubs. To find the latest version of the JAR file, you can check the Maven search results.

If you're using Gradle, you can add the following line to the dependencies section of your build.gradle file:

implementation 'com.microsoft.signalr:signalr:7.0.0'

If you're using Maven, you can add the following lines inside the <dependencies> element of your pom.xml file:

<dependency> <groupId>com.microsoft.signalr</groupId> <artifactId>signalr</artifactId> <version>1.0.0</version> </dependency>         

Connecting to a hub

To establish a HubConnection, you should use the HubConnectionBuilder. You can configure the hub URL and log level while building a connection, and then start the connection with start. Here's an example:

HubConnection hubConnection = HubConnectionBuilder.create(input) .build();         

Calling hub methods from client

To call a hub method, you can use the send method. Pass the hub method name and any arguments defined in the hub method to send. Here's an example:

hubConnection.send("Send", input);         

Note that calling hub methods from a client is only supported when using the Azure SignalR Service in Default mode. For more information, see the Frequently Asked Questions on the azure-signalr GitHub repository.

Calling client methods from hub

You can define methods on the client that the hub can call using the on method of HubConnection. Define the methods after building but before starting the connection. Here's an example:

hubConnection.on("Send", (message) -> { System.out.println("New Message: " + message); }, String.class);         

Adding logging

The SignalR Java client uses the SLF4J library for logging, which is a high-level logging API that allows users of the library to choose their own specific logging implementation by bringing in a specific logging dependency. Here's an example of how to use java.util.logging with the SignalR Java client:

implementation 'org.slf4j:slf4j-jdk14:1.7.25'         

If you don't configure logging in your dependencies, SLF4J loads a default no-operation logger with the following warning message:

SLF4J: Failed to load class "org.slf4j.impl.StaticLoggerBinder". 
SLF4J: Defaulting to no-operation (NOP) logger implementation 
SLF4J: See https://www.slf4j.org/codes.html#StaticLoggerBinder for further details.         

Android development notes

If you're developing an Android app, you should keep in mind the following items when specifying your target Android SDK version:

• The SignalR Java Client will run on Android API Level 16 and later.
• Connecting through the Azure SignalR Service will require Android API Level 20 and later.

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