ASP.NET Core SignalR Java client

By Mikael Mengistu

The Java client enables connecting to an ASP.NET Core SignalR server from Java code, including Android apps. Like the JavaScript client and the .NET client, the Java client enables you to receive and send messages to a hub in real time. The Java client is available in ASP.NET Core 2.2 and later.

The sample Java console app referenced in this article uses the SignalR Java client.

View or download sample code (how to download)

Install the SignalR Java client package

The signalr-1.0.0 JAR file allows clients to connect to SignalR hubs. To find the latest JAR file version number, see the Maven search results.

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

implementation 'com.microsoft.signalr:signalr:1.0.0'

If using Maven, 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>

Connect to a hub

To establish a HubConnection, the HubConnectionBuilder should be used. The hub URL and log level can be configured while building a connection. Configure any required options by calling any of the HubConnectionBuilder methods before build. Start the connection with start.

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

Call hub methods from client

A call to send invokes a hub method. Pass the hub method name and any arguments defined in the hub method to send.

hubConnection.send("Send", input);

Note

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

Call client methods from hub

Use hubConnection.on to define methods on the client that the hub can call. Define the methods after building but before starting the connection.

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

Add logging

The SignalR Java client uses the SLF4J library for logging. It's a high-level logging API that allows users of the library to chose their own specific logging implementation by bringing in a specific logging dependency. The following code snippet shows 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 http://www.slf4j.org/codes.html#StaticLoggerBinder for further details.

This can safely be ignored.

Android development notes

With regards to Android SDK compatibility for the SignalR client features, consider the following items when specifying your target Android SDK version:

Configure bearer token authentication

In the SignalR Java client, you can configure a bearer token to use for authentication by providing an "access token factory" to the HttpHubConnectionBuilder. Use withAccessTokenFactory to provide an RxJava Single<String>. With a call to Single.defer, you can write logic to produce access tokens for your client.

HubConnection hubConnection = HubConnectionBuilder.create("YOUR HUB URL HERE")
    .withAccessTokenProvider(Single.defer(() -> {
        // Your logic here.
        return Single.just("An Access Token");
    })).build();

Passing Class information in Java

When calling the on, invoke, or stream methods of HubConnection in the Java client, users should pass a Type object rather than a Class<?> object to describe any generic Object passed to the method. A Type can be acquired using the provided TypeReference class. For example, using a custom generic class named Foo<T>, the following code gets the Type:

Type fooType = new TypeReference<Foo<String>>() { }).getType();

For non-generics, such as primitives or other non-parameterized types like String, you can simply use the built-in .class.

When calling one of these methods with one or more object types, use the generics syntax when invoking the method. For example, when registering an on handler for a method named func, which takes as arguments a String and a Foo<String> object, use the following code to set an action to print the arguments:

hubConnection.<String, Foo<String>>on("func", (param1, param2) ->{
    System.out.println(param1);
    System.out.println(param2);
}, String.class, fooType);

This convention is necessary because we can not retrieve complete information about complex types with the Object.getClass method due to type erasure in Java. For example, calling getClass on an ArrayList<String> would not return Class<ArrayList<String>>, but rather Class<ArrayList>, which does not give the deserializer enough information to correctly deserialize an incoming message. The same is true for custom objects.

Known limitations

  • Transport fallback and the Server Sent Events transport aren't supported.
  • Transport fallback and the Server Sent Events transport aren't supported.
  • Only the JSON protocol is supported.
  • Only the JSON protocol is supported.
  • Only the WebSockets transport is supported.
  • Streaming isn't supported yet.

Additional resources