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{snippet:id=e2|lang=java|url=camel/trunk/tests/camel-itest/src/test/java/org/apache/camel/itest/async/HttpSyncTest.java}

Using the Async API with callbacks

Suppose we want to call a Http service but it is usually slow and thus we do not want to block and wait for the response, but instead let a callback gather the response. This allows us to send multiple requests without waiting for the replies before we can send the next request.

First we define a route in Camel for the Http service where we simulate a slow server as it takes at least 1 second to reply.

{snippet:id=e1|lang=java|url=camel/trunk/tests/camel-itest/src/test/java/org/apache/camel/itest/async/HttpAsyncCallbackTest.java}

Then we define our callback where we gather the responses. As this is based on an unit test it just gathers the responses in a list. This is a shared callback we use for every request we send in, but you can use your own individual or use an anonymous callback. The callback supports different methods, but we use onDone that is invoked regardless if the Exchange was processed successfully or failed. The org.apache.camel.spi.Synchronization API provides fine grained methods for onCompletion and onFailure for the two situations.

{snippet:id=e2|lang=java|url=camel/trunk/tests/camel-itest/src/test/java/org/apache/camel/itest/async/HttpAsyncCallbackTest.java}

And then we have the client API where we call the Http service using asyncCallback 3 times with different input. As the invocation is Async the client will send 3 requests right after each other, so we have 3 concurrent exchanges in progress. The response is gathered by our callback so we do not have to care how to get the response.

{snippet:id=e3|lang=java|url=camel/trunk/tests/camel-itest/src/test/java/org/apache/camel/itest/async/HttpAsyncCallbackTest.java}

Using the Async API with the Camel classic API

When using the Camel API to create a producer and send an Exchange we do it like this:

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