Java Messaging : Difference between ActiveMQ, Mule, ServiceMix and Camel

ActiveMQ is a message broker which implements the JMS API and supports a number of cross language clients and network protocols. It lets you implement queues or topics and write listeners or subscribers to respond to queue events.

Mule and ServiceMix are open source ESB (enterprise service bus). An ESB has capabilities beyond JMS: queuing is key, but so are transformation, routing, logging, security, etc.

Apache Camel is an implementation of the patterns in Enterprise Integration Patterns. It can use JMS and other components to put the ideas from that book into place.

JMS is fundamental to the other technologies, like JDBC is the foundation for Hibernate, iBatis, etc.

JMS is a Java API and a specification and TCK (part of Java EE). ActiveMQ is a particular implementation of it.

Camel can use ActiveMQ (and Camel is bundled inside the ActiveMQ broker so you can easily route to and from JMS to the other components Camel supports).

Camel doesn't use Mule or ServiceMix directly; though ServiceMix uses Camel as its preferred routing engine and EIP implementation. Camel does have a ton of different components though that use various different technologies.

First let's define

JMS is a Java Messaging Service protocol specification.
ESB is Enterprise Service Bus.
JBI is Java Business Integration.

Now we can answer in details:

Apache ActiveMQ is an implementation of the above JMS (Java Messaging Service).

Apache Camel is a message routing engine implementing Enterprise Integration Patterns.
It provides a lot of predefined components.
One of its key component supports JMS (Java Messaging Service).

Apache ServiceMix is an implementation of the above ESB (Enterprise Service Bus)
compatible with the JBI (Java Business Integration) specification.
It also provides many infrastructural features not available in Camel (like services OSGI bundle support).
SM makes heavy use of Camel.

Mule is another implementation of ESB (Enterprise Service Bus), but not related to the Camel/ServiceMix family.