What performance overhead do IoC containers involve?

Question

Loose coupling is wonderful of course, but I have often wondered what overhead wiring up dynamically using an IoC container (for example Castle Windsor) has over a tightly coupl

Answer 1

The best way to understand how complex an IoC container is comes from analyzing it.

In a particular experience, once I took an entire afternoon debugging some simple 'Hello World' code using plexus, which Maven is based upon (and here is a helpful link to browse its source code). It kinda came up (by looking at defaultPlexusContainer) as:

• Classpath Configuration (via classworlds)
• Creation of a Runtime Context Variable (basically a map), in order to store properties and variables
• Configuration Parsing (discovery of modules metadata on classpath, etc)
• Initialization:
  • Construction / Instantiation of Services
• Firing additional ComponentDiscoverers
• Firing of additional ComponentDiscovererListeners

This leaves an important aspect, deep into the steps above: Looking up a component. In plexus, the Phase concept wraps the steps for the construction of an Object, and those Phases are usually bound to a Personality concept. However, for the default setting, this is done by executing the following phases:

• Object Instantiation (i.e., new Object())
• Log Enabling (i.e., setting a logger for the object)
• Composition: i.e, dependency lookup and setting
  • The setter strategy is an interesting point, but I will leave the details for now
• The passing of the Context into the created object
• The object additional startup procedure

Most of those steps are optional, and usually involve identifying a given interface and calling it on the target object - this is the default for the plexus personality, note that.

Also, each object might be bound to a lifecycle manager, which mostly makes the difference between a new object and a singleton.

In my particular record: The most difficult part is actually parsing the configuration, and booting the container. After that, you're likely to notice no further difference in performance.