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5.5. Conditional installation

The @Install annotation controls conditional installation of components that are required in some deployment scenarios and not in others. This is useful when you want to:
  • mock out an infrastructural component in a test,
  • change a component's implementation in certain deployment scenarios, or
  • install some components only if their dependencies are available. (This is useful for framework authors.)
@Install lets you specify precedence and dependencies.
The precedence of a component is a number that Seam uses to decide which component to install when there are multiple classes with the same component name in the classpath. Seam will choose the component with the higher precendence. Some predefined precedence values are (in ascending order):
  1. BUILT_IN — the lowest precedence components are the components built in to Seam.
  2. FRAMEWORK — components defined by third-party frameworks may override built-in components, but are overridden by application components.
  3. APPLICATION — the default precedence. This is appropriate for most application components.
  4. DEPLOYMENT — for application components which are deployment-specific.
  5. MOCK — for mock objects used in testing.
Suppose we have a component named messageSender that talks to a JMS queue. 
@Name("messageSender") 
public class MessageSender { 

  public void sendMessage() { 
    //do something with JMS 
  } 
}
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In our unit tests, there is no available JMS queue, so we would like to stub out this method. We'll create a mock component that exists in the classpath when unit tests are running, but is never deployed with the application: 
@Name("messageSender") 
@Install(precedence=MOCK) 
public class MockMessageSender extends MessageSender { 
  public void sendMessage() { 
    //do nothing! 
  } 
}
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The precedence helps Seam decide which version to use when it finds both components in the classpath.
If we are able to control precisely which classes are in the classpath, this works well. But if we are writing a reuseable framework with many dependencies, we do not want to have to break that framework across multiple jars. We want to be able to decide which components to install based on other installed components, and classes available in the classpath. The @Install annotation also controls this functionality. Seam uses this mechanism internally to enable the conditional installation of many built-in components.
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