Component Singleton Lifestyle

Singleton Lifestyles

This example demonstrates the uniquenes of a singleton instances across multiple threads. Unlike earlier examples of TRANSIENT and THREAD lifestyle policy - the instances returned within any given thread and any given part access invocation is the same instance.

Component Example (unchanged from thread demo)

tutorial/components/singleton:

Demo.java	A component that that established n threads, and makes multiple requests against the internal parts instance. The number of threads and per-thread hits is controlled by respective context values.
Gizmo.java	A test component we will use as the solution to the demo context criteria.
DemoTestCase.java	The testcase.

Deployment Solution

<component xmlns="dpml:metro" class="org.acme.Demo" name="demo">
  <context>
    <entry key="threadCount" value="3"/>
    <entry key="accessCount" value="2"/>
  </context>
  <parts>
    <component key="gizmo" type="org.acme.Gizmo" lifestyle="singleton"/>
  </parts>
</component>

Testing the component

The following debug level logging illustrates the thread name and resolved instance. Analysis of the results demonstrates the resolution of the same instance irrespective of request and/or thread.

test:
    [junit] Executing forked test.
    [junit] Running org.acme.test.DemoTestCase
    [junit] [13279] [INFO   ] (demo):  gizmo (Thread[0,5,main]) [3916915]
    [junit] [13279] [INFO   ] (demo):  gizmo (Thread[2,5,main]) [3916915]
    [junit] [13279] [INFO   ] (demo):  gizmo (Thread[1,5,main]) [3916915]
    [junit] [13279] [INFO   ] (demo):  gizmo (Thread[1,5,main]) [3916915]
    [junit] [13279] [INFO   ] (demo):  gizmo (Thread[0,5,main]) [3916915]
    [junit] [13279] [INFO   ] (demo):  gizmo (Thread[2,5,main]) [3916915]
    [junit] Tests run: 1, Failures: 0, Errors: 0, Time elapsed: 0.891 sec

Summary

Components assiated with a singleton sifestyle are shared across thread boundaries and as such need to deal with schronization concerns when managing internal state.