Component Lifestyle

A component lifestyle policy is used to govern the decision to construct new instances. Three lifestyle policies are supported by the Metro runtime including SINGLETON, THREAD and TRANSIENT. In the previouse examples we have not declared a lifestyle and as a consequence the default per-thread lifestyle is implied.

For reference - a brief description of the respective lifestyle is included in the following table:

SINGLETON A single instance of the component is shared across all consumers - as such singleton instances must be implemented with the assumption of multiple concurrent requests from different threads. A singleton lifestyle handler may be associated with a HARD, SOFT, WEAK or SYSTEM collection policy. If the collection policy is SYSTEM and the compoent is a top-level component, a HARD collection policy is assumed, otherwise the default SOFT policy is applied.
THREAD A per-thread policy ensures that a unique instance of the component is established for each unique consumer thread. As such, per-thread components may assume sequential access. Per-thread references are maintained as HARD references relative to the associated thread (i.e. finalization of the thread will result in finalization of the instance).
TRANSIENT The transient policy will establish a new instance per consumer. All transient instance references are maintained as WEAK references.
Component Example

To demonstrate the impact of the lifestyle policy we can update our project defintition and declare an explicit lifestyle. In this case we assign the TRANSIENT lifestyle policy to the widget component.

component definition::

<component xmlns="dpml:metro" class="org.acme.Demo" name="demo">
    <component key="widget" type="org.acme.Widget" name="widget" lifestyle="transient">
        <entry key="color" method="BLUE"/>
    <component key="gizmo" type="org.acme.Gizmo" name="gizmo">
        <entry lookup="org.acme.Widget"/>
Testing the component

The following debug level logging reflects the establishment of the transient lifestyle policy and automatic disposal of the Widget instance.

    [junit] Executing forked test.
    [junit] Running org.acme.test.DemoTestCase
    [junit] [96928] [FINE   ] (demo): established per-thread lifestyle handler for [org.acme.Demo]
    [junit] [96928] [FINE   ] (demo): building internal parts
    [junit] [96928] [FINE   ] (demo.widget): established transient lifestyle handler for [org.acme.Widget]
    [junit] [96928] [FINE   ] (demo.gizmo): established per-thread lifestyle handler for [org.acme.Gizmo]
    [junit] [96928] [FINE   ] (demo): commissioning internal parts
    [junit] [96928] [FINE   ] (demo.gizmo): instantiated [27165481 ]
    [junit] [96928] [FINE   ] (demo.gizmo): activated [27165481 ]
    [junit] [96928] [FINE   ] (demo): mediating lookup for [org.acme.Widget]
    [junit] [96928] [FINE   ] (demo.widget): instantiated [849515   ]
    [junit] [96928] [FINE   ] (demo.widget): activated [849515   ]
    [junit] [96928] [INFO   ] (demo): located the color java.awt.Color[r=0,g=0,b=255]
    [junit] [96928] [FINE   ] (demo): instantiated [23438274 ]
    [junit] [96928] [FINE   ] (demo): activated [23438274 ]
    [junit] [96928] [FINE   ] (demo.widget): instance disposal [849515   ]
    [junit] Tests run: 1, Failures: 0, Errors: 0, Time elapsed: 0.953 sec

Lifestyle policies have a direct impact on the availability of services and the extent to which instances may be shared between components. The default THREAD policy is suitable for most scenarios, however - applications using a component across multiple threads of execution will need to declare a singleton lifestyle in order to ensure that a single service instance is maintained.