Android Service Patterns

Android Service Patterns AIDL Services Aren’t That Hard…

Shree Kumar InnoMinds Software

Motivation • Writing services is useful & fun

– Not to mention, painful ! • Documentation is present, but…

– Scattered – Deals mostly with mechanics

• Can a 45 minute session make a difference ? Android Service Patterns | DroidCon India 2011

Objectives • Clarification on the term “Patterns”

– NOT design patterns a-la Grady Booch! • This talk gives you

– Quick overview of AIDL based Services – Features & Pains – Writing useful services as patterns

Android Service Patterns | DroidCon India 2011

Intro to Android Services • The “Service” concept

– Application component – Typically used for long-running operations – Does not automatically create

• new processes or threads • Can be roughly classified based on

– Interaction Mechanism : Started, Bound – Application Boundaries : In-process, IPC


AIDL based IPC Services • Android Interface Definition Language • Inter-Process Communication to

– Expose interfaces to other applications • More complicated than “Messenger”


Why all the pain? • “natural” interactions with the service

– Calls & callbacks – With complex objects as parameters

• And all these across process boundaries!


Hello, AIDL Service! • Implements onBind()

– Exposes interface by implementing stubs • Exports via AndroidManifest.xml • Client apps bind via Intents

– Use interface methods – Callbacks require app to implement stubs

corresponding to callback interface Android Service Patterns | DroidCon India 2011

What’s not-so-nice there ? • No control over binding

– onBind() return value is cached! • No control over threading • Code messier than in-process interface

– No support for exceptions – Dead remote object exceptions => messy code – Callbacks not invoked in UI context

• No concept of versioning


Common Service Patterns • Exclusive access to shared resource

– The “lock” pattern • Simultaneous access to shared resource

– “serialize”, “broadcast” patterns • Less common

– Service versioning – Multiple objects of the same type


The “lock” pattern • Exclusive access using “locks”

– Take lock before use – Release lock after use

• Looks simple ? – Multi-app access – Appropriate threading model – Improper app lock handling => starvation


Locked Service Implementation • Associate locks with calling process • Worker thread for serialization

– Started on service creation – Producer-consumer modeling

• Java.util.concurrent provides ample support • Partial solution for starvation

– Evict clients on DeadObjectException Android Service Patterns | DroidCon India 2011

Shared Access Service • “broadcast” method

– Shares a state update with multiple listeners – Broadcast receivers don’t make the cut

• Can’t bind a Service inside them • Implementation

– Reuse “listener” concept • Clients add/remove listeners

– Service maintains list of listeners • Updates listeners when needed • Removes dead objects

– Reference : MindTheRobot Android Architecture Tutorial


Questions ? Shree Kumar

[email protected] http:://www.shreekumar.in/

Android Service Patterns

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