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Introduction to Spring Framework and Dependency Injection Aaron Zeckoski azeckoski@gmail.com

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Spring Framework A popular and stable Java application framework for enterprise development Ubiquitous for Java development Well established in enterprise Java apps Time tested and proven reliable A primary purpose is to reduce dependencies and even introduce negative dependencies Different from almost every other framework out there Part of the reason it has been adopted so quickly

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Spring code structure Spring code base is proven to be well structured (possibly the best) Analysis using Structure 101 139 packages No dependency cycles

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More Spring Considered an alternative / replacement for the Enterprise JavaBean (EJB) model Flexible Programmers decide how to program Not exclusive to Java (e.g. .NET) Solutions to typical coding busywork JDBC LDAP Web Services

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What does Spring offer? Dependency Injection Also known as IoC (Inversion of Control) Aspect Oriented Programming Runtime injection-based Portable Service Abstractions The rest of spring ORM, DAO, Web MVC, Web, etc. Allows access to these without knowing how they actually work

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Dependency Injection defined Method to create needed dependencies or look them up somehow without doing it in the dependent code Often called Inversion of Control (IoC) IoC injects needed dependencies into the object instead Setters or Contructor Primary goal is reduction of dependencies in code an excellent goal in any case This is the central part of Spring

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Aspect Oriented Programming defined Attempts to separate concerns, increase modularity, and decrease redundancy Separation of Concerns (SoC) Break up features to minimize overlap Don’t Repeat Yourself (DRY) Minimize code duplication Cross-Cutting Concerns Program aspects that affect many others (e.g. logging) AspectJ is the top AOP package Java like syntax, IDE integration

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Portable Service Abstractions defined Services that easily move between systems without heavy reworking Ideally easy to run on any system Abstraction without exposing service dependencies LDAP access without knowing what LDAP is Database access without typical JDBC hoops Basically everything in Spring that is not IoC or AOP

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What is a bean? Typical java bean with a unique id In spring there are basically two types Singleton One instance of the bean created and referenced each time it is requested Prototype (non-singleton) New bean created each time Same as new ClassName() Beans are normally created by Spring as late as possible

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What is a bean definition? Defines a bean for Spring to manage Key attributes class (required): fully qualified java class name id: the unique identifier for this bean configuration: (singleton, init-method, etc.) constructor-arg: arguments to pass to the constructor at creation time property: arguments to pass to the bean setters at creation time Collaborators: other beans needed in this bean (a.k.a dependencies), specified in property or constructor-arg Typically defined in an XML file

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Sample bean definition

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What is a bean factory? Often seen as an ApplicationContext BeanFactory is not used directly often ApplicationContext is a complete superset of bean factory methods Same interface implemented Offers a richer set of features Spring uses a BeanFactory to create, manage and locate “beans” which are basically instances of a class Typical usage is an XML bean factory which allows configuration via XML files

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How are beans created? Beans are created in order based on the dependency graph Often they are created when the factory loads the definitions Can override this behavior in bean You can also override this in the factory or context but this is not recommended Spring will instantiate beans in the order required by their dependencies app scope singleton - eagerly instantiated at container startup lazy dependency - created when dependent bean created VERY lazy dependency - created when accessed in code

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How are beans injected? A dependency graph is constructed based on the various bean definitions Beans are created using constructors (mostly no-arg) or factory methods Dependencies that were not injected via constructor are then injected using setters Any dependency that has not been created is created as needed

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Multiple bean config files There are 3 ways to load multiple bean config files (allows for logical division of beans) Load multiple config files from web.xml contextConfigLocation classpath:/WEB-INF/spring-config.xml, classpath:/WEB-INF/applicationContext.xml Use the import tag Load multiple config files using Resources in the application context constructor Recommended by the spring team Not always possible though ClassPathXmlApplicationContext appContext = new ClassPathXmlApplicationContext( new String[] {"applicationContext.xml", "applicationContext-part2.xml"});

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Bean properties? The primary method of dependency injection Can be another bean, value, collection, etc.

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Anonymous vs ID Beans that do not need to be referenced elsewhere can be defined anonymously This bean is identified (has an id) and can be accessed to inject it into another bean

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What is an inner bean? It is a way to define a bean needed by another bean in a shorthand way Always anonymous (id is ignored) Always prototype (non-singleton)

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Bean init-method The init method runs AFTER all bean dependencies are loaded Constructor loads when the bean is first instantiated Allows the programmer to execute code once all dependencies are present

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Bean values Spring can inject more than just other beans Values on beans can be of a few types Direct value (string, int, etc.) Collection (list, set, map, props) Bean Compound property

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Abstract (parent) beans Allows definition of part of a bean which can be reused many times in other bean definitions

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AOP in Spring Provides way to create declarative services and custom aspects Transaction management is the most common aspect (or concern) Spring handles AOP via advisors or interceptors Interception point is a joinpoint A set of joinpoints are called a pointcut pointcuts are key to Spring AOP, they allow intercepts without explicit knowledge of the OO hierarchy Action taken by an interceptor is called advice

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AOP advice types Around Most common and powerful Execute code before and after joinpoint Before Executes before joinpoint, cannot stop execution Throws Executes code if exception is thrown After return Executes code after normal joinpoint execution

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Spring AOP key points Pure java implementation Allows method interception No field or property intercepts yet AOP advice is specified using typical bean definitions Closely integrates with Spring IoC Proxy based AOP J2SE dynamic proxies or CGLIB proxies Not a replacement for AspectJ

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Example transaction proxy This wraps a transaction interceptor around a DAO

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Working example Let’s look at some example code pre and post spring Simple application that allows a user to add, remove, and list a set of strings Pre spring code Programmers Cafe - Example App Post spring code Programmers Cafe - Example App Spring

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Example App The example app is a simple command line Java app which is meant to demonstrate a reasonable dependency structure This app allows a user to save, delete, and list a set of strings associated with their username

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Example App Structure Alpha is the main class Bravo handles user interaction Charlie handles application logic Delta handles data access Dependency graph is non-cyclical No A => B => C => A

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Non-spring version Involves using new to create needed dependencies Each class must know about the dependencies that it needs Singletons have to be created and handed to the classes that need them at the same time or you need a static way to access them (or a framework) Tightly coupled code structure

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Spring version No more new use Classes only have to know about the interface or class if no interface available Singletons easy to handle Loose coupling allows flexible changes