Introduction to the Spring Framework 2.5

Since I last revised this article in May, 2005, the Spring Framework has continued to grow in popularity, and has become the de facto standard for enterprise Java development. It has progressed from version 1.2 to the present 2.5, and has been adopted in an even wider range of industries and projects. In this article, I'll try to explain what Spring sets out to achieve, and how I believe it can help you to develop enterprise Java applications.

Why Spring?

I believe that Spring is unique, for several reasons:

* It addresses important areas that other popular frameworks don't. Spring focuses around providing a way to manage your business objects.

* Spring is comprehensive and modular. Spring has a layered architecture, meaning that you can choose to use just about any part of it in isolation, yet its architecture is internally consistent. So you get maximum value from your learning curve. You might choose to use Spring only to simplify use of JDBC, for example, or you might choose to use Spring to manage all your business objects. And it's easy to introduce Spring incrementally into existing projects.

* Spring is designed from the ground up to help you write code that's easy to test. Spring is an ideal framework for test driven projects.

* Spring is an increasingly important integration technology, its role recognized by vendors large and small.

* The Spring Framework is the core of the Spring Portfolio , an increasingly complete solution for enterprise Java development, exhibiting the same consistency of approach developed in the Spring Framework itself.

Spring addresses most infrastructure concerns of typical applications. It also goes places other frameworks don't.

An open source project since February 2003, Spring has a long heritage. The open source project started from infrastructure code published with my book, Expert One-on-One J2EE Design and Development, in late 2002. Expert One-on-One J2EE laid out the basic architectural thinking behind Spring. However, the core architectural concepts go back to early 2000, and reflect my experience in developing infrastructure for a series of successful commercial projects.

There are now almost 40 developers, with the leading contributors devoted full-time to Spring development and support at Interface21. The flourishing open source community has helped it evolve into far more than could have been achieved by any individual.

Architectural benefits of Spring

Before we get down to specifics, let's look at some of the benefits Spring can bring to a project:

* Spring can effectively organize your middle tier objects. Spring takes care of plumbing that would be left up to you if you use only Struts or other frameworks geared to particular J2EE APIs. And Spring's configuration management services can be used in any architectural layer, in whatever runtime environment.

* Spring can eliminate the proliferation of Singletons seen on many projects. In my experience, this is a major problem, reducing testability and object orientation.

* Spring eliminates the need to use a variety of custom properties file formats, by handling configuration in a consistent way throughout applications and projects. Ever wondered what magic property keys or system properties a particular class looks for, and had to read the Javadoc or even source code? With Spring you simply look at the class's JavaBean properties or constructor arguments. The use of Inversion of Control and Dependency Injection (discussed below) helps achieve this simplification.

* Spring facilitates good programming practice by reducing the cost of programming to interfaces, rather than classes, almost to zero.

* Spring is designed so that applications built with it depend on as few of its APIs as possible. Most business objects in Spring applications have no dependency on Spring.

* Applications built using Spring are very easy to test. For certain unit testing scenarios, the Spring Framework provides mock objects and testing support classes. Spring also provides unique “integration testing” functionality in the form of the Spring TestContext Framework and legacy JUnit 3.8 support classes that enable you to test your code quickly and easily, even while accessing a staging database.

* Spring helps you solve problems with the most lightweight possible infrastructure. Spring provides an alternative to EJB that's appropriate for many applications. For example, Spring can use AOP to deliver declarative transaction management without using an EJB container; even without a JTA implementation, if you only need to work with a single database, or want to avoid two phase commit.

* Spring provides a consistent framework for data access, whether using JDBC or an O/R mapping product such as TopLink, Hibernate or a JPA or JDO implementation.

* Spring provides a consistent, simple programming model in many areas, making it ideal architectural "glue." You can see this consistency in the Spring approach to JDBC, JMS, JavaMail, JNDI and many other important APIs.

Spring is essentially a technology dedicated to enabling you to build applications using Plain Old Java Objects (POJOs). It enables you to develop components as POJOs containing only your business logic, while the framework takes care of the many value adds you need to build enterprise applications — even in areas that you may not have considered when initially authoring the application. This goal requires a sophisticated framework, which conceals much complexity from the developer. Because your business logic is abstracted from infrastructure concerns, it’s also likely to enjoy a longer life, improving the return on investment of writing it. Business logic should change at the pace of your business; only if it is abstracted from infrastructure concerns can the impact on your code base of inevitable infrastructure change (such as choice of application server) be minimized.

Thus Spring can enable you to implement the simplest possible solution to your problems. And that's worth a lot.

What does Spring do?

Spring provides a lot of functionality. So I'll quickly review each major area in turn.

Mission statement

Spring's main aim is to make enterprise Java easier to use and promote good programming practice. It does this by enabling a POJO-based programming model that is applicable in a wide range of environments. We believe that Spring provides the ultimate programming model for modern enterprise Java.

Spring does not reinvent the wheel. Thus you'll find no logging packages in Spring, no connection pools, no distributed transaction coordinator. All these things are provided by open source projects (such as Commons Logging, which we use for all our log output, or Commons DBCP), or by your application server or web container. For the same reason, we don't provide an O/R mapping layer. There are good solutions to this problem such as TopLink, Hibernate, JPA and JDO.

Spring does aim to make existing technologies easier to use and does aim to provide a unified, simple yet powerful programming model. For example, although we are not in the business of low-level transaction coordination, we provide an abstraction layer over JTA or any other transaction strategy that is more portable, easier to use and makes code easier to test.

Spring benefits from internal consistency. All the developers are singing from the same hymn sheet, whose fundamental ideas remain faithful to those of Expert One-on-One J2EE Design and Development. And we've been able to use some central concepts, such as Inversion of Control, across multiple areas.

Spring is portable between application servers and web containers. (Indeed, its core functionality does not require another container.) Of course ensuring portability is always a challenge, but we avoid anything platform-specific or non-standard in the developer's view, and support users on WebLogic, Tomcat, Resin, JBoss, Jetty, Geronimo, WebSphere and other application servers. Spring's non-invasive, POJO approach enables us to take advantage of environment-specific features without sacrificing portability, as in the case of enhanced WebLogic, WebSphere and OC4J transaction management functionality that uses BEA and IBM proprietary APIs under the covers.

Inversion of control container

The core of Spring is the org.springframework.beans package, designed for working with POJOs. This package typically isn't used directly by users, but underpins much Spring functionality.

The next higher layer of abstraction is the bean factory. A Spring bean factory is a generic factory that enables configured objects to be retrieved by name, and which can manage relationships between objects.

A word on the term “bean”: very early versions of Spring were intended to configure only JavaBean objects. Since 1.0, Spring has been able to configure just about any Java object, regardless of whether it uses accessor and mutator methods, and in 2.5 it has become still more flexible. Nevertheless the term “Spring Bean” has remained common parlance. “Spring-managed object” is a more accurate term, conveying also the fact that Spring does not merely configure objects, but often continues to manage them at runtime -- for example, to apply enterprise services on every invocation.

Bean factories support three modes of object lifecycle:

* Singleton: in this case, there's one shared instance of the object with a particular name, which will be retrieved on lookup. This is the default, and most often used, mode. It's ideal for stateless service objects.

* Prototype or non-singleton: in this case, each retrieval will result in the creation of an independent object. For example, this could be used to allow each caller to have a distinct object reference.

* Custom object “scopes”, which typically interact with a store outside the control of the container. Some of these come out of the box, such as request and session (for web applications). Others come with third party products, such as clustered caches. It is easy to define custom scopes in the event that none of those provided out of the box is sufficient, through implementing a simple interface.

Because the Spring container manages relationships between objects, it can add value where necessary through services such as transparent pooling for managed POJOs, and support for hot swapping, where the container introduces a level of indirection that allows the target of a reference to be swapped at runtime without affecting callers and without loss of thread safety. One of the beauties of Dependency Injection (discussed shortly) is that all this is possible transparently, with no API involved.

As org.springframework.beans.factory.BeanFactory is a simple interface, it can be implemented in different ways. The BeanDefinitionReader interface separates the metadata format from BeanFactory implementations themselves, so the generic BeanFactory implementations Spring provides can be used with different types of metadata. You could easily implement your own BeanFactory or BeanDefinitionReader, although few users find a need to. The most commonly used BeanFactory definitions are:

* XmlBeanFactory. This parses a simple, intuitive XML structure defining the classes and properties of named objects.

* DefaultListableBeanFactory: This provides the ability to parse bean definitions in properties files, and create BeanFactories programmatically.

Each bean definition can be a POJO (defined by class name and JavaBean initialization properties or constructor arguments), or a FactoryBean. The FactoryBean interface adds a level of indirection. Typically this is used to create proxied objects using AOP or other approaches: for example, proxies that add declarative transaction management. This is conceptually similar to EJB interception, but works out much simpler in practice, and is more powerful.

BeanFactories can optionally participate in a hierarchy, "inheriting" definitions from their ancestors. This enables the sharing of common configuration across a whole application, while individual resources such as controller Servlets also have their own independent set of objects.

This motivation for the use of JavaBeans is described in Chapter 4 of Expert One-on-One J2EE Design and Development, which is available on the ServerSide as a free PDF.

Through its bean factory concept, Spring is an Inversion of Control container. (I don't much like the term container, as it conjures up visions of heavyweight containers such as EJB containers. A Spring BeanFactory is a container that can be created in a single line of code, and requires no special deployment steps.) Spring is most closely identified with a flavor of Inversion of Control known as Dependency Injection – a name coined by Martin Fowler, Rod Johnson and the PicoContainer team in late 2003.

The concept behind Inversion of Control is often expressed in the Hollywood Principle: "Don't call me, I'll call you." IoC moves the responsibility for making things happen into the framework, and away from application code. Whereas your code calls a traditional class library, an IoC framework calls your code. Lifecycle callbacks in many APIs, such as the setSessionContext() method for session EJBs, demonstrate this approach.

Dependency Injection is a form of IoC that removes explicit dependence on container APIs. Ordinary Java methods are used to inject dependencies such as collaborating objects or configuration values into application object instances. Where configuration is concerned this means that while in traditional container architectures such as EJB, a component might call the container to say "where's object X, which I need to do my work", with Dependency Injection the container figures out that the component needs an X object, and provides it to it at runtime. The container does this figuring out based on method signatures (usually JavaBean properties or constructors) and, possibly, configuration data such as XML.

The two major flavors of Dependency Injection are Setter Injection (injection via JavaBean setters); and Constructor Injection (injection via constructor arguments). Spring provides sophisticated support for both, and even allows you to mix the two when configuring the one object.

As well as supporting all forms of Dependency Injection, Spring also provides a range of callback events, and an API for traditional lookup where necessary. However, we recommend a pure Dependency Injection approach in general.

Dependency Injection has important benefits. For example:

* Because components don't need to look up collaborators at runtime, they're much simpler to write and maintain. In Spring's version of IoC, components express their dependency on other components via exposing JavaBean setter methods or through constructor arguments. For example, there is no need for JNDI lookups, which require the developer to write code that makes environmental assumptions.

* For the same reasons, application code is much easier to test. For example, JavaBean properties are simple, core Java and easy to test: simply write a self-contained JUnit or TestNG test method that creates the object and sets the relevant properties.

* A good IoC implementation preserves strong typing. If you need to use a generic factory to look up collaborators, you have to cast the results to the desired type. This isn't a major problem, but it is inelegant. With IoC you express strongly typed dependencies in your code and the framework is responsible for type casts. This means that type mismatches will be raised as errors when the framework configures the application; you don't have to worry about class cast exceptions in your code.

* Dependencies are explicit. For example, if an application class tries to load a properties file or connect to a database on instantiation, the environmental assumptions may not be obvious without reading the code (complicating testing and reducing deployment flexibility). With a Dependency Injection approach, dependencies are explicit, and evident in constructors or JavaBean properties.

* Most business objects don't depend on IoC container APIs. This makes it easy to use legacy code, and easy to use objects either inside or outside the IoC container. For example, Spring users often configure the Jakarta Commons DBCP DataSource as a Spring bean: there's no need to write any custom code to do this. We say that an IoC container isn't invasive: using it won't invade your code with dependency on its APIs. Almost any POJO can become a component in a Spring bean factory. Existing JavaBeans or objects with multi-argument constructors work particularly well, but Spring also provides unique support for instantiating objects from static factory methods or even methods on other objects managed by the IoC container.

This last point deserves emphasis. Dependency Injection is unlike traditional container architectures, such as EJB, in this minimization of dependency of application code on a container. This means that your business objects can potentially be run in different Dependency Injection frameworks - or outside any framework - without code changes.

In my experience and that of Spring users, it's hard to overemphasize the benefits that IoC -- and, especially, Dependency Injection -- brings to application code.

Spring BeanFactories are very lightweight. Users have successfully used them inside applets, as well as standalone Swing applications. There are no special deployment steps and no detectable startup time associated with the container itself (although certain objects configured by the container may of course take time to initialize). This ability to instantiate a container almost instantly in any tier of an application can be very valuable.

The Spring BeanFactory concept is used throughout Spring, and is a key reason that Spring is so internally consistent. Spring is also unique among IoC containers in that it uses IoC as a basic concept throughout a full-featured framework.

Most importantly for application developers, one or more BeanFactories provide a well-defined layer of business objects. This is analogous to, but simpler (yet more powerful), than a layer of local session beans. Having a well-defined layer of business objects is very important to a successful architecture.

A Spring ApplicationContext is a subinterface of BeanFactory, which provides support for:

* Message lookup, supporting internationalization

* An event mechanism, allowing application objects to publish and optionally register to be notified of events

* Automatic recognition of special application-specific or generic bean definitions that customize container behavior

* Portable file and resource access

XmlBeanFactory example

Spring users traditionally configure their applications in XML "bean definition" files, although other forms of configuration, including source-level annotations, properties files and Java code, can also be used, and Spring will merge the results of the different configuration sources.

The root of a Spring XML bean definition document is a element. The element contains one or more definitions. We normally specify the class and properties of each bean definition. We normally also specify the id, which will be the name that we'll use this bean with in our code.

Let's look at a simple example, which configures three application objects with relationships commonly seen in enterprise Java applications:

* A DataSource used to connect to a relational database

* A data access object (DAO) that uses the DataSource

* A business object that uses the DAO in the course of its work

In the following example, we use a BasicDataSource from the Jakarta Commons DBCP project. ComboPooledDataSource from the C3PO project is also an excellent option. BasicDataSource, like many other existing classes, can easily be used in a Spring bean factory, as it offers JavaBean-style configuration. The close() method that needs to be called on shutdown can be registered via Spring's "destroy-method" attribute, to avoid the need for BasicDataSource to implement any Spring interface.

All the properties of BasicDataSource we're interested in are Strings, so we specify their values using the "p:" attribute prefix, a special, non-validated Spring namespace that allows the use of bean property names as XML attributes. This shortcut was introduced in Spring 2.0 as a convenient alternative to the "value" attribute or subelement, which is usable even for values that are problematic in XML attributes. Spring uses the standard JavaBean PropertyEditor mechanism to convert String representations to other types if necessary.

Now we define the DAO, which has a bean reference to the DataSource. Relationships between beans are specified using a combination of the "p:" prefix and "-ref" suffix, the "ref" attribute, or the element:

The business object has a reference to the DAO, and an int property (exampleParam):

Relationships between objects are normally set explicitly in configuration, as in this example. We consider this to be a Good Thing in most cases. However, Spring also provides two kinds of what we call "autowire" support, where it figures out the dependencies between beans: autowire by type, and autowire by name

The limitation with autowiring by type is that if there are multiple beans of a particular type it's impossible to work out which instance a dependency of that type should be resolved to. Unsatisfied dependencies are caught when the factory is initialized. (Spring also offers an optional dependency check for explicit configuration, which verifies that all properties have been set.)

This limitation in autowiring by type can often be overcome by autowiring by name. When using autowire by name, property names are used instead of types. For example, if a bean expresses a dependency by defining a setMaster method, Spring will try to find a bean with the name "master" within the BeanFactory and inject that bean to satisfy the dependency. While autowire by type works with either constructors or setter methods, autowire by name works automatically only with setter methods: a result of the fact that Java reflection does not expose the names of constructor or other method arguments. Spring 2.5 allows autowiring by name to be used for constructors via the @Qualifier parameter annotation. See the “Beyond XML” section below for further details.

We could use the autowire by type feature as follows in the above example, if we didn't want to code these relationships explicitly:

With this usage, Spring will work out that the dataSource property of exampleBusinessObject should be set to the implementation of DataSource it finds in the present BeanFactory. It's an error if there is none, or more than one bean of the required type in the present BeanFactory. We still need to set the exampleParam property, as it's not a reference.

Autowire support has the advantage of reducing the volume of configuration, especially when used as an optional attribute on the root element, which activates autowiring for all beans managed by Spring. It also means that the container learns about application structure using reflection, so if you add an additional constructor argument of JavaBean property, it may be successfully populated without any need to change configuration. The tradeoffs around autowiring should be evaluated carefully.

Externalizing relationships from Java code often has great benefit over hard coding them, as it's possible to change XML files without changing a line of Java code. For example, we could simply change the myDataSource bean definition to refer to a different bean class to use an alternative connection pool, or a test data source. We could use JNDI to get a data source from an application server in a single alternative XML stanza, as follows. There would be no impact on Java code or any other bean definitions.

Now let's look at the Java code for the example business object. Note that there are no Spring dependencies in the code listing below. A Spring IoC container is not invasive: you don't normally need to code awareness of it into application objects.

public class ExampleBusinessObject implements MyBusinessObject {

private ExampleDataAccessObject dao;

private int exampleParam;

public void setDataAccessObject(ExampleDataAccessObject dao) {

this.dao = dao;

}

public void setExampleParam(int exampleParam) {

this.exampleParam = exampleParam;