Field-Based Injection in Java

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Listing 28
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public class LoanComponent
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@Reference protected CreditService CreditService; // }
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Field-injection follows the basic pattern set by method-injection except that they may be private and public or protected In the absence of a name attribute declared on @Reference, the eld name is used as the name of the reference Again, the preceding example would be con gured using the same composite syntax as the previous examples A major advantage of eld-based injection is that it is concise (Methods do not need to be created for each reference) It also avoids long constructor parameter lists The main disadvantage of eld-based injection is it is dif cult to unit test; component classes must either be subclassed to expose reference elds or those elds must be set through Java re ection
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A major advantage of eld-based injection is that it is concise The main disadvantage of eld-based injection is it is dif cult to unit test
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Perspective: What s the Best Injection Style
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Several years ago, setter- versus constructor-based injection was an area of contention among advocates of various Java-based IoC frameworks, notably Spring and PicoContainer Most modern IoC frameworks now support both approaches, as does SCA In the process of writing this book, we debated between ourselves about the best injection style Jim favors constructor injection because it makes service dependencies explicit Mike prefers eld-based injection because it limits verbosity In the end, like the debates among the various IoC frameworks a few years back, we went
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around in circles and were unable to convince one another This led us to agree on an important point: Choosing an injection style is largely a matter of personal preference Pick the one that best suits the project requirements or the one project developers are used to and stay consistent That said, there is one important difference between eld and setter versus constructor injection in SCA Namely, eld and setter injection can be dynamic As we will cover in 7, eld- and setter-based references may be reinjected if a reference changes after a component has been instantiated In contrast, constructorbased references cannot be changed If a reference may change, you need to use eld- or setter-based injection On the other hand, if a reference must be immutable, use constructor injection
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De ning Properties Consider the case where we want to add the capability to set con guration parameters on the CreditComponent component, such as minimum and maximum scores SCA supports con guration through component properties, which in Java are speci ed using the @Property annotation CreditComponent is modi ed to take maximum and minimum scores in Listing 29
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public void CreditComponent implements CreditService { private int min; private int max; public CreditComponent (@Property(name="min") int min, @Property(name="max") int max) { thismin = min; thismax = max; } // }
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Like a reference, a property name is speci ed using the name attribute, whereas the required attribute determines whether a property value must be provided in the composite le (that is, when it is set to true) or it is optional (that is, it is set to false, the default)
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In addition, properties follow the same injection guidelines as references: constructor-, method-, and eld-based injection are supported Given that most IoC frameworks do not distinguish between properties and references, why does SCA The short answer is they are different References provide access to services, whereas properties provide con guration data Differentiating properties and references makes it clear to someone con guring a component whether a property value needs to be supplied or a reference wired to a service Further, as we will see in later chapters, references may have various qualities of service applied to them, such as reliability, transactions, and security The bene ts of distinguishing properties and references also extends to tooling: Knowing if a particular value is a property or reference makes for better validation and visual feedback, such as displaying speci c icons in graphical tooling
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