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What is the use of method injection?

Published in Dependency Injection 3 mins read

Method injection is a dependency injection (DI) pattern that provides dependencies to a class through its methods. It offers flexibility and granular control over dependency provisioning, especially when not all methods within a class require the same set of dependencies.

Benefits of Method Injection

Here's a breakdown of the uses and benefits of method injection:

  • Flexibility: Method injection shines when a class doesn't need all of its dependencies all the time. It allows dependencies to be provided only when and where they are needed, enhancing modularity and reducing unnecessary object creation.
  • Selective Injection: This is a core advantage. Different methods can receive entirely different sets of dependencies. This leads to more fine-grained control over dependencies and makes it easier to understand what a method relies on.

Scenarios where Method Injection is Useful

  • Optional Dependencies: When a method requires a dependency only under certain conditions, method injection avoids injecting it into the constructor (and thus, the class itself) when it's not needed.
  • Transient Dependencies: If a dependency is needed only for the duration of a single method call and shouldn't be stored as a class member, method injection is a suitable solution.
  • Loosely Coupled Components: It promotes loose coupling by allowing classes to operate without needing to know the details of how their dependencies are created or managed.
  • Dynamic Dependency Needs: If a method's dependencies are determined at runtime, method injection provides a way to supply them dynamically.

Example Scenario

Imagine a ReportGenerator class that creates different types of reports. The methods to generate specific reports require different data sources or formatting services.

public class ReportGenerator {

    public void generateSalesReport(SalesReportData data, SalesFormatter formatter) {
        // Use 'data' and 'formatter' to generate a sales report.
    }

    public void generateInventoryReport(InventoryData data, InventoryAnalyzer analyzer) {
        // Use 'data' and 'analyzer' to generate an inventory report.
    }
}

In this example, generateSalesReport requires SalesReportData and SalesFormatter, while generateInventoryReport requires InventoryData and InventoryAnalyzer. Using method injection, each method receives only the dependencies it needs, avoiding unnecessary dependencies at the class level. This demonstrates the principle of "Selective Injection" where different methods receive different dependencies, leading to cleaner and more maintainable code.

Advantages summarized:

Feature Description
Flexibility Provides dependencies only when needed, avoiding unnecessary object creation.
Selective Injection Allows different methods to receive different sets of dependencies, enhancing modularity.
Loose Coupling Reduces dependencies at the class level, improving component independence.
Dynamic Needs Supports runtime determination of dependencies.

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