What is the purpose of the @RequestParam annotation?

Table of Contents

Introduction

In Spring MVC and Spring Boot, the @RequestParam annotation is used to extract query parameters from an HTTP request and bind them to the method parameters of a controller. It is one of the most commonly used annotations for handling HTTP GET requests, especially when dealing with parameters passed in the URL.

When building web applications or REST APIs, you often need to capture user input or request details. These inputs might be sent via URL parameters (query parameters) or form data. The @RequestParam annotation makes it easy to access and work with these parameters in your controller methods.

1. Binding Query Parameters to Method Parameters

The primary purpose of @RequestParam is to bind query parameters from the HTTP request to method parameters in a Spring controller. Query parameters are typically included in the URL after the ? symbol, and they follow the pattern key=value.

Example: Simple Use of @RequestParam

URL Example:

  • http://localhost:8080/search?query=laptop

Explanation:

  • **@RequestParam String query**: The query parameter from the URL (e.g., ?query=laptop) is automatically mapped to the query parameter of the search method.
  • The response will be "Searching for: laptop".

2. Handling Optional Query Parameters

In many cases, a query parameter may be optional. If a parameter is not provided in the URL, Spring can assign a default value or leave the parameter as null (if the required attribute is set to false).

Example: Optional Query Parameters

URL Examples:

  • http://localhost:8080/search?query=laptop → "Searching for: laptop"
  • http://localhost:8080/search → "Searching for: all"

Explanation:

  • **required = false**: Makes the query parameter optional.
  • **defaultValue = "all"**: Provides a default value for query if it is not included in the URL. In this case, the default value is "all".

3. Handling Multiple Query Parameters

When handling multiple query parameters, you can add multiple @RequestParam annotations to the method, each corresponding to a different query parameter.

Example: Multiple Query Parameters

URL Example:

  • http://localhost:8080/products?category=electronics&limit=5

Explanation:

  • **category** and **limit**: These two parameters are mapped from the query string to the corresponding method parameters. The URL includes both category=electronics and limit=5, which are mapped to the category and limit method parameters, respectively.

4. Binding Multiple Values to a Single Parameter

@RequestParam can also handle cases where the same parameter appears multiple times in the URL (e.g., when you have an array of values). Spring automatically maps multiple occurrences of the same parameter to a List or Set.

Example: Multiple Values for a Query Parameter

URL Example:

  • http://localhost:8080/categories?categories=electronics&categories=clothing&categories=home

Explanation:

  • **List<String> categories**: Spring will collect all the values passed with the categories query parameter into a List. The resulting list will contain ["electronics", "clothing", "home"], and the response will be "Categories: electronics, clothing, home".

5. Binding Query Parameters to Custom Objects

For more complex query parameter handling, you can bind multiple query parameters into a custom Java object using the @ModelAttribute annotation. Although @RequestParam is not directly used for this, it's worth mentioning as a powerful technique for handling multiple parameters.

Example: Binding Query Parameters to a Custom Object

URL Example:

  • http://localhost:8080/search?category=electronics&minPrice=100&maxPrice=500

Explanation:

  • **@ModelAttribute**: This binds multiple query parameters (category, minPrice, maxPrice) to the fields of the ProductSearchCriteria object. Spring automatically maps the query parameters to the object fields.

6. Customizing Parameter Binding

Spring allows customization in parameter binding, such as using a custom converter or validating parameters before they are processed.

Example: Custom Conversion for Query Parameters

URL Example:

  • http://localhost:8080/products?minPrice=100&maxPrice=500

Explanation:

  • Spring automatically converts the query string parameters (minPrice and maxPrice) to Double type, which is passed to the controller method.

7. Conclusion

The @RequestParam annotation is a powerful and essential tool for handling query parameters in Spring MVC. It simplifies the process of extracting data from the URL and binding it to Java method parameters. Key features of @RequestParam include:

  • Binding query parameters to method parameters.
  • Making parameters optional or providing default values.
  • Handling multiple query parameters.
  • Supporting collections (e.g., lists) for parameters with multiple values.
  • Binding complex query data to custom objects.

By using @RequestParam, you can efficiently process client input, making it easier to create dynamic, user-driven web applications and APIs.es. C

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