@RequestParam List — Multi-Value Query Parameters in Spring

Direct Answer

@RequestParam List<String> ids can collect repeated values such as /items?ids=1&ids=2&ids=3. With Spring's normal conversion service, comma-delimited input can also be converted for suitable collection element types, but conversion rules/custom converters can change this. The parameter is required by default; use defaultValue, Optional, or required=false when absence has a defined meaning.

Code Example

@RestController
@RequestMapping("/api/products")
public class ProductController {
    private final ProductService productService;

    @GetMapping
    public List<Product> getProducts(
            @RequestParam(required = false) List<String> categories,
            @RequestParam(required = false) List<Long> ids) {
        if (ids != null && !ids.isEmpty()) {
            return productService.findByIds(ids); // batch fetch
        }
        if (categories != null && !categories.isEmpty()) {
            return productService.findByCategories(categories); // multi-filter
        }
        return productService.findAll();
    }

    @DeleteMapping("/batch")
    public void deleteProducts(@RequestParam List<Long> ids) {
        productService.deleteByIds(ids); // bulk delete
    }

    @GetMapping("/tags")
    public List<Product> getByTags(@RequestParam("tag") String[] tags) {
        return productService.findByTags(List.of(tags)); // array alternative
    }

    @GetMapping("/unique-categories")
    public List<Product> getByUniqueCategories(@RequestParam Set<String> categories) {
        return productService.findByCategories(categories); // Set removes duplicates
    }
}

Common Mistakes & Pitfalls

• Manually parsing comma-separated strings: Freshers often declare @RequestParam String ids then split it with ids.split(","). Spring can bind to List directly — no manual parsing needed. Just use @RequestParam List<String> ids and it handles both repeated params and comma-separated values.
• Using primitive arrays: Writing @RequestParam long[] ids causes type conversion issues. Use boxed types: @RequestParam List<Long> ids or @RequestParam Long[] ids. Spring's conversion works better with wrapper classes.
• Forgetting required=false for optional lists: If the parameter is optional, you must set @RequestParam(required = false) List<String> categories, otherwise Spring throws an exception when the param is missing. Then check for null or empty in your code before using the list.