N Ways to Dynamically Switch Spring Boot Implementations and Ditch if‑else

In Spring Boot projects a single interface often has several implementations (e.g., PDF and CSV exporters). Traditional dependency injection binds a fixed bean, leading to redundant code and poor extensibility. The article explores flexible injection strategies that allow runtime selection of the appropriate implementation.

Environment: Spring Boot 3.5.0. The example defines a DataExporter interface and two beans: @Component("pdf") class PdfExporter implements DataExporter returning "Pdf格式导出" and @Component("csv") class CsvExporter implements DataExporter returning "Csv格式导出".

2.2 Qualifier annotation – The most common static approach. By annotating a constructor parameter with @Qualifier("pdf") (or "csv") the desired bean is fixed at compile time. This cannot switch dynamically based on request data.

private final DataExporter exporter;</code>
<code>public ExportController(@Qualifier("pdf") DataExporter exporter) {</code>
<code>    this.exporter = exporter;</code>
<code>}

2.3 Manual bean retrieval – Inject ApplicationContext and obtain the bean whose name matches a request parameter.

@RestController</code>
<code>@RequestMapping("/export")</code>
<code>public class ExportController {</code>
<code>    private final ApplicationContext context;</code>
<code>    public ExportController(ApplicationContext context) {</code>
<code>        this.context = context;</code>
<code>    }</code>
<code>    @GetMapping</code>
<code>    public String export(String format) {</code>
<code>        DataExporter exporter = this.context.getBean(format, DataExporter.class);</code>
<code>        return exporter.export();</code>
<code>    }</code>
<code>}

The format query parameter (e.g., "pdf" or "csv") determines which exporter runs, as shown in the accompanying result screenshots.

2.4 Map injection – Inject a Map<String, DataExporter> containing all exporter beans. The map key is the bean name, enabling dynamic lookup.

private final Map<String, DataExporter> exporters;</code>
<code>public ExportController(Map<String, DataExporter> exporters) {</code>
<code>    this.exporters = exporters;</code>
<code>}</code>
<code>@GetMapping</code>
<code>public String export(String format) {</code>
<code>    DataExporter exporter = this.exporters.get(format);</code>
<code>    return exporter.export();</code>
<code>}

2.5 FactoryBean with dynamic proxy – Create a DataExporterFactoryBean that implements FactoryBean<DataExporter>. It receives ApplicationContext and HttpServletRequest, builds a JDK proxy, and delegates each method call to the bean whose name matches the request parameter format. Marked @Primary, the proxy can be injected wherever DataExporter is required.

@Component</code>
<code>@Primary</code>
<code>public class DataExporterFactoryBean implements FactoryBean<DataExporter> {</code>
<code>    private final ApplicationContext context;</code>
<code>    private final HttpServletRequest request;</code>
<code>    public DataExporterFactoryBean(ApplicationContext context, HttpServletRequest request) {</code>
<code>        this.context = context;</code>
<code>        this.request = request;</code>
<code>    }</code>
<code>    @Override</code>
<code>    public DataExporter getObject() throws Exception {</code>
<code>        return (DataExporter) Proxy.newProxyInstance(</code>
<code>            DataExporterFactoryBean.class.getClassLoader(),</code>
<code>            new Class<?>[] { DataExporter.class },</code>
<code>            (proxy, method, args) -> {</code>
<code>                DataExporter instance = context.getBean(request.getParameter("format"), DataExporter.class);</code>
<code>                return method.invoke(instance, args);</code>
<code>            });</code>
<code>    }</code>
<code>    @Override</code>
<code>    public Class<?> getObjectType() {</code>
<code>        return DataExporter.class;</code>
<code>    }</code>
<code>}

Controller can now inject the proxy as a regular DataExporter and call export() without knowing the concrete implementation.

private final DataExporter exporter;</code>
<code>public ExportController(DataExporter exporter) {</code>
<code>    this.exporter = exporter;</code>
<code>}</code>
<code>@GetMapping</code>
<code>public String export() {</code>
<code>    return this.exporter.export();</code>
<code>}

All four approaches achieve dynamic bean selection, with the FactoryBean solution offering the most transparent usage while keeping the concrete implementations hidden.