To perform HTTP requests from a Java backend, include the FastJSON library in your project dependencies.

<dependency>
    <groupId>com.alibaba</groupId>
    <artifactId>fastjson</artifactId>
    <version>1.2.56</version>
</dependency>

Here's a practical implementation using Apache HttpClient to fetch and process JSON data from an extenral API endpoint.

import org.apache.http.HttpEntity;
import org.apache.http.client.methods.CloseableHttpResponse;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.CloseableHttpClient;
import org.apache.http.impl.client.HttpClients;
import org.apache.http.util.EntityUtils;
import com.alibaba.fastjson.JSONArray;
import com.alibaba.fastjson.JSONObject;
import org.springframework.web.bind.annotation.*;

import java.io.IOException;
import java.util.*;

@RestController
@RequestMapping("/api")
public class ExternalApiClient {

    @GetMapping("/fetchWorkflowTemplates")
    public List<Map<String, Object>> retrieveFilteredTemplates(@RequestParam(value = "criteria", required = false) String criteria) {
        String endpoint = "http://api.example.com:8080/workflow/templates";
        if (criteria != null && !criteria.trim().isEmpty()) {
            endpoint += "?filter=" + criteria;
        }
        
        CloseableHttpClient httpClient = HttpClients.createDefault();
        HttpGet request = new HttpGet(endpoint);
        String responseContent = null;
        
        try (CloseableHttpResponse response = httpClient.execute(request)) {
            HttpEntity responseEntity = response.getEntity();
            if (responseEntity != null) {
                responseContent = EntityUtils.toString(responseEntity);
            }
        } catch (IOException e) {
            e.printStackTrace();
            return Collections.emptyList();
        }
        
        List<Map<String, Object>> resultList = new ArrayList<>();
        if (responseContent != null) {
            JSONArray jsonArray = JSONArray.parseArray(responseContent);
            for (int i = 0; i < jsonArray.size(); i++) {
                JSONObject jsonObj = jsonArray.getJSONObject(i);
                resultList.add(convertJsonToMap(jsonObj));
            }
        }
        return resultList;
    }
    
    private Map<String, Object> convertJsonToMap(JSONObject jsonObject) {
        Map<String, Object> mapResult = new HashMap<>();
        for (String key : jsonObject.keySet()) {
            mapResult.put(key, jsonObject.get(key));
        }
        return mapResult;
    }
}

The controller method retrieveFilteredTemplates constructs a URL with an optional filter parameter, executes an HTTP GET request, and converts the JSON response into a list of maps. The convertJsonToMap helper method tarnsforms individual JSON objects into Java Map instances.

For more complex JSON structures with nested objects and arrays, consider this enhanced conversion method:

private Map<String, Object> convertComplexJsonToMap(JSONObject jsonObject) {
    Map<String, Object> resultMap = new HashMap<>();
    
    for (String key : jsonObject.keySet()) {
        Object value = jsonObject.get(key);
        
        if (value instanceof JSONArray) {
            value = convertJsonArrayToList((JSONArray) value);
        } else if (value instanceof JSONObject) {
            value = convertComplexJsonToMap((JSONObject) value);
        }
        resultMap.put(key, value);
    }
    return resultMap;
}

private List<Object> convertJsonArrayToList(JSONArray jsonArray) {
    List<Object> listResult = new ArrayList<>();
    
    for (int i = 0; i < jsonArray.size(); i++) {
        Object element = jsonArray.get(i);
        
        if (element instanceof JSONArray) {
            element = convertJsonArrayToList((JSONArray) element);
        } else if (element instanceof JSONObject) {
            element = convertComplexJsonToMap((JSONObject) element);
        }
        listResult.add(element);
    }
    return listResult;
}