How to make parallel calls to multiple services to generate a report?

For parallel invocation of multiple services in Java, you can use CompletableFuture. This allows you to run asynchronous tasks and then combine their results.

Example:

import java.util.concurrent.*;
import java.util.*;

public class ReportService {

    private ExecutorService executor = Executors.newFixedThreadPool(10);

    public Report buildReport() throws ExecutionException, InterruptedException {
        CompletableFuture<String> service1 = CompletableFuture.supplyAsync(() -> callService1(), executor);
        CompletableFuture<String> service2 = CompletableFuture.supplyAsync(() -> callService2(), executor);
        CompletableFuture<String> service3 = CompletableFuture.supplyAsync(() -> callService3(), executor);

        // Wait for all to complete
        CompletableFuture<Void> allDone = CompletableFuture.allOf(service1, service2, service3);

        // After completion, gather results
        allDone.get();

        String result1 = service1.get();
        String result2 = service2.get();
        String result3 = service3.get();

        // Assemble report from results
        return assembleReport(result1, result2, result3);
    }

    private String callService1() {
        // call first service
        return "data1";
    }

    private String callService2() {
        // call second service
        return "data2";
    }

    private String callService3() {
        // call third service
        return "data3";
    }

    private Report assembleReport(String r1, String r2, String r3) {
        // report assembly logic
        return new Report(r1, r2, r3);
    }

    static class Report {
        String part1, part2, part3;
        public Report(String p1, String p2, String p3) {
            this.part1 = p1;
            this.part2 = p2;
            this.part3 = p3;
        }
    }
}

This approach allows efficient use of threads and does not block the main thread, waiting for each service separately. You can also handle errors and timeouts using CompletableFuture methods.