How to execute multiple sql query parallel in java

Execute multiple queries in parallel via Streams works for your task. Here is a sample code which demonstrates it:

public static void main(String[] args) {
    // Create Stream of tasks:
    Stream<Supplier<List<String>>> tasks = Stream.of(
            () -> getServerListFromDB(),
            () -> getAppListFromDB(),
            () -> getUserFromDB());

    List<List<String>> lists = tasks
            // Supply all the tasks for execution and collect CompletableFutures
            .map(CompletableFuture::supplyAsync).collect(Collectors.toList())
            // Join all the CompletableFutures to gather the results
            .stream()
            .map(CompletableFuture::join).collect(Collectors.toList());
    System.out.println(lists);
}

private static List<String> getUserFromDB() {
    try {
        TimeUnit.SECONDS.sleep((long) (Math.random() * 3));
    } catch (InterruptedException e) {
        e.printStackTrace();
    }
    System.out.println(Thread.currentThread().getName() + " getUser");
    return Arrays.asList("User1", "User2", "User3");
}

private static List<String> getAppListFromDB() {
    try {
        TimeUnit.SECONDS.sleep((long) (Math.random() * 3));
    } catch (InterruptedException e) {
        e.printStackTrace();
    }

    System.out.println(Thread.currentThread().getName() + " getAppList");
    return Arrays.asList("App1", "App2", "App3");
}

private static List<String> getServerListFromDB() {
    try {
        TimeUnit.SECONDS.sleep((long) (Math.random() * 3));
    } catch (InterruptedException e) {
        e.printStackTrace();
    }

    System.out.println(Thread.currentThread().getName() + " getServer");
    return Arrays.asList("Server1", "Server2", "Server3");
}

The output is:

ForkJoinPool.commonPool-worker-1 getServer
ForkJoinPool.commonPool-worker-3 getUser
ForkJoinPool.commonPool-worker-2 getAppList
[[Server1, Server2, Server3], [App1, App2, App3], [User1, User2, User3]]

You can see that default ForkJoinPool.commonPool is used and each get* method is executed from separate thread in this pool. You just need to run your SQL queries inside these get* methods

The solution from Alex won't work in an expected way. CompletableFuture executes effects one after another. You will have to run parallel streams for this or use CompletableFuture with parallelStream