How to execute multiple sql query parallel in java - concurrency

Question

I have 3 method which is returning a List of result and my sql query execute in each method and returning a list of result .I want to execute all 3 method parallel so it will not wait to complete of one and another . I saw one stachoverflow post but its not working.
that link is [How to execute multiple queries in parallel instead of sequentially?
[Execute multiple queries in parallel via Streams
I want to solve using java 8 features.
But the above link how I can call multiple method please tell me.

Answer 1

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

Answer 2

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