In my previous articles, I discussed the Spring Async concept
and How to use it effectively. If you want to revisit, below are the
links, Also for new visitors I recommend, please go through previous
parts after reading this one.
Part 1:: How Spring
Async work internally, and how to use wisely so it assigns a task to a new
thread.
Part2:: How to
handles Exception, If something goes wrong while executing a task.
In this part, we will discuss How Spring Async work with the
web.
I am very excited to share an experience with you about Spring
Async and HttpRequest, as an interesting incident happened in one of my
projects, and I believe by sharing it I can save some valuable time of yours in
future.
Let me try to depict the scenario in Crisp word,
Objective: The objective was to pass some information from UI to a backend controller,
which will do some work and eventually calls an async mail service for triggers
a mail.
One of my Juniors did the following code(Try to replicate the
code intention with below code snippet, not the actual code ), can you spot
where the problem lies?
The Controller:: Which
collect information from UI as a form HTTP Servelet request, do some
operation and pass it to async Mail service.
package com.example.demo;
import javax.servlet.http.HttpServletRequest;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RestController;
@RestController
public class GreetController {
@Autowired
private AsyncMailTrigger greeter;
@RequestMapping(value = "/greet", method =
RequestMethod.GET)
public String greet(HttpServletRequest request) throws Exception
{
String name = request.getParameter("name");
greeter.asyncGreet(request);
System.out.println(Thread.currentThread() + " Says Name is
" + name);
System.out.println(Thread.currentThread().getName() + "
Hashcode" + request.hashCode());
return name;
}
}
The Async Mail Service, I marked it as
@Component you can easily change it to @Service. Here I have one method called
asyncGreet which takes the HttpRequest, fetch the information from there and
say, trigger the mail(this part is omitted for simplicity). Notice I put a
Thread.sleep() here for a purpose, I will discuss the same later.
package com.example.demo;
import java.util.Map;
import javax.servlet.http.HttpServletRequest;
import org.springframework.scheduling.annotation.Async;
import org.springframework.stereotype.Component;
@Component
public class AsyncMailTrigger {
@Async
public void asyncGreet(HttpServletRequest request) throws
Exception {
System.out.println("Trigger mail in a New Thread ::
" +
Thread.currentThread().getName());
System.out.println(Thread.currentThread().getName() + "
greets before sleep" + request.getParameter("name"));
Thread.sleep(1000);
System.out.println(Thread.currentThread().getName() + "
greets" + request.getParameter("name"));
System.out.println(Thread.currentThread().getName() + "
Hashcode" + request.hashCode());
}
}
Now the main class
package com.example.demo;
import org.springframework.boot.SpringApplication;
import
org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.scheduling.annotation.EnableAsync;
@SpringBootApplication
@EnableAsync
public class SpringAsyncWebApplication {
public static void main(String[] args) {
SpringApplication.run(SpringAsyncWebApplication.class, args);
}
}
If I run this program output will very similar
to below
Thread[http-nio-8080-exec-1,5,main] Says Name is Shamik
http-nio-8080-exec-1 Hashcode 821691136
Trigger mail in a New Thread:: task-1
task-1 greets before sleep Shamik
task-1 greets null task-1 Hashcode 821691136
Pay attention to the output, the request has the information
still before sleep but after that, it magically disappears? Strange
isn't? But It is the same request object hashcode proves the same.
What happened ? what is the reason behind the disappearance of
the information from Request? That was happening to my junior, the mail
recipients, recipients name disappear from the request and mail is not
triggered.
Let's put Sherlock hats to
investigate the problem.
It is a very common problem with the request, to understand the
problem have a look, how a request lifecycle works.
The request is created by the servlet container right before the
call to servlet service method, In Spring then request passes through
dispatcher servlet, in the Dispatcher servlet identifies the controller by
request mapping and calls the desired method in the controller, and when the
request has been served, servlet container either delete the request object or
reset the state of the request object. (This totally depends on the container
implementation, actually it maintains a pool of request). However, I am not
going to the deep dive how container maintains the request object.
But keep one thing in mind once the request has been served and
response is committed, container reset its state or destroy the request
object.
Now put Spring Async part into the consideration, What async
did, it picks one thread from the thread pool and assigns the task to it,
In our case, we pass the request object to the async thread and in the
asyncGreet method, we are trying to extract info directly from the request.
But as this is async our main thread (Controller part)
will be not waiting for this thread to complete, so it's print the statement
and commits the response, and refresh the state of the request object.
Ironically we pass the request object
directly to the async thread, still, the point where the response is not committed
in the main thread, request holds the data, I explicitly put a sleep a statement so in main thread response can be committed and refresh the
request state, so after sleep we experience there is no data in the request, it
vanishes, a great experiment to prove the incident.
What we will learn from this
experiment?
Never pass Request object or any
object related to Request/Response(headers), directly while using async you
never know when your response will be committed and refresh the state If you do
you will face an intermittent error.
What can be done?
If you need to pass a bunch of information from request create a
value object and set the information and pass the value object to Spring Async,
in this manner, you can create a concrete solution.
RequestVO Object
package com.example.demo;
public class RequestVO {
String name;
public String getName() {
return name;
}
public void setName(String name) {
this.name = name;
}
}
Async Mail service
package com.example.demo;
import java.util.Map;
import javax.servlet.http.HttpServletRequest;
import org.springframework.scheduling.annotation.Async;
import org.springframework.stereotype.Component;
@Component
public class AsyncMailTrigger {
@Async
public void asyncGreet(RequestVO reqVO) throws Exception {
System.out.println("Trigger mail in a New Thread ::
" +
Thread.currentThread().getName());
System.out.println(Thread.currentThread().getName() + "
greets before sleep" + reqVO.getName());
Thread.sleep(1000);
System.out.println(Thread.currentThread().getName() + "
greets" + reqVO.getName());
}
}
Greet Controller
package com.example.demo;
import javax.servlet.http.HttpServletRequest;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.PathVariable;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RequestMethod;
import org.springframework.web.bind.annotation.RestController;
@RestController
public class GreetController {
@Autowired
private AsyncMailTrigger greeter;
@RequestMapping(value = "/greet", method =
RequestMethod.GET)
public String greet(HttpServletRequest request) throws Exception
{
String name = request.getParameter("name");
RequestVO vo = new RequestVO();
vo.setName(name);
//greeter.asyncGreet(request);
greeter.asyncGreet(vo);
System.out.println(Thread.currentThread() + " Says Name is
" + name);
System.out.println(Thread.currentThread().getName() + "
Hashcode" + request.hashCode());
return name;
}
}
Output
Thread[http-nio-8080-exec-1,5,main] Says Name is Shamik
http-nio-8080-exec-1 Hashcode 1669579896
Trigger mail in a New Thread:: task-1
task-1 greets before sleep Shamik
task-1 greets Shamik
Conclusion: Hope you enjoyed the article If you have questions, please
put your questions on the comment box.
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