Java servlet with threads send response to client - multithreading

Question

I have an HTML page with 3 input fields, when the user clicks the submit button, a AJAX request is made using jQuery to a servlet with the conetnts of the 3 fields.
When the requests arrives in the server (my servlet), I need to create a thread for each input field, and manage this thread pool, put them in order.
For example; If thread 1 finishes earlier, then thread 2 should print the response to the client, before the next thread. I'm currently using a sleep method in each thread with a random sleep time, to determine the time each thread should take.
On the client side, in the HTML, I've an element to show the response.
To send the information to the client, I just need to call PrintWriter.write on the servlet side.
The problem that I'm having is that the information of each thread appears at client side at the same time.
The output only appears on client side after all the threads have finished.
For example; I have thread a which took 200ms, a thread b which took 300ms, and a thread c which took 100ms.
The result on client side will only appear after 300ms, for example the result would be the string "cab".
So I have the following JavaScript function on my HTML page:
$.ajax({
type : 'get',
url : 'ServletResponse2',
data : {
"name1" : name1,"name2" : name2, "name3" : name3
},
success : function(xhr,data) {
console.log("xhr:"+xhr);
console.log("xhr text:"+xhr.responseText);
console.log("data:"+data);
console.log("data2:"+data.data);
$("#listResponse2").append("<li>" + xhr);
$("#listResponse2").append("</li>");
},
complete : function(xhr,data) {
response = xhr.responseText;
console.log("exception:"+response);
$("#listResponse").append("<li>" + response);
$("#listResponse").append("</li>");
},
error : function(e) {
console.log("exception:"+e);
}
});
Then, in the servlet, I have this get method calling a executor service, which calls a Class Worker Thread which implements Runnable:
protected void doGet(HttpServletRequest request,HttpServletResponse response)
{
ExecutorService executor= Executors.newFixedThreadPool(3);
PrintWriter out=null;
response.setCharacterEncoding("UTF-8");
String name1 = request.getParameter("name1");
String name2 = request.getParameter("name2");
String name3 = request.getParameter("name3");
String name="";
for (int i = 1; i < 4; i++) {
name=request.getParameter("name"+i);
try {
out = response.getWriter();
Runnable worker = new WorkerThread("" + name,response);
executor.execute(worker);
} catch (IOException e) {
e.printStackTrace();
}
}
executor.shutdown();
while (!executor.isTerminated()) {
}
System.out.println("Finished all threads");
out.print(name);
}
In the WorkerThread.java, I created a run() method which is writing the response for each servlet, I even used the out.flush() and response.flushbuffer() methods.
public class WorkerThread implements Runnable {
private String command;
PrintWriter out;
HttpServletResponse response;
public WorkerThread(String s,HttpServletResponse sresponse){
this.command=s;
this.response=sresponse;
}
#Override
public void run() {
System.out.println(Thread.currentThread().getName()+" Start. Command = "+command);
try {
out = response.getWriter();
out.write(command);
out.flush();
response.flushBuffer();
} catch (IOException e) {
e.printStackTrace();
}
processCommand();
System.out.println(Thread.currentThread().getName()+" End.");
}
private void processCommand() {
try {
Long time=(long)(Math.random() * 5000);
Thread.sleep(time);
System.out.println("Thread.getName():"+Thread.currentThread().getName()+"|comand:"+command+"|time:"+ time);
} catch (InterruptedException e) {
e.printStackTrace();
}
}
#Override
public String toString(){
return this.command;
}
}
I also tried to use a Callabe in the executor service instead of a Runnable, but the result was the same. Can anyone tell me what I'm doing wrong, and why the result of the threads is arriving at the same time to the client?

Answer 1

Well no matter what you do in your servlet you are using HTTP protocol between a client, the browser and a server, the java application.
The protocole says : browser prepares and send a request, and the servers gets the request, and elaborates and send a response : one response per request.
The three threads write in the same reponse. Hopefully, your the 3 threads run one at a time and each write little to the response, if not you could have output of threads intermixed. And your servlet kindly waits the end of the threads to terminate, if not the servlet container would have closed the response before the threads have time to write anything.
You did not say what you were trying to achieve, I suppose you wanted to test asynchronous loading of information. For that you need multiple requests : one to initialize the operation, and one for each piece of information, the servlet using the session to register what it has transmitted and what remains, or the client side identifying the pieces it wants. But a detailed workthrough is far beyond what I can put in this answer ...
In fact when in a browser you see pages coming pieces by pieces it is exactly what happens : a first request for the HTML part, then optionally others for css, jsp, images, videos.