如何使主线程等待所有线程完成

Question

I have a program in which I've created 10 threads to run. I want the main thread to wait until everything has finished before it completes. I'm trying to use the .join command but it doesn't seem to be working. Additionally, I'm trying to display the runtime of all the threads but it isn't working correctly. Below is my code.

//File: CohanThread.java
//Author: Ryan A. Cohan
//Date: August 4, 2016
//Purpose: To create, run, and analyze a thread based program. IOBound runs IO intensive
//operations in the form of printing 1000 times to the console. CPUBound runs CPU
//intensive operations by computing an equation and printing 1000 times.

package cohanthread;

import java.text.*;

public class CohanThread{

    public static void main(String args[]) throws InterruptedException{
        NumberFormat formatter = new DecimalFormat("#0.00000");

        IOBound io1 = new IOBound();
        IOBound io2 = new IOBound();
        IOBound io3 = new IOBound();
        IOBound io4 = new IOBound();
        IOBound io5 = new IOBound();

        CPUBound cpu1 = new CPUBound();
        CPUBound cpu2 = new CPUBound();
        CPUBound cpu3 = new CPUBound();
        CPUBound cpu4 = new CPUBound();
        CPUBound cpu5 = new CPUBound();

        long scheduleStart = System.currentTimeMillis();
        io1.start();
        io2.join();
        io3.join();
        io4.join();
        io5.join();
        cpu1.join();
        cpu2.join();
        cpu3.join();
        cpu4.join();
        cpu5.join();
        long scheduleEnd = System.currentTimeMillis();
        System.out.println("Runtime of all threads: " + formatter.format((scheduleEnd - scheduleStart) / 1000d));
        System.out.println("Processes complete.");
    }
}

class IOBound extends Thread{
    NumberFormat formatter = new DecimalFormat("#0.00000");
    @Override
    public void run(){
        long start = System.currentTimeMillis();
        for(int i = 0; i < 1000; i++){
            System.out.println("Thread number is: " + i);
        }
        long end = System.currentTimeMillis();
        System.out.println("IO Thread runtime: " + formatter.format((end - start) / 1000d));
    }
}

class CPUBound extends Thread{
    NumberFormat formatter = new DecimalFormat("#0.00000");
    @Override
    public void run(){
        String binary = "";
        long start = System.currentTimeMillis();
        for(int i = 0; i < 1000; i++){
            while(i > 0){
                int remainder = i % 2;
                binary = remainder + binary;
                i = i / 2;
            }
            System.out.println("Binary number: " + binary);
            long end = System.currentTimeMillis();
            System.out.println("CPU Thread runtime: " + formatter.format((end - start) / 1000d));
        }
    }
}

Answer 1

Join does not start computation, you must start all threads and use join to wait for completion...

https://docs.oracle.com/javase/tutorial/essential/concurrency/join.html

Use this main....

public static void main(String args[]) throws InterruptedException{
    NumberFormat formatter = new DecimalFormat("#0.00000");

    IOBound io1 = new IOBound();
    IOBound io2 = new IOBound();
    IOBound io3 = new IOBound();
    IOBound io4 = new IOBound();
    IOBound io5 = new IOBound();

    CPUBound cpu1 = new CPUBound();
    CPUBound cpu2 = new CPUBound();
    CPUBound cpu3 = new CPUBound();
    CPUBound cpu4 = new CPUBound();
    CPUBound cpu5 = new CPUBound();

    long scheduleStart = System.currentTimeMillis();
    io1.start();
    io2.start();
    io3.start();
    io4.start();
    io5.start();
    cpu1.start();
    cpu2.start();
    cpu3.start();
    cpu4.start();
    cpu5.start();


    io1.join();
    io2.join();
    io3.join();
    io4.join();
    io5.join();
    cpu1.join();
    cpu2.join();
    cpu3.join();
    cpu4.join();
    cpu5.join();
    long scheduleEnd = System.currentTimeMillis();
    System.out.println("Runtime of all threads: " + formatter.format((scheduleEnd - scheduleStart) / 1000d));
    System.out.println("Processes complete.");
}

Answer 2

you can use java java.util.concurrent package;

public static void main(String[] args) {
        ExecutorService executor = Executors.newFixedThreadPool(10);
        List<Future<?>> futureList = new ArrayList<>();
        futureList.add(executor.submit(new IOBound()));
        futureList.add(executor.submit(new IOBound()));
        futureList.add(executor.submit(new IOBound()));
        futureList.add(executor.submit(new IOBound()));
        futureList.add(executor.submit(new IOBound()));

        futureList.add(executor.submit(new CPUBound()));
        futureList.add(executor.submit(new CPUBound()));
        futureList.add(executor.submit(new CPUBound()));
        futureList.add(executor.submit(new CPUBound()));
        futureList.add(executor.submit(new CPUBound()));

        for (Future future:futureList) {
            try {
                //this method will blocking until task finished
                future.get();
            } catch (InterruptedException e) {
                e.printStackTrace();
            } catch (ExecutionException e) {
                e.printStackTrace();
            }
        }
    }

Answer 3

ExecutorService is the way to go!!

You can use the executor service here with list of threads to be executed.

I have updated the program to use Executor Service.

import java.text.*;
import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

public class CohanThread{

    public static void main(String args[]) throws InterruptedException{
        NumberFormat formatter = new DecimalFormat("#0.00000");

        ExecutorService es = Executors.newFixedThreadPool(5);
        List<Callable<Object>> threadList = new ArrayList<Callable<Object>>();

        IOBound io1 = new IOBound();
        IOBound io2 = new IOBound();
        IOBound io3 = new IOBound();
        IOBound io4 = new IOBound();
        IOBound io5 = new IOBound();
        threadList.add(Executors.callable(io1));
        threadList.add(Executors.callable(io2));
        threadList.add(Executors.callable(io3));
        threadList.add(Executors.callable(io4));
        threadList.add(Executors.callable(io5));

        CPUBound cpu1 = new CPUBound();
        CPUBound cpu2 = new CPUBound();
        CPUBound cpu3 = new CPUBound();
        CPUBound cpu4 = new CPUBound();
        CPUBound cpu5 = new CPUBound();
        threadList.add(Executors.callable(cpu1));
        threadList.add(Executors.callable(cpu2));
        threadList.add(Executors.callable(cpu3));
        threadList.add(Executors.callable(cpu4));
        threadList.add(Executors.callable(cpu5));

        long scheduleStart = System.currentTimeMillis();
        List<Future<Object>> results = es.invokeAll(threadList);
        long scheduleEnd = System.currentTimeMillis();
        System.out.println("Runtime of all threads: " + formatter.format((scheduleEnd - scheduleStart) / 1000d));
        System.out.println("Processes complete.");
    }
}

class IOBound extends Thread{
    NumberFormat formatter = new DecimalFormat("#0.00000");
    @Override
    public void run(){
        long start = System.currentTimeMillis();
        for(int i = 0; i < 1000; i++){
            System.out.println("Thread number is: " + i);
        }
        long end = System.currentTimeMillis();
        System.out.println("IO Thread runtime: " + formatter.format((end - start) / 1000d));
    }
}

class CPUBound extends Thread{
    NumberFormat formatter = new DecimalFormat("#0.00000");
    @Override
    public void run(){
        String binary = "";
        long start = System.currentTimeMillis();
        for(int i = 0; i < 1000; i++){
            while(i > 0){
                int remainder = i % 2;
                binary = remainder + binary;
                i = i / 2;
            }
            System.out.println("Binary number: " + binary);
            long end = System.currentTimeMillis();
            System.out.println("CPU Thread runtime: " + formatter.format((end - start) / 1000d));
        }
    }
}

Note that the List<Future<Object>> answers = es.invokeAll(threadList); will wait untill all the threads have completed execution.

Hope it helps. Happy Coding Cheers!