Multithreaded instance code (Demo)

What is multithreading ？

When introducing multithreading , We first need to know what threads are , And to understand threads, you have to understand processes .
1. process ： One of the programs being executed , Each process execution has an execution order , This sequence is an execution path , Or a control unit .
2. Threads ： An independent control unit in the process , Threads are controlling the execution of processes . At least one thread in a process .
3. Multithreading ： There is more than one thread in a process .
eg： For example, we drive from Beijing to Shanghai , The process can be understood as we are on our way to Shanghai , Going by plane is equivalent to a process , We can not only fly but also take high-speed rail , Self driving, etc. This is the multithreading we mentioned above .

principle ：
Multithreading is to apply the principle of concurrent execution mechanism in the operating system to a program , Divide a program into several subtasks , Multiple subtasks are executed concurrently , Every task is a thread .

Multithreading is a concurrent execution mechanism .

advantage
1、 Convenient communication and data exchange

2、 Make more efficient use of CPU

3、 The difference between thread and process

Threads Is an execution path in the process , Share a memory space , You can switch between threads freely , Concurrent execution . One process is the least There is a thread , Threads are actually further divided on the basis of processes , After a process starts , Several execution paths inside can be divided into several threads .

process An application running in memory , Each process has a separate memory space

Why should we use multithreading ？

In order to make better use of CPU Resources for , If there is only one thread , When we have multiple tasks, we must wait for the last task to be completed . Multithreading can perform other tasks while the main thread executes without waiting .
Data cannot be shared between processes , however Threads can .

The system creation process needs to reallocate system resources for this process , Creating threads is less expensive .
Java Language built-in multithreading support , To simplify the Java Multithreaded programming .

CompletionService What is it? ？
Callable+Future Can achieve more than one task Parallel execution , But if you encounter the previous task When the execution is slow, you need to block and wait for the previous task After execution task To achieve results .

CompletionService Its main function is to generate tasks on one side , While getting the return value of the task . Separate the two things , Tasks don't block each other , You can get the first result after the first execution , No longer rely on task order .

He has only one implementation class ：ExecutorCompletionService

CompletionService principle
Internally through the blocking queue +FutureTask, The task is completed first, and the priority can be obtained , That is, the results are sorted according to the order of completion , There is a first in first out blocking queue inside , Used to save completed execution of Future, By calling its take Method or poll Method can get a completed execution of Future, And then by calling Future Interface implementation class get Method to get the final result .

CompletionService Application scenarios of
When you need to submit asynchronous tasks in batch, we suggest that you use CompletionService：CompletionService Put thread pool Executor And blocking queues BlockingQueue It's a combination of functions , It can make the management of batch asynchronous tasks easier .

CompletionService It can make the execution results of asynchronous tasks orderly ： First of all, execute the first in the blocking queue , Take advantage of this feature , You can easily achieve the order of subsequent processing , Avoid unnecessary waiting , At the same time, it can also quickly realize such as Forking Cluster Such a need .

Thread pool isolation ：CompletionService Support to create thread pools by yourself , This isolation avoids the risk of several particularly time-consuming tasks dragging down the entire application .

Although the use of thread pool improves the overall execution efficiency , But traverse these Future, call Future Interface implementation class get Method is blocked , This and the present Future When the associated computing task is actually completed ,get Method to return the result , If the current calculation task is not completed , And there are others Future The associated calculation task has been executed , A lot of waiting time will be wasted , So it's best to get the first result when traversing , This saves a lot of waiting time .

and ExecutorCompletionService Can achieve this effect , It has a first in, first out blocking queue inside , Used to save completed execution of Future, By calling its take Method or poll Method can get a completed execution of Future, And then by calling Future Interface implementation class get Method to get the final result

ExecutorCompletionService Realized CompletionService Interface , stay CompletionService The following methods are defined in the interface ：

Future submit(Callable task): To submit a Callable Types of tasks , And return the task execution results associated with Future;

Future submit(Runnable task,V result): To submit a Runnable Types of tasks , And return the task execution results associated with Future;

Future take(): Get and remove the first completed task from the internal blocking queue , Blocking , Until something's done ;

Future poll(): Get and remove the first completed task from the internal blocking queue , If not, return null, Don't block ;

Future poll(long timeout, TimeUnit unit): Get and remove the first completed task from the internal blocking queue , The blocking time is timeout, If not, return null;

package com.example.yanwc;

import java.util.Random;
import java.util.concurrent.*;

public class Deadlock {



    public static void main(String[] args) throws ExecutionException, InterruptedException {

        //  Creating a thread pool 
        ExecutorService executor = Executors.newFixedThreadPool(5);
        //  establish CompletionService
        CompletionService<Integer> cs = new ExecutorCompletionService<>(executor);
        //  Turn on 5 A mission 
        for (int i = 0; i < 5; i++) {
            cs.submit(new CompletionTask());
        }
        //  Asynchronously save the inquiry result to the database 
        for (int i = 0; i < 5; i++) {
            System.out.println(Thread.currentThread().getName() + "  Get task results ：" + cs.take().get());
        }
    }
}

/** *  Custom tasks  */
class CompletionTask implements Callable<Integer> {
    @Override
    public Integer call() throws Exception {
        //  Get the random world 
        int randomSleepTime = new Random().nextInt(10) * 1000;
        //  Start execution 
        System.out.println(Thread.currentThread().getName() + "  Start execution , The current task needs to wait " + randomSleepTime + " millisecond ");
        //  Thread waiting , Simulation is in progress 
        TimeUnit.MILLISECONDS.sleep(randomSleepTime);
        //  End to perform 
        System.out.println(Thread.currentThread().getName() + " End to perform , The current task needs to wait " + randomSleepTime + " millisecond ");
        //  Return to the time he waited 
        return randomSleepTime;
    }
}

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com.example.yanwc.Deadlock
pool-1-thread-4  Start execution , The current task needs to wait 2000 millisecond 
pool-1-thread-3  Start execution , The current task needs to wait 0 millisecond 
pool-1-thread-3 End to perform , The current task needs to wait 0 millisecond 
pool-1-thread-2  Start execution , The current task needs to wait 2000 millisecond 
pool-1-thread-5  Start execution , The current task needs to wait 2000 millisecond 
pool-1-thread-1  Start execution , The current task needs to wait 7000 millisecond 
main  Get task results ：0
pool-1-thread-2 End to perform , The current task needs to wait 2000 millisecond 
pool-1-thread-4 End to perform , The current task needs to wait 2000 millisecond 
pool-1-thread-5 End to perform , The current task needs to wait 2000 millisecond 
main  Get task results ：2000
main  Get task results ：2000
main  Get task results ：2000
pool-1-thread-1 End to perform , The current task needs to wait 7000 millisecond 
main  Get task results ：7000

package com.example;

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.*;

public class DeadLock2 {

    public static void main(String[] args) throws ExecutionException, InterruptedException {
        Integer i = 1;
        Integer ii = 2;
        Integer iii = 3;
        Integer iiii = 4;
        List<Integer> list = new ArrayList<>();
        // Pass relevant parameters into list aggregate 
        list.add(i);
        list.add(ii);
        list.add(iii);
        list.add(iiii);

        System.out.println(" Before conversion " + list);
        // The thread returns the object 
        List<Future<Integer>> l = new ArrayList<>();
        //  Create a thread pool waiting queue 
        BlockingQueue q = new LinkedBlockingQueue();
        // Create a thread pool and set up related parameters 
        //  Creating thread objects   Enable multithreading 
        ThreadPoolExecutor executor = new ThreadPoolExecutor(4,6,3, TimeUnit.SECONDS,q);
        // Threads   To carry out a task 
        CompletionService<Integer> completionService = new ExecutorCompletionService(executor);
        //for loop 
        for (Integer integer : list) {
            Future<Integer> future = completionService.submit(wc(integer));
            l.add(future);
        }

        List<Integer> ll = new ArrayList<>();
        // Instantiate data 
        for (Future<Integer> future : l) {
            ll.add(future.get());
        }
        System.out.println(" After the transformation " + ll);

    }

    private static Callable<Integer> wc(Integer integer) {
        Callable<Integer> callable = new Callable<Integer>() {
            @Override
            public Integer call() throws Exception {
                return integer + 1;
            }
        };
        return callable;

    }


}

Running results ：

 Before conversion [1, 2, 3, 4]
 After the transformation [2, 3, 4, 5]

Construction method analysis ：
CompletionService Construction method of source code analysis
/**

• Construction method of incoming thread pool
  */

public ExecutorCompletionService(Executor executor) {

    //  Thread pool is null, Throw an exception directly 
    if (executor == null)
        throw new NullPointerException();
    //  Assign a value to the thread pool used 
    this.executor = executor;
    //  Judgment is not AbstractExecutorService Subclasses of 
    this.aes = (executor instanceof AbstractExecutorService) ?
        (AbstractExecutorService) executor : null;
    //  Default only LinkedBlockingQueue Store the results 
    this.completionQueue = new LinkedBlockingQueue<Future<V>>();
}

/** *  The construction method of incoming thread pool and the queue used  */
public ExecutorCompletionService(Executor executor,
                                 BlockingQueue<Future<V>> completionQueue) {
    //  Thread pool is null Or the queue used is empty , Throw an exception directly 
    if (executor == null || completionQueue == null)
        throw new NullPointerException();
    //  Assign a value to the thread pool used 
    this.executor = executor;
    //  Judgment is not AbstractExecutorService Subclasses of 
    this.aes = (executor instanceof AbstractExecutorService) ?
        (AbstractExecutorService) executor : null;
    //  Use the incoming queue to store the results 
    this.completionQueue = completionQueue;
}
CompletionService Of submit Method source code analysis 
/** * CompletionService Task submission method  */
public Future<V> submit(Runnable task, V result) {
    //  Task is empty , Throws a null pointer exception 
    if (task == null) throw new NullPointerException();
    //  Create a specific task 
    RunnableFuture<V> f = newTaskFor(task, result);
    //  Perform this task ： Encapsulate the , Rewrote done Method , Put the running results in the queue .
    executor.execute(new QueueingFuture(f));
    //  Return the result of execution 
    return f;
}

/** * QueueingFuture class ： He rewrote it done Method  */
private class QueueingFuture extends FutureTask<Void> {
    QueueingFuture(RunnableFuture<V> task) {
        super(task, null);
        this.task = task;
    }
    //  Execute the completed method ： Put the running results into the queue .
    protected void done() { completionQueue.add(task); }
    private final Future<V> task;
}
CompletionService Of take Method source code analysis 
/** * CompletionService Out of the team take Method ： Directly call the blocking out method of the queue . */
public Future<V> take() throws InterruptedException {
    return completionQueue.take();
}