并发编程与高并发解决方案学习(线程池)

继承Thread的弊端

1.每次new Thread的时候都需要新建一个线程，性能差

2.线程缺乏统一管理，可能无限制的新建线程，相互竞争，有可能占用过多系统资源导致死机或者OOM

3.Thread类缺少更多功能，比如更多的执行、定期执行、线程中断。

线程池的好处

1.重用存在的线程，减少对象创建、消亡的开销、性能佳

2.可以有效的控制最大并发线程数，提高系统资源利用率，同时可以避免过多资源竞争，避免阻塞。

3.提供定时执行、定期执行、单线程、并发数控制等功能。

线程池-ThreadPoolExecutor

corePoolSize:核心线程数

maximumPoolSize:线程最大线程数

workQueue : 阻塞队列，存储等待执行的任务，很重要，会对线程池运行过程产生重大影响

※如果运行的线程数小于corePoolSize,直接创建新线程来处理任务，即使线程池中的其他线程是空闲的；

※如果线程池中的线程数量大于等于corePoolSize且小于maximumPoolSize的时候后，则只有当workQueue满的时候才创建新的线程来处理任务

※如果corePoolSize等于maximumPoolSize的时候，那么创建的线程池大小是确定的，这个时候如果有新任务提交如果workQueue没满的时候就把请求放在workQueue里面，等待有空闲的线程从里面取出任务来处理

如果运行的线程数目大于maximumPoolSize的时候，这个时候如果workQueue也已经满了那么需要通过拒绝策略处理任务。

keepAliveTime : 线程没有任何任务执行时，最多保持多久时间终止。

ThreadPoolExecutor执行execute方法分下面4种情况。
1）如果当前运行的线程少于corePoolSize，则创建新线程来执行任务（注意，执行这一步骤
需要获取全局锁）。
2）如果运行的线程等于或多于corePoolSize，则将任务加入BlockingQueue。
3）如果无法将任务加入BlockingQueue（队列已满），则创建新的线程来处理任务（注意，执
行这一步骤需要获取全局锁）。
4）如果创建新线程将使当前运行的线程超出maximumPoolSize，任务将被拒绝，并调用
RejectedExecutionHandler.rejectedExecution()方法。
ThreadPoolExecutor采取上述步骤的总体设计思路，是为了在执行execute()方法时，尽可能
地避免获取全局锁（那将会是一个严重的可伸缩瓶颈）。在ThreadPoolExecutor完成预热之后
（当前运行的线程数大于等于corePoolSize），几乎所有的execute()方法调用都是执行步骤2，而
步骤2不需要获取全局锁

源码分析

/**
     * Executes the given task sometime in the future.  The task
     * may execute in a new thread or in an existing pooled thread.
     *
     * If the task cannot be submitted for execution, either because this
     * executor has been shutdown or because its capacity has been reached,
     * the task is handled by the current {@code RejectedExecutionHandler}.
     *
     * @param command the task to execute
     * @throws RejectedExecutionException at discretion of
     *         {@code RejectedExecutionHandler}, if the task
     *         cannot be accepted for execution
     * @throws NullPointerException if {@code command} is null
     */
    public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
        /*
         * Proceed in 3 steps:
         *
         * 1. If fewer than corePoolSize threads are running, try to
         * start a new thread with the given command as its first
         * task.  The call to addWorker atomically checks runState and
         * workerCount, and so prevents false alarms that would add
         * threads when it shouldn't, by returning false.
         *
         * 2. If a task can be successfully queued, then we still need
         * to double-check whether we should have added a thread
         * (because existing ones died since last checking) or that
         * the pool shut down since entry into this method. So we
         * recheck state and if necessary roll back the enqueuing if
         * stopped, or start a new thread if there are none.
         *
         * 3. If we cannot queue task, then we try to add a new
         * thread.  If it fails, we know we are shut down or saturated
         * and so reject the task.
         */
        int c = ctl.get();
        if (workerCountOf(c) < corePoolSize) {
            if (addWorker(command, true))
                return;
            c = ctl.get();
        }
        if (isRunning(c) && workQueue.offer(command)) {
            int recheck = ctl.get();
            if (! isRunning(recheck) && remove(command))
                reject(command);
            else if (workerCountOf(recheck) == 0)
                addWorker(null, false);
        }
        else if (!addWorker(command, false))
            reject(command);
    }

unit : keepAliveTime的时间单位

threadFactory:线程工厂，用来创建线程

rejectHandler:当拒绝处理任务时的策略

ThreadPoolExecutor方法

execute()：提交任务，交给线程池执行

submit():提交任务，能够返回执行结果 execute + future

shutdown()关闭线程池，等待任务都执行完了

shutdownNow():关闭线程池，不等待任务执行完

getTaskCount():线程池已执行和未执行的任务总数

getCompletedTaskCount():已完成的任务数

getPoolSize():线程池当前的线程数

getActiveCount():当前线程池中正在执行任务的线程数量

newCachedThreadPool

import lombok.extern.slf4j.Slf4j;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

@Slf4j
public class ThreadPoolExample1 {
    public static void main(String[] args) {
        ExecutorService executorService = Executors.newCachedThreadPool();
        for (int i = 0; i < 10; i++) {
            final int index = i;
            executorService.execute(new Runnable() {
                @Override
                public void run() {
                    log.info("task:{}", index);
                }
            });
        }
        executorService.shutdown();
    }
}

newFixedThreadPool

import lombok.extern.slf4j.Slf4j;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

@Slf4j
public class ThreadPoolExample2 {

    public static void main(String[] args) {

        ExecutorService executorService = Executors.newFixedThreadPool(3);

        for (int i = 0; i < 10; i++) {
            final int index = i;
            executorService.execute(new Runnable() {
                @Override
                public void run() {
                    log.info("task:{}", index);
                }
            });
        }
        executorService.shutdown();
    }
}

newSingleThreadExecutor

import lombok.extern.slf4j.Slf4j;

import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;

@Slf4j
public class ThreadPoolExample3 {

    public static void main(String[] args) {

        ExecutorService executorService = Executors.newSingleThreadExecutor();

        for (int i = 0; i < 10; i++) {
            final int index = i;
            executorService.execute(new Runnable() {
                @Override
                public void run() {
                    log.info("task:{}", index);
                }
            });
        }
        executorService.shutdown();
    }
}

newScheduledThreadPool

import lombok.extern.slf4j.Slf4j;

import java.util.Date;
import java.util.Timer;
import java.util.TimerTask;
import java.util.concurrent.Executors;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.TimeUnit;

@Slf4j
public class ThreadPoolExample4 {

    public static void main(String[] args) {

        ScheduledExecutorService executorService = Executors.newScheduledThreadPool(1);

//        executorService.schedule(new Runnable() {
//            @Override
//            public void run() {
//                log.warn("schedule run");
//            }
//        }, 3, TimeUnit.SECONDS);

        executorService.scheduleAtFixedRate(new Runnable() {
            @Override
            public void run() {
                log.warn("schedule run");
            }
        }, 1, 3, TimeUnit.SECONDS);
//        executorService.shutdown();

        Timer timer = new Timer();
        timer.schedule(new TimerTask() {
            @Override
            public void run() {
                log.warn("timer run");
            }
        }, new Date(), 5 * 1000);
    }
}