为什么要引入线程池

• 降低资源消耗。通过重复利用已创建的线程降低线程创建和销毁造成的消耗
• 提高响应速度。当任务到达时，任务可以不需要等到线程创建就能立即执行
• 提高线程的可管理性。线程是稀缺资源，如果无限制的创建，不仅会消耗系统的资源，还会降低系统的稳定性，使用线程池可以进行统一的分配，调优和监控

Java中几种默认的线程池

• 如何创建线程池
  
  
• JUC包下Executors提供了几种线程池

            //单一线程数，同时只有一个线程存活，但线程等待队列无界
            Executors.newSingleThreadExecutor();
            //线程可复用线程池，核心线程数是0，最大可创建的线程数为Interger.Max，线程复用存活时间为60s
            Executors.newCachedThreadPool();
            //固定线程数量的线程池
            Executors.newFixedThreadPool(int corePoolSize);
            //可执行定时任务，延迟任务的线程池
            Executors.newScheduledThreadPool(int corePoolSize);

• 线程池的重要方法
  
• 线程池状态流转
  
• 线程池的执行流程
  大家可以参考下面这篇博客，笔者觉得写得很通俗易懂。
  https://juejin.cn/post/6866054685044768782

封装一个高扩展性的线程池框架

• 支持任务优先级
• 支持线程池暂停、恢复、关闭
• 支持异步任务结果回调

package com.example.hilibrary.executor

import android.os.Handler
import android.os.Looper
import android.util.Log
import androidx.annotation.IntRange
import java.util.PriorityQueue
import java.util.concurrent.BlockingQueue
import java.util.concurrent.PriorityBlockingQueue
import java.util.concurrent.ThreadFactory
import java.util.concurrent.ThreadPoolExecutor
import java.util.concurrent.TimeUnit
import java.util.concurrent.atomic.AtomicLong
import java.util.concurrent.locks.Condition
import java.util.concurrent.locks.ReentrantLock

//支持按任务的优先级去执行，支持线程池暂停，恢复，异步结果主动回调主线程
object HCommonExecutor {
    private val TAG:String = "HCommonExecutor"
    private var isPaused: Boolean = false
    private var hiExecutor: ThreadPoolExecutor
    private var lock:ReentrantLock
    private var pauseCondition:Condition
    private val mainHandler = Handler(Looper.myLooper()!!)

    init {
        lock = ReentrantLock();
        pauseCondition = lock.newCondition()
        val cpuCount = Runtime.getRuntime().availableProcessors()
        val corePoolSize = cpuCount + 1
        val maxPoolSize = cpuCount * 2 + 1;
        val blockingQueue: PriorityBlockingQueue<out Runnable> = PriorityBlockingQueue()
        val keepAliveTime = 30L
        val unit = TimeUnit.SECONDS

        val seq = AtomicLong()
        val threadFactory = ThreadFactory {
            val thread = Thread(it)
            thread.name = "hi-executor-" + seq.getAndIncrement()
            return@ThreadFactory thread
        }
        hiExecutor = object : ThreadPoolExecutor(
            corePoolSize,
            maxPoolSize,
            keepAliveTime,
            unit,
            blockingQueue as BlockingQueue<Runnable>,
            threadFactory
        ){
            override fun beforeExecute(t: Thread?, r: Runnable?) {
                super.beforeExecute(t, r)
                if(isPaused){
                    lock.lock()
                    try {
                        pauseCondition.await()
                    }finally {
                        lock.unlock()
                    }
                }
            }

            override fun afterExecute(r: Runnable?, t: Throwable?) {
                super.afterExecute(r, t)
                //监控线程池耗时任务，线程创建数量，正在运行的数量
                Log.e(TAG,"已执行完的任务的优先级是： "+(r as PriorityRunnable).priority)
            }
        }
    }

    @JvmOverloads
    fun execute(@IntRange(from = 0, to = 10)priority:Int=0,runnable: Runnable){
        hiExecutor.execute(PriorityRunnable(priority, runnable))
    }

    @JvmOverloads
    fun execute(@IntRange(from = 0, to = 10)priority:Int=0,runnable: Callable<*>){
        hiExecutor.execute(PriorityRunnable(priority, runnable))
    }

    abstract class Callable<T>:Runnable{
        override fun run() {
            mainHandler.post { onPrepare() }

            val t = onBackground()

            mainHandler.post { onCompleted(t) }
        }

        open fun onPrepare(){
            //转菊花
        }

        abstract fun onBackground():T
        abstract fun onCompleted(t:T)
    }

    class PriorityRunnable(val priority:Int,val runnable:Runnable):Runnable,Comparable<PriorityRunnable>{
        override fun run() {
            runnable.run()
        }

        override fun compareTo(other: PriorityRunnable): Int {
            return if(this.priority<other.priority) 1 else if(this.priority>other.priority) -1 else 0
        }
    }

    @Synchronized
    fun pause(){
        isPaused = true
        Log.e(TAG,"hcexecutor is pause")
    }

    @Synchronized
    fun resume(){
        isPaused = false
        lock.lock()
        try {
            pauseCondition.signalAll()
        }finally {
            lock.unlock()
        }
        Log.e(TAG,"hcexecutor is resume")
    }
}