线程池为什么要有它：
线程创建要开辟虚拟机栈，释放线程要垃圾回收的。
server端要并发访问数据库的。
服务器启动有线程池放着。
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线程池的概念：
1.任务队列
2.拒绝策略(抛出异常，直接丢弃，阻塞，放在临时队列)
3.初始化值init(min),初始线程池大小
3.active
5.max线程池线程的最大个数
min<=active<=max
quartz:定时任务
代码：
package chapter13;
import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;
public class SimpleThreadPoolMy {
private final int size;
private final static int DEFAULT_SIZE = 10;
private static volatile int seq = 0;
private final static String THREAD_PREFIX = "SIMPLE_THREAD_POOL-";
private final static ThreadGroup GROUP = new ThreadGroup("Pool_Group");
private final static List<WorkerTask> THREAD_QUEUE = new ArrayList<>();
/**
* task queue
*/
private final static LinkedList<Runnable> TASK_QUEUE = new LinkedList<>();
public SimpleThreadPoolMy() {
this(DEFAULT_SIZE);
}
public SimpleThreadPoolMy(int size) {
this.size = size;
init();
}
private void init() {
for (int i = 0; i < size; i++) {
createWorkTask();
}
}
private void createWorkTask() {
WorkerTask task = new WorkerTask(GROUP, THREAD_PREFIX + (seq++));
task.start();
THREAD_QUEUE.add(task);
}
private enum TaskState {
FREE, RUNNING, BLOCKEED, DEAD
}
public void submit(Runnable runnable) {
synchronized (TASK_QUEUE) {//别的方法有读的操作，这里有写的操作，所以你要加锁的
TASK_QUEUE.addLast(runnable);
TASK_QUEUE.notifyAll();
}
}
/**
* Thread in ThreadPool
*/
private static class WorkerTask extends Thread {//为什么定义为private呢就是为了不想让别人知道我是什么
private volatile TaskState taskState = TaskState.FREE;
public WorkerTask(ThreadGroup group, String name) {//构造方法，调用了Thread的构造
super(group, name);
}
public TaskState getTaskState() {
return this.taskState;
}
public void close() {
this.taskState = TaskState.DEAD;
}
public void run() {//重写外汇返佣保证执行完之后不能挂掉
OUTER:
while (this.taskState != TaskState.DEAD) {//当前的线程没有死掉
Runnable runnable;
synchronized (TASK_QUEUE) {//去任务队列调取任务
while (TASK_QUEUE.isEmpty()) {
try {
taskState = TaskState.BLOCKEED;
TASK_QUEUE.wait();//没有任务就释放锁，这个线程在TASK_QUEUE上等待 wait会被打断 打断的话就break出来
} catch (InterruptedException e) {
break OUTER;//https://blog.csdn.net/zhaoheng2017/article/details/78385973 被打断了就接着去调取任务
}
}
runnable = TASK_QUEUE.removeFirst();//队列 先进先出 任务队列只被消费一次 比如数据库线程池，取出来我执行连接就可以了
}
if (runnable != null) {
taskState = TaskState.RUNNING;
runnable.run();
taskState = TaskState.FREE;
}
}
}
}
}
原文链接：https://blog.csdn.net/qq_28764557/article/details/103575987