Java线程池在实际的应用开发中十分广泛。虽然Java1.5之后在JUC包中提供了内置线程池可以拿来就用，但是这之前仍有许多老的应用和系统是需要程序员自己开发的。因此，基于线程池的需求背景、技术要求了解线程池原理和实现，一方面可以更为深刻理解Java多线程开发，有助于解决业务系统中因为线程问题所产生的bug；另一方面也有利于对遗留代码的重构。
如果需要先行了解Java并发编程的基础知识，可参考以下随笔：
1. Java并发编程之线程创建和启动(Thread、Runnable、Callable和Future)
2. Java并发编程之线程生命周期、守护线程、优先级、关闭和join、sleep、yield、interrupt
线程池原理
所谓的线程池，跟JDBC连接池、代理池等一样，属于一种“池”的设计模式。在设计好的数据结构中存放一定数量的线程，并且根据任务需要自动调整线程数量的多少，直到峰值。具体说来，线程池需要满足若干条件：
1. 任务队列：用于缓存提交的任务
2. QueueSize：任务队列存放的Runnable任务实例的数量，需要有限制值防止内存溢出。
3. 线程数量管理：创建线程时初始的数量init；线程池自动扩充时最大的线程数量max；空闲时的活跃线程或核心线程数量core。三者满足init<=core<=max
4. 工作线程队列：用于存储工作线程，并统计工作线程的数量。
5. 任务拒绝策略：如果线程数量达到上限且任务队列已满，需要有拒绝策略通知任务提交者，这个在工程实践中非常重要。
6. 线程工厂：用于个性化定制线程，如设置守护线程、线程名称等。
7. Keepedalive时间：线程各个重要参数自动维护的时间间隔。
线程池原理图如下：
线程池实现
“模块设计，类图先行”。明确了线程池需要实现的功能之后，就可以画出线程池的草图了，核心接口及实现类如下：
ThreadPool接口
ThreadPool接口主要定义一个线程池的基本属性，如任务提交、初始容量、最大容量、核心容量等。实现代码如下：
public interface ThreadPool {
//submit tasks to thread pool
void execute(Runnable runnable);
//close pool
void shutdown();
//get the initial size of pool
int getInitSize();
//get the max size of pool
int getMaxSize();
//get the core size of pool
int getCoreSize();
//get the cache tasks queue of pool
int getQueueSize();
//get the active thread volume of pool
int getActiveCount();
//check if pool has been shutdown
boolean isShutdown();
}
RunnableQueue接口
这个接口的作用与BlockingQueue接口一样，用于存储提交的Runnable实现类任务。
public interface RunnableQueue {
//提交任务到队列
void offer(Runnable runnable);
//从队列中获取任务
Runnable take() throws InterruptedException;
//返回队列中任务数
int size();
}
ThreadFactory接口
定义了个性化创建线程的工厂方法
@FunctionalInterface
public interface ThreadFactory { Thread createThread(Runnable runnable); }
DenyPolicy接口
定义了线程池的拒绝策略，即当任务队列达到上限时，采取何种措施拒绝。注意接口内定义了内部类作为外围接口的实现类（该类自动为public和static，像这种嵌套类的实现，可查询《Java编程思想》）。
@FunctionalInterface
public interface DenyPolicy { void reject(Runnable runnable, ThreadPool threadPool);
//定义嵌套类作为拒绝策略的实现类
//1.拒绝并丢弃任务
class DiscardDenyPolicy implements DenyPolicy{ @Override
public void reject(Runnable runnable, ThreadPool threadPool) { }
} //2.拒绝并抛出自定义异常
class AbortDenyPolicy implements DenyPolicy{ @Override
public void reject(Runnable runnable, ThreadPool threadPool) {
throw new RunnableDenyException("The runnable " + runnable + " will abort.");
}
} //3.拒绝, 使用提交者所在线程来完成线程任务.
class RunnerDenyPolicy implements DenyPolicy{ @Override
public void reject(Runnable runnable, ThreadPool threadPool) { if(!threadPool.isShutdown()) {
runnable.run();
}
}
}
}
其实实现了自定义异常类RunnableDenyException：
public class RunnableDenyException extends RuntimeException {
private static final long serialVersionUID = 112311231231412L;
public RunnableDenyException(String message) {
super(message);
}
}
InternalTask实现类
Runnable的实现类，会使用到RunnableQueue，它的作用其实是封装了一个任务实例，把Runnable任务的run方法封装到自己的Run方法实现中，并且提供了一个stop方法，用于在线程池销毁或数量维护时停止当前线程。
public class InternalTask implements Runnable {
//组合一个RunnableQueue的引用
private final RunnableQueue runnableQueue;
//使用volatile关键字修饰开关变量
private volatile boolean running = true;
public InternalTask(RunnableQueue runnableQueue) {
this.runnableQueue = runnableQueue;
}
@Override
public void run() {
// if current task match "both running and isInterrupt" are true
// continue to take runnable from queue and run
while(running && !Thread.currentThread().isInterrupted()) {
try {
Runnable task = runnableQueue.take();
task.run();
} catch (Exception e) {
running = false;
break;
}
}
}
//停止线程的开关方法
public void stop() {
this.running = false;
}
}
到这里，一个基本线程池的骨架就搭好了，接下来主要是实现各接口，实现具体的方法。
1. 队列的实现类LinkedRunnableQueue
public class LinkedRunnableQueue implements RunnableQueue {
//设置队列上限
private final int limit;
//设置拒绝策略的引用
private final DenyPolicy denyPolicy;
//使用LinkedList作为队列的具体实现类
private final LinkedList<Runnable> runnableList = new LinkedList<>();
//设置线程池的引用
private final ThreadPool threadPool;
//构造方法时赋初始值
public LinkedRunnableQueue(int limit, DenyPolicy denyPolicy, ThreadPool threadPool) {
this.limit = limit;
this.denyPolicy = denyPolicy;
this.threadPool = threadPool;
}
@Override
public void offer(Runnable runnable) {
//使用同步锁, 确保入队的线程安全
synchronized (runnableList) {
//当达到队列上限, 调用拒绝策略;否则加入队尾, 并唤醒阻塞中的线程.
if(runnableList.size() >= limit) {
denyPolicy.reject(runnable, threadPool);
}else {
runnableList.addLast(runnable);
runnableList.notifyAll();
}
}
}
@Override
public Runnable take() throws InterruptedException {
synchronized (runnableList) {
while(runnableList.isEmpty()) {
try {
//如果队列中没有可执行任务, 线程挂起, 进入runnableList关联的monitor waitset中等待被唤醒
runnableList.wait();
} catch (InterruptedException e) {
//如果被中断, 需要抛出异常
throw e;
}
}
return runnableList.removeFirst();
}
}
@Override
public int size() {
synchronized (runnableList) {
//返回队列中的任务数量
return runnableList.size();
}
}
}
2. 线程工厂的实现
public class DefaultThreadFactory implements ThreadFactory {
//定义原子类的Integer作为线程组的计数
private static final AtomicInteger GROUP_COUNTER = new AtomicInteger(1);
//定义线程组对象
private static final ThreadGroup group = new ThreadGroup("MyThreadPool-"+ GROUP_COUNTER.getAndDecrement());
//定义生产的线程计数
private static final AtomicInteger COUNTER = new AtomicInteger(0);
@Override
public Thread createThread(Runnable runnable) {
return new Thread(group, runnable, "thread-pool-" + COUNTER.getAndDecrement());
}
}
3. 线程池的实现
线程池的实现相对比较复杂， 运用了多种设计模式的思想，核心的要点包括：
1. 使用私有内部类的方式来复用Thread类，防止向外暴露Thread类的方法；
2. 核心组成部分主要是LinkedList实现的任务队列和ArrayDeque实现的工作线程队列，构成了主要的存储主体。
3. 核心的扩容机制需要RunnableQueue + InternalTask + ThreadFactory的结合， 简单说来就是通过判定任务数是否达到阈值，然后增加工作线程的数量。
public class BasicThreadPool implements ThreadPool {
//为了不暴露Thread类的方法, 使用私有内部类WorkThread来继承Thread类
private WorkThread workThread;
//线程池的基本属性
private final int initSize;
private final int maxSize;
private final int coreSize;
private int activeCount;
//线程工厂引用
private final ThreadFactory threadFactory;
//队列引用
private final RunnableQueue runnableQueue;
//线程池销毁标识
private volatile boolean isShutdown = false;
//工作线程的队列, 使用ArrayDeque实现
private final Queue<ThreadTask> threadQueue = new ArrayDeque<>();
//定义了一个默认的拒绝策略
private final static DenyPolicy DEFAULT_DENY_POLICY = new DenyPolicy.DiscardDenyPolicy();
//定义了一个默认的工厂对象
private final static ThreadFactory DEFAULT_THREAD_FACTORY = new DefaultThreadFactory();
private final long keepAliveTime;
private final TimeUnit timeUnit;
//默认的构造器, 只需要传入初始容量, 最大容量, 核心容量和队列上限
public BasicThreadPool(int initSize, int maxSize, int coreSize, int queueSize) {
this(initSize, maxSize, coreSize, queueSize, DEFAULT_THREAD_FACTORY,
DEFAULT_DENY_POLICY,10,TimeUnit.SECONDS);
}
//完整构造器
public BasicThreadPool(int initSize, int maxSize, int coreSize, int queueSize, ThreadFactory threadFactory,
DenyPolicy denyPolicy,long keepAliveTime, TimeUnit timeUnit) {
this.workThread = new WorkThread();
this.initSize = initSize;
this.maxSize = maxSize;
this.coreSize = coreSize;
this.threadFactory = threadFactory;
this.runnableQueue = new LinkedRunnableQueue(queueSize, denyPolicy, this);
this.keepAliveTime = keepAliveTime;
this.timeUnit = timeUnit;
this.init();
}
//线程池的初始化方法, 在构造器中被调用, 用于启动工作线程
private void init() {
workThread.start();
for(int i = 0; i < initSize; i++) {
newThread();
}
}
//封装了工作线程的启动方法:
//1. 使用InternalTask封装RunnableQueue对象
//2. 通过工厂方法制造工作线程并启动
//3. 工作线程入队, 工作线程队列计数器+1
private void newThread() {
InternalTask internalTask = new InternalTask(runnableQueue);
Thread thread = this.threadFactory.createThread(internalTask);
ThreadTask threadTask = new ThreadTask(thread, internalTask);
threadQueue.offer(threadTask);
this.activeCount++;
thread.start();
}
//工作线程出队的方法
private void removeThread() {
ThreadTask threadTask = threadQueue.remove();
threadTask.internalTask.stop();
this.activeCount--;
}
//核心:通过内部类继承Thread方法, 设计了自动扩容的机制.
//为了防止过快增加到Max容量, 使用continue来退出循环
private class WorkThread extends Thread{
@Override
public void run() {
while(!isShutdown && !isInterrupted()) {
try {
timeUnit.sleep(keepAliveTime);
} catch (InterruptedException e) {
isShutdown = true;
break;
}
synchronized (this) {
if(isShutdown) {
break;
}
if(runnableQueue.size() > 0 && activeCount < coreSize) {
for(int i = initSize; i<coreSize;i++) {
newThread();
}
continue;
}
if(runnableQueue.size() > 0 && activeCount < maxSize) {
for(int i = coreSize; i<maxSize;i++) {
newThread();
}
}
if(runnableQueue.size()==0 && activeCount > coreSize) {
for(int i = coreSize; i < activeCount; i++) {
removeThread();
}
}
}
}
}
}
@Override
public void execute(Runnable runnable) {
//如果线程池已经销毁, 将抛出异常
if(this.isShutdown) {
throw new IllegalStateException("the thread pool is destoried");
}
this.runnableQueue.offer(runnable);
}
@Override
public void shutdown() {
synchronized(this) {
//防止重复销毁
if(isShutdown) {
return;
}
//重置关闭标识
isShutdown = true;
//关闭任务工作线程
threadQueue.forEach(threadTask -> {
threadTask.internalTask.stop();
threadTask.thread.interrupt();
});
//关闭线程池的工作线程
this.workThread.interrupt();
}
}
@Override
public int getInitSize() {
if(isShutdown) {
throw new IllegalStateException("The thread pool is destroy");
}
return this.initSize;
}
@Override
public int getMaxSize() {
if(isShutdown) {
throw new IllegalStateException("The thread pool is destroy");
}
return this.maxSize;
}
@Override
public int getCoreSize() {
if(isShutdown) {
throw new IllegalStateException("The thread pool is destroy");
}
return this.coreSize;
}
@Override
public int getQueueSize() {
if(isShutdown) {
throw new IllegalStateException("The thread pool is destroy");
}
return runnableQueue.size();
}
@Override
public int getActiveCount() {
synchronized(this) {
return this.activeCount;
}
}
@Override
public boolean isShutdown() {
return this.isShutdown;
}
}
线程池的测试
编写一个简单的测试类，同时启动20个任务，测试线程池的活动状态：
public class ThreadPoolTest {
public static void main(String[] args) throws InterruptedException {
final ThreadPool threadPool = new BasicThreadPool(2, 6, 4, 1000);
for(int i = 0; i < 20; i++) {
threadPool.execute(() -> {
try {
TimeUnit.SECONDS.sleep(10);
System.out.println(Thread.currentThread().getName() + "is Running and done");
} catch (InterruptedException e) {
e.printStackTrace();
}
});
}
while(true) {
System.out.println("getActiveCount: " + threadPool.getActiveCount());
System.out.println("getQueueSize: " + threadPool.getQueueSize());
System.out.println("getCoreSize: " + threadPool.getCoreSize());
System.out.println("getMaxSize: " + threadPool.getMaxSize());
System.out.println("================================================");
TimeUnit.SECONDS.sleep(5);
}
}
}
输出结果如下
thread-pool--1is Running and done
thread-pool-0is Running and done
getActiveCount: 4
getQueueSize: 14
getCoreSize: 4
getMaxSize: 6
================================================
getActiveCount: 4
getQueueSize: 14
getCoreSize: 4
getMaxSize: 6
================================================
thread-pool--3is Running and done
thread-pool--2is Running and done
thread-pool--1is Running and done
thread-pool-0is Running and done
getActiveCount: 6
getQueueSize: 8
getCoreSize: 4
getMaxSize: 6