ThreadPoolTaskExecutor 和 ThreadPoolExecutor 的区别

ThreadPoolExecutor是Java原生的线程池类，而ThreadPoolTaskExecutor是Spring推出的线程池工具

一、从核心参数看两者关系

ThreadPoolExecutor（java.util.concurrent）

public ThreadPoolExecutor(int corePoolSize,
                          int maximumPoolSize,
                          long keepAliveTime,
                          TimeUnit unit,
                          BlockingQueue<Runnable> workQueue,
                          ThreadFactory threadFactory,
                          RejectedExecutionHandler handler) {
    if (corePoolSize < 0 ||
        maximumPoolSize <= 0 ||
        maximumPoolSize < corePoolSize ||
        keepAliveTime < 0)
        throw new IllegalArgumentException();
    if (workQueue == null || threadFactory == null || handler == null)
        throw new NullPointerException();
    this.acc = System.getSecurityManager() == null ?
        null :
    AccessController.getContext();
    this.corePoolSize = corePoolSize;
    this.maximumPoolSize = maximumPoolSize;
    this.workQueue = workQueue;
    this.keepAliveTime = unit.toNanos(keepAliveTime);
    this.threadFactory = threadFactory;
    this.handler = handler;
}

ThreadPoolTaskExecutor（org.springframework.scheduling.concurrent）

public class ThreadPoolTaskExecutor extends ExecutorConfigurationSupport implements AsyncListenableTaskExecutor, SchedulingTaskExecutor {
    private final Object poolSizeMonitor = new Object();
    private int corePoolSize = 1;
    private int maxPoolSize = 2147483647;
    private int keepAliveSeconds = 60;
    private int queueCapacity = 2147483647;
    private boolean allowCoreThreadTimeOut = false;
    @Nullable
    private TaskDecorator taskDecorator;
    @Nullable
    private ThreadPoolExecutor threadPoolExecutor;
    private final Map<Runnable, Object> decoratedTaskMap;

    public ThreadPoolTaskExecutor() {
        this.decoratedTaskMap = new ConcurrentReferenceHashMap(16, ReferenceType.WEAK);
    }
}

ThreadPoolTaskExecutor的核心参数共有四个，分别是corePoolSize，maxPoolSize，keepAliveSeconds以及queueCapacity。

• corePoolSize：核心线程数量（对应ThreadPoolExecutor的corePoolSize）
• maxPoolSize：最大线程数量（对应ThreadPoolExecutor的maximumPoolSize）
• keepAliveSeconds：线程池维护线程所允许的空闲时间（对应ThreadPoolExecutor的keepAliveTime）
• queueCapacity：工作队列容量（对应ThreadPoolExecutor的workQueue的容量）

从ThreadPoolTaskExecutor的唯一带参构造方法可以看出，似乎并没有对上述四个核心参数做自定义初始化的工作，实际上，ThreadPoolTaskExecutor在底层依然依赖ThreadPoolExecutor本身，也就是说该工具更关注于扩展的内容，执行任务依然交由ThreadPoolExecutor去处理。
​

二、底层原生线程池依赖解析

从上述代码中可以看到，ThreadPoolTaskExecutor继承了ExecutorConfigurationSupport类，该类不仅体现了Spring自身的设计思想，也是ThreadPoolTaskExecutor底层调用ThreadPoolExecutor的具体实现。

ExecutorConfigurationSupport

在ExecutorConfigurationSupport中有这么两个关键参数：threadFactory和rejectedExecutionHandler

public abstract class ExecutorConfigurationSupport extends CustomizableThreadFactory implements BeanNameAware, InitializingBean, DisposableBean {
    private ThreadFactory threadFactory = this;
    private RejectedExecutionHandler rejectedExecutionHandler = new AbortPolicy();
}

• threadFactory：线程工厂（对应ThreadPoolExecutor的threadFactory）
• rejectedExecutionHandler：拒绝策略（对应ThreadPoolExecutor的rejectedExecutionHandler）

threadFactory之所以初始化为this，是因为ExecutorConfigurationSupport本身就继承了ThreadFactory的实现类之一CustomizableThreadFactory。
自此，ThreadPoolTaskExecutor继承ExecutorConfigurationSupport之后，与ThreadPoolExecutor已经极为接近了，但还差最为关键的一步，如何创建ThreadPoolExecutor来执行任务？

InitializeExecutor()

事实上，ThreadPoolTaskExecutor有两种方式来创建ThreadPoolExecutor，一种是手动创建，一种是注解装配。

手动创建：ThreadPoolTaskExecutor.initialize()

public abstract class ExecutorConfigurationSupport extends CustomizableThreadFactory implements BeanNameAware, InitializingBean, DisposableBean {
    
    public void initialize() {
        if (this.logger.isDebugEnabled()) {
            this.logger.debug("Initializing ExecutorService" + (this.beanName != null ? " '" + this.beanName + "'" : ""));
        }

        if (!this.threadNamePrefixSet && this.beanName != null) {
            this.setThreadNamePrefix(this.beanName + "-");
        }

        this.executor = this.initializeExecutor(this.threadFactory, this.rejectedExecutionHandler);
    }
    
    protected abstract ExecutorService initializeExecutor(ThreadFactory var1, RejectedExecutionHandler var2);
}

可以看到，ThreadPoolTaskExecutor执行父类的initialize方法时，将threadFactory和rejectedExecutionHander作为传参，而initializeExecutor方法是一个抽象方法，在ThreadPoolTaskExecutor有以下实现：

public class ThreadPoolTaskExecutor extends ExecutorConfigurationSupport implements AsyncListenableTaskExecutor, SchedulingTaskExecutor {
    
    protected ExecutorService initializeExecutor(ThreadFactory threadFactory, RejectedExecutionHandler rejectedExecutionHandler) {
        BlockingQueue<Runnable> queue = this.createQueue(this.queueCapacity);
        ThreadPoolExecutor executor;
        if (this.taskDecorator != null) {
            executor = new ThreadPoolExecutor(this.corePoolSize, this.maxPoolSize, (long)this.keepAliveSeconds, TimeUnit.SECONDS, queue, threadFactory, rejectedExecutionHandler) {
                public void execute(Runnable command) {
                    Runnable decorated = ThreadPoolTaskExecutor.this.taskDecorator.decorate(command);
                    if (decorated != command) {
                        ThreadPoolTaskExecutor.this.decoratedTaskMap.put(decorated, command);
                    }

                    super.execute(decorated);
                }
            };
        } else {
            executor = new ThreadPoolExecutor(this.corePoolSize, this.maxPoolSize, (long)this.keepAliveSeconds, TimeUnit.SECONDS, queue, threadFactory, rejectedExecutionHandler);
        }

        if (this.allowCoreThreadTimeOut) {
            executor.allowCoreThreadTimeOut(true);
        }

        this.threadPoolExecutor = executor;
        return executor;
    }
    
    protected BlockingQueue<Runnable> createQueue(int queueCapacity) {
        return (BlockingQueue)(queueCapacity > 0 ? new LinkedBlockingQueue(queueCapacity) : new SynchronousQueue());
    }
}

通过initializeExecutor方法，创建一个容量为queueCapacity的阻塞队列作为工作队列，再以这六个参数（外加一个TimeUnit恒定为SECONDS）来构造ThreadPoolExecutor。

注解装配：@Bean与InitializingBean

对于Spring而言，更简洁的方式无疑是通过注解来实现自动装配，而ExecutorConfigurationSupport恰好实现了InitailizingBean接口，因此也实现了该接口唯一的一个方法：afterPropertiesSet()

public abstract class ExecutorConfigurationSupport extends CustomizableThreadFactory implements BeanNameAware, InitializingBean, DisposableBean {
    
    public void afterPropertiesSet() {
        this.initialize();
    }   
}

这意味着，当初始化代码中配置的ThreadPoolTaskExecutor这个bean时，便会调用这个方法，从而实现自动创建ThreadPoolExecutor。
​

三、SpringBoot线程池配置示例

接下来，本文将实现一个简单的线程池配置类，采用的是读取配置文件+注解配置bean注入的方式来创建Spring线程池。

application.yml

# 线程池配置
thread:
  pool:
    # 核心线程数
    core-pool-size: 50
    # 最大线程数
    max-pool-size: 200
    # 工作队列容量
    queue-capacity: 1000
    # 线程池维护线程所允许的空闲时间
    keep-alive-seconds: 300
    # 拒绝策略
    rejected-execution-handler: CallerRunsPolicy

ThreadPoolConfig.java

package com.ponder.core.config;

import com.ponder.common.utils.ThreadUtil;
import lombok.Data;
import org.apache.commons.lang3.concurrent.BasicThreadFactory;
import org.springframework.boot.context.properties.ConfigurationProperties;
import org.springframework.context.annotation.Bean;
import org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor;
import org.springframework.stereotype.Component;

import java.lang.reflect.InvocationTargetException;
import java.util.concurrent.RejectedExecutionHandler;
import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ScheduledThreadPoolExecutor;
import java.util.concurrent.ThreadPoolExecutor;

/**
 * 线程池配置
 *
 * @author Ponder Yao
 * @version 1.0.0  2021/7/29 13:39
 */
@Data
@Component
@ConfigurationProperties(prefix = "thread.pool")
public class ThreadPoolConfig {

    /** 核心线程数 */
    private int corePoolSize;

    /** 最大线程数 */
    private int maxPoolSize;

    /** 工作队列容量 */
    private int queueCapacity;

    /** 线程池维护线程所允许的空闲时间 */
    private int keepAliveSeconds;

    /** 拒绝策略 */
    private String rejectedExecutionHandler;

    @Bean
    public ThreadPoolTaskExecutor threadPoolTaskExecutor() {
        ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
        executor.setCorePoolSize(this.corePoolSize);
        executor.setMaxPoolSize(this.maxPoolSize);
        executor.setQueueCapacity(this.queueCapacity);
        executor.setKeepAliveSeconds(this.keepAliveSeconds);
        try {
            // 反射加载拒绝策略类
            Class clazz = Class.forName("java.util.concurrent.ThreadPoolExecutor$" + this.rejectedExecutionHandler);
            executor.setRejectedExecutionHandler((RejectedExecutionHandler) clazz.newInstance());
        } catch (ClassNotFoundException | IllegalAccessException | InstantiationException e) {
            e.printStackTrace();
            // 默认使用CallerRunsPolicy策略：直接在execute方法的调用线程中运行被拒绝的任务
            executor.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy());
        }
        return executor;
    }

}

没错，就是这么简单，轻轻松松搞定线程池初始化工作，后面就可以随时从Spring容器取出使用啦
​

四、总结

所以，为何Spring要自己写一个ThreadPoolTaskExecutor并推荐代替直接使用ThreadPoolExecutor呢？

其实最主要的原因很直观：ThreadPoolExecutor是一个不受Spring管理生命周期、参数装配的Java类，而有了ThreadPoolTaskExecutor的封装，线程池才有Spring“内味”。

当然，本文只是简要分析了一下两个类的本质区别与关系，很多具体细节还未探讨，而从这次分析中也可以得到以下结论：ThreadPoolTaskExecutor本质依然是ThreadPoolExecutor来实现基本的线程池工作，不同的是前者更关注自己实现的增强扩展部分，让线程池具有更多特性可供使用。