Tomcat 线程池学习总结

前提

Tomcat 10.1.x

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Tomcat线程池介绍

Tomcat线程池，源于JAVA JDK自带线程池。由于JAVA JDK线程池策略，比较适合处理 CPU 密集型任务，但是对于 I/O 密集型任务，如数据库查询，rpc 请求调用等，不是很友好，所以Tomcat在其基础上进行了扩展。

任务处理流程

扩展线程池相关源码简析

Tomcat中定义了一个StandardThreadExecutor类，该类实现了org.apache.catalina.Executor，org.apache.tomcat.util.threads.ResizableExecutor接口

该类内部定义了namePrefix(创建的线程名称前缀，默认值tomcat-exec-)，maxThreads(最大线程数，默认值 200)，minSpareThreads(最小线程数，即核心线程数，默认值 25)，maxIdleTime（线程最大空闲时间，毫秒为单位，默认值60秒），maxQueueSize(最大队列大小，默认值 Integer.MAX_VALUE)等属性，此外，还定义了一个org.apache.tomcat.util.threads.ThreadPoolExecutor类型的执行器对象，一个execute(Runnable command)方法

当execute(Runnable command)方法被调用时，会调用上述ThreadPoolExecutor类对象的execute方法

org.apache.catalina.core.StandardThreadExecutor.java

import org.apache.catalina.Executor;
import org.apache.catalina.LifecycleException;
import org.apache.catalina.LifecycleState;
import org.apache.catalina.util.LifecycleMBeanBase;
import org.apache.tomcat.util.res.StringManager;
import org.apache.tomcat.util.threads.ResizableExecutor;
import org.apache.tomcat.util.threads.TaskQueue;
import org.apache.tomcat.util.threads.TaskThreadFactory;
import org.apache.tomcat.util.threads.ThreadPoolExecutor;

public class StandardThreadExecutor extends LifecycleMBeanBase
        implements Executor, ResizableExecutor {

    protected static final StringManager sm = StringManager.getManager(StandardThreadExecutor.class);

    // ---------------------------------------------- Properties
    /**
     * Default thread priority
     */
    protected int threadPriority = Thread.NORM_PRIORITY;

    /**
     * Run threads in daemon or non-daemon state
     */
    protected boolean daemon = true;

    /**
     * Default name prefix for the thread name
     */
    protected String namePrefix = "tomcat-exec-";

    /**
     * max number of threads
     */
    protected int maxThreads = 200;

    /**
     * min number of threads
     */
    protected int minSpareThreads = 25;

    /**
     * idle time in milliseconds
     */
    protected int maxIdleTime = 60000;

    /**
     * The executor we use for this component
     */
    protected ThreadPoolExecutor executor = null;

    /**
     * the name of this thread pool
     */
    protected String name;

    /**
     * The maximum number of elements that can queue up before we reject them
     */
    protected int maxQueueSize = Integer.MAX_VALUE;

    /**
     * After a context is stopped, threads in the pool are renewed. To avoid
     * renewing all threads at the same time, this delay is observed between 2
     * threads being renewed.
     */
    protected long threadRenewalDelay =
        org.apache.tomcat.util.threads.Constants.DEFAULT_THREAD_RENEWAL_DELAY;

    private TaskQueue taskqueue = null;
    // ---------------------------------------------- Constructors
    public StandardThreadExecutor() {
        //empty constructor for the digester
    }
    
    //....此处代码已省略

    @Override
    public void execute(Runnable command) {
        if (executor != null) {
            // Note any RejectedExecutionException due to the use of TaskQueue
            // will be handled by the o.a.t.u.threads.ThreadPoolExecutor
            executor.execute(command);
        } else {
            throw new IllegalStateException(sm.getString("standardThreadExecutor.notStarted"));
        }
    }
    
    //....此处代码已省略

}

当org.apache.tomcat.util.threads.ThreadPoolExecuto类对象的execute(Runnable command)方法被调用时，会调用该类定义的一个executeInternal方法，并在捕获到RejectedExecutionException异常时，尝试再次将任务放入工作队列中。

executeInternal方法中，通过代码可知，当前线程数小于核心线程池大小时，会创建新线程，否则，会调用workQueue对象（org.apache.tomcat.util.threads.TaskQueue类型）的offer方法，将任务进行排队。Tomcat通过控制workQueue.offer()方法的返回值，实现了当前线程数超过核心线程池大小时，优先创建线程，而不是让任务排队。

org.apache.tomcat.util.threads.ThreadPoolExecutor

public class ThreadPoolExecutor extends AbstractExecutorService {
    //...此处代码已省略
    @Override
    public void execute(Runnable command) {
        submittedCount.incrementAndGet();
        try {
            executeInternal(command);
        } catch (RejectedExecutionException rx) {
            if (getQueue() instanceof TaskQueue) {
                // If the Executor is close to maximum pool size, concurrent
                // calls to execute() may result (due to Tomcat's use of
                // TaskQueue) in some tasks being rejected rather than queued.
                // If this happens, add them to the queue.
                final TaskQueue queue = (TaskQueue) getQueue();
                if (!queue.force(command)) {
                    submittedCount.decrementAndGet();
                    throw new RejectedExecutionException(sm.getString("threadPoolExecutor.queueFull"));
                }
            } else {
                submittedCount.decrementAndGet();
                throw rx;
            }
        }
    }


    /**
     * 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 {@link 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
     */
    private void executeInternal(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) { // 当前线程数小于核心线程数时，
            if (addWorker(command, true)) { // 创建线程
                return;
            }
            c = ctl.get();
        }
        if (isRunning(c) && workQueue.offer(command)) { //workQueue.offer(command)为false时，会走以下的else if分支，创建线程
            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);
        }
    }
    
    //...此处代码已省略

}

org.apache.tomcat.util.threads.TaskQueue继承于java.util.concurrent.LinkedBlockingQueue，并重写了offer（排队任务的方法），该方法中，当当前线程数大于核心线程数，小于最大线程数时，返回false，导致上述executeInternal方法中workQueue.offer(command)为false，进而导致该分支代码不被执行，执行addWorker(command, false)方法，创建新线程。

org.apache.tomcat.util.threads.TaskQueue

import java.util.Collection;
import java.util.concurrent.LinkedBlockingQueue;
import java.util.concurrent.RejectedExecutionException;
import java.util.concurrent.TimeUnit;

import org.apache.tomcat.util.res.StringManager;

/**
 * As task queue specifically designed to run with a thread pool executor. The
 * task queue is optimised to properly utilize threads within a thread pool
 * executor. If you use a normal queue, the executor will spawn threads when
 * there are idle threads and you won't be able to force items onto the queue
 * itself.
 */
public class TaskQueue extends LinkedBlockingQueue {

    //...此处代码已省略
    
    /**
     * Used to add a task to the queue if the task has been rejected by the Executor.
     *
     * @param o         The task to add to the queue
     *
     * @return          {@code true} if the task was added to the queue,
     *                      otherwise {@code false}
     */
    public boolean force(Runnable o) {
        if (parent == null || parent.isShutdown()) {
            throw new RejectedExecutionException(sm.getString("taskQueue.notRunning"));
        }
        return super.offer(o); //forces the item onto the queue, to be used if the task is rejected
    }


    @Override
    public boolean offer(Runnable o) {
      //we can't do any checks
        if (parent==null) {
            return super.offer(o);
        }
        //we are maxed out on threads, simply queue the object
        if (parent.getPoolSize() == parent.getMaximumPoolSize()) {
            return super.offer(o);
        }
        //we have idle threads, just add it to the queue
        if (parent.getSubmittedCount()<=(parent.getPoolSize())) {
            return super.offer(o);
        }
        //if we have less threads than maximum force creation of a new thread
        if (parent.getPoolSize()