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Tomcat 线程池学习总结
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前提
Tomcat 10.1.x
相关问答
Q1: 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<Runnable> {
//...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()<parent.getMaximumPoolSize()) {
return false;
}
//if we reached here, we need to add it to the queue
return super.offer(o);
}
//...此处代码已省略
}
Q2: 参考链接
https://gitee.com/apache/tomcat/blob/10.1.x/java/org/apache/catalina/core/StandardThreadExecutor.java
