• executes()源码


        /**
         * 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 {@code 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
         */
        public void execute(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.
             * 1、如果少于corePoolSize数量的线程正在运行,尝试利用给定的Runnable实例command开启一个新的线程作为它的第一个任务。
             * addWorker()方法的调用会对线程池运行状态runState、worker线程数量workerCount进行原子性检测,返回值为启动新线程结果。
             *
             * 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.
             * 2、如果一个任务可以成功地进入队列,然后我们还需要再次检查(即双份检查)自从进入这个方法后,我们是否应该添加一个线程
             * (因为自从上一次检查以来可能存在死亡情况),
             * 所以我们重新检查状态,如果有必要的话,即线程池已停止,回滚之前的入队操作,或者在没有线程时启动一个新线程。
             *
             * 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.
             * 3、如果我们不能入列一个任务,那么我们尝试添加一个新线程。
             * 如果添加失败,我们知道线程池可能已被关闭或者数量饱和,所以我们会拒绝这个任务。
             */
            
            // 获取ctl的值c
            int c = ctl.get();
            
            // 如果c中有效线程数目小于corePoolSize大小,尝试添加新的worker线程处理任务command:
            // 从c中获取有效线程数目调用的是workerCountOf()方法,
            // 添加新的worker线程处理任务command调用的是addWorker()方法,
            // 线程数的判断利用corePoolSize作为边界约束条件
            // 方法返回值是标志添加worker是否成功的标志位,ture表示成功,false表示失败,
            // 如果为true,则直接返回,否则重新获取ctl的值c
            if (workerCountOf(c) < corePoolSize) {
                if (addWorker(command, true))
                    return;
                
                // 添加work线程失败则再次获取ctl的值
                c = ctl.get();
            }
            
            // 根据c判断当前线程池的状态是否为RUNNING状态,即既可以接受新任务,又会处理队列任务的状态,
            // 并且通过offer()方法,尝试将commond添加到队列workQueue中
            // BlockingQueue的offer()方法表示如果可能的话,将参数对象加到BlockingQueue里,
            // 即如果BlockingQueue可以容纳,则返回true,否则返回false
            
            if (isRunning(c) && workQueue.offer(command)) {
                
                // 如果当前线程池处于RUNNING状态,且workQueue能够容纳command,并添加成功的话,
                // 再次获取ctl的值recheck,
                int recheck = ctl.get();
                
                // 如果当前线程池的状态不是RUNNING,并且从队列workQueue移除command成功的话,
                // 调用reject()方法拒绝任务command,
                if (! isRunning(recheck) && remove(command))
                    reject(command);
                
                // 否则如果当前工作线程woker数目为0,尝试添加新的worker线程,但是不携带任务
                else if (workerCountOf(recheck) == 0)
                    addWorker(null, false);
            }
            // 如果尝试添加新的worker线程处理任务command失败,
            // 调用reject()方法拒绝任务command,线程数的判断利用maximumPoolSize作为边界约束条件
            else if (!addWorker(command, false))
                reject(command);
        }
        /**
         * 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 {@code 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
         */
        public void execute(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.
             * 1、如果少于corePoolSize数量的线程正在运行,尝试利用给定的Runnable实例command开启一个新的线程作为它的第一个任务。
             * addWorker()方法的调用会对线程池运行状态runState、worker线程数量workerCount进行原子性检测,返回值为启动新线程结果。
             *
             * 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.
             * 2、如果一个任务可以成功地进入队列,然后我们还需要再次检查(即双份检查)自从进入这个方法后,我们是否应该添加一个线程
             * (因为自从上一次检查以来可能存在死亡情况),
             * 所以我们重新检查状态,如果有必要的话,即线程池已停止,回滚之前的入队操作,或者在没有线程时启动一个新线程。
             *
             * 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.
             * 3、如果我们不能入列一个任务,那么我们尝试添加一个新线程。
             * 如果添加失败,我们知道线程池可能已被关闭或者数量饱和,所以我们会拒绝这个任务。
             */
            
            // 获取ctl的值c
            int c = ctl.get();
            
            // 如果c中有效线程数目小于corePoolSize大小,尝试添加新的worker线程处理任务command:
            // 从c中获取有效线程数目调用的是workerCountOf()方法,
            // 添加新的worker线程处理任务command调用的是addWorker()方法,
            // 线程数的判断利用corePoolSize作为边界约束条件
            // 方法返回值是标志添加worker是否成功的标志位,ture表示成功,false表示失败,
            // 如果为true,则直接返回,否则重新获取ctl的值c
            if (workerCountOf(c) < corePoolSize) {
                if (addWorker(command, true))
                    return;
                
                // 添加work线程失败则再次获取ctl的值
                c = ctl.get();
            }
            
            // 根据c判断当前线程池的状态是否为RUNNING状态,即既可以接受新任务,又会处理队列任务的状态,
            // 并且通过offer()方法,尝试将commond添加到队列workQueue中
            // BlockingQueue的offer()方法表示如果可能的话,将参数对象加到BlockingQueue里,
            // 即如果BlockingQueue可以容纳,则返回true,否则返回false
            
            if (isRunning(c) && workQueue.offer(command)) {
                
                // 如果当前线程池处于RUNNING状态,且workQueue能够容纳command,并添加成功的话,
                // 再次获取ctl的值recheck,
                int recheck = ctl.get();
                
                // 如果当前线程池的状态不是RUNNING,并且从队列workQueue移除command成功的话,
                // 调用reject()方法拒绝任务command,
                if (! isRunning(recheck) && remove(command))
                    reject(command);
                
                // 否则如果当前工作线程woker数目为0,尝试添加新的worker线程,但是不携带任务
                else if (workerCountOf(recheck) == 0)
                    addWorker(null, false);
            }
            // 如果尝试添加新的worker线程处理任务command失败,
            // 调用reject()方法拒绝任务command,线程数的判断利用maximumPoolSize作为边界约束条件
            else if (!addWorker(command, false))
                reject(command);
        }
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  • 原文地址:https://www.cnblogs.com/da-peng/p/10032431.html
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