Java concurrency线程池之线程池原理(四)_动力节点Java学院整理

发布于 2020-11-22|标签java
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摘记: 拒绝策略介绍 线程池的拒绝策略,是指当任务添加到线程池中被拒绝,而采取的处理措施。 当任务添加到线程池中之所以被拒绝,可能是由于:第一,线程池异常关闭。第二,任务数量超过线程池的最大限制。 ..
拒绝策略介绍线程池的拒绝策略,是指当任务添加到线程池中被拒绝,而采取的处理措施。 当任务添加到线程池中之所以被拒绝,可能是由于:第一,线程池异常关闭。第二,任务数量超过线程池的最大限制。 线程池共包括4种拒绝策略,它们分别是:AbortPolicy, CallerRunsPolicy, DiscardOldestPolicy和DiscardPolicy。 AbortPolicy         -- 当任务添加到线程池中被拒绝时,它将抛出 RejectedExecutionException 异常。 CallerRunsPolicy    -- 当任务添加到线程池中被拒绝时,会在线程池当前正在运行的Thread线程池中处理被拒绝的任务。 DiscardOldestPolicy -- 当任务添加到线程池中被拒绝时,线程池会放弃等待队列中最旧的未处理任务,然后将被拒绝的任务添加到等待队列中。 DiscardPolicy       -- 当任务添加到线程池中被拒绝时,线程池将丢弃被拒绝的任务。 线程池默认的处理策略是AbortPolicy! 拒绝策略对比和示例 下面通过示例,分别演示线程池的4种拒绝策略。1. DiscardPolicy 示例 ```java import java.lang.reflect.Field; import java.util.concurrent.ArrayBlockingQueue; import java.util.concurrent.ThreadPoolExecutor; import java.util.concurrent.TimeUnit; import java.util.concurrent.ThreadPoolExecutor.DiscardPolicy; public class DiscardPolicyDemo { private static final int THREADS_SIZE = 1; private static final int CAPACITY = 1; public static void main(String[] args) throws Exception { // 创建线程池。线程池的"最大池大小"和"核心池大小"都为1(THREADS_SIZE),"线程池"的阻塞队列容量为1(CAPACITY)。 ThreadPoolExecutor pool = new ThreadPoolExecutor(THREADS_SIZE, THREADS_SIZE, 0, TimeUnit.SECONDS, new ArrayBlockingQueue(CAPACITY)); // 设置线程池的拒绝策略为"丢弃" pool.setRejectedExecutionHandler(new ThreadPoolExecutor.DiscardPolicy()); // 新建10个任务,并将它们添加到线程池中。 for (int i = 0; i 运行结果: ```plain task-0 is running. task-1 is running. ``` 结果说明:线程池pool的"最大池大小"和"核心池大小"都为1(THREADS_SIZE),这意味着"线程池能同时运行的任务数量最大只能是1"。 线程池pool的阻塞队列是ArrayBlockingQueue,ArrayBlockingQueue是一个有界的阻塞队列,ArrayBlockingQueue的容量为1。这也意味着线程池的阻塞队列只能有一个线程池阻塞等待。 根据""中分析的execute()代码可知:线程池中共运行了2个任务。第1个任务直接放到Worker中,通过线程去执行;第2个任务放到阻塞队列中等待。其他的任务都被丢弃了! 2. DiscardOldestPolicy 示例 ```java import java.lang.reflect.Field; import java.util.concurrent.ArrayBlockingQueue; import java.util.concurrent.ThreadPoolExecutor; import java.util.concurrent.TimeUnit; import java.util.concurrent.ThreadPoolExecutor.DiscardOldestPolicy; public class DiscardOldestPolicyDemo { private static final int THREADS_SIZE = 1; private static final int CAPACITY = 1; public static void main(String[] args) throws Exception { // 创建线程池。线程池的"最大池大小"和"核心池大小"都为1(THREADS_SIZE),"线程池"的阻塞队列容量为1(CAPACITY)。 ThreadPoolExecutor pool = new ThreadPoolExecutor(THREADS_SIZE, THREADS_SIZE, 0, TimeUnit.SECONDS, new ArrayBlockingQueue(CAPACITY)); // 设置线程池的拒绝策略为"DiscardOldestPolicy" pool.setRejectedExecutionHandler(new ThreadPoolExecutor.DiscardOldestPolicy()); // 新建10个任务,并将它们添加到线程池中。 for (int i = 0; i 运行结果: ```plain task-0 is running. task-9 is running. ``` 结果说明:将"线程池的拒绝策略"由DiscardPolicy修改为DiscardOldestPolicy之后,当有任务添加到线程池被拒绝时,线程池会丢弃阻塞队列中末尾的任务,然后将被拒绝的任务添加到末尾。 3. AbortPolicy 示例 ```java import java.lang.reflect.Field; import java.util.concurrent.ArrayBlockingQueue; import java.util.concurrent.ThreadPoolExecutor; import java.util.concurrent.TimeUnit; import java.util.concurrent.ThreadPoolExecutor.AbortPolicy; import java.util.concurrent.RejectedExecutionException; public class AbortPolicyDemo { private static final int THREADS_SIZE = 1; private static final int CAPACITY = 1; public static void main(String[] args) throws Exception { // 创建线程池。线程池的"最大池大小"和"核心池大小"都为1(THREADS_SIZE),"线程池"的阻塞队列容量为1(CAPACITY)。 ThreadPoolExecutor pool = new ThreadPoolExecutor(THREADS_SIZE, THREADS_SIZE, 0, TimeUnit.SECONDS, new ArrayBlockingQueue(CAPACITY)); // 设置线程池的拒绝策略为"抛出异常" pool.setRejectedExecutionHandler(new ThreadPoolExecutor.AbortPolicy()); try { // 新建10个任务,并将它们添加到线程池中。 for (int i = 0; i (某一次)运行结果: ```plain java.util.concurrent.RejectedExecutionException at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:1774) at java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:768) at java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:656) at AbortPolicyDemo.main(AbortPolicyDemo.java:27) task-0 is running. task-1 is running. ``` 结果说明:将"线程池的拒绝策略"由DiscardPolicy修改为AbortPolicy之后,当有任务添加到线程池被拒绝时,会抛出RejectedExecutionException。 4. CallerRunsPolicy 示例 ```java import java.lang.reflect.Field; import java.util.concurrent.ArrayBlockingQueue; import java.util.concurrent.ThreadPoolExecutor; import java.util.concurrent.TimeUnit; import java.util.concurrent.ThreadPoolExecutor.CallerRunsPolicy; public class CallerRunsPolicyDemo { private static final int THREADS_SIZE = 1; private static final int CAPACITY = 1; public static void main(String[] args) throws Exception { // 创建线程池。线程池的"最大池大小"和"核心池大小"都为1(THREADS_SIZE),"线程池"的阻塞队列容量为1(CAPACITY)。 ThreadPoolExecutor pool = new ThreadPoolExecutor(THREADS_SIZE, THREADS_SIZE, 0, TimeUnit.SECONDS, new ArrayBlockingQueue(CAPACITY)); // 设置线程池的拒绝策略为"CallerRunsPolicy" pool.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy()); // 新建10个任务,并将它们添加到线程池中。 for (int i = 0; i (某一次)运行结果: ```plain task-2 is running. task-3 is running. task-4 is running. task-5 is running. task-6 is running. task-7 is running. task-8 is running. task-9 is running. task-0 is running. task-1 is running. ```
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