通過之前三篇關於Spring Boot非同步任務實現的博文，我們分別學會了用@Async建立非同步任務、為非同步任務配置執行緒池、使用多個執行緒池隔離不同的非同步任務。今天這篇，我們繼續對上面的知識進行完善和優化！
如果你已經看過上面幾篇內容並已經掌握之後，一起來思考下面這個問題：
假設，執行緒池配置為核心執行緒數2、最大執行緒數2、緩衝佇列長度2。此時，有5個非同步任務同時開始，會發生什麼？
場景重現
我們先來把上面的假設用程式碼實現一下：
第一步：建立Spring Boot應用，根據上面的假設寫好執行緒池配置。
@EnableAsync
@SpringBootApplication
public class Chapter78Application {
public static void main(String[] args) {
SpringApplication.run(Chapter78Application.class, args);
}
@EnableAsync
@Configuration
class TaskPoolConfig {
@Bean
public Executor taskExecutor1() {
ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
executor.setCorePoolSize(2);
executor.setMaxPoolSize(2);
executor.setQueueCapacity(2);
executor.setKeepAliveSeconds(60);
executor.setThreadNamePrefix("executor-1-");
return executor;
}
}
}第二步：用@Async註解實現一個部分任務
@Slf4j
@Component
public class AsyncTasks {
public static Random random = new Random();
@Async("taskExecutor1")
public CompletableFuture<String> doTaskOne(String taskNo) throws Exception {
log.info("開始任務：{}", taskNo);
long start = System.currentTimeMillis();
Thread.sleep(random.nextInt(10000));
long end = System.currentTimeMillis();
log.info("完成任務：{}，耗時：{} 毫秒", taskNo, end - start);
return CompletableFuture.completedFuture("任務完成");
}
}第三步：編寫測試用例
@Slf4j
@SpringBootTest
public class Chapter78ApplicationTests {
@Autowired
private AsyncTasks asyncTasks;
@Test
public void test2() throws Exception {
// 執行緒池配置：core-2,max-2,queue=2，同時有5個任務，出現下面異常：
// org.springframework.core.task.TaskRejectedException: Executor [java.util.concurrent.ThreadPoolExecutor@59901c4d[Running, pool size = 2,
// active threads = 0, queued tasks = 2, completed tasks = 4]] did not accept task: java.util.concurrent.CompletableFuture$AsyncSupply@408e96d9
long start = System.currentTimeMillis();
// 執行緒池1
CompletableFuture<String> task1 = asyncTasks.doTaskOne("1");
CompletableFuture<String> task2 = asyncTasks.doTaskOne("2");
CompletableFuture<String> task3 = asyncTasks.doTaskOne("3");
CompletableFuture<String> task4 = asyncTasks.doTaskOne("4");
CompletableFuture<String> task5 = asyncTasks.doTaskOne("5");
// 一起執行
CompletableFuture.allOf(task1, task2, task3, task4, task5).join();
long end = System.currentTimeMillis();
log.info("任務全部完成，總耗時：" + (end - start) + "毫秒");
}
}執行一下，可以類似下面這樣的日誌資訊：
2021-09-22 17:33:08.159 INFO 21119 --- [ executor-1-2] com.didispace.chapter78.AsyncTasks : 開始任務：2
2021-09-22 17:33:08.159 INFO 21119 --- [ executor-1-1] com.didispace.chapter78.AsyncTasks : 開始任務：1
org.springframework.core.task.TaskRejectedException: Executor [java.util.concurrent.ThreadPoolExecutor@3e1a3801[Running, pool size = 2, active threads = 2, queued tasks = 2, completed tasks = 0]] did not accept task: java.util.concurrent.CompletableFuture$AsyncSupply@64968732
at org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor.execute(ThreadPoolTaskExecutor.java:324)
at java.util.concurrent.CompletableFuture.asyncSupplyStage(CompletableFuture.java:1604)
at java.util.concurrent.CompletableFuture.supplyAsync(CompletableFuture.java:1830)
at org.springframework.aop.interceptor.AsyncExecutionAspectSupport.doSubmit(AsyncExecutionAspectSupport.java:274)
at org.springframework.aop.interceptor.AsyncExecutionInterceptor.invoke(AsyncExecutionInterceptor.java:129)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:186)
at org.springframework.aop.framework.CglibAopProxy$CglibMethodInvocation.proceed(CglibAopProxy.java:750)
at org.springframework.aop.framework.CglibAopProxy$DynamicAdvisedInterceptor.intercept(CglibAopProxy.java:692)
at com.didispace.chapter78.AsyncTasks$$EnhancerBySpringCGLIB$$c7e8d57b.doTaskOne(<generated>)
at com.didispace.chapter78.Chapter78ApplicationTests.test2(Chapter78ApplicationTests.java:51)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
at java.lang.reflect.Method.invoke(Method.java:498)
at org.junit.platform.commons.util.ReflectionUtils.invokeMethod(ReflectionUtils.java:688)
at org.junit.jupiter.engine.execution.MethodInvocation.proceed(MethodInvocation.java:60)
at org.junit.jupiter.engine.execution.InvocationInterceptorChain$ValidatingInvocation.proceed(InvocationInterceptorChain.java:131)
at org.junit.jupiter.engine.extension.TimeoutExtension.intercept(TimeoutExtension.java:149)
at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestableMethod(TimeoutExtension.java:140)
at org.junit.jupiter.engine.extension.TimeoutExtension.interceptTestMethod(TimeoutExtension.java:84)
at org.junit.jupiter.engine.execution.ExecutableInvoker$ReflectiveInterceptorCall.lambda$ofVoidMethod$0(ExecutableInvoker.java:115)
at org.junit.jupiter.engine.execution.ExecutableInvoker.lambda$invoke$0(ExecutableInvoker.java:105)
at org.junit.jupiter.engine.execution.InvocationInterceptorChain$InterceptedInvocation.proceed(InvocationInterceptorChain.java:106)
at org.junit.jupiter.engine.execution.InvocationInterceptorChain.proceed(InvocationInterceptorChain.java:64)
at org.junit.jupiter.engine.execution.InvocationInterceptorChain.chainAndInvoke(InvocationInterceptorChain.java:45)
at org.junit.jupiter.engine.execution.InvocationInterceptorChain.invoke(InvocationInterceptorChain.java:37)
at org.junit.jupiter.engine.execution.ExecutableInvoker.invoke(ExecutableInvoker.java:104)
at org.junit.jupiter.engine.execution.ExecutableInvoker.invoke(ExecutableInvoker.java:98)
at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.lambda$invokeTestMethod$6(TestMethodTestDescriptor.java:210)
at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.invokeTestMethod(TestMethodTestDescriptor.java:206)
at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:131)
at org.junit.jupiter.engine.descriptor.TestMethodTestDescriptor.execute(TestMethodTestDescriptor.java:65)
at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:139)
at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$7(NodeTestTask.java:129)
at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:137)
at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:127)
at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:126)
at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:84)
at java.util.ArrayList.forEach(ArrayList.java:1255)
at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.invokeAll(SameThreadHierarchicalTestExecutorService.java:38)
at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:143)
at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$7(NodeTestTask.java:129)
at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:137)
at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:127)
at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:126)
at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:84)
at java.util.ArrayList.forEach(ArrayList.java:1255)
at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.invokeAll(SameThreadHierarchicalTestExecutorService.java:38)
at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$5(NodeTestTask.java:143)
at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$7(NodeTestTask.java:129)
at org.junit.platform.engine.support.hierarchical.Node.around(Node.java:137)
at org.junit.platform.engine.support.hierarchical.NodeTestTask.lambda$executeRecursively$8(NodeTestTask.java:127)
at org.junit.platform.engine.support.hierarchical.ThrowableCollector.execute(ThrowableCollector.java:73)
at org.junit.platform.engine.support.hierarchical.NodeTestTask.executeRecursively(NodeTestTask.java:126)
at org.junit.platform.engine.support.hierarchical.NodeTestTask.execute(NodeTestTask.java:84)
at org.junit.platform.engine.support.hierarchical.SameThreadHierarchicalTestExecutorService.submit(SameThreadHierarchicalTestExecutorService.java:32)
at org.junit.platform.engine.support.hierarchical.HierarchicalTestExecutor.execute(HierarchicalTestExecutor.java:57)
at org.junit.platform.engine.support.hierarchical.HierarchicalTestEngine.execute(HierarchicalTestEngine.java:51)
at org.junit.platform.launcher.core.EngineExecutionOrchestrator.execute(EngineExecutionOrchestrator.java:108)
at org.junit.platform.launcher.core.EngineExecutionOrchestrator.execute(EngineExecutionOrchestrator.java:88)
at org.junit.platform.launcher.core.EngineExecutionOrchestrator.lambda$execute$0(EngineExecutionOrchestrator.java:54)
at org.junit.platform.launcher.core.EngineExecutionOrchestrator.withInterceptedStreams(EngineExecutionOrchestrator.java:67)
at org.junit.platform.launcher.core.EngineExecutionOrchestrator.execute(EngineExecutionOrchestrator.java:52)
at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:96)
at org.junit.platform.launcher.core.DefaultLauncher.execute(DefaultLauncher.java:75)
at com.intellij.junit5.JUnit5IdeaTestRunner.startRunnerWithArgs(JUnit5IdeaTestRunner.java:71)
at com.intellij.rt.junit.IdeaTestRunner$Repeater.startRunnerWithArgs(IdeaTestRunner.java:33)
at com.intellij.rt.junit.JUnitStarter.prepareStreamsAndStart(JUnitStarter.java:235)
at com.intellij.rt.junit.JUnitStarter.main(JUnitStarter.java:54)
Caused by: java.util.concurrent.RejectedExecutionException: Task java.util.concurrent.CompletableFuture$AsyncSupply@64968732 rejected from java.util.concurrent.ThreadPoolExecutor@3e1a3801[Running, pool size = 2, active threads = 2, queued tasks = 2, completed tasks = 0]
at java.util.concurrent.ThreadPoolExecutor$AbortPolicy.rejectedExecution(ThreadPoolExecutor.java:2063)
at java.util.concurrent.ThreadPoolExecutor.reject(ThreadPoolExecutor.java:830)
at java.util.concurrent.ThreadPoolExecutor.execute(ThreadPoolExecutor.java:1379)
at org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor.execute(ThreadPoolTaskExecutor.java:321)
... 74 more從異常資訊org.springframework.core.task.TaskRejectedException: Executor [java.util.concurrent.ThreadPoolExecutor@3e1a3801[Running, pool size = 2, active threads = 2, queued tasks = 2, completed tasks = 0]] did not accept task: 中，可以很明確的知道，第5個任務因為超過了執行執行緒+緩衝佇列長度，而被拒絕了。
所有，預設情況下，執行緒池的拒絕策略是：當執行緒池佇列滿了，會丟棄這個任務，並丟擲異常。
配置拒絕策略
雖然執行緒池有預設的拒絕策略，但實際開發過程中，有些業務場景，直接拒絕的策略往往並不適用，有時候我們可能會選擇捨棄最早開始執行而未完成的任務、也可能會選擇捨棄剛開始執行而未完成的任務等更貼近業務需要的策略。所以，為執行緒池配置其他拒絕策略或自定義拒絕策略是很常見的需求，那麼這個要怎麼實現呢？
下面就來具體說說今天的正題，如何為執行緒池配置拒絕策略、如何自定義拒絕策略。
看下面這段程式碼的最後一行，setRejectedExecutionHandler方法就是為執行緒池設定拒絕策略的方法：
ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
//...其他執行緒池配置
executor.setRejectedExecutionHandler(new ThreadPoolExecutor.AbortPolicy());在ThreadPoolExecutor中提供了4種執行緒的策略可以供開發者直接使用，你只需要像下面這樣設定即可：
// AbortPolicy策略
executor.setRejectedExecutionHandler(new ThreadPoolExecutor.AbortPolicy());
// DiscardPolicy策略
executor.setRejectedExecutionHandler(new ThreadPoolExecutor.DiscardPolicy());
// DiscardOldestPolicy策略
executor.setRejectedExecutionHandler(new ThreadPoolExecutor.DiscardOldestPolicy());
// CallerRunsPolicy策略
executor.setRejectedExecutionHandler(new ThreadPoolExecutor.CallerRunsPolicy());這四個策略對應的含義分別是：
- AbortPolicy策略：預設策略，如果執行緒池佇列滿了丟掉這個任務並且丟擲RejectedExecutionException異常。
- DiscardPolicy策略：如果執行緒池佇列滿了，會直接丟掉這個任務並且不會有任何異常。
- DiscardOldestPolicy策略：如果佇列滿了，會將最早進入佇列的任務刪掉騰出空間，再嘗試加入佇列。
- CallerRunsPolicy策略：如果新增到執行緒池失敗，那麼主執行緒會自己去執行該任務，不會等待執行緒池中的執行緒去執行。
而如果你要自定義一個拒絕策略，那麼可以這樣寫：
executor.setRejectedExecutionHandler(new RejectedExecutionHandler() {
@Override
public void rejectedExecution(Runnable r, ThreadPoolExecutor executor) {
// 拒絕策略的邏輯
}
});當然如果你喜歡用Lamba表示式，也可以這樣寫：
executor.setRejectedExecutionHandler((r, executor1) -> {
// 拒絕策略的邏輯
});好了，今天的學習就到這裡！
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程式碼示例
本文的完整工程可以檢視下面倉庫中2.x目錄下的chapter7-8工程：
- Github：https://github.com/dyc87112/SpringBoot-Learning/
- Gitee：https://gitee.com/didispace/SpringBoot-Learning/
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