Field xxxx required a single bean, but 2 were found

在一次配置多線程bean的過程中報瞭如下錯誤：

Field xxx required a single bean, but 2 were found:

• async-task-pool: defined by method ‘asyncTaskPool’ in class path resource
• async-task-executor: defined by method ‘asyncTaskExecutor’ in class path resource

Action:

Consider marking one of the beans as @Primary, updating the consumer
to accept multiple beans, or using @Qualifier to identify the bean
that should be consumed

上代碼：

import lombok.extern.slf4j.Slf4j;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor;

import java.util.concurrent.Executor;

/**
 * 線程池配置
 */
@Slf4j
@Configuration
public class ExecutorConfig {

    /**
     * 任務執行器BeanName
     */
    public static final String TASK_EXECUTOR = "task-executor";


    /**
     * 任務執行器BeanName
     */
    public static final String TASK_POOL = "task-pool";

    /**
     * 配置任務執行器
     */
    @Bean(TASK_POOL)
    public ThreadPoolTaskExecutor asyncTaskPool() {
        ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
        executor.setCorePoolSize(20);
        executor.setMaxPoolSize(30);
        executor.setQueueCapacity(1000);
        executor.setKeepAliveSeconds(60);
        executor.setRejectedExecutionHandler((r, currentExecutor) -> {
            log.error("任務執行器隊列已佔滿,請及時處理...");
        });
        initDefaultThread(executor, TASK_POOL);
        executor.initialize();
        return executor;
    }

    /**
     * 配置任務執行器
     */
    @Bean(TASK_EXECUTOR)
    public Executor asyncTaskExecutor() {
        ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
        executor.setCorePoolSize(10);
        executor.setMaxPoolSize(20);
        executor.setQueueCapacity(1000);
        executor.setKeepAliveSeconds(60);
        executor.setRejectedExecutionHandler((r, currentExecutor) -> log.error("任務執行器隊列已佔滿,請及時處理..."));
        initDefaultThread(executor, TASK_EXECUTOR);
        executor.initialize();
        return executor;
    }

    private void initDefaultThread(ThreadPoolTaskExecutor executor, String taskExecutorName) {
        executor.setThreadFactory(r -> {
            Thread thread = new Thread(r);
            thread.setName(taskExecutorName + "-" + thread.getId());
            thread.setUncaughtExceptionHandler((t, e) -> log.error("線程異常,線程名:{}", t.getName()));
            return thread;
        });
    }
}

先看下報錯的源碼

@Override
protected FailureAnalysis analyze(Throwable rootFailure,
		NoUniqueBeanDefinitionException cause, String description) {
	if (description == null) {
		return null;
	}
	String[] beanNames = extractBeanNames(cause);
	if (beanNames == null) {
		return null;
	}
	StringBuilder message = new StringBuilder();
	message.append(String.format("%s required a single bean, but %d were found:%n",
			description, beanNames.length));
	for (String beanName : beanNames) {
		buildMessage(message, beanName);
	}
	return new FailureAnalysis(message.toString(),
			"Consider marking one of the beans as @Primary, updating the consumer to"
					+ " accept multiple beans, or using @Qualifier to identify the"
					+ " bean that should be consumed",
			cause);
}

異常爲Bean定義非唯一異常，爲啥報錯呢？

我們注入Bean通常會用@Resource和 @Autowired：
@Autowired 按 byType 自動注入，
@Resource默認按 byName 自動注入。
@Autowired是通過byType形式，Spring容器將無法確定到底要用哪一個 Bean，因此異常發生了。

解決方案報錯提示中給出了：
使用@Primary註解在其中一個bean上，或者消費的時候@Qualifier指定

 /**
     * 配置任務執行器
     */
    @Bean(TASK_EXECUTOR)
    @Primary
    public Executor asyncTaskExecutor() {
        ThreadPoolTaskExecutor executor = new ThreadPoolTaskExecutor();
        executor.setCorePoolSize(10);
        executor.setMaxPoolSize(20);
        executor.setQueueCapacity(1000);
        executor.setKeepAliveSeconds(60);
        executor.setRejectedExecutionHandler((r, currentExecutor) -> log.error("任務執行器隊列已佔滿,請及時處理..."));
        initDefaultThread(executor, TASK_EXECUTOR);
        executor.initialize();
        return executor;
    }

再次重啓完美解決。
附上Primary源碼：

 /*
 * Copyright 2002-2016 the original author or authors.
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package org.springframework.context.annotation;

import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * Indicates that a bean should be given preference when multiple candidates
 * are qualified to autowire a single-valued dependency. If exactly one
 * 'primary' bean exists among the candidates, it will be the autowired value.
 *
 * <p>This annotation is semantically equivalent to the {@code <bean>} element's
 * {@code primary} attribute in Spring XML.
 *
 * <p>May be used on any class directly or indirectly annotated with
 * {@code @Component} or on methods annotated with @{@link Bean}.
 *
 * <h2>Example</h2>
 * <pre class="code">
 * &#064;Component
 * public class FooService {
 *
 *     private FooRepository fooRepository;
 *
 *     &#064;Autowired
 *     public FooService(FooRepository fooRepository) {
 *         this.fooRepository = fooRepository;
 *     }
 * }
 *
 * &#064;Component
 * public class JdbcFooRepository {
 *
 *     public JdbcFooService(DataSource dataSource) {
 *         // ...
 *     }
 * }
 *
 * &#064;Primary
 * &#064;Component
 * public class HibernateFooRepository {
 *
 *     public HibernateFooService(SessionFactory sessionFactory) {
 *         // ...
 *     }
 * }
 * </pre>
 *
 * <p>Because {@code HibernateFooRepository} is marked with {@code @Primary},
 * it will be injected preferentially over the jdbc-based variant assuming both
 * are present as beans within the same Spring application context, which is
 * often the case when component-scanning is applied liberally.
 *
 * <p>Note that using {@code @Primary} at the class level has no effect unless
 * component-scanning is being used. If a {@code @Primary}-annotated class is
 * declared via XML, {@code @Primary} annotation metadata is ignored, and
 * {@code <bean primary="true|false"/>} is respected instead.
 *
 * @author Chris Beams
 * @since 3.0
 * @see Lazy
 * @see Bean
 * @see ComponentScan
 * @see org.springframework.stereotype.Component
 */
@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Inherited
@Documented
public @interface Primary {

}