使用 Spring 中的 @Lazy 注解

@Lazy注解用于标识 bean 是否需要延迟加载：

@Target({ElementType.TYPE, ElementType.METHOD, ElementType.CONSTRUCTOR, ElementType.PARAMETER, ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Lazy {
    /**
     * Whether lazy initialization should occur.
     */
    boolean value() default true;
}

只有一个参数，默认是true，也就是说只要加了这个注解就会延迟加载。
@Lazy注解则必须在第一次调用的时候才会加载。
@Lazy注解注解的作用主要是减少springIOC容器启动的加载时间。

@Lazy注解可以用来解决 Requested bean is currently in creation 的问题 (https://www.cnblogs.com/zjhgx/p/12158698.html)。

完整源码如下：

/*
 * Copyright 2002-2013 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
 *
 *      https://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.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * Indicates whether a bean is to be lazily initialized.
 *
 * <p>May be used on any class directly or indirectly annotated with {@link
 * org.springframework.stereotype.Component @Component} or on methods annotated with
 * {@link Bean @Bean}.
 *
 * <p>If this annotation is not present on a {@code @Component} or {@code @Bean} definition,
 * eager initialization will occur. If present and set to {@code true}, the {@code @Bean} or
 * {@code @Component} will not be initialized until referenced by another bean or explicitly
 * retrieved from the enclosing {@link org.springframework.beans.factory.BeanFactory
 * BeanFactory}. If present and set to {@code false}, the bean will be instantiated on
 * startup by bean factories that perform eager initialization of singletons.
 *
 * <p>If Lazy is present on a {@link Configuration @Configuration} class, this
 * indicates that all {@code @Bean} methods within that {@code @Configuration}
 * should be lazily initialized. If {@code @Lazy} is present and false on a {@code @Bean}
 * method within a {@code @Lazy}-annotated {@code @Configuration} class, this indicates
 * overriding the 'default lazy' behavior and that the bean should be eagerly initialized.
 *
 * <p>In addition to its role for component initialization, this annotation may also be placed
 * on injection points marked with {@link org.springframework.beans.factory.annotation.Autowired}
 * or {@link javax.inject.Inject}: In that context, it leads to the creation of a
 * lazy-resolution proxy for all affected dependencies, as an alternative to using
 * {@link org.springframework.beans.factory.ObjectFactory} or {@link javax.inject.Provider}.
 *
 * @author Chris Beams
 * @author Juergen Hoeller
 * @since 3.0
 * @see Primary
 * @see Bean
 * @see Configuration
 * @see org.springframework.stereotype.Component
 */
@Target({ElementType.TYPE, ElementType.METHOD, ElementType.CONSTRUCTOR, ElementType.PARAMETER, ElementType.FIELD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Lazy {

    /**
     * Whether lazy initialization should occur.
     */
    boolean value() default true;

}