如果你没听过这个类，笨比，那还犹豫什么，赶紧进来看看。
自动配置中的ObjectProvider
在Spring Boot自动配置源码中关于Tomcat的配置时，有这样的自动配置配置源代码。
@Configuration(proxyBeanMethods = false)
@ConditionalOnClass({ Servlet.class, Tomcat.class, UpgradeProtocol.class })
@ConditionalOnMissingBean(value = ServletWebServerFactory.class, search = SearchStrategy.CURRENT)
public static class EmbeddedTomcat {
@Bean
public TomcatServletWebServerFactory tomcatServletWebServerFactory(
ObjectProvider<TomcatConnectorCustomizer> connectorCustomizers,
ObjectProvider<TomcatContextCustomizer> contextCustomizers,
ObjectProvider<TomcatProtocolHandlerCustomizer<?>> protocolHandlerCustomizers) {
TomcatServletWebServerFactory factory = new TomcatServletWebServerFactory();
factory.getTomcatConnectorCustomizers()
.addAll(connectorCustomizers.orderedStream().collect(Collectors.toList()));
factory.getTomcatContextCustomizers()
.addAll(contextCustomizers.orderedStream().collect(Collectors.toList()));
factory.getTomcatProtocolHandlerCustomizers()
.addAll(protocolHandlerCustomizers.orderedStream().collect(Collectors.toList()));
return factory;
}
}
这就是一个常规的基于Java的配置类。
Spring的注入
在介绍ObjectProvider的使用之前，我们先来回顾一下注入相关的知识。
在Spring的使用过程中，我们可以通过多种形式将一个类注入到另外一个类当中，比如通过@Autowired和@Resources注解，这里不过多赘述。
而@Autowired又可以注解在不同的地方来达到注入的效果，比如：
注解在构造函数上
@Service
public class UserService {
private final UserRepository repository;
@Autowired
public UserService(UserRepository repository) {
this.repository = repository
}
}
注解在属性上
@Service
public class UserService {
@Autowired
private final UserRepository repository;
}
注解在setter方法上
@Service
public class UserService {
private final UserRepository repository;
@Autowired
public void setUserRepository(UserRepository repository) {
this.repository = repository
}
}
Spring4.3新特性
上面是最常见的注入方式，如果忘记写@Autowired注解，那么在启动的时候就会抛出异常。
但在spring 4.3之后，引入了一个新特性：当构造方法的参数为单个构造参数时，可以不使用@Autowired进行注解。
因此，上面的代码可变为如下形式：
@Service
public class UserService {
private final UserRepository repository;
public void setUserRepository(UserRepository repository) {
this.repository = repository
}
}
使用此种形式便会显得优雅一些。该特性，在Spring Boot的自动配置类中被大量使用。
依赖关系的改进
同样是在Spring 4.3版本中，不仅隐式的注入了单构造参数的属性，还引入了ObjectProvider接口。
ObjectProvider接口是ObjectFactory接口的扩展，专门为注入点设计的，可以让注入变得更加宽松和更具有可选项。
那么什么时候使用ObjectProvider接口？
如果待注入参数的Bean为空或有多个时，便是ObjectProvider发挥作用的时候了。
在我们以前写例如@Autowired的时候，如果找不到这个Bean注入，就会抛出异常。
如果注入实例为空时，使用ObjectProvider则避免了强依赖导致的依赖对象不存在异常；
如果有多个实例，ObjectProvider的方法会根据Bean实现的Ordered接口或@Order注解指定的先后顺序获取一个Bean。从而了提供了一个更加宽松的依赖注入方式。
Spring 5.1之后提供了基于Stream的orderedStream方法来获取有序的Stream的方法。
使用ObjectProvider之后，上面的代码便变为如下方式：
@Service
public class UserService {
private final UserRepository repository;
public UserService(ObjectProvider<UserRepository> repositoryProvider) {
this.repository = repositoryProvider.getIfUnique();
}
}
这样的好处很显然，当容器中不存在UserRepository或存在多个时，可以从容处理。
但坏处也很明显，如果UserRepository不能为null，则可能将异常从启动阶段转移到业务运行阶段。
ObjectProvider源码
/*
* Copyright 2002-2018 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.beans.factory;
import java.util.Iterator;
import java.util.function.Consumer;
import java.util.function.Supplier;
import java.util.stream.Stream;
import org.springframework.beans.BeansException;
import org.springframework.lang.Nullable;
/**
* A variant of {@link ObjectFactory} designed specifically for injection points,
* allowing for programmatic optionality and lenient not-unique handling.
*
* <p>As of 5.1, this interface extends {@link Iterable} and provides {@link Stream}
* support. It can be therefore be used in {@code for} loops, provides {@link #forEach}
* iteration and allows for collection-style {@link #stream} access.
*
* @author Juergen Hoeller
* @since 4.3
* @param <T> the object type
* @see BeanFactory#getBeanProvider
* @see org.springframework.beans.factory.annotation.Autowired
*/
public interface ObjectProvider<T> extends ObjectFactory<T>, Iterable<T> {
/**
* Return an instance (possibly shared or independent) of the object
* managed by this factory.
* <p>Allows for specifying explicit construction arguments, along the
* lines of {@link BeanFactory#getBean(String, Object...)}.
* @param args arguments to use when creating a corresponding instance
* @return an instance of the bean
* @throws BeansException in case of creation errors
* @see #getObject()
*/
// 返回指定类型的bean, 如果容器中不存在, 抛出NoSuchBeanDefinitionException异常
// 如果容器中有多个此类型的bean, 抛出NoUniqueBeanDefinitionException异常
T getObject(Object... args) throws BeansException;
/**
* Return an instance (possibly shared or independent) of the object
* managed by this factory.
* @return an instance of the bean, or {@code null} if not available
* @throws BeansException in case of creation errors
* @see #getObject()
*/
// 如果指定类型的bean注册到容器中, 返回 bean 实例, 否则返回 null
@Nullable
T getIfAvailable() throws BeansException;
/**
* Return an instance (possibly shared or independent) of the object
* managed by this factory.
* @param defaultSupplier a callback for supplying a default object
* if none is present in the factory
* @return an instance of the bean, or the supplied default object
* if no such bean is available
* @throws BeansException in case of creation errors
* @since 5.0
* @see #getIfAvailable()
*/
// 如果返回对象不存在，则进行回调，回调对象由Supplier传入
default T getIfAvailable(Supplier<T> defaultSupplier) throws BeansException {
T dependency = getIfAvailable();
return (dependency != null ? dependency : defaultSupplier.get());
}
/**
* Consume an instance (possibly shared or independent) of the object
* managed by this factory, if available.
* @param dependencyConsumer a callback for processing the target object
* if available (not called otherwise)
* @throws BeansException in case of creation errors
* @since 5.0
* @see #getIfAvailable()
*/
// 消费对象的一个实例（可能是共享的或独立的），如果存在通过Consumer回调消耗目标对象。
default void ifAvailable(Consumer<T> dependencyConsumer) throws BeansException {
T dependency = getIfAvailable();
if (dependency != null) {
dependencyConsumer.accept(dependency);
}
}
/**
* Return an instance (possibly shared or independent) of the object
* managed by this factory.
* @return an instance of the bean, or {@code null} if not available or
* not unique (i.e. multiple candidates found with none marked as primary)
* @throws BeansException in case of creation errors
* @see #getObject()
*/
// 如果不可用或不唯一（没有指定primary）则返回null。否则，返回对象。
@Nullable
T getIfUnique() throws BeansException;
/**
* Return an instance (possibly shared or independent) of the object
* managed by this factory.
* @param defaultSupplier a callback for supplying a default object
* if no unique candidate is present in the factory
* @return an instance of the bean, or the supplied default object
* if no such bean is available or if it is not unique in the factory
* (i.e. multiple candidates found with none marked as primary)
* @throws BeansException in case of creation errors
* @since 5.0
* @see #getIfUnique()
*/
// 如果存在唯一对象，则调用Supplier的回调函数
default T getIfUnique(Supplier<T> defaultSupplier) throws BeansException {
T dependency = getIfUnique();
return (dependency != null ? dependency : defaultSupplier.get());
}
/**
* Consume an instance (possibly shared or independent) of the object
* managed by this factory, if unique.
* @param dependencyConsumer a callback for processing the target object
* if unique (not called otherwise)
* @throws BeansException in case of creation errors
* @since 5.0
* @see #getIfAvailable()
*/
// 如果存在唯一对象，则消耗掉该对象
default void ifUnique(Consumer<T> dependencyConsumer) throws BeansException {
T dependency = getIfUnique();
if (dependency != null) {
dependencyConsumer.accept(dependency);
}
}
/**
* Return an {@link Iterator} over all matching object instances,
* without specific ordering guarantees (but typically in registration order).
* @since 5.1
* @see #stream()
*/
// 返回符合条件的对象的Iterator，没有特殊顺序保证（一般为注册顺序）
@Override
default Iterator<T> iterator() {
return stream().iterator();
}
/**
* Return a sequential {@link Stream} over all matching object instances,
* without specific ordering guarantees (but typically in registration order).
* @since 5.1
* @see #iterator()
* @see #orderedStream()
*/
// 返回符合条件对象的连续的Stream，没有特殊顺序保证（一般为注册顺序）
default Stream<T> stream() {
throw new UnsupportedOperationException("Multi element access not supported");
}
/**
* Return a sequential {@link Stream} over all matching object instances,
* pre-ordered according to the factory's common order comparator.
* <p>In a standard Spring application context, this will be ordered
* according to {@link org.springframework.core.Ordered} conventions,
* and in case of annotation-based configuration also considering the
* {@link org.springframework.core.annotation.Order} annotation,
* analogous to multi-element injection points of list/array type.
* @since 5.1
* @see #stream()
* @see org.springframework.core.OrderComparator
*/
// 返回符合条件对象的连续的Stream。在标注Spring应用上下文中采用@Order注解或实现Order接口的顺序
default Stream<T> orderedStream() {
throw new UnsupportedOperationException("Ordered element access not supported");
}
}
其中，在BeanFactory中也使用了该接口来定义方法的返回值：
public interface BeanFactory {
...
<T> ObjectProvider<T> getBeanProvider(Class<T> requiredType);
<T> ObjectProvider<T> getBeanProvider(ResolvableType requiredType);
...
}
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