条件化bean

天行健，君子以自强不息。——《周易》

应用场景

　　假设你希望一个或多个bean只有在应用的类路径下包含特定的库时才创建，希望某个bean只有当某个特定的bean也声明了之后才会创建，或者只有某个特定的环境变量设置之后才会创建bean。Spring 4 引入了@Conditional注解，它可以用到带有@Bean注解的方法上，如果给定的条件满足才会创建bean，反之则会被忽略。

　　条件化配置bean的代码如下:

package chapter3;

import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Conditional;
import org.springframework.context.annotation.Configuration;

@Configuration
public class MagicConfig {
    
    @Bean
    @Conditional(MagicExistsCondition.class)
    public MagicBean magicBean() {
        return new MagicBean();
    }
}

　　设置给@Conditional的类可以使任意实现了Condition接口的类，如下MagicExistsCondition.java：

package chapter3;

import org.springframework.context.annotation.Condition;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.core.env.Environment;
import org.springframework.core.type.AnnotatedTypeMetadata;

public class MagicExistsCondition implements Condition {

    public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
        Environment environment = context.getEnvironment();
        return environment.containsProperty("magic");
    }

}

　　ConditionContext是一个接口，通过该接口我们可以做到以下几点：

 * Copyright 2002-2017 the original author or authors.

package org.springframework.context.annotation;

import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
import org.springframework.beans.factory.support.BeanDefinitionRegistry;
import org.springframework.core.env.Environment;
import org.springframework.core.io.ResourceLoader;

/**
 * Context information for use by {@link Condition}s.
 *
 * @author Phillip Webb
 * @author Juergen Hoeller
 * @since 4.0
 */
public interface ConditionContext {

    /**
     * Return the {@link BeanDefinitionRegistry} that will hold the bean definition
     * should the condition match, or {@code null} if the registry is not available.
　　　* 借助返回的BeanDefinitionRegistry检查bean的定义
     */
    BeanDefinitionRegistry getRegistry();

    /**
     * Return the {@link ConfigurableListableBeanFactory} that will hold the bean
     * definition should the condition match, or {@code null} if the bean factory
     * is not available.
　　  * 检查bean是否存在，甚至探查bean的属性
     */
    ConfigurableListableBeanFactory getBeanFactory();

    /**
     * Return the {@link Environment} for which the current application is running,
     * or {@code null} if no environment is available.
　　  * 检查环境变量是否存在以及它的值是什么
     */
    Environment getEnvironment();

    /**
     * Return the {@link ResourceLoader} currently being used, or {@code null} if
     * the resource loader cannot be obtained.
     * 读取并探查返回的ResourceLoader所加载的资源
     */
    ResourceLoader getResourceLoader();

    /**
     * Return the {@link ClassLoader} that should be used to load additional classes,
     * or {@code null} if the default classloader should be used.
     * 借助返回的ClassLoader加载并检查类是否存在
     */
    ClassLoader getClassLoader();

}

　　AnnotatedTypeMetadata也是一个接口，通过该接口我们可以：

 * Copyright 2002-2015 the original author or authors.

package org.springframework.core.type;

import java.util.Map;

import org.springframework.util.MultiValueMap;

/**
 * Defines access to the annotations of a specific type ({@link AnnotationMetadata class}
 * or {@link MethodMetadata method}), in a form that does not necessarily require the
 * class-loading.
 *
 * @author Juergen Hoeller
 * @author Mark Fisher
 * @author Mark Pollack
 * @author Chris Beams
 * @author Phillip Webb
 * @author Sam Brannen
 * @since 4.0
 * @see AnnotationMetadata
 * @see MethodMetadata
 */
public interface AnnotatedTypeMetadata {

    /**
     * Determine whether the underlying element has an annotation or meta-annotation
     * of the given type defined.
     * <p>If this method returns {@code true}, then
     * {@link #getAnnotationAttributes} will return a non-null Map.
     * @param annotationName the fully qualified class name of the annotation
     * type to look for
     * @return whether a matching annotation is defined
     * 判断带有@Bean注解的方法是否还有其他特定的注解
     */
    boolean isAnnotated(String annotationName);

    /**
     * Retrieve the attributes of the annotation of the given type, if any (i.e. if
     * defined on the underlying element, as direct annotation or meta-annotation),
     * also taking attribute overrides on composed annotations into account.
     * @param annotationName the fully qualified class name of the annotation
     * type to look for
     * @return a Map of attributes, with the attribute name as key (e.g. "value")
     * and the defined attribute value as Map value. This return value will be
     * {@code null} if no matching annotation is defined.
     * 检查@Bean注解的方法上其他注解的属性
     */
    Map<String, Object> getAnnotationAttributes(String annotationName);

    /**
     * Retrieve the attributes of the annotation of the given type, if any (i.e. if
     * defined on the underlying element, as direct annotation or meta-annotation),
     * also taking attribute overrides on composed annotations into account.
     * @param annotationName the fully qualified class name of the annotation
     * type to look for
     * @param classValuesAsString whether to convert class references to String
     * class names for exposure as values in the returned Map, instead of Class
     * references which might potentially have to be loaded first
     * @return a Map of attributes, with the attribute name as key (e.g. "value")
     * and the defined attribute value as Map value. This return value will be
     * {@code null} if no matching annotation is defined.
     * 检查@Bean注解的方法上其他注解的属性
     */
    Map<String, Object> getAnnotationAttributes(String annotationName, boolean classValuesAsString);

    /**
     * Retrieve all attributes of all annotations of the given type, if any (i.e. if
     * defined on the underlying element, as direct annotation or meta-annotation).
     * Note that this variant does <i>not</i> take attribute overrides into account.
     * @param annotationName the fully qualified class name of the annotation
     * type to look for
     * @return a MultiMap of attributes, with the attribute name as key (e.g. "value")
     * and a list of the defined attribute values as Map value. This return value will
     * be {@code null} if no matching annotation is defined.
     * @see #getAllAnnotationAttributes(String, boolean)
     */
    MultiValueMap<String, Object> getAllAnnotationAttributes(String annotationName);

    /**
     * Retrieve all attributes of all annotations of the given type, if any (i.e. if
     * defined on the underlying element, as direct annotation or meta-annotation).
     * Note that this variant does <i>not</i> take attribute overrides into account.
     * @param annotationName the fully qualified class name of the annotation
     * type to look for
     * @param classValuesAsString  whether to convert class references to String
     * @return a MultiMap of attributes, with the attribute name as key (e.g. "value")
     * and a list of the defined attribute values as Map value. This return value will
     * be {@code null} if no matching annotation is defined.
     * @see #getAllAnnotationAttributes(String)
     */
    MultiValueMap<String, Object> getAllAnnotationAttributes(String annotationName, boolean classValuesAsString);

}

　　Spring4中，@Profile注解实现代码如下：

/*
 * Copyright 2002-2017 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.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

import org.springframework.core.env.AbstractEnvironment;
import org.springframework.core.env.ConfigurableEnvironment;

/**
 * Indicates that a component is eligible for registration when one or more
 * {@linkplain #value specified profiles} are active.
 *
 * <p>A <em>profile</em> is a named logical grouping that may be activated
 * programmatically via {@link ConfigurableEnvironment#setActiveProfiles} or declaratively
 * by setting the {@link AbstractEnvironment#ACTIVE_PROFILES_PROPERTY_NAME
 * spring.profiles.active} property as a JVM system property, as an
 * environment variable, or as a Servlet context parameter in {@code web.xml}
 * for web applications. Profiles may also be activated declaratively in
 * integration tests via the {@code @ActiveProfiles} annotation.
 *
 * <p>The {@code @Profile} annotation may be used in any of the following ways:
 * <ul>
 * <li>as a type-level annotation on any class directly or indirectly annotated with
 * {@code @Component}, including {@link Configuration @Configuration} classes</li>
 * <li>as a meta-annotation, for the purpose of composing custom stereotype annotations</li>
 * <li>as a method-level annotation on any {@link Bean @Bean} method</li>
 * </ul>
 *
 * <p>If a {@code @Configuration} class is marked with {@code @Profile}, all of the
 * {@code @Bean} methods and {@link Import @Import} annotations associated with that class
 * will be bypassed unless one or more of the specified profiles are active. This is
 * analogous to the behavior in Spring XML: if the {@code profile} attribute of the
 * {@code beans} element is supplied e.g., {@code <beans profile="p1,p2">}, the
 * {@code beans} element will not be parsed unless at least profile 'p1' or 'p2' has been
 * activated. Likewise, if a {@code @Component} or {@code @Configuration} class is marked
 * with {@code @Profile({"p1", "p2"})}, that class will not be registered or processed unless
 * at least profile 'p1' or 'p2' has been activated.
 *
 * <p>If a given profile is prefixed with the NOT operator ({@code !}), the annotated
 * component will be registered if the profile is <em>not</em> active &mdash; for example,
 * given {@code @Profile({"p1", "!p2"})}, registration will occur if profile 'p1' is active
 * or if profile 'p2' is <em>not</em> active.
 *
 * <p>If the {@code @Profile} annotation is omitted, registration will occur regardless
 * of which (if any) profiles are active.
 *
 * <p><b>NOTE:</b> With {@code @Profile} on {@code @Bean} methods, a special scenario may
 * apply: In the case of overloaded {@code @Bean} methods of the same Java method name
 * (analogous to constructor overloading), an {@code @Profile} condition needs to be
 * consistently declared on all overloaded methods. If the conditions are inconsistent,
 * only the condition on the first declaration among the overloaded methods will matter.
 * {@code @Profile} can therefore not be used to select an overloaded method with a
 * particular argument signature over another; resolution between all factory methods
 * for the same bean follows Spring's constructor resolution algorithm at creation time.
 * <b>Use distinct Java method names pointing to the same {@link Bean#name bean name}
 * if you'd like to define alternative beans with different profile conditions</b>;
 * see {@code ProfileDatabaseConfig} in {@link Configuration @Configuration}'s javadoc.
 *
 * <p>When defining Spring beans via XML, the {@code "profile"} attribute of the
 * {@code <beans>} element may be used. See the documentation in the
 * {@code spring-beans} XSD (version 3.1 or greater) for details.
 *
 * @author Chris Beams
 * @author Phillip Webb
 * @author Sam Brannen
 * @since 3.1
 * @see ConfigurableEnvironment#setActiveProfiles
 * @see ConfigurableEnvironment#setDefaultProfiles
 * @see AbstractEnvironment#ACTIVE_PROFILES_PROPERTY_NAME
 * @see AbstractEnvironment#DEFAULT_PROFILES_PROPERTY_NAME
 * @see Conditional
 * @see org.springframework.test.context.ActiveProfiles
 */
@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional(ProfileCondition.class)
public @interface Profile {

    /**
     * The set of profiles for which the annotated component should be registered.
     */
    String[] value();

}

　　从上面@Profile注解的实现代码可发现@Profile也使用了@Conditional注解，并且引用ProfileCondition作为Condition实现，其中ProfileCondition实现如下：

/*
 * 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
 *
 *      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 org.springframework.core.type.AnnotatedTypeMetadata;
import org.springframework.util.MultiValueMap;

/**
 * {@link Condition} that matches based on the value of a {@link Profile @Profile}
 * annotation.
 *
 * @author Chris Beams
 * @author Phillip Webb
 * @author Juergen Hoeller
 * @since 4.0
 */
class ProfileCondition implements Condition {

    @Override
    public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
        if (context.getEnvironment() != null) {
            MultiValueMap<String, Object> attrs = metadata.getAllAnnotationAttributes(Profile.class.getName());
            if (attrs != null) {
                for (Object value : attrs.get("value")) {
                    if (context.getEnvironment().acceptsProfiles(((String[]) value))) {
                        return true;
                    }
                }
                return false;
            }
        }
        return true;
    }

}