深入理解SpringBoot之装配条件

　　我们知道自动装配是SpringBoot微服务化的核心，它会把META-INF/spring.factoires里配置的EnableAutoConfiguration注册到IOC容器里。但是，请大家考虑一个问题，根据需求我们要配置一个tomcat的内嵌容器，可是当前的运行环境里都没有servlet的相关API或者说当前的ApplicationContext不是一个WebApplicationContext，如果这样的话，那么创建tomcat的内嵌容器还有什么意义上呢？如果根据需求我们想自动装配一个Mybatis的SqlSessionFactory，可是运行环境里连DataSource都没有，恐怕要自动装配Mybatis的愿望也会落空吧！针对这种问题，SpringBoot早都考虑到了，下面我们来看看SpringBoot是怎么解决的。

一、关于@Conditional

　　conditional中文的意思为条件，其本身是Springframework提供的核心注解，通常情况下该注解可以加在类上或者方法上与@Configuration或者@Bean配合使用，当和@Configuration配合使用时，那么该类下所有@Bean方法 或者@Import 或者 @ComponentScan都会受到其配置条件的影响，我们先看一下其源码：

/*
 * 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;

/**
 * Indicates that a component is only eligible for registration when all
 * {@linkplain #value specified conditions} match.
 *
 * <p>A <em>condition</em> is any state that can be determined programmatically
 * before the bean definition is due to be registered (see {@link Condition} for details).
 *
 * <p>The {@code @Conditional} 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 @Conditional},
 * all of the {@code @Bean} methods, {@link Import @Import} annotations, and
 * {@link ComponentScan @ComponentScan} annotations associated with that
 * class will be subject to the conditions.
 *
 * <p><strong>NOTE</strong>: Inheritance of {@code @Conditional} annotations
 * is not supported; any conditions from superclasses or from overridden
 * methods will not be considered. In order to enforce these semantics,
 * {@code @Conditional} itself is not declared as
 * {@link java.lang.annotation.Inherited @Inherited}; furthermore, any
 * custom <em>composed annotation</em> that is meta-annotated with
 * {@code @Conditional} must not be declared as {@code @Inherited}.
 *
 * @author Phillip Webb
 * @author Sam Brannen
 * @since 4.0
 * @see Condition
 */
@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Conditional {

    /**
     * All {@link Condition}s that must {@linkplain Condition#matches match}
     * in order for the component to be registered.
     */
    Class<? extends Condition>[] value();

}

在这里文档注释提醒我们去看Condition接口：

/*
 * 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.beans.factory.config.BeanFactoryPostProcessor;
import org.springframework.core.type.AnnotatedTypeMetadata;

/**
 * A single {@code condition} that must be {@linkplain #matches matched} in order
 * for a component to be registered.
 *
 * <p>Conditions are checked immediately before the bean-definition is due to be
 * registered and are free to veto registration based on any criteria that can
 * be determined at that point.
 *
 * <p>Conditions must follow the same restrictions as {@link BeanFactoryPostProcessor}
 * and take care to never interact with bean instances. For more fine-grained control
 * of conditions that interact with {@code @Configuration} beans consider the
 * {@link ConfigurationCondition} interface.
 *
 * @author Phillip Webb
 * @since 4.0
 * @see ConfigurationCondition
 * @see Conditional
 * @see ConditionContext
 */
public interface Condition {

    /**
     * Determine if the condition matches.
     * @param context the condition context
     * @param metadata metadata of the {@link org.springframework.core.type.AnnotationMetadata class}
     * or {@link org.springframework.core.type.MethodMetadata method} being checked.
     * @return {@code true} if the condition matches and the component can be registered
     * or {@code false} to veto registration.
     */
    boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata);

}

该接口就有一个方法：matches方法。它定义了最基本的匹配规则，该方法传入两个参数一个是ConditionContext ，该接口定义了若干个方法来获取spring核心接口的方法：

/*
 * 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 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.
     */
    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.
     */
    ConfigurableListableBeanFactory getBeanFactory();

    /**
     * Return the {@link Environment} for which the current application is running,
     * or {@code null} if no environment is available.
     */
    Environment getEnvironment();

    /**
     * Return the {@link ResourceLoader} currently being used, or {@code null} if
     * the resource loader cannot be obtained.
     */
    ResourceLoader getResourceLoader();

    /**
     * Return the {@link ClassLoader} that should be used to load additional classes,
     * or {@code null} if the default classloader should be used.
     */
    ClassLoader getClassLoader();

}

在这里我们能获取到BeanFactory，ResourceLoader，Enviroment等。而另外一个参数是AnnotatedTypeMetadata接口，该接口主要获取该类上标记的注解。在这里我先写一个简单的例子，来试验一下：

MyTestConditional:

package com.hzgj.lyrk.autoconfigure;

import org.springframework.context.annotation.ConditionContext;
import org.springframework.context.annotation.ConfigurationCondition;
import org.springframework.core.type.AnnotatedTypeMetadata;

import java.util.Map;

public class MyTestConditional implements ConfigurationCondition {
    @Override
    public ConfigurationPhase getConfigurationPhase() {
        return ConfigurationPhase.REGISTER_BEAN;
    }

    @Override
    public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
        Map<String, Object> map = metadata.getAnnotationAttributes("org.springframework.context.annotation.Description");
        System.out.println(map);
        return false;
    }
}

　　这个是自定义的Conditional，该类实现了ConfigurationCondition接口，该接口继承了Condition，只不过它多添加了一个用于设置解析Condition阶段的方法，在这里有两个阶段进行解析：

　　1）PARSE_CONFIGURATION：会在解析@Configuration时进行condition的解析

　　2）REGISTER_BEAN：会在注册Bean的时候进行condition的解析

ServerAutoConfiguration:

package com.hzgj.lyrk.autoconfigure;

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

@Configuration
public class ServerAutoConfiguration {

    @Configuration
    @Conditional(MyTestConditional.class)
    @Description(value = "student")
    public static class StudentAutoConfiguration {
        @Bean
        public Student student() {
            System.out.println("student create....");
            return new Student();
        }

    }

    @Configuration
    @Conditional(MyTestConditional.class)
    @Description(value = "teacher")
    public static class TeacherAutoConfiguration {
        @Bean
        public Teacher teacher() {
            System.out.println("teacher create.....");
            return new Teacher();
        }
    }
}

此时由于自定义的Conditional的match方法返回值是false，因此不能注册@Bean配置的对象：

当值改为true时，则能注册@Bean配置的对象：

二、SpringBoot中对Conditional的扩展

　　在SpringBoot中定义了一个SpringBootCondition类对Condition进行了扩展，该类源代码如下：

/*
 * Copyright 2012-2017 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.boot.autoconfigure.condition;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import org.springframework.context.annotation.Condition;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.core.type.AnnotatedTypeMetadata;
import org.springframework.core.type.AnnotationMetadata;
import org.springframework.core.type.ClassMetadata;
import org.springframework.core.type.MethodMetadata;
import org.springframework.util.ClassUtils;
import org.springframework.util.StringUtils;

/**
 * Base of all {@link Condition} implementations used with Spring Boot. Provides sensible
 * logging to help the user diagnose what classes are loaded.
 *
 * @author Phillip Webb
 * @author Greg Turnquist
 */
public abstract class SpringBootCondition implements Condition {

    private final Log logger = LogFactory.getLog(getClass());

    @Override
    public final boolean matches(ConditionContext context,
            AnnotatedTypeMetadata metadata) {
        String classOrMethodName = getClassOrMethodName(metadata);
        try {
            ConditionOutcome outcome = getMatchOutcome(context, metadata);
            logOutcome(classOrMethodName, outcome);
            recordEvaluation(context, classOrMethodName, outcome);
            return outcome.isMatch();
        }
        catch (NoClassDefFoundError ex) {
            throw new IllegalStateException(
                    "Could not evaluate condition on " + classOrMethodName + " due to "
                            + ex.getMessage() + " not "
                            + "found. Make sure your own configuration does not rely on "
                            + "that class. This can also happen if you are "
                            + "@ComponentScanning a springframework package (e.g. if you "
                            + "put a @ComponentScan in the default package by mistake)",
                    ex);
        }
        catch (RuntimeException ex) {
            throw new IllegalStateException(
                    "Error processing condition on " + getName(metadata), ex);
        }
    }

    private String getName(AnnotatedTypeMetadata metadata) {
        if (metadata instanceof AnnotationMetadata) {
            return ((AnnotationMetadata) metadata).getClassName();
        }
        if (metadata instanceof MethodMetadata) {
            MethodMetadata methodMetadata = (MethodMetadata) metadata;
            return methodMetadata.getDeclaringClassName() + "."
                    + methodMetadata.getMethodName();
        }
        return metadata.toString();
    }

    private static String getClassOrMethodName(AnnotatedTypeMetadata metadata) {
        if (metadata instanceof ClassMetadata) {
            ClassMetadata classMetadata = (ClassMetadata) metadata;
            return classMetadata.getClassName();
        }
        MethodMetadata methodMetadata = (MethodMetadata) metadata;
        return methodMetadata.getDeclaringClassName() + "#"
                + methodMetadata.getMethodName();
    }

    protected final void logOutcome(String classOrMethodName, ConditionOutcome outcome) {
        if (this.logger.isTraceEnabled()) {
            this.logger.trace(getLogMessage(classOrMethodName, outcome));
        }
    }

    private StringBuilder getLogMessage(String classOrMethodName,
            ConditionOutcome outcome) {
        StringBuilder message = new StringBuilder();
        message.append("Condition ");
        message.append(ClassUtils.getShortName(getClass()));
        message.append(" on ");
        message.append(classOrMethodName);
        message.append(outcome.isMatch() ? " matched" : " did not match");
        if (StringUtils.hasLength(outcome.getMessage())) {
            message.append(" due to ");
            message.append(outcome.getMessage());
        }
        return message;
    }

    private void recordEvaluation(ConditionContext context, String classOrMethodName,
            ConditionOutcome outcome) {
        if (context.getBeanFactory() != null) {
            ConditionEvaluationReport.get(context.getBeanFactory())
                    .recordConditionEvaluation(classOrMethodName, this, outcome);
        }
    }

    /**
     * Determine the outcome of the match along with suitable log output.
     * @param context the condition context
     * @param metadata the annotation metadata
     * @return the condition outcome
     */
    public abstract ConditionOutcome getMatchOutcome(ConditionContext context,
            AnnotatedTypeMetadata metadata);

    /**
     * Return true if any of the specified conditions match.
     * @param context the context
     * @param metadata the annotation meta-data
     * @param conditions conditions to test
     * @return {@code true} if any condition matches.
     */
    protected final boolean anyMatches(ConditionContext context,
            AnnotatedTypeMetadata metadata, Condition... conditions) {
        for (Condition condition : conditions) {
            if (matches(context, metadata, condition)) {
                return true;
            }
        }
        return false;
    }

    /**
     * Return true if any of the specified condition matches.
     * @param context the context
     * @param metadata the annotation meta-data
     * @param condition condition to test
     * @return {@code true} if the condition matches.
     */
    protected final boolean matches(ConditionContext context,
            AnnotatedTypeMetadata metadata, Condition condition) {
        if (condition instanceof SpringBootCondition) {
            return ((SpringBootCondition) condition).getMatchOutcome(context, metadata)
                    .isMatch();
        }
        return condition.matches(context, metadata);
    }

}

　　在这里，我们需要重写getMatchOutcome方法来进行，匹配结果的过滤，下面我们列举一下常见的Conditional:

2.1、Class Conditions

　　常见的有ConditionalOnClass，ConditionalOnMissingClass

　　ConditionalOnClass:表明当前classpath有对应指定的类型才去创建Bean,我们来看一下源代码：

/*
 * Copyright 2012-2017 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.boot.autoconfigure.condition;

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.context.annotation.Conditional;

/**
 * {@link Conditional} that only matches when the specified classes are on the classpath.
 *
 * @author Phillip Webb
 */
@Target({ ElementType.TYPE, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional(OnClassCondition.class)
public @interface ConditionalOnClass {

    /**
     * The classes that must be present. Since this annotation is parsed by loading class
     * bytecode, it is safe to specify classes here that may ultimately not be on the
     * classpath, only if this annotation is directly on the affected component and
     * <b>not</b> if this annotation is used as a composed, meta-annotation. In order to
     * use this annotation as a meta-annotation, only use the {@link #name} attribute.
     * @return the classes that must be present
     */
    Class<?>[] value() default {};

    /**
     * The classes names that must be present.
     * @return the class names that must be present.
     */
    String[] name() default {};

}

　　根据注释我们去寻找一下：OnClassCondition这个类，我贴出部分代码：

    @Override
    public ConditionOutcome getMatchOutcome(ConditionContext context,
            AnnotatedTypeMetadata metadata) {
        ClassLoader classLoader = context.getClassLoader();
        ConditionMessage matchMessage = ConditionMessage.empty();
        List<String> onClasses = getCandidates(metadata, ConditionalOnClass.class);
        if (onClasses != null) {
            List<String> missing = getMatches(onClasses, MatchType.MISSING, classLoader);
            if (!missing.isEmpty()) {
                return ConditionOutcome
                        .noMatch(ConditionMessage.forCondition(ConditionalOnClass.class)
                                .didNotFind("required class", "required classes")
                                .items(Style.QUOTE, missing));
            }
            matchMessage = matchMessage.andCondition(ConditionalOnClass.class)
                    .found("required class", "required classes").items(Style.QUOTE,
                            getMatches(onClasses, MatchType.PRESENT, classLoader));
        }
        List<String> onMissingClasses = getCandidates(metadata,
                ConditionalOnMissingClass.class);
        if (onMissingClasses != null) {
            List<String> present = getMatches(onMissingClasses, MatchType.PRESENT,
                    classLoader);
            if (!present.isEmpty()) {
                return ConditionOutcome.noMatch(
                        ConditionMessage.forCondition(ConditionalOnMissingClass.class)
                                .found("unwanted class", "unwanted classes")
                                .items(Style.QUOTE, present));
            }
            matchMessage = matchMessage.andCondition(ConditionalOnMissingClass.class)
                    .didNotFind("unwanted class", "unwanted classes").items(Style.QUOTE,
                            getMatches(onMissingClasses, MatchType.MISSING, classLoader));
        }
        return ConditionOutcome.match(matchMessage);
    }

在这里我们关注一下getMatches方法：

private List<String> getMatches(Collection<String> candidates, MatchType matchType,
            ClassLoader classLoader) {
        List<String> matches = new ArrayList<String>(candidates.size());
        for (String candidate : candidates) {
            if (matchType.matches(candidate, classLoader)) {
                matches.add(candidate);
            }
        }
        return matches;
    }

//.....

private enum MatchType {

        PRESENT {

            @Override
            public boolean matches(String className, ClassLoader classLoader) {
                return isPresent(className, classLoader);
            }

        },

        MISSING {

            @Override
            public boolean matches(String className, ClassLoader classLoader) {
                return !isPresent(className, classLoader);
            }

        };

        private static boolean isPresent(String className, ClassLoader classLoader) {
            if (classLoader == null) {
                classLoader = ClassUtils.getDefaultClassLoader();
            }
            try {
                forName(className, classLoader);
                return true;
            }
            catch (Throwable ex) {
                return false;
            }
        }

        private static Class<?> forName(String className, ClassLoader classLoader)
                throws ClassNotFoundException {
            if (classLoader != null) {
                return classLoader.loadClass(className);
            }
            return Class.forName(className);
        }

　　我们可以看到这里是通过ClassLoader或者Class.forName来加载类的

2.2、Bean Conditionals

　　在这里常见的是ConditionalOnBean和ConditionalOnMissingBean，只有当BeanFactory里(不)包含指定的Bean时，才能通过匹配。注意：官网建议我们在AutoConfiguration里使用此注解，因为受到bean装配顺序影响，很有可能不能达到我们的预期效果。

package com.hzgj.lyrk.autoconfigure;

import org.springframework.boot.autoconfigure.condition.ConditionalOnBean;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Conditional;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Description;

@Configuration
public class ServerAutoConfiguration {

    @Configuration
    @ConditionalOnBean(Teacher.class)
    public static class StudentAutoConfiguration {
        @Bean
        public Student student() {
            System.out.println("student create....");
            return new Student();
        }

    }

    @Configuration
    public static class TeacherAutoConfiguration {
        @Bean
        public Teacher teacher() {
            System.out.println("teacher create.....");
            return new Teacher();
        }
    }
}

比如说如上代码，运行后将得到如下结果：

我们可以看到此时Student并未创建。因为受其顺序影响当注册Student时，IOC容器里并没有Teacher，我在这里贴出OnBeanCondition的关键代码：

@Override
    public ConditionOutcome getMatchOutcome(ConditionContext context,
            AnnotatedTypeMetadata metadata) {
        ConditionMessage matchMessage = ConditionMessage.empty();
        if (metadata.isAnnotated(ConditionalOnBean.class.getName())) {
            BeanSearchSpec spec = new BeanSearchSpec(context, metadata,
                    ConditionalOnBean.class);
            List<String> matching = getMatchingBeans(context, spec);
            if (matching.isEmpty()) {
                return ConditionOutcome.noMatch(
                        ConditionMessage.forCondition(ConditionalOnBean.class, spec)
                                .didNotFind("any beans").atAll());
            }
            matchMessage = matchMessage.andCondition(ConditionalOnBean.class, spec)
                    .found("bean", "beans").items(Style.QUOTE, matching);
        }
        if (metadata.isAnnotated(ConditionalOnSingleCandidate.class.getName())) {
            BeanSearchSpec spec = new SingleCandidateBeanSearchSpec(context, metadata,
                    ConditionalOnSingleCandidate.class);
            List<String> matching = getMatchingBeans(context, spec);
            if (matching.isEmpty()) {
                return ConditionOutcome.noMatch(ConditionMessage
                        .forCondition(ConditionalOnSingleCandidate.class, spec)
                        .didNotFind("any beans").atAll());
            }
            else if (!hasSingleAutowireCandidate(context.getBeanFactory(), matching,
                    spec.getStrategy() == SearchStrategy.ALL)) {
                return ConditionOutcome.noMatch(ConditionMessage
                        .forCondition(ConditionalOnSingleCandidate.class, spec)
                        .didNotFind("a primary bean from beans")
                        .items(Style.QUOTE, matching));
            }
            matchMessage = matchMessage
                    .andCondition(ConditionalOnSingleCandidate.class, spec)
                    .found("a primary bean from beans").items(Style.QUOTE, matching);
        }
        if (metadata.isAnnotated(ConditionalOnMissingBean.class.getName())) {
            BeanSearchSpec spec = new BeanSearchSpec(context, metadata,
                    ConditionalOnMissingBean.class);
            List<String> matching = getMatchingBeans(context, spec);
            if (!matching.isEmpty()) {
                return ConditionOutcome.noMatch(ConditionMessage
                        .forCondition(ConditionalOnMissingBean.class, spec)
                        .found("bean", "beans").items(Style.QUOTE, matching));
            }
            matchMessage = matchMessage.andCondition(ConditionalOnMissingBean.class, spec)
                    .didNotFind("any beans").atAll();
        }
        return ConditionOutcome.match(matchMessage);
    }

　　@SuppressWarnings("deprecation")
    private List<String> getMatchingBeans(ConditionContext context,
            BeanSearchSpec beans) {
        ConfigurableListableBeanFactory beanFactory = context.getBeanFactory();
        if (beans.getStrategy() == SearchStrategy.PARENTS
                || beans.getStrategy() == SearchStrategy.ANCESTORS) {
            BeanFactory parent = beanFactory.getParentBeanFactory();
            Assert.isInstanceOf(ConfigurableListableBeanFactory.class, parent,
                    "Unable to use SearchStrategy.PARENTS");
            beanFactory = (ConfigurableListableBeanFactory) parent;
        }
        if (beanFactory == null) {
            return Collections.emptyList();
        }
        List<String> beanNames = new ArrayList<String>();
        boolean considerHierarchy = beans.getStrategy() != SearchStrategy.CURRENT;
        for (String type : beans.getTypes()) {
            beanNames.addAll(getBeanNamesForType(beanFactory, type,
                    context.getClassLoader(), considerHierarchy));
        }
        for (String ignoredType : beans.getIgnoredTypes()) {
            beanNames.removeAll(getBeanNamesForType(beanFactory, ignoredType,
                    context.getClassLoader(), considerHierarchy));
        }
        for (String annotation : beans.getAnnotations()) {
            beanNames.addAll(Arrays.asList(getBeanNamesForAnnotation(beanFactory,
                    annotation, context.getClassLoader(), considerHierarchy)));
        }
        for (String beanName : beans.getNames()) {
            if (containsBean(beanFactory, beanName, considerHierarchy)) {
                beanNames.add(beanName);
            }
        }
        return beanNames;
    }

　　在这里我们关注getMatchingBeans方法，此方法从当前的BeanFactory找所需要的Bean。由于BeanFactory层次化的关系，因此在ConditionalOn(Missing)Bean里有相关属性来配置寻找策略：

/**
     * Strategy to decide if the application context hierarchy (parent contexts) should be
     * considered.
     * @return the search strategy
     */
    SearchStrategy search() default SearchStrategy.ALL;

/*
 * Copyright 2012-2018 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
 *
 *      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.boot.autoconfigure.condition;

/**
 * Some named search strategies for beans in the bean factory hierarchy.
 *
 * @author Dave Syer
 */
public enum SearchStrategy {

    /**
     * Search only the current context.
     */
    CURRENT,

    /**
     * Search all parents and ancestors, but not the current context.
     * @deprecated as of 1.5 in favor of {@link SearchStrategy#ANCESTORS}
     */
    @Deprecated
    PARENTS,

    /**
     * Search all ancestors, but not the current context.
     */
    ANCESTORS,

    /**
     * Search the entire hierarchy.
     */
    ALL

}

2.3、Property Conditions

　　常见的注解为@ConditionalOnProperty，该注解会在Spring的Environment里面找对应的PropertySource，如果存在对应的属性值并且对应的值不为false时则匹配，我贴出关键代码部分：

　　@Override
    public ConditionOutcome getMatchOutcome(ConditionContext context,
            AnnotatedTypeMetadata metadata) {
  //.....

          for (AnnotationAttributes annotationAttributes : allAnnotationAttributes) {
            ConditionOutcome outcome = determineOutcome(annotationAttributes,
                    context.getEnvironment());
            (outcome.isMatch() ? match : noMatch).add(outcome.getConditionMessage());
        }
//.....

}

private static class Spec {

      // .....

      private void collectProperties(PropertyResolver resolver, List<String> missing,
                List<String> nonMatching) {
            if (this.relaxedNames) {
                resolver = new RelaxedPropertyResolver(resolver, this.prefix);
            }
            for (String name : this.names) {
                String key = (this.relaxedNames ? name : this.prefix + name);
                if (resolver.containsProperty(key)) {
                    if (!isMatch(resolver.getProperty(key), this.havingValue)) {
                        nonMatching.add(name);
                    }
                }
                else {
                    if (!this.matchIfMissing) {
                        missing.add(name);
                    }
                }
            }
        }

        private boolean isMatch(String value, String requiredValue) {
            if (StringUtils.hasLength(requiredValue)) {
                return requiredValue.equalsIgnoreCase(value);
            }
            return !"false".equalsIgnoreCase(value);
        }
//.....

}    

在这里通过ConditionContext拿到当前的Environment对象，在通过PropertyResover获取其配置的值。isMatch方法表明如果值存在且不等于false的情况下条件才生效

2.4、Resource Conditions

　　常见的注解为：@ConditionalOnResource，只有存在指定的资源文件时才生效，默认情况下是classpath,当然我们也可以指定其绝对路径，例如：file:/home/user/test.dat，源代码如下：

package org.springframework.boot.autoconfigure.condition;

import java.util.ArrayList;
import java.util.List;

import org.springframework.boot.autoconfigure.condition.ConditionMessage.Style;
import org.springframework.context.annotation.Condition;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.core.Ordered;
import org.springframework.core.annotation.Order;
import org.springframework.core.io.DefaultResourceLoader;
import org.springframework.core.io.ResourceLoader;
import org.springframework.core.type.AnnotatedTypeMetadata;
import org.springframework.util.Assert;
import org.springframework.util.MultiValueMap;

/**
 * {@link Condition} that checks for specific resources.
 *
 * @author Dave Syer
 * @see ConditionalOnResource
 */
@Order(Ordered.HIGHEST_PRECEDENCE + 20)
class OnResourceCondition extends SpringBootCondition {

    private final ResourceLoader defaultResourceLoader = new DefaultResourceLoader();

    @Override
    public ConditionOutcome getMatchOutcome(ConditionContext context,
            AnnotatedTypeMetadata metadata) {
        MultiValueMap<String, Object> attributes = metadata
                .getAllAnnotationAttributes(ConditionalOnResource.class.getName(), true);
        ResourceLoader loader = context.getResourceLoader() == null
                ? this.defaultResourceLoader : context.getResourceLoader();
        List<String> locations = new ArrayList<String>();
        collectValues(locations, attributes.get("resources"));
        Assert.isTrue(!locations.isEmpty(),
                "@ConditionalOnResource annotations must specify at "
                        + "least one resource location");
        List<String> missing = new ArrayList<String>();
        for (String location : locations) {
            String resource = context.getEnvironment().resolvePlaceholders(location);
            if (!loader.getResource(resource).exists()) {
                missing.add(location);
            }
        }
        if (!missing.isEmpty()) {
            return ConditionOutcome.noMatch(ConditionMessage
                    .forCondition(ConditionalOnResource.class)
                    .didNotFind("resource", "resources").items(Style.QUOTE, missing));
        }
        return ConditionOutcome
                .match(ConditionMessage.forCondition(ConditionalOnResource.class)
                        .found("location", "locations").items(locations));
    }

    private void collectValues(List<String> names, List<Object> values) {
        for (Object value : values) {
            for (Object item : (Object[]) value) {
                names.add((String) item);
            }
        }
    }

}

在这里我们可以看到，它是用DefaultResourceLoader来加载资源文件的，它会根据路径前缀来判断根据classpath加载或是url加载，其相关代码如下：

　　@Override
    public Resource getResource(String location) {
        Assert.notNull(location, "Location must not be null");

        for (ProtocolResolver protocolResolver : this.protocolResolvers) {
            Resource resource = protocolResolver.resolve(location, this);
            if (resource != null) {
                return resource;
            }
        }

        if (location.startsWith("/")) {
            return getResourceByPath(location);
        }
        else if (location.startsWith(CLASSPATH_URL_PREFIX)) {
            return new ClassPathResource(location.substring(CLASSPATH_URL_PREFIX.length()), getClassLoader());
        }
        else {
            try {
                // Try to parse the location as a URL...
                URL url = new URL(location);
                return new UrlResource(url);
            }
            catch (MalformedURLException ex) {
                // No URL -> resolve as resource path.
                return getResourceByPath(location);
            }
        }
    }

2.5、WebApplication Conditions

　　最常见的是@ConditionalOnWebApplication或者@ConditionalOnNotWebApplication，此注解的含义为：判断当前的ApplicationContext是否为WebApplicationContext。其关键代码如下:

private ConditionOutcome isWebApplication(ConditionContext context,
            AnnotatedTypeMetadata metadata, boolean required) {
        ConditionMessage.Builder message = ConditionMessage.forCondition(
                ConditionalOnWebApplication.class, required ? "(required)" : "");
        if (!ClassUtils.isPresent(WEB_CONTEXT_CLASS, context.getClassLoader())) {
            return ConditionOutcome
                    .noMatch(message.didNotFind("web application classes").atAll());
        }
        if (context.getBeanFactory() != null) {
            String[] scopes = context.getBeanFactory().getRegisteredScopeNames();
            if (ObjectUtils.containsElement(scopes, "session")) {
                return ConditionOutcome.match(message.foundExactly("'session' scope"));
            }
        }
        if (context.getEnvironment() instanceof StandardServletEnvironment) {
            return ConditionOutcome
                    .match(message.foundExactly("StandardServletEnvironment"));
        }
        if (context.getResourceLoader() instanceof WebApplicationContext) {
            return ConditionOutcome.match(message.foundExactly("WebApplicationContext"));
        }
        return ConditionOutcome.noMatch(message.because("not a web application"));
    }

在此我们可以看到它的判断依据有如下几个方面：

　　1）是否为WebApplicationContext

　　2) 是否包含session的scope

　　3) 当前的Environment是否为StandardServletEnvironment

三、基于@Configuration注册Bean分析

　　我们知道解析@Configuration的最主要的类是ConfigurationClassPostProcessor,这个类下有一个属性叫ConfigurationClassBeanDefinitionReader，与接口BeanDefiinitiaonReader类似，一看这个类我们就能联想到它的loadBeanDefinitions，其方法会调用loadBeanDefinitionsForConfigurationClass，我们来看一看这个方法：

/**
     * Read a particular {@link ConfigurationClass}, registering bean definitions
     * for the class itself and all of its {@link Bean} methods.
     */
    private void loadBeanDefinitionsForConfigurationClass(ConfigurationClass configClass,
            TrackedConditionEvaluator trackedConditionEvaluator) {

        if (trackedConditionEvaluator.shouldSkip(configClass)) {
            String beanName = configClass.getBeanName();
            if (StringUtils.hasLength(beanName) && this.registry.containsBeanDefinition(beanName)) {
                this.registry.removeBeanDefinition(beanName);
            }
            this.importRegistry.removeImportingClass(configClass.getMetadata().getClassName());
            return;
        }

        if (configClass.isImported()) {
            registerBeanDefinitionForImportedConfigurationClass(configClass);
        }
        for (BeanMethod beanMethod : configClass.getBeanMethods()) {
            loadBeanDefinitionsForBeanMethod(beanMethod);
        }
        loadBeanDefinitionsFromImportedResources(configClass.getImportedResources());
        loadBeanDefinitionsFromRegistrars(configClass.getImportBeanDefinitionRegistrars());
    }

在这里大家关注一下loadBeanDefinitionsForBeanMethod，此方法是加载@Bean注解所配置的Bean,我们来看一下其源码，我贴出关键部分：

private void loadBeanDefinitionsForBeanMethod(BeanMethod beanMethod) {
        ConfigurationClass configClass = beanMethod.getConfigurationClass();
        MethodMetadata metadata = beanMethod.getMetadata();
        String methodName = metadata.getMethodName();

        // Do we need to mark the bean as skipped by its condition?
        if (this.conditionEvaluator.shouldSkip(metadata, ConfigurationPhase.REGISTER_BEAN)) {
            configClass.skippedBeanMethods.add(methodName);
            return;
        }
        if (configClass.skippedBeanMethods.contains(methodName)) {
            return;
        }

            //.....省略后续代码
}

　　在这里会调用conditionEvaluator的shouldSkip方法 如果为true，则return，那么shuoldSkip方法又是怎么样的呢？我们来追踪一下：

/**
     * Determine if an item should be skipped based on {@code @Conditional} annotations.
     * @param metadata the meta data
     * @param phase the phase of the call
     * @return if the item should be skipped
     */
    public boolean shouldSkip(AnnotatedTypeMetadata metadata, ConfigurationPhase phase) {
        if (metadata == null || !metadata.isAnnotated(Conditional.class.getName())) {
            return false;
        }

        if (phase == null) {
            if (metadata instanceof AnnotationMetadata &&
                    ConfigurationClassUtils.isConfigurationCandidate((AnnotationMetadata) metadata)) {
                return shouldSkip(metadata, ConfigurationPhase.PARSE_CONFIGURATION);
            }
            return shouldSkip(metadata, ConfigurationPhase.REGISTER_BEAN);
        }

        List<Condition> conditions = new ArrayList<Condition>();
        for (String[] conditionClasses : getConditionClasses(metadata)) {
            for (String conditionClass : conditionClasses) {
                Condition condition = getCondition(conditionClass, this.context.getClassLoader());
                conditions.add(condition);
            }
        }

        AnnotationAwareOrderComparator.sort(conditions);

        for (Condition condition : conditions) {
            ConfigurationPhase requiredPhase = null;
            if (condition instanceof ConfigurationCondition) {
                requiredPhase = ((ConfigurationCondition) condition).getConfigurationPhase();
            }
            if (requiredPhase == null || requiredPhase == phase) {
                if (!condition.matches(this.context, metadata)) {
                    return true;
                }
            }
        }

        return false;
    }

　　那么至此，终于和先前的Condition所关联了。

四、总结

　　SpringBoot通过扩展Conditional来设置装配Bean的条件，通过Condition接口的matches方法的返回值来判断是否向IOC容器里注册Bean