深入理解SpringCloud之引导程序应用上下文

　　tips:我希望通过这篇文章来给对于bootstrap还不理解的朋友带来帮助。当然这篇文章不仅仅是讲解知识，我更希望给广大朋友带来学习与理解官方文档的一种思路。阅读本文前，建议大家对SpringBoot的启动机制与Environment的作用有大致的了解。关于SpringBoot的启动机制我们可以参考：SpringBoot学习之启动探究

　　SpringCloud为我们提供了bootstrap.properties的属性文件，我们可以在该属性文件里做我们的服务配置。可是，我们知道SpringBoot已经为我们提供了做服务配置的属性文件application.properties，那么这两个配置文件有什么区别呢？在SpringCloud里是否能用bootstrap代替application做服务的配置？要解决这个问题，我们必须先讨论一下SpringCloud的引导。

一、ConfigurableApplicationContext 的层级结构

1.1、层次结构的代码分析

　　ConfigurableApplicationContext是ApplicationContext的子接口，这里面有一个方法叫setParent(), 该方法就的作用是设置它的父级ApplicationContext ，注意一旦设置了它的父上下文，后面就不能再次调用setParent方法了。究竟调用这个方法会产生什么效果呢？下面我们来看一下源代码：

　　AbstractApplicationContext的setParent：

/**
     * Set the parent of this application context.
     * <p>The parent {@linkplain ApplicationContext#getEnvironment() environment} is
     * {@linkplain ConfigurableEnvironment#merge(ConfigurableEnvironment) merged} with
     * this (child) application context environment if the parent is non-{@code null} and
     * its environment is an instance of {@link ConfigurableEnvironment}.
     * @see ConfigurableEnvironment#merge(ConfigurableEnvironment)
     */
    @Override
    public void setParent(ApplicationContext parent) {
        this.parent = parent;
        if (parent != null) {
            Environment parentEnvironment = parent.getEnvironment();
            if (parentEnvironment instanceof ConfigurableEnvironment) {
                getEnvironment().merge((ConfigurableEnvironment) parentEnvironment);
            }
        }
    }

　　我们可以通过源代码得知：一旦设置设置父上下文，当前的Environment会合并父上下文的Environment。

　　GenericApplicationContext：

//.......
/**
     * Create a new GenericApplicationContext with the given parent.
     * @param parent the parent application context
     * @see #registerBeanDefinition
     * @see #refresh
     */
    public GenericApplicationContext(ApplicationContext parent) {
        this();
        setParent(parent);
    }

// .....

/**
     * Set the parent of this application context, also setting
     * the parent of the internal BeanFactory accordingly.
     * @see org.springframework.beans.factory.config.ConfigurableBeanFactory#setParentBeanFactory
     */
    @Override
    public void setParent(ApplicationContext parent) {
        super.setParent(parent);
        this.beanFactory.setParentBeanFactory(getInternalParentBeanFactory());
    }

　　通过源代码得知：该类不仅会合并Environment还会把父上下文的BeanFactory"借用过来" ，我们常用的ClasspathXmlApplicationContext是AbstractApplicationContext的子类，而AnnotationConfigApplicationContext是GenericApplicationContext的子类

1.2、演示示例

　　首先我们先建一个属性文件application.properties，在属性文件里配置：

jdbc.user=root

　　然后我们按照如下目录建立好相关文件：

　　

　　StudentConfig:

package org.hzgj.spring.study.student;

import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.PropertySource;

@Configuration
@ComponentScan
@PropertySource("application.properties")
public class StudentConfig {
}

　　TeacherConfig:

package org.hzgj.spring.study.teacher;

import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.Configuration;

@Configuration
@ComponentScan
public class TeacherConfig {
}

　　Student:

package org.hzgj.spring.study.student;

import org.hzgj.spring.study.teacher.Teacher;
import org.springframework.beans.factory.annotation.Value;
import org.springframework.stereotype.Component;

@Component
public class Student {

    @Value("${jdbc.user}")
    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    private int age=20;

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

}

　　Teacher:

package org.hzgj.spring.study.teacher;

import org.springframework.stereotype.Component;

@Component
public class Teacher {

    private String name = "张老师";

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

　　Main方法：

package org.hzgj.spring.study;

import org.hzgj.spring.study.student.StudentConfig;
import org.hzgj.spring.study.student.Student;
import org.hzgj.spring.study.teacher.TeacherConfig;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;

import javax.naming.NamingException;
import java.io.IOException;

public class Main {
    public static void main(String[] args) throws IOException, NamingException {

        AnnotationConfigApplicationContext studentApplicationContext = new AnnotationConfigApplicationContext(StudentConfig.class);
        AnnotationConfigApplicationContext teacherApplicationContext = new AnnotationConfigApplicationContext(TeacherConfig.class);
        teacherApplicationContext.setParent(studentApplicationContext);
        Student student = teacherApplicationContext.getBean(Student.class);
        System.out.println("获取student对象的name属性:" + student.getName());
        System.out.println(studentApplicationContext.getEnvironment().getProperty("jdbc.user"));
    }
}

　　在这里我们将Teacher的父级上下文设置成student的，运行得到如下结果：

二、SpringCloud引导上下文

　　我在这里先贴出官方文档的一段描述：

引导应用程序上下文

　　一个Spring Cloud应用程序通过创建一个“引导”上下文来进行操作，这个上下文是主应用程序的父上下文。开箱即用，负责从外部源加载配置属性，还解密本地外部配置文件中的属性。这两个上下文共享一个Environment，这是任何Spring应用程序的外部属性的来源。Bootstrap属性的优先级高，因此默认情况下不能被本地配置覆盖。

引导上下文使用与主应用程序上下文不同的外部配置约定，因此使用bootstrap.yml application.yml（或.properties）代替引导和主上下文的外部配置。例：bootstrap.yml

spring:
  application:
    name: foo
  cloud:
    config:
      uri: ${SPRING_CONFIG_URI:http://localhost:8888}

如果您的应用程序需要服务器上的特定于应用程序的配置，那么设置spring.application.name（在bootstrap.yml或application.yml）中是个好主意。

您可以通过设置spring.cloud.bootstrap.enabled=false（例如在系统属性中）来完全禁用引导过程。

　　初看这段话的朋友，可能会比较蒙圈，没关系我来解释几个关键点：

　　2.1、关于引导上下文在哪里

　　      引导上下文，这个是什么意思呢？我们可以把这个理解为springcloud的"bios"。我们可以先看一下这个引导到底在哪里：

　　

　　在这里我们可以发现几个关键的类,其中BootstrapApplicationListener是核心中的核心：我在这里贴一下源代码：

/*
 * Copyright 2013-2014 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.cloud.bootstrap;

import java.lang.reflect.Field;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.LinkedHashSet;
import java.util.List;
import java.util.Map;
import java.util.Set;

import org.springframework.beans.factory.ListableBeanFactory;
import org.springframework.boot.Banner.Mode;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.builder.ParentContextApplicationContextInitializer;
import org.springframework.boot.builder.SpringApplicationBuilder;
import org.springframework.boot.context.event.ApplicationEnvironmentPreparedEvent;
import org.springframework.boot.logging.LoggingApplicationListener;
import org.springframework.cloud.bootstrap.encrypt.EnvironmentDecryptApplicationInitializer;
import org.springframework.context.ApplicationContextInitializer;
import org.springframework.context.ApplicationListener;
import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.core.Ordered;
import org.springframework.core.annotation.AnnotationAwareOrderComparator;
import org.springframework.core.annotation.Order;
import org.springframework.core.env.CompositePropertySource;
import org.springframework.core.env.ConfigurableEnvironment;
import org.springframework.core.env.EnumerablePropertySource;
import org.springframework.core.env.MapPropertySource;
import org.springframework.core.env.MutablePropertySources;
import org.springframework.core.env.PropertySource;
import org.springframework.core.env.StandardEnvironment;
import org.springframework.core.env.SystemEnvironmentPropertySource;
import org.springframework.core.io.support.SpringFactoriesLoader;
import org.springframework.util.ClassUtils;
import org.springframework.util.ReflectionUtils;
import org.springframework.util.StringUtils;

/**
 * A listener that prepares a SpringApplication (e.g. populating its Environment) by
 * delegating to {@link ApplicationContextInitializer} beans in a separate bootstrap
 * context. The bootstrap context is a SpringApplication created from sources defined in
 * spring.factories as {@link BootstrapConfiguration}, and initialized with external
 * config taken from "bootstrap.properties" (or yml), instead of the normal
 * "application.properties".
 *
 * @author Dave Syer
 *
 */
public class BootstrapApplicationListener
        implements ApplicationListener<ApplicationEnvironmentPreparedEvent>, Ordered {

    public static final String BOOTSTRAP_PROPERTY_SOURCE_NAME = "bootstrap";

    public static final int DEFAULT_ORDER = Ordered.HIGHEST_PRECEDENCE + 5;

    public static final String DEFAULT_PROPERTIES = "defaultProperties";

    private int order = DEFAULT_ORDER;

    @Override
    public void onApplicationEvent(ApplicationEnvironmentPreparedEvent event) {
        ConfigurableEnvironment environment = event.getEnvironment();
        if (!environment.getProperty("spring.cloud.bootstrap.enabled", Boolean.class,
                true)) {
            return;
        }
        // don't listen to events in a bootstrap context
        if (environment.getPropertySources().contains(BOOTSTRAP_PROPERTY_SOURCE_NAME)) {
            return;
        }
        ConfigurableApplicationContext context = null;
        String configName = environment
                .resolvePlaceholders("${spring.cloud.bootstrap.name:bootstrap}");
        for (ApplicationContextInitializer<?> initializer : event.getSpringApplication()
                .getInitializers()) {
            if (initializer instanceof ParentContextApplicationContextInitializer) {
                context = findBootstrapContext(
                        (ParentContextApplicationContextInitializer) initializer,
                        configName);
            }
        }
        if (context == null) {
            context = bootstrapServiceContext(environment, event.getSpringApplication(),
                    configName);
        }
        apply(context, event.getSpringApplication(), environment);
    }

    private ConfigurableApplicationContext findBootstrapContext(
            ParentContextApplicationContextInitializer initializer, String configName) {
        Field field = ReflectionUtils
                .findField(ParentContextApplicationContextInitializer.class, "parent");
        ReflectionUtils.makeAccessible(field);
        ConfigurableApplicationContext parent = safeCast(
                ConfigurableApplicationContext.class,
                ReflectionUtils.getField(field, initializer));
        if (parent != null && !configName.equals(parent.getId())) {
            parent = safeCast(ConfigurableApplicationContext.class, parent.getParent());
        }
        return parent;
    }

    private <T> T safeCast(Class<T> type, Object object) {
        try {
            return type.cast(object);
        }
        catch (ClassCastException e) {
            return null;
        }
    }

    private ConfigurableApplicationContext bootstrapServiceContext(
            ConfigurableEnvironment environment, final SpringApplication application,
            String configName) {
        StandardEnvironment bootstrapEnvironment = new StandardEnvironment();
        MutablePropertySources bootstrapProperties = bootstrapEnvironment
                .getPropertySources();
        for (PropertySource<?> source : bootstrapProperties) {
            bootstrapProperties.remove(source.getName());
        }
        String configLocation = environment
                .resolvePlaceholders("${spring.cloud.bootstrap.location:}");
        Map<String, Object> bootstrapMap = new HashMap<>();
        bootstrapMap.put("spring.config.name", configName);
        if (StringUtils.hasText(configLocation)) {
            bootstrapMap.put("spring.config.location", configLocation);
        }
        bootstrapProperties.addFirst(
                new MapPropertySource(BOOTSTRAP_PROPERTY_SOURCE_NAME, bootstrapMap));
        for (PropertySource<?> source : environment.getPropertySources()) {
            bootstrapProperties.addLast(source);
        }
        ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
        // Use names and ensure unique to protect against duplicates
        List<String> names = SpringFactoriesLoader
                .loadFactoryNames(BootstrapConfiguration.class, classLoader);
        for (String name : StringUtils.commaDelimitedListToStringArray(
                environment.getProperty("spring.cloud.bootstrap.sources", ""))) {
            names.add(name);
        }
        // TODO: is it possible or sensible to share a ResourceLoader?
        SpringApplicationBuilder builder = new SpringApplicationBuilder()
                .profiles(environment.getActiveProfiles()).bannerMode(Mode.OFF)
                .environment(bootstrapEnvironment)
                .properties("spring.application.name:" + configName)
                .registerShutdownHook(false).logStartupInfo(false).web(false);
        if (environment.getPropertySources().contains("refreshArgs")) {
            // If we are doing a context refresh, really we only want to refresh the
            // Environment, and there are some toxic listeners (like the
            // LoggingApplicationListener) that affect global static state, so we need a
            // way to switch those off.
            builder.application()
                    .setListeners(filterListeners(builder.application().getListeners()));
        }
        List<Class<?>> sources = new ArrayList<>();
        for (String name : names) {
            Class<?> cls = ClassUtils.resolveClassName(name, null);
            try {
                cls.getDeclaredAnnotations();
            }
            catch (Exception e) {
                continue;
            }
            sources.add(cls);
        }
        AnnotationAwareOrderComparator.sort(sources);
        builder.sources(sources.toArray(new Class[sources.size()]));
        final ConfigurableApplicationContext context = builder.run();
        // Make the bootstrap context a parent of the app context
        addAncestorInitializer(application, context);
        // It only has properties in it now that we don't want in the parent so remove
        // it (and it will be added back later)
        bootstrapProperties.remove(BOOTSTRAP_PROPERTY_SOURCE_NAME);
        mergeDefaultProperties(environment.getPropertySources(), bootstrapProperties);
        return context;
    }

    private Collection<? extends ApplicationListener<?>> filterListeners(
            Set<ApplicationListener<?>> listeners) {
        Set<ApplicationListener<?>> result = new LinkedHashSet<>();
        for (ApplicationListener<?> listener : listeners) {
            if (!(listener instanceof LoggingApplicationListener)
                    && !(listener instanceof LoggingSystemShutdownListener)) {
                result.add(listener);
            }
        }
        return result;
    }

    private void mergeDefaultProperties(MutablePropertySources environment,
            MutablePropertySources bootstrap) {
        String name = DEFAULT_PROPERTIES;
        if (!bootstrap.contains(name)) {
            return;
        }
        PropertySource<?> source = bootstrap.get(name);
        if (source instanceof MapPropertySource) {
            Map<String, Object> map = ((MapPropertySource) source).getSource();
            // The application name is "bootstrap" (by default) at this point and
            // we don't want that to appear in the parent context at all.
            map.remove("spring.application.name");
        }
        if (!environment.contains(name)) {
            environment.addLast(source);
        }
        else {
            PropertySource<?> target = environment.get(name);
            if (target instanceof MapPropertySource) {
                Map<String, Object> targetMap = ((MapPropertySource) target).getSource();
                if (target == source) {
                    return;
                }
                if (source instanceof MapPropertySource) {
                    Map<String, Object> map = ((MapPropertySource) source).getSource();
                    for (String key : map.keySet()) {
                        if (!target.containsProperty(key)) {
                            targetMap.put(key, map.get(key));
                        }
                    }
                }
            }
        }
        mergeAdditionalPropertySources(environment, bootstrap);
    }

    private void mergeAdditionalPropertySources(MutablePropertySources environment,
            MutablePropertySources bootstrap) {
        PropertySource<?> defaultProperties = environment.get(DEFAULT_PROPERTIES);
        ExtendedDefaultPropertySource result = defaultProperties instanceof ExtendedDefaultPropertySource
                ? (ExtendedDefaultPropertySource) defaultProperties
                : new ExtendedDefaultPropertySource(defaultProperties.getName(),
                        defaultProperties);
        for (PropertySource<?> source : bootstrap) {
            if (!environment.contains(source.getName())) {
                result.add(source);
            }
        }
        for (String name : result.getPropertySourceNames()) {
            bootstrap.remove(name);
        }
        environment.replace(DEFAULT_PROPERTIES, result);
        bootstrap.replace(DEFAULT_PROPERTIES, result);
    }

    private void addAncestorInitializer(SpringApplication application,
            ConfigurableApplicationContext context) {
        boolean installed = false;
        for (ApplicationContextInitializer<?> initializer : application
                .getInitializers()) {
            if (initializer instanceof AncestorInitializer) {
                installed = true;
                // New parent
                ((AncestorInitializer) initializer).setParent(context);
            }
        }
        if (!installed) {
            application.addInitializers(new AncestorInitializer(context));
        }

    }

    private void apply(ConfigurableApplicationContext context,
            SpringApplication application, ConfigurableEnvironment environment) {
        @SuppressWarnings("rawtypes")
        List<ApplicationContextInitializer> initializers = getOrderedBeansOfType(context,
                ApplicationContextInitializer.class);
        application.addInitializers(initializers
                .toArray(new ApplicationContextInitializer[initializers.size()]));
        addBootstrapDecryptInitializer(application);
    }

    private void addBootstrapDecryptInitializer(SpringApplication application) {
        DelegatingEnvironmentDecryptApplicationInitializer decrypter = null;
        for (ApplicationContextInitializer<?> initializer : application
                .getInitializers()) {
            if (initializer instanceof EnvironmentDecryptApplicationInitializer) {
                @SuppressWarnings("unchecked")
                ApplicationContextInitializer<ConfigurableApplicationContext> delegate = (ApplicationContextInitializer<ConfigurableApplicationContext>) initializer;
                decrypter = new DelegatingEnvironmentDecryptApplicationInitializer(
                        delegate);
            }
        }
        if (decrypter != null) {
            application.addInitializers(decrypter);
        }
    }

    private <T> List<T> getOrderedBeansOfType(ListableBeanFactory context,
            Class<T> type) {
        List<T> result = new ArrayList<T>();
        for (String name : context.getBeanNamesForType(type)) {
            result.add(context.getBean(name, type));
        }
        AnnotationAwareOrderComparator.sort(result);
        return result;
    }

    public void setOrder(int order) {
        this.order = order;
    }

    @Override
    public int getOrder() {
        return this.order;
    }

    private static class AncestorInitializer implements
            ApplicationContextInitializer<ConfigurableApplicationContext>, Ordered {

        private ConfigurableApplicationContext parent;

        public AncestorInitializer(ConfigurableApplicationContext parent) {
            this.parent = parent;
        }

        public void setParent(ConfigurableApplicationContext parent) {
            this.parent = parent;
        }

        @Override
        public int getOrder() {
            // Need to run not too late (so not unordered), so that, for instance, the
            // ContextIdApplicationContextInitializer runs later and picks up the merged
            // Environment. Also needs to be quite early so that other initializers can
            // pick up the parent (especially the Environment).
            return Ordered.HIGHEST_PRECEDENCE + 5;
        }

        @Override
        public void initialize(ConfigurableApplicationContext context) {
            while (context.getParent() != null && context.getParent() != context) {
                context = (ConfigurableApplicationContext) context.getParent();
            }
            reorderSources(context.getEnvironment());
            new ParentContextApplicationContextInitializer(this.parent)
                    .initialize(context);
        }

        private void reorderSources(ConfigurableEnvironment environment) {
            PropertySource<?> removed = environment.getPropertySources()
                    .remove(DEFAULT_PROPERTIES);
            if (removed instanceof ExtendedDefaultPropertySource) {
                ExtendedDefaultPropertySource defaultProperties = (ExtendedDefaultPropertySource) removed;
                environment.getPropertySources().addLast(new MapPropertySource(
                        DEFAULT_PROPERTIES, defaultProperties.getSource()));
                for (PropertySource<?> source : defaultProperties.getPropertySources()
                        .getPropertySources()) {
                    if (!environment.getPropertySources().contains(source.getName())) {
                        environment.getPropertySources().addBefore(DEFAULT_PROPERTIES,
                                source);
                    }
                }
            }
        }

    }

    /**
     * A special initializer designed to run before the property source bootstrap and
     * decrypt any properties needed there (e.g. URL of config server).
     */
    @Order(Ordered.HIGHEST_PRECEDENCE + 9)
    private static class DelegatingEnvironmentDecryptApplicationInitializer
            implements ApplicationContextInitializer<ConfigurableApplicationContext> {

        private ApplicationContextInitializer<ConfigurableApplicationContext> delegate;

        public DelegatingEnvironmentDecryptApplicationInitializer(
                ApplicationContextInitializer<ConfigurableApplicationContext> delegate) {
            this.delegate = delegate;
        }

        @Override
        public void initialize(ConfigurableApplicationContext applicationContext) {
            this.delegate.initialize(applicationContext);
        }

    }

    private static class ExtendedDefaultPropertySource
            extends SystemEnvironmentPropertySource {

        private final CompositePropertySource sources;
        private final List<String> names = new ArrayList<>();

        public ExtendedDefaultPropertySource(String name,
                PropertySource<?> propertySource) {
            super(name, findMap(propertySource));
            this.sources = new CompositePropertySource(name);
        }

        public CompositePropertySource getPropertySources() {
            return this.sources;
        }

        public List<String> getPropertySourceNames() {
            return this.names;
        }

        public void add(PropertySource<?> source) {
            if (source instanceof EnumerablePropertySource
                    && !this.names.contains(source.getName())) {
                this.sources.addPropertySource(source);
                this.names.add(source.getName());
            }
        }

        @Override
        public Object getProperty(String name) {
            if (this.sources.containsProperty(name)) {
                return this.sources.getProperty(name);
            }
            return super.getProperty(name);
        }

        @Override
        public boolean containsProperty(String name) {
            if (this.sources.containsProperty(name)) {
                return true;
            }
            return super.containsProperty(name);
        }

        @Override
        public String[] getPropertyNames() {
            List<String> names = new ArrayList<>();
            names.addAll(Arrays.asList(this.sources.getPropertyNames()));
            names.addAll(Arrays.asList(super.getPropertyNames()));
            return names.toArray(new String[0]);
        }

        @SuppressWarnings("unchecked")
        private static Map<String, Object> findMap(PropertySource<?> propertySource) {
            if (propertySource instanceof MapPropertySource) {
                return (Map<String, Object>) propertySource.getSource();
            }
            return new LinkedHashMap<String, Object>();
        }

    }

}

　　这个类是一个监听器，它用于监听ApplicationEnvironmentPreparedEvent事件，而EventPublishingRunListener在SpringBoot启动时会触发该事件。如果不理解的这个类的朋友请务必先了解SpringBoot启动过程

　　2.2、这个上下文是主应用程序的父上下文

　　　　这个工作主要分为两个层面：1.创建上下文引导 2.设置为其为当前程序的父级上下文

　　　   1) 我们先看看onApplicationEvent方法，该方法首先读取spring.cloud.bootstrap.enabled的属性值如果为false，那么就直接return。这也就是官方文档里的说明可以用此属性禁用引导的理由。

　　　   2)紧接着它会从当前应用程序SpringApplication试着在所有的ApplicationInitializer中获取ParentContextApplicationContextInitializer，如果找到的话就把该类下的parent做为引导上下文。

　　　   3)如果没有找到ParentContextApplicationContextInitializer，则通过 bootstrapServiceContext方法来创建引导上下文，其中如下代码请大家留意下：

    List<String> names = SpringFactoriesLoader
                .loadFactoryNames(BootstrapConfiguration.class, classLoader);

　　       看到SpringFactoriesLoader不用想一定会在META-INF/spring.factoies里找配置的BootstrapConfiguration的进行实例化

4）通过如下代码创建引导上下文对象：

SpringApplicationBuilder builder = new SpringApplicationBuilder()
                .profiles(environment.getActiveProfiles()).bannerMode(Mode.OFF)
                .environment(bootstrapEnvironment)
                .properties("spring.application.name:" + configName)
                .registerShutdownHook(false).logStartupInfo(false).web(false);
        if (environment.getPropertySources().contains("refreshArgs")) {
            // If we are doing a context refresh, really we only want to refresh the
            // Environment, and there are some toxic listeners (like the
            // LoggingApplicationListener) that affect global static state, so we need a
            // way to switch those off.
            builder.application()
                    .setListeners(filterListeners(builder.application().getListeners()));
        }
        List<Class<?>> sources = new ArrayList<>();
        for (String name : names) {
            Class<?> cls = ClassUtils.resolveClassName(name, null);
            try {
                cls.getDeclaredAnnotations();
            }
            catch (Exception e) {
                continue;
            }
            sources.add(cls);
        }
        AnnotationAwareOrderComparator.sort(sources);
        builder.sources(sources.toArray(new Class[sources.size()]));
        final ConfigurableApplicationContext context = builder.run();

　　5）最后通过如下方法设置引导上下文为当前应用程序的上下文：

// Make the bootstrap context a parent of the app context
        addAncestorInitializer(application, context);

　　2.3、开箱即用，负责从外部源加载配置属性，还解密本地外部配置文件中的属性。

　　　开箱即用，理解起来很简单。通过2.2分析，引导程序在SpringBoot的启动前就帮我们创建好了，当然也就开箱即用了。

　　　下面我们看一下spring-cloud-context.jar下的META-INF/spring.factoies文件:

# AutoConfiguration
org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.cloud.autoconfigure.ConfigurationPropertiesRebinderAutoConfiguration,\
org.springframework.cloud.autoconfigure.RefreshAutoConfiguration,\
org.springframework.cloud.autoconfigure.RefreshEndpointAutoConfiguration,\
org.springframework.cloud.autoconfigure.LifecycleMvcEndpointAutoConfiguration

# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.cloud.bootstrap.BootstrapApplicationListener,\
org.springframework.cloud.bootstrap.LoggingSystemShutdownListener,\
org.springframework.cloud.context.restart.RestartListener

# Bootstrap components
org.springframework.cloud.bootstrap.BootstrapConfiguration=\
org.springframework.cloud.bootstrap.config.PropertySourceBootstrapConfiguration,\
org.springframework.cloud.bootstrap.encrypt.EncryptionBootstrapConfiguration,\
org.springframework.cloud.autoconfigure.ConfigurationPropertiesRebinderAutoConfiguration,\
org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration

　　  我们来看一下  BootstrapConfiguration下面配置的引导程序类：

　　  org.springframework.cloud.bootstrap.config.PropertySourceBootstrapConfiguration：这个类主要解析加载外部化配置属性

　　  org.springframework.cloud.bootstrap.encrypt.EncryptionBootstrapConfiguration：主要配置文件中前缀为{cipher}的相关解密，熟悉spring-boot-starter-security在springcloud应用的朋友一定不陌生

　　  org.springframework.cloud.autoconfigure.ConfigurationPropertiesRebinderAutoConfiguration：主要是监听EnvironmentChangeEvent事件用于刷新@ConfigurationProperties标记的配置

　　  org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration：主要解析配置文件中的${}占位符

　 2.4、这两个上下文共享一个Environment，这是任何Spring应用程序的外部属性的来源。

　　  既然引导上下文为当前主程序的父级上下文，那么就可以确定他们共享Environment，至于为什么请阅读文章第一部分

　 2.5、Bootstrap属性的优先级高，因此默认情况下不能被本地配置覆盖。

　　　　要解释这个我们必须用代码来演示了，结构图：

　　　　

　　　　注意：MyBootstrapAutoConfiguration是我们自定义的引导类，该类一定不能被@SpringBootApplication注解ComponentScan到,否则引导必然就会被主程序所覆盖。因此我用包把他们区分开来

　　　　MyBootstrapAutoConfiguration代码：

package com.bdqn.lyrk.bootstrap.config;

import com.bdqn.lyrk.bootstrap.server.BootStrapConfig;
import com.bdqn.lyrk.bootstrap.server.Student;
import org.springframework.boot.context.properties.EnableConfigurationProperties;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;

@Configuration
@EnableConfigurationProperties(BootStrapConfig.class)
public class MyBootstrapAutoConfiguration {
    @Bean
    public Student student(BootStrapConfig bootStrapConfig){
        Student student = new Student();
        student.setName(bootStrapConfig.getName());
        return student;
    }
}

　　　　BootstrapConfig:

package com.bdqn.lyrk.bootstrap.server;

import org.springframework.boot.context.properties.ConfigurationProperties;

@ConfigurationProperties("student")
public class BootStrapConfig {
    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

　　　　Student:

package com.bdqn.lyrk.bootstrap.server;

public class Student {

    private String name;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }
}

　　　　application.yml:

student:
  name: application

　　　　bootstrap.yml:

student:
  name: bootstrap

　　　　spring.factories:

org.springframework.cloud.bootstrap.BootstrapConfiguration=\
  com.bdqn.lyrk.bootstrap.config.MyBootstrapAutoConfiguration

　　　   启动类代码：

package com.bdqn.lyrk.bootstrap.server;

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ConfigurableApplicationContext;

@SpringBootApplication()
public class BootstrapServer {

    public static void main(String[] args) {
        ConfigurableApplicationContext applicationContext = SpringApplication.run(BootstrapServer.class, args);
        Student student = applicationContext.getBean(Student.class);
        System.out.println(student.getName());
    }
}

　　运行后得到结果：

　因此我们可以看到对于引导程序bootstrap.yml比application.yml优先级更高，更不可能被application.yml文件里的所覆盖

　　　　

三、总结

　　1）引导程序上下文在prepareEnvironment的阶段就会被创建，创建时会读取bootstrap.properties|yml 在内容作为引导配置, 因此bootstrap优先于application加载。引导程序非常类似于bios，而bootstrap.application就相当于设置bios的相关参数

　　2）boostrap的属性文件在以下情景下会使用：

　　　　配置中心：config-server里请用bootstrap属性文件

　　　   解密属性文件时，最好使用bootstrap属性文件

　　　　需要自定义引导程序时使用bootstrap属性文件，主要一定不要被我们主程序扫描到

　　3）application会覆盖bootstrap中的非引导配置，因此不建议两种类型配置文件同时存在。简单粗暴的做法是在springcloud应用中用bootstrap属性文件代替application一统江湖嘛，毕竟Envrionment是共享的。

　　4)  在阅读官方文档时，一定要结合源代码深入分析，才能更好的理解其用意