spring boot源码分析之SpringApplication

spring boot提供了sample程序，学习spring boot之前先跑一个最简单的示例：

/*
 * Copyright 2012-2016 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 sample.simple;
 
import sample.simple.ExitException;
import sample.simple.service.HelloWorldService;
 
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.CommandLineRunner;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
 
@SpringBootApplication
public class SampleSimpleApplication implements CommandLineRunner {
 
    // Simple example shows how a command line spring application can execute an
    // injected bean service. Also demonstrates how you can use @Value to inject
    // command line args ('--name=whatever') or application properties
 
    @Autowired
    private HelloWorldService helloWorldService;
 
    public void run(String... args) {
        System.out.println(this.helloWorldService.getHelloMessage());
        if (args.length > 0 && args[0].equals("exitcode")) {
            throw new ExitException();
        }
    }
 
    public static void main(String[] args) throws Exception {
        SpringApplication application = new SpringApplication(
                SampleSimpleApplication.class);
        application.setApplicationContextClass(AnnotationConfigApplicationContext.class);
        SpringApplication.run(SampleSimpleApplication.class, args);
    }
 
}

可以发现在主方法main里启动了一个SpringApplication，启动方法是run方法。

SpringApplication用来从java main方法启动一个spring应用，默认的启动步骤如下：

1)创建一个合适的ApplicationContext实例,这个实例取决于classpath。

2)注册一个CommandLinePropertySource，以spring属性的形式来暴露命令行参数。

3)刷新ApplicationContext，加载所有的单例bean。

4)触发所有的命令行CommanLineRunner来执行bean。

大部分场景下，可以从你的application的main方法中直接调用它的run()静态方法。示例如下：

 @Configuration
 @EnableAutoConfiguration
 public class MyApplication  {
 
 // ... Bean definitions
 
 public static void main(String[] args) throws Exception {
   SpringApplication.run(MyApplication.class, args);
 }

定制则可以这样：

 public static void main(String[] args) throws Exception {
   SpringApplication app = new SpringApplication(MyApplication.class);
   // ... customize app settings here
   app.run(args)
 }

springApplication可以读取不同种类的源文件：

• 类- java类由AnnotatedBeanDefinitionReader加载。
• Resource - xml资源文件由XmlBeanDefinitionReader读取, 或者groovy脚本由GroovyBeanDefinitionReader读取
• Package - java包文件由ClassPathBeanDefinitionScanner扫描读取。
• CharSequence - 字符序列可以是类名、资源文件、包名，根据不同方式加载。如果一个字符序列不可以解析程序到类，也不可以解析到资源文件，那么就认为它是一个包。

1.初始化过程

    public SpringApplication(Object... sources) {
        initialize(sources);
    }
    private void initialize(Object[] sources) {
        if (sources != null && sources.length > 0) {
            this.sources.addAll(Arrays.asList(sources));
        }
        this.webEnvironment = deduceWebEnvironment();
        setInitializers((Collection) getSpringFactoriesInstances(
                ApplicationContextInitializer.class));
        setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
        this.mainApplicationClass = deduceMainApplicationClass();
    }

2.运行方法run

/**
     * Run the Spring application, creating and refreshing a new
     * {@link ApplicationContext}.
     * @param args the application arguments (usually passed from a Java main method)
     * @return a running {@link ApplicationContext}
     */
    public ConfigurableApplicationContext run(String... args) {
        StopWatch stopWatch = new StopWatch();
        stopWatch.start();
        ConfigurableApplicationContext context = null;
        configureHeadlessProperty();
        SpringApplicationRunListeners listeners = getRunListeners(args);
        listeners.started();
        try {
            ApplicationArguments applicationArguments = new DefaultApplicationArguments(
                    args);
            context = createAndRefreshContext(listeners, applicationArguments);
            afterRefresh(context, applicationArguments);
            listeners.finished(context, null);
            stopWatch.stop();
            if (this.logStartupInfo) {
                new StartupInfoLogger(this.mainApplicationClass)
                        .logStarted(getApplicationLog(), stopWatch);
            }
            return context;
        }
        catch (Throwable ex) {
            handleRunFailure(context, listeners, ex);
            throw new IllegalStateException(ex);
        }
    }

2.1 配置属性

    private void configureHeadlessProperty() {
        System.setProperty(SYSTEM_PROPERTY_JAVA_AWT_HEADLESS, System.getProperty(
                SYSTEM_PROPERTY_JAVA_AWT_HEADLESS, Boolean.toString(this.headless)));
    }

2.2 获取监听器

    private SpringApplicationRunListeners getRunListeners(String[] args) {
        Class<?>[] types = new Class<?>[] { SpringApplication.class, String[].class };
        return new SpringApplicationRunListeners(logger, getSpringFactoriesInstances(
                SpringApplicationRunListener.class, types, this, args));
    }

2.3 启动监听器

    public void started() {
        for (SpringApplicationRunListener listener : this.listeners) {
            listener.started();
        }
    }

listener最终会被初始化为ParentContextCloserApplicationListener，FileEncodingApplicationListener，AnsiOutputApplicationListener，ConfigFileApplicationListener，DelegatingApplicationListener，LiquibaseServiceLocatorApplicationListener，ClasspathLoggingApplicationListener，LoggingApplicationListener这几个类的对象组成的list。

下图画出了加载的ApplicationListener，并说明了他们的作用。

2.4 创建并刷新容器(重点)

private ConfigurableApplicationContext createAndRefreshContext(
            SpringApplicationRunListeners listeners,
            ApplicationArguments applicationArguments) {
        ConfigurableApplicationContext context;
        // Create and configure the environment
        ConfigurableEnvironment environment = getOrCreateEnvironment();
        configureEnvironment(environment, applicationArguments.getSourceArgs());
        listeners.environmentPrepared(environment);
        if (isWebEnvironment(environment) && !this.webEnvironment) {
            environment = convertToStandardEnvironment(environment);
        }
 
        if (this.bannerMode != Banner.Mode.OFF) {
            printBanner(environment);
        }
 
        // Create, load, refresh and run the ApplicationContext
        context = createApplicationContext();
        context.setEnvironment(environment);
        postProcessApplicationContext(context);
        applyInitializers(context);
        listeners.contextPrepared(context);
        if (this.logStartupInfo) {
            logStartupInfo(context.getParent() == null);
            logStartupProfileInfo(context);
        }
 
        // Add boot specific singleton beans
        context.getBeanFactory().registerSingleton("springApplicationArguments",
                applicationArguments);
 
        // Load the sources
        Set<Object> sources = getSources();
        Assert.notEmpty(sources, "Sources must not be empty");
        load(context, sources.toArray(new Object[sources.size()]));
        listeners.contextLoaded(context);
 
        // Refresh the context
        refresh(context);
        if (this.registerShutdownHook) {
            try {
                context.registerShutdownHook();
            }
            catch (AccessControlException ex) {
                // Not allowed in some environments.
            }
        }
        return context;
    }

2.4.1 获取或者创建环境

    private ConfigurableEnvironment getOrCreateEnvironment() {
        if (this.environment != null) {
            return this.environment;
        }
        if (this.webEnvironment) {
            return new StandardServletEnvironment();
        }
        return new StandardEnvironment();
    }

若是有指定环境，则返回指定的ConfigurableEnvironment

public interface ConfigurableEnvironment extends Environment, ConfigurablePropertyResolver

使用示例如下：

以最高搜索级别增加一个属性

   ConfigurableEnvironment environment = new StandardEnvironment();
   MutablePropertySources propertySources = environment.getPropertySources();
   Map myMap = new HashMap();
   myMap.put("xyz", "myValue");
   propertySources.addFirst(new MapPropertySource("MY_MAP", myMap));

移除默认系统属性

   MutablePropertySources propertySources = environment.getPropertySources();
   propertySources.remove(StandardEnvironment.SYSTEM_PROPERTIES_PROPERTY_SOURCE_NAME)

测试环境mock系统属性

   MutablePropertySources propertySources = environment.getPropertySources();
   MockPropertySource mockEnvVars = new MockPropertySource().withProperty("xyz", "myValue");
   propertySources.replace(StandardEnvironment.SYSTEM_ENVIRONMENT_PROPERTY_SOURCE_NAME, mockEnvVars);

若是web环境，则使用StandardServletEnvironment

public class StandardServletEnvironment extends StandardEnvironment implements ConfigurableWebEnvironment

它用于基于server相关的web应用，所有web相关(Servlet相关)Application类默认初始化一个实例。

默认返回StandardEnvironment

public class StandardEnvironment extends AbstractEnvironment

StandardEnvironment例如非web环境等

2.4.2 配置环境

    /**
     * Template method delegating to
     * {@link #configurePropertySources(ConfigurableEnvironment, String[])} and
     * {@link #configureProfiles(ConfigurableEnvironment, String[])} in that order.
     * Override this method for complete control over Environment customization, or one of
     * the above for fine-grained control over property sources or profiles, respectively.
     * @param environment this application's environment
     * @param args arguments passed to the {@code run} method
     * @see #configureProfiles(ConfigurableEnvironment, String[])
     * @see #configurePropertySources(ConfigurableEnvironment, String[])
     */
    protected void configureEnvironment(ConfigurableEnvironment environment,
            String[] args) {
        configurePropertySources(environment, args);
        configureProfiles(environment, args);
    }

2.4.3 创建ApplicationContext

/**
     * Strategy method used to create the {@link ApplicationContext}. By default this
     * method will respect any explicitly set application context or application context
     * class before falling back to a suitable default.
     * @return the application context (not yet refreshed)
     * @see #setApplicationContextClass(Class)
     */
    protected ConfigurableApplicationContext createApplicationContext() {
        Class<?> contextClass = this.applicationContextClass;
        if (contextClass == null) {
            try {
                contextClass = Class.forName(this.webEnvironment
                        ? DEFAULT_WEB_CONTEXT_CLASS : DEFAULT_CONTEXT_CLASS);
            }
            catch (ClassNotFoundException ex) {
                throw new IllegalStateException(
                        "Unable create a default ApplicationContext, "
                                + "please specify an ApplicationContextClass",
                        ex);
            }
        }
        return (ConfigurableApplicationContext) BeanUtils.instantiate(contextClass);
    }

2.4.4 加载bean到ApplicationContext

    /**
     * Load beans into the application context.
     * @param context the context to load beans into
     * @param sources the sources to load
     */
    protected void load(ApplicationContext context, Object[] sources) {
        if (logger.isDebugEnabled()) {
            logger.debug(
                    "Loading source " + StringUtils.arrayToCommaDelimitedString(sources));
        }
        BeanDefinitionLoader loader = createBeanDefinitionLoader(
                getBeanDefinitionRegistry(context), sources);
        if (this.beanNameGenerator != null) {
            loader.setBeanNameGenerator(this.beanNameGenerator);
        }
        if (this.resourceLoader != null) {
            loader.setResourceLoader(this.resourceLoader);
        }
        if (this.environment != null) {
            loader.setEnvironment(this.environment);
        }
        loader.load();
    }

2.4.5 刷新ApplicationContext

    /**
     * Refresh the underlying {@link ApplicationContext}.
     * @param applicationContext the application context to refresh
     */
    protected void refresh(ApplicationContext applicationContext) {
        Assert.isInstanceOf(AbstractApplicationContext.class, applicationContext);
        ((AbstractApplicationContext) applicationContext).refresh();
    }

小结：

上面仅仅是入门，若有谬误，请指正。后面随着学习的深入会修改。

参考文献：

【1】http://www.cnblogs.com/java-zhao/p/5540309.html

【2】http://zhaox.github.io/java/2016/03/22/spring-boot-start-flow