【附3】springboot源码解析 - 构建SpringApplication

package com.microservice.framework;

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

@SpringBootApplication
public class MySpringAplication {

    public void run(String[] args) {
        SpringApplication sa = new SpringApplication(MySpringAplication.class);
        sa.run(args);
    }

}

SpringBoot启动过程：

1、构建SpringApplication对象

2、执行run()

一、构建SpringApplication对象

     /**
      * The application context will load beans from the specified sources
      */
     public SpringApplication(Object... sources) {
         initialize(sources);
     }

说明：

• 实例化该类的时候会加载bean到applicationContext中去
• 这里的入参是MySpringApplication.class这样一个Class<com.microservice.framework.MySpringApplication>对象

private final Set<Object> sources = new LinkedHashSet<Object>();
    private boolean webEnvironment;
    private Class<?> mainApplicationClass;

    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();
    }

步骤：

• 将传入的MySpringApplication.class对象放入Set集合
• 判断是否是web环境
• 创建ApplicationInitializer列表
• 初始化ApplicationListener列表
• 初始化主类mainApplicationClass

1.1、将传入的MySpringApplication.class对象放入Set集合

1.2、判断是否是web环境：

private static final String[] WEB_ENVIRONMENT_CLASSES = { "javax.servlet.Servlet",
            "org.springframework.web.context.ConfigurableWebApplicationContext" };

    private boolean deduceWebEnvironment() {
        for (String className : WEB_ENVIRONMENT_CLASSES) {
            if (!ClassUtils.isPresent(className, null)) {
                return false;
            }
        }
        return true;
    }

说明：通过在classpath中查看是否存在WEB_ENVIRONMENT_CLASSES这个数组中所包含的所有类（实际上就是2个类），如果存在那么当前程序即是一个Web应用程序，反之则不然。

1.3、创建ApplicationContextInitializer列表

private List<ApplicationContextInitializer<?>> initializers;

    public void setInitializers(
            Collection<? extends ApplicationContextInitializer<?>> initializers) {
        this.initializers = new ArrayList<ApplicationContextInitializer<?>>();
        this.initializers.addAll(initializers);
    }

    private <T> Collection<? extends T> getSpringFactoriesInstances(Class<T> type) {
        return getSpringFactoriesInstances(type, new Class<?>[] {});
    }

    private <T> Collection<? extends T> getSpringFactoriesInstances(Class<T> type,
            Class<?>[] parameterTypes, Object... args) {
        ClassLoader classLoader = Thread.currentThread().getContextClassLoader();

        // Use names and ensure unique to protect against duplicates
        Set<String> names = new LinkedHashSet<String>(
                SpringFactoriesLoader.loadFactoryNames(type, classLoader));
        List<T> instances = new ArrayList<T>(names.size());

        // Create instances from the names
        for (String name : names) {
            try {
                Class<?> instanceClass = ClassUtils.forName(name, classLoader);
                Assert.isAssignable(type, instanceClass);
                Constructor<?> constructor = instanceClass.getConstructor(parameterTypes);
                T instance = (T) constructor.newInstance(args);
                instances.add(instance);
            }
            catch (Throwable ex) {
                throw new IllegalArgumentException(
                        "Cannot instantiate " + type + " : " + name, ex);
            }
        }

        AnnotationAwareOrderComparator.sort(instances);
        return instances;
    }

步骤：

• 调用SpringFactoriesLoader.loadFactoryNames(type, classLoader)来获取所有Spring Factories的名字，（这里是获取了四个ApplicationContextInitializer实现类的全类名，见下边）
• 为每一个Spring Factories根据读取到的名字创建其对象。（这里创建了4个对象）
• 将创建好的对象列表排序并返回。

其中，SpringFactoriesLoader.loadFactoryNames(type, classLoader)如下：

/**
     * The location to look for factories.
     * <p>Can be present in multiple JAR files.
     */
    public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";

    /**
     * Load the fully qualified class names of factory implementations of the
     * given type from {@value #FACTORIES_RESOURCE_LOCATION}, using the given
     * class loader.
     */
    public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) {
        String factoryClassName = factoryClass.getName();
        try {
            Enumeration<URL> urls = (classLoader != null ? classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
                    ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
            List<String> result = new ArrayList<String>();
            while (urls.hasMoreElements()) {
                URL url = urls.nextElement();
                Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url));
                String factoryClassNames = properties.getProperty(factoryClassName);
                result.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(factoryClassNames)));
            }
            return result;
        }
        catch (IOException ex) {
            throw new IllegalArgumentException("Unable to load [" + factoryClass.getName() +
                    "] factories from location [" + FACTORIES_RESOURCE_LOCATION + "]", ex);
        }
    }

META-INF/spring-factories

1 # Application Context Initializers
2 org.springframework.context.ApplicationContextInitializer=\
3 org.springframework.boot.context.ConfigurationWarningsApplicationContextInitializer,\
4 org.springframework.boot.context.ContextIdApplicationContextInitializer,\
5 org.springframework.boot.context.config.DelegatingApplicationContextInitializer,\
6 org.springframework.boot.context.web.ServerPortInfoApplicationContextInitializer

说明：

• 从所有jar获取所有的META-INF/spring-factories文件。（这里只有spring-boot-1.3.0.RELEASE.jar下有一个）
• 遍历每一个spring-factories文件，并获取其下key为factoryClass.getName()（这里是入参

  org.springframework.context.ApplicationContextInitializer）的value（这里有以上四个ApplicationContextInitializer实现类）

以上四个类的作用：

至此，设置ApplicationContextInitialize就完成了。

总结：整个setInitializers实际上就是初始化了SpringApplication的属性List<ApplicationContextInitializer<?>> initializers为一个ArrayList列表，该列表中有四个实例：

• ConfigurationWarningsApplicationContextInitializer的实例
• ContextIdApplicationContextInitializer的实例
• DelegatingApplicationContextInitializer实例
• ServerPortInfoApplicationContextInitializer实例

1.4、初始化ApplicationListener列表

private List<ApplicationListener<?>> listeners;    

        /**
     * Sets the {@link ApplicationListener}s that will be applied to the SpringApplication
     * and registered with the {@link ApplicationContext}.
     * @param listeners the listeners to set
     */
    public void setListeners(Collection<? extends ApplicationListener<?>> listeners) {
        this.listeners = new ArrayList<ApplicationListener<?>>();
        this.listeners.addAll(listeners);
    }

META-INF/spring-factories

# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.builder.ParentContextCloserApplicationListener,\
org.springframework.boot.context.FileEncodingApplicationListener,\
org.springframework.boot.context.config.AnsiOutputApplicationListener,\
org.springframework.boot.context.config.ConfigFileApplicationListener,\
org.springframework.boot.context.config.DelegatingApplicationListener,\
org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener,\
org.springframework.boot.logging.ClasspathLoggingApplicationListener,\
org.springframework.boot.logging.LoggingApplicationListener

以上八个listener的作用如下：

至此，整个setListeners方法结束，初始化了一个包含以上8个ApplicationListener实例的List集合。

1.5、初始化主类mainApplicationClass

private Class<?> mainApplicationClass;

    private Class<?> deduceMainApplicationClass() {
        try {
            StackTraceElement[] stackTrace = new RuntimeException().getStackTrace();
            for (StackTraceElement stackTraceElement : stackTrace) {
                if ("main".equals(stackTraceElement.getMethodName())) {
                    return Class.forName(stackTraceElement.getClassName());
                }
            }
        }
        catch (ClassNotFoundException ex) {
            // Swallow and continue
        }
        return null;
    }

说明：获取main()方法所在的主类Class对象，并赋值给SpringApplication的mainApplicationClass属性。

至此，SpringApplication对象初始化完成了。

总结：整个SpringApplication初始化的过程，就是初始化了

• 一个包含入参MySpringApplication.class的sources的Set<Object>
• 一个当前环境是否是web环境的boolean webEnvironment
• 一个包含4个ApplicationContextInitializer实例的List
• 一个包含8个ApplicationListener实例的List
• 一个main方法所在的主类的Class对象。

注意：

本文基本参照http://zhaox.github.io/java/2016/03/22/spring-boot-start-flow完成，该文的作者已经解析的很好了，我这里再抄一遍，只是为了加深记忆！！！