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springboot源码解析 - 构建SpringApplication

叁叁肆2018-12-02 13:58

此文已由作者赵计刚薪授权网易云社区发布。

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 1 package com.microservice.framework;
 2 
 3 import org.springframework.boot.SpringApplication;
 4 import org.springframework.boot.autoconfigure.SpringBootApplication;
 5 
 6 @SpringBootApplication
 7 public class MySpringAplication {
 8 
 9     public void run(String[] args) {
10         SpringApplication sa = new SpringApplication(MySpringAplication.class);
11         sa.run(args);
12     }
13 
14 }

SpringBoot启动过程：

1、构建SpringApplication对象

2、执行run()

一、构建SpringApplication对象

1     /**
2      * The application context will load beans from the specified sources 
3      */
4     public SpringApplication(Object... sources) {
5         initialize(sources);
6     }

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

 1     private List<ApplicationContextInitializer<?>> initializers;
 2 
 3     public void setInitializers(
 4             Collection<? extends ApplicationContextInitializer<?>> initializers) {
 5         this.initializers = new ArrayList<ApplicationContextInitializer<?>>();
 6         this.initializers.addAll(initializers);
 7     }
 8 
 9     private <T> Collection<? extends T> getSpringFactoriesInstances(Class<T> type) {
10         return getSpringFactoriesInstances(type, new Class<?>[] {});
11     }
12 
13     private <T> Collection<? extends T> getSpringFactoriesInstances(Class<T> type,
14             Class<?>[] parameterTypes, Object... args) {
15         ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
16 
17         // Use names and ensure unique to protect against duplicates
18         Set<String> names = new LinkedHashSet<String>(
19                 SpringFactoriesLoader.loadFactoryNames(type, classLoader));
20         List<T> instances = new ArrayList<T>(names.size());
21 
22         // Create instances from the names
23         for (String name : names) {
24             try {
25                 Class<?> instanceClass = ClassUtils.forName(name, classLoader);
26                 Assert.isAssignable(type, instanceClass);
27                 Constructor<?> constructor = instanceClass.getConstructor(parameterTypes);
28                 T instance = (T) constructor.newInstance(args);
29                 instances.add(instance);
30             }
31             catch (Throwable ex) {
32                 throw new IllegalArgumentException(
33                         "Cannot instantiate " + type + " : " + name, ex);
34             }
35         }
36 
37         AnnotationAwareOrderComparator.sort(instances);
38         return instances;
39     }

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：

 1     /**
 2      * The location to look for factories.
 3      * <p>Can be present in multiple JAR files.
 4      */
 5     public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";
 6 
 7     /**
 8      * Load the fully qualified class names of factory implementations of the
 9      * given type from {@value #FACTORIES_RESOURCE_LOCATION}, using the given
10      * class loader.
11      */
12     public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) {
13         String factoryClassName = factoryClass.getName();
14         try {
15             Enumeration<URL> urls = (classLoader != null ? classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
16                     ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
17             List<String> result = new ArrayList<String>();
18             while (urls.hasMoreElements()) {
19                 URL url = urls.nextElement();
20                 Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url));
21                 String factoryClassNames = properties.getProperty(factoryClassName);
22                 result.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(factoryClassNames)));
23             }
24             return result;
25         }
26         catch (IOException ex) {
27             throw new IllegalArgumentException("Unable to load [" + factoryClass.getName() +
28                     "] factories from location [" + FACTORIES_RESOURCE_LOCATION + "]", ex);
29         }
30     }

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

 1     private List<ApplicationListener<?>> listeners;    
 2 
 3         /**
 4      * Sets the {@link ApplicationListener}s that will be applied to the SpringApplication
 5      * and registered with the {@link ApplicationContext}.
 6      * @param listeners the listeners to set
 7      */
 8     public void setListeners(Collection<? extends ApplicationListener<?>> listeners) {
 9         this.listeners = new ArrayList<ApplicationListener<?>>();
10         this.listeners.addAll(listeners);
11     }

META-INF/spring-factories

 1 # Application Listeners
 2 org.springframework.context.ApplicationListener=\
 3 org.springframework.boot.builder.ParentContextCloserApplicationListener,\
 4 org.springframework.boot.context.FileEncodingApplicationListener,\
 5 org.springframework.boot.context.config.AnsiOutputApplicationListener,\
 6 org.springframework.boot.context.config.ConfigFileApplicationListener,\
 7 org.springframework.boot.context.config.DelegatingApplicationListener,\
 8 org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener,\
 9 org.springframework.boot.logging.ClasspathLoggingApplicationListener,\
10 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

 1     private Class<?> mainApplicationClass;
 2 
 3     private Class<?> deduceMainApplicationClass() {
 4         try {
 5             StackTraceElement[] stackTrace = new RuntimeException().getStackTrace();
 6             for (StackTraceElement stackTraceElement : stackTrace) {
 7                 if ("main".equals(stackTraceElement.getMethodName())) {
 8                     return Class.forName(stackTraceElement.getClassName());
 9                 }
10             }
11         }
12         catch (ClassNotFoundException ex) {
13             // Swallow and continue
14         }
15         return null;
16     }

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