前言

我们生成一个spring boot 项目时,会自带一个启动类. 代码如下:

@SpringBootApplication
public class DemoApplication {

    public static void main(String[] args) {
        SpringApplication.run(DemoApplication.class, args);
    }
}

就是这么简单的代码,构成了spring boot的世界. 那么代码中只有一个@SpringBootApplication 注解和调用了SpringApplication#run方法.那么我们先来解析SpringApplication的run方法.

解析

首先调用了org.springframework.boot.SpringApplication#run(Object, String…) 方法.代码如下:

public static ConfigurableApplicationContext run(Object source, String... args) {
    return run(new Object[] { source }, args);
}

接着调用如下代码:

public static ConfigurableApplicationContext run(Object[] sources, String[] args) {
    return new SpringApplication(sources).run(args);
}

可以发现首先初始化了SpringApplication,然后调用其实例方法:run.

在 SpringApplication 的构造器中,调用了 initialize 方法.

public SpringApplication(Object... sources) {
    initialize(sources);
}

SpringApplication#initialize方法代码如下:

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();
}

可以看到做了如下5件事:

如果sources长度大于0的话,加入到SpringApplication的sources中,该sources是一个LinkedHashSet.
调用deduceWebEnvironment方法判断是否是web环境
设置initializers.
设置Listeners.
设置mainApplicationClass.

deduceWebEnvironment代码如下:

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;
}

可以发现会调用ClassUtils类的isPresent方法，检查classpath中是否存在javax.servlet.Servlet类和org.springframework.web.context.ConfigurableWebApplicationContext类,如果存在的话,返回true.否则返回false.

在设置Initializers时首先调用getSpringFactoriesInstances方法加载ApplicationContextInitializer.然后直接赋值给initializers.代码如下:

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

转而调用如下代码:

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
    // 使用Set保存names来避免重复元素
    Set<String> names = new LinkedHashSet<String>(
            SpringFactoriesLoader.loadFactoryNames(type, classLoader));
    // 根据names来进行实例化
    List<T> instances = createSpringFactoriesInstances(type, parameterTypes,
            classLoader, args, names);
    // 对实例进行排序
    AnnotationAwareOrderComparator.sort(instances);
    return instances;
}

该方法逻辑如下:

首先获得ClassLoader.
调用SpringFactoriesLoader#loadFactoryNames进行加载,然后放入到LinkedHashSet进行去重.
调用createSpringFactoriesInstances进行初始化
排序

其中SpringFactoriesLoader#loadFactoryNames代码如下:

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);
    }
}

逻辑如下:

获得factoryClassName,对于当前来说factoryClassName =org.springframework.context.ApplicationContextInitializer.
通过传入的classLoader加载META-INF/spring.factories文件.
通过调用PropertiesLoaderUtils#loadProperties将其转为Properties.
获得factoryClassName对应的值进行返回.

对于当前来说,在spring-boot/META-INF/spring.factories中.org.springframework.context.ApplicationContextInitializer值如下:

org.springframework.context.ApplicationContextInitializer=org.springframework.boot.context.ConfigurationWarningsApplicationContextInitializer,org.springframework.boot.context.ContextIdApplicationContextInitializer,org.springframework.boot.context.config.DelegatingApplicationContextInitializer,org.springframework.boot.context.embedded.ServerPortInfoApplicationContextInitializer

SpringApplication#createSpringFactoriesInstances方法如下:

private <T> List<T> createSpringFactoriesInstances(Class<T> type,
        Class<?>[] parameterTypes, ClassLoader classLoader, Object[] args,
        Set<String> names) {
    List<T> instances = new ArrayList<T>(names.size());
    for (String name : names) {
        try {
            Class<?> instanceClass = ClassUtils.forName(name, classLoader);
            Assert.isAssignable(type, instanceClass);
            Constructor<?> constructor = instanceClass
                    .getDeclaredConstructor(parameterTypes);
            T instance = (T) BeanUtils.instantiateClass(constructor, args);
            instances.add(instance);
        }
        catch (Throwable ex) {
            throw new IllegalArgumentException(
                    "Cannot instantiate " + type + " : " + name, ex);
        }
    }
    return instances;
}

逻辑如下:遍历传入的names,也就是之前通过SpringFactoriesLoader加载的类名.通过遍历,依次调用其构造器进行初始化.加入到instances.然后进行返回.

对于当前场景来说:
ConfigurationWarningsApplicationContextInitializer,DelegatingApplicationContextInitializer,ServerPortInfoApplicationContextInitializer初始化没有做任何事.

ContextIdApplicationContextInitializer在初始化时.会获得spring boot的应用名.搜索路径如下:

spring.application.name
vcap.application.name
spring.config.name
如果都没有配置的话,返回application.

代码如下:

private static final String NAME_PATTERN = "${spring.application.name:${vcap.application.name:${spring.config.name:application}}}";

public ContextIdApplicationContextInitializer() {
    this(NAME_PATTERN);
}

public ContextIdApplicationContextInitializer(String name) {
    this.name = name;
}

设置SpringApplication#setListeners时,还是同样的套路.调用getSpringFactoriesInstances加载META-INF/spring.factories中配置的org.springframework.context.ApplicationListener.
对于当前来说.加载的类如下:

org.springframework.context.ApplicationListener=org.springframework.boot.ClearCachesApplicationListener,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

这些类在构造器中都没有做任何事.

调用SpringApplication#deduceMainApplicationClass方法.获得应用的启动类.该方法通过获取当前方法调用栈，找到main函数的类.代码如下:

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;
}

流程图如下:

(2条消息) spring boot 源码解析2

前言

解析

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