原始碼分析

本文程式碼基於 Springboot 2.1.0

執行環境屬性"context.initializer.classes"指定的初始化器，這裡其實是給springboot應用程式使用者提供了一個機會指定自己實現的ApplicationContextInitializer對應用程式做一些初始化工作。

package org.springframework.boot.context.config;

import java.util.ArrayList;
import java.util.List;

import org.springframework.
 
beans.BeanUtils;
import org.springframework.context.ApplicationContextException;
import org.springframework.context.ApplicationContextInitializer;
import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.core.GenericTypeResolver;
import org.springframework.core.Ordered;
 

import org.springframework.core.annotation.AnnotationAwareOrderComparator;
import org.springframework.core.env.ConfigurableEnvironment;
import org.springframework.util.Assert;
import org.springframework.util.ClassUtils;
import org.springframework.util.StringUtils;

/**
 * ApplicationContextInitializer that delegates to other initializers that are
 * specified under a "context.initializer.classes" environment property.
 * 
 * 執行環境屬性"context.initializer.classes"指定的初始化器
 * @author Dave Syer
 * @author Phillip Webb
 */
 

public class DelegatingApplicationContextInitializer implements
		ApplicationContextInitializer<ConfigurableApplicationContext>, Ordered {

	// NOTE: Similar to org.springframework.web.context.ContextLoader

	private static final String PROPERTY_NAME = "context.initializer.classes";

	private int order = 0;

	@Override
	public void initialize(ConfigurableApplicationContext context) {
		ConfigurableEnvironment environment = context.getEnvironment();
		// 從環境屬性context.initializer.classes分析出各個ApplicationContextInitializer實現類
		// 並應用它們，也就是例項化它們並呼叫它們的initialize(context)方法
		List<Class<?>> initializerClasses = getInitializerClasses(environment);
		if (!initializerClasses.isEmpty()) {
			applyInitializerClasses(context, initializerClasses);
		}
	}

	private List<Class<?>> getInitializerClasses(ConfigurableEnvironment env) {
	// 環境屬性context.initializer.classes是一個字串，以逗號分割,
	// 分隔開的每一部分是一個類的全名稱，並且該類應該是一個ApplicationContextInitializer實現類
		String classNames = env.getProperty(PROPERTY_NAME);
		List<Class<?>> classes = new ArrayList<>();
		if (StringUtils.hasLength(classNames)) {
			for (String className : StringUtils.tokenizeToStringArray(classNames, ",")) {
				// 獲取每一個類名稱，分析其對對應的ApplicationContextInitializer實現類,
				// 如果某個類名稱字串並不對應classpath上的某個類，或者不是一個ApplicationContextInitializer
				// 實現類，這裡會丟擲ClassNotFoundException異常
				classes.add(getInitializerClass(className));
			}
		}
		return classes;
	}

	private Class<?> getInitializerClass(String className) throws LinkageError {
		try {
			Class<?> initializerClass = ClassUtils.forName(className,
					ClassUtils.getDefaultClassLoader());
			Assert.isAssignable(ApplicationContextInitializer.class, initializerClass);
			return initializerClass;
		}
		catch (ClassNotFoundException ex) {
			throw new ApplicationContextException(
					"Failed to load context initializer class [" + className + "]", ex);
		}
	}

	private void applyInitializerClasses(ConfigurableApplicationContext context,
			List<Class<?>> initializerClasses) {
		Class<?> contextClass = context.getClass();
		List<ApplicationContextInitializer<?>> initializers = new ArrayList<>();
		for (Class<?> initializerClass : initializerClasses) {
			initializers.add(instantiateInitializer(contextClass, initializerClass));
		}
		applyInitializers(context, initializers);
	}

	private ApplicationContextInitializer<?> instantiateInitializer(Class<?> contextClass,
			Class<?> initializerClass) {
		Class<?> requireContextClass = GenericTypeResolver.resolveTypeArgument(
				initializerClass, ApplicationContextInitializer.class);
		Assert.isAssignable(requireContextClass, contextClass,
				String.format(
						"Could not add context initializer [%s]"
								+ " as its generic parameter [%s] is not assignable "
								+ "from the type of application context used by this "
								+ "context loader [%s]: ",
						initializerClass.getName(), requireContextClass.getName(),
						contextClass.getName()));
		return (ApplicationContextInitializer<?>) BeanUtils
				.instantiateClass(initializerClass);
	}

	@SuppressWarnings({ "unchecked", "rawtypes" })
	private void applyInitializers(ConfigurableApplicationContext context,
			List<ApplicationContextInitializer<?>> initializers) {
		// 應用這些ApplicationContextInitializer之前先對他們進行排序	
		initializers.sort(new AnnotationAwareOrderComparator());
		for (ApplicationContextInitializer initializer : initializers) {
			initializer.initialize(context);
		}
	}

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

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

}

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