死磕源码系列【DelegatingApplicationContextInitializer源码分析】

骑猪看日落 2022-11-21 03:34 203阅读 0赞

> DelegatingApplicationContextInitializer初始化器实现类是ApplicationContextInitializer接口的一个代理实现类，提供了一个属性key(context.initializer.classes)在配置文件中配置自定义的初始化器实现类；当然，也可以使用SPI方式在spring.factories配置文件配置org.springframework.context.ApplicationContextInitializer作为key来配置，两种方式都可以，本文重点讲解第一种方式。

DelegatingApplicationContextInitializer源码分析：

public class DelegatingApplicationContextInitializer
    		implements ApplicationContextInitializer<ConfigurableApplicationContext>, Ordered { 
    
    	// 配置属性key
    
    	private static final String PROPERTY_NAME = "context.initializer.classes";
    	//默认优先级，在所有的初始化器中默认是最高的
    	private int order = 0;
    	//初始化器回调方法
    	@Override
    	public void initialize(ConfigurableApplicationContext context) { 
    		ConfigurableEnvironment environment = context.getEnvironment();
        //获取配置的自定义初始化器实例集合
    		List<Class<?>> initializerClasses = getInitializerClasses(environment);
    		if (!initializerClasses.isEmpty()) { 
    			applyInitializerClasses(context, initializerClasses);
    		}
    	}
    
    	private List<Class<?>> getInitializerClasses(ConfigurableEnvironment env) { 
        //获取配置文件中配置的自定义初始化器
    		String classNames = env.getProperty(PROPERTY_NAME);
    		List<Class<?>> classes = new ArrayList<>();
    		if (StringUtils.hasLength(classNames)) { 
          //使用逗号分割初始化器
    			for (String className : StringUtils.tokenizeToStringArray(classNames, ",")) { 
            //将初始化器class实例添加到集合
    				classes.add(getInitializerClass(className));
    			}
    		}
    		return classes;
    	}
    	//将指定初始化器className实例化
    	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) { 
          //将初始化器的class实例转换成具体的bean对象
    			initializers.add(instantiateInitializer(contextClass, initializerClass));
    		}
        //调用初始化器的初始化方法
    		applyInitializers(context, initializers);
    	}
    	//将初始化器的class实例转换成具体的bean对象
    	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);
    	}
    	//调用初始化器的初始化方法
    	@SuppressWarnings({  "unchecked", "rawtypes" })
    	private void applyInitializers(ConfigurableApplicationContext context,
    			List<ApplicationContextInitializer<?>> initializers) { 
    		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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GitHub地址：[https://github.com/mingyang66/spring-parent][https_github.com_mingyang66_spring-parent]

[https_github.com_mingyang66_spring-parent]: https://github.com/mingyang66/spring-parent