Spring Framework--ApplicationComtext(2)以ClassPathXmlApplicationContext看ApplicationContext

最新推荐文章于 2020-12-03 12:09:51 发布

原创最新推荐文章于 2020-12-03 12:09:51 发布 · 1k 阅读

0 ·

CC 4.0 BY-SA版权

文章标签：

#spring #applicationcontext-xml #spring源码

springCode 专栏收录该内容

3 篇文章

订阅专栏

本文详细介绍了Spring框架中的ClassPathXmlApplicationContext类，包括其继承关系、构造方法及核心功能，如设置XML配置文件路径、初始化上下文等，并通过实战示例帮助读者深入理解。

摘要生成于 C知道，由 DeepSeek-R1 满血版支持，前往体验 >

前言

上篇博客我们简单介绍了ApplicationContext，说实话，讲得太糙了，自己都看不下去了。所以打算在本文和后面的文章以稍微详细的说明来弥补之前的不足。本文将以debug ClassPathXmlApplicationContext的方式一步一步去了解Application。

1. 概述

首先让我们来看个spring的测试用例：

    private static final String PATH = "/org/springframework/context/support/";
    private static final String CONTEXT_A = "test/contextA.xml";
    private static final String CONTEXT_B = "test/contextB.xml";
    private static final String CONTEXT_C = "test/contextC.xml";
    private static final String FQ_CONTEXT_A = PATH + CONTEXT_A;
    private static final String FQ_CONTEXT_B = PATH + CONTEXT_B;
    private static final String FQ_CONTEXT_C = PATH + CONTEXT_C;

@Test
    public void testMultipleConfigLocations() {
      // 根据多个xml路径实例化beans
        ClassPathXmlApplicationContext ctx = new ClassPathXmlApplicationContext(
                FQ_CONTEXT_B, FQ_CONTEXT_C, FQ_CONTEXT_A);
      // 断言，BeanFacoty（containsBean是ClassPathXmlApplication的纠结父类BeanFantocty的方法）里面是否包含传入参数的方法。
        assertTrue(ctx.containsBean("service"));
        assertTrue(ctx.containsBean("logicOne"));
        assertTrue(ctx.containsBean("logicTwo"));

      // 通过getBean获取相应的实例化bean
        // re-refresh (after construction refresh)
        Service service = (Service) ctx.getBean("service");
        ctx.refresh();
        assertTrue(service.isProperlyDestroyed());

        // regular close call
        service = (Service) ctx.getBean("service");
        ctx.close();
        assertTrue(service.isProperlyDestroyed());

        // re-activating and re-closing the context (SPR-13425)
        ctx.refresh();
        service = (Service) ctx.getBean("service");
        ctx.close();
        assertTrue(service.isProperlyDestroyed());
    }

上面是一个ClassPathXmlApplicationContext的应用，通过xml的路径，获取xml下的bean配置，从而获取实例化bean。

1.1 初识ClassPathXmlApplicationContext

下面看看Spring中开发者对ClassPathXmlApplicationContext的讲解

/**
 * Standalone XML application context, taking the context definition files
 * from the class path, interpreting plain paths as class path resource names
 * that include the package path (e.g. "mypackage/myresource.txt"). Useful for
 * test harnesses as well as for application contexts embedded within JARs.
 * 独立XML应用程序上下文，从类路径中获取上下文定义文件，将纯路径解释为包含程序包路径的类路径资源名称（例如“mypackage / myresource.txt”）。对测试工具以及嵌入JAR的应用程序上下文非常有用。

 * <p>The config location defaults can be overridden via {@link #getConfigLocations},
 * Config locations can either denote concrete files like "/myfiles/context.xml"
 * or Ant-style patterns like "/myfiles/*-context.xml" (see the
 * {@link org.springframework.util.AntPathMatcher} javadoc for pattern details).
 *
 * <p>Note: In case of multiple config locations, later bean definitions will
 * override ones defined in earlier loaded files. This can be leveraged to
 * deliberately override certain bean definitions via an extra XML file.
 *
 * <p><b>This is a simple, one-stop shop convenience ApplicationContext.
 * Consider using the {@link GenericApplicationContext} class in combination
 * with an {@link org.springframework.beans.factory.xml.XmlBeanDefinitionReader}
 * for more flexible context setup.</b>
 *
 * @author Rod Johnson
 * @author Juergen Hoeller
 * @see #getResource
 * @see #getResourceByPath
 * @see GenericApplicationContext
 */

下面是ClassPathXmlApplicationContext类图! 这里写图片描述

由图可知，ClassPathXmlApplicationContext继承关系为：

@startuml

ApplicationContext<|—AbstactApplicationContext<|—AbstractRefreshableApplicationContext<—AbstractRefreshableConfigApplicationContext<|—AbstractXmlApplicationContext<|–ClassPathXmlApplicationContext

@enduml

下图为ClassPathXmlApplicationContext所包含的方法：

这里写图片描述

又上图可知，ClassPathXmlApplicationContext几乎全是构造方法的重载（Overload）

/** 
    *   不难看出，其造函数主要分为两类：
    *       1.指定xml文件配置路径，不指定需要获取的bean实例化对象。
    *       2.指定xml文件配置路径，指定需要获取的bean实例化对象。
    *  分别为一下两种方法。
    */
/**
     * Create a new ClassPathXmlApplicationContext with the given parent,
     * loading the definitions from the given XML files.
     * @param configLocations array of resource locations
     * @param refresh whether to automatically refresh the context,
     * loading all bean definitions and creating all singletons.
     * Alternatively, call refresh manually after further configuring the context.
     * @param parent the parent context
     * @throws BeansException if context creation failed
     * @see #refresh()
     */
public ClassPathXmlApplicationContext(
        String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)
            throws BeansException {

        super(parent);
        setConfigLocations(configLocations);
        if (refresh) {
            refresh();
        }
    }


/**
     * Create a new ClassPathXmlApplicationContext with the given parent,
     * loading the definitions from the given XML files and automatically
     * refreshing the context.
     * @param paths array of relative (or absolute) paths within the class path
     * @param clazz the class to load resources with (basis for the given paths)
     * @param parent the parent context
     * @throws BeansException if context creation failed
     * @see org.springframework.core.io.ClassPathResource#ClassPathResource(String, Class)
     * @see org.springframework.context.support.GenericApplicationContext
     * @see org.springframework.beans.factory.xml.XmlBeanDefinitionReader
     */
public ClassPathXmlApplicationContext(String[] paths, Class<?> clazz, @Nullable ApplicationContext parent)
    throws BeansException {

        super(parent);
        Assert.notNull(paths, "Path array must not be null");
        Assert.notNull(clazz, "Class argument must not be null");
        this.configResources = new Resource[paths.length];
        for (int i = 0; i < paths.length; i++) {
            this.configResources[i] = new ClassPathResource(paths[i], clazz);
        }
        refresh();
    }

2. 一起学习源码—debug spring test

下面将debug org.springframework.context.support.ClassPathXmlApplicationContextTests 中的testConfigLocationPattern，以此了解ApplicationContext中的ClassPathXmlApplicationContext的初始化过程。

private static final String PATH = "/org/springframework/context/support/";
private static final String CONTEXT_WILDCARD = PATH + "test/context*.xml";

@Test
    public void testConfigLocationPattern() {
        ClassPathXmlApplicationContext ctx = new ClassPathXmlApplicationContext(CONTEXT_WILDCARD);
        assertTrue(ctx.containsBean("service"));
        assertTrue(ctx.containsBean("logicOne"));
        assertTrue(ctx.containsBean("logicTwo"));
        Service service = (Service) ctx.getBean("service");
        ctx.close();
        assertTrue(service.isProperlyDestroyed());
    }

可知，此处是一个通过通配符获取test目录下所有已context打头的所有xml配置文件，以此初始化ClassPathXmlApplicationContext，从而实例化beans。

以下一段只是觉得搞笑，当做注释吧！

哈哈，太好玩了。当我进入到

ClassPathXmlApplicationContext ctx = new ClassPathXmlApplicationContext(CONTEXT_WILDCARD);

这个函数时，没想到第一步居然是跳到了AbstractApplicationContext下的一段静态代码块

static {
        // Eagerly load the ContextClosedEvent class to avoid weird classloader issues
        // on application shutdown in WebLogic 8.1. (Reported by Dustin Woods.)
        ContextClosedEvent.class.getName();
    }

注意看注释，他说，这么急切的加载ContextClosedEvent类，是为了避免WebLogic 8.1中在关闭应用程序的时候出现奇怪的类加载器问题。

~QAQ~ 莫名的喜感

然后进入到ClassPathXmlApplicationContext的构造函数中，最终会调用以下这个构造函数

public ClassPathXmlApplicationContext(
                    String[] configLocations,
                    boolean refresh, 
                    @Nullable ApplicationContext parent)throws BeansException {
        super(parent);
        setConfigLocations(configLocations);
        if (refresh) {
            refresh();
        }
    }

（1）Super(parent)—>首先会去加载父类的构造方法；

org/springframework/context/support/AbstractApplicationContext.java

/**
     * Create a new AbstractApplicationContext with the given parent context.
     * @param parent the parent context
     */
public AbstractApplicationContext(@Nullable ApplicationContext parent) {
        this();
        setParent(parent);
    }

    //---------------------------------------------------------------------
    // Implementation of ConfigurableApplicationContext interface
    //---------------------------------------------------------------------

    /**
     * Set the parent of this application context.
     * <p>The parent {@linkplain ApplicationContext#getEnvironment() environment} is
     * {@linkplain ConfigurableEnvironment#merge(ConfigurableEnvironment) merged} with
     * this (child) application context environment if the parent is non-{@code null} and
     * its environment is an instance of {@link ConfigurableEnvironment}.
     * @see ConfigurableEnvironment#merge(ConfigurableEnvironment)
     */
    @Override
    public void setParent(@Nullable ApplicationContext parent) {
        this.parent = parent;
        if (parent != null) {
            Environment parentEnvironment = parent.getEnvironment();
            if (parentEnvironment instanceof ConfigurableEnvironment) {
                getEnvironment().merge((ConfigurableEnvironment) parentEnvironment);
            }
        }
    }

已知这里通过传进来的ApplicationContext获取配置环境,但是显然，我们调用的ClassPathXmlApplicationContext的构造方法传入参数为null。

（2） setConfigLocations(configLocations);——设置spring的配置文件

public void setConfigLocations(@Nullable String... locations) {
        if (locations != null) {
            Assert.noNullElements(locations, "Config locations must not be null");
            this.configLocations = new String[locations.length];
            for (int i = 0; i < locations.length; i++) {
                this.configLocations[i] = resolvePath(locations[i]).trim();
            }
        }
        else {
            this.configLocations = null;
        }
    }

易知，这里根据传入的xml路径，去解析beans路径。

这里通过resolvePath去获取一个StandardEnvironment环境的。

最后调用的是org.springframework.util.PropertyPlaceholderHelper 里的parseStringValue方法.—-这里会去解析路径中的${}就不太懂了。。。

org.springframework.util.PropertyPlaceholderHelper

protected String parseStringValue(String value,
                                  PlaceholderResolver placeholderResolver,
                                  Set<String> visitedPlaceholders) {

        StringBuilder result = new StringBuilder(value);

        int startIndex = value.indexOf(this.placeholderPrefix);
        while (startIndex != -1) {
            int endIndex = findPlaceholderEndIndex(result, startIndex);
            if (endIndex != -1) {
                String placeholder = result.substring(startIndex + this.placeholderPrefix.length(), endIndex);
                String originalPlaceholder = placeholder;
                if (!visitedPlaceholders.add(originalPlaceholder)) {
                    throw new IllegalArgumentException(
                            "Circular placeholder reference '" + originalPlaceholder + "' in property definitions");
                }
                // Recursive invocation, parsing placeholders contained in the placeholder key.
                placeholder = parseStringValue(placeholder, placeholderResolver, visitedPlaceholders);
                // Now obtain the value for the fully resolved key...
                String propVal = placeholderResolver.resolvePlaceholder(placeholder);
                if (propVal == null && this.valueSeparator != null) {
                    int separatorIndex = placeholder.indexOf(this.valueSeparator);
                    if (separatorIndex != -1) {
                        String actualPlaceholder = placeholder.substring(0, separatorIndex);
                        String defaultValue = placeholder.substring(separatorIndex + this.valueSeparator.length());
                        propVal = placeholderResolver.resolvePlaceholder(actualPlaceholder);
                        if (propVal == null) {
                            propVal = defaultValue;
                        }
                    }
                }
                if (propVal != null) {
                    // Recursive invocation, parsing placeholders contained in the
                    // previously resolved placeholder value.
                    propVal = parseStringValue(propVal, placeholderResolver, visitedPlaceholders);
                    result.replace(startIndex, endIndex + this.placeholderSuffix.length(), propVal);
                    if (logger.isTraceEnabled()) {
                        logger.trace("Resolved placeholder '" + placeholder + "'");
                    }
                    startIndex = result.indexOf(this.placeholderPrefix, startIndex + propVal.length());
                }
                else if (this.ignoreUnresolvablePlaceholders) {
                    // Proceed with unprocessed value.
                    startIndex = result.indexOf(this.placeholderPrefix, endIndex + this.placeholderSuffix.length());
                }
                else {
                    throw new IllegalArgumentException("Could not resolve placeholder '" +
                            placeholder + "'" + " in value \"" + value + "\"");
                }
                visitedPlaceholders.remove(originalPlaceholder);
            }
            else {
                startIndex = -1;
            }
        }

        return result.toString();
    }

（3）refresh();——//调用父类的refresh函数，进行一系列初始化

/**
     * Load or refresh the persistent representation of the configuration,
     * which might an XML file, properties file, or relational database schema.
     * <p>As this is a startup method, it should destroy already created singletons
     * if it fails, to avoid dangling resources. In other words, after invocation
     * of that method, either all or no singletons at all should be instantiated.
     * @throws BeansException if the bean factory could not be initialized
     * @throws IllegalStateException if already initialized and multiple refresh
     * attempts are not supported
     */
@Override
    public void refresh() throws BeansException, IllegalStateException {
        synchronized (this.startupShutdownMonitor) {
            // Prepare this context for refreshing.
            prepareRefresh();

            // Tell the subclass to refresh the internal bean factory.
            ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

            // Prepare the bean factory for use in this context.
            prepareBeanFactory(beanFactory);

            try {
                // Allows post-processing of the bean factory in context subclasses.
                postProcessBeanFactory(beanFactory);

                // Invoke factory processors registered as beans in the context.
                invokeBeanFactoryPostProcessors(beanFactory);

                // Register bean processors that intercept bean creation.
                registerBeanPostProcessors(beanFactory);

                // Initialize message source for this context.
                initMessageSource();

                // Initialize event multicaster for this context.
                initApplicationEventMulticaster();

                // Initialize other special beans in specific context subclasses.
                onRefresh();

                // Check for listener beans and register them.
                registerListeners();

                // Instantiate all remaining (non-lazy-init) singletons.
                finishBeanFactoryInitialization(beanFactory);

                // Last step: publish corresponding event.
                finishRefresh();
            }

            catch (BeansException ex) {
                if (logger.isWarnEnabled()) {
                    logger.warn("Exception encountered during context initialization - " +
                            "cancelling refresh attempt: " + ex);
                }

                // Destroy already created singletons to avoid dangling resources.
                destroyBeans();

                // Reset 'active' flag.
                cancelRefresh(ex);

                // Propagate exception to caller.
                throw ex;
            }

            finally {
                // Reset common introspection caches in Spring's core, since we
                // might not ever need metadata for singleton beans anymore...
                resetCommonCaches();
            }
        }
    }