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关于spring的几个问题

几个开发中可能会遇到的问题。

本文都是先说结论，然后以源码做注解。

前提:基于spring 4.3.1,使用XmlWebApplicationContext,使用默认策略

xml声明的bean和注解声明的bean的覆盖规则是怎样的？

  1. 同一容器内，xml会覆盖之前声明的同beanName的beanDefinition（包括注解和xml）
  2. 同一容器内，注解不会覆盖之前声明的同beanName的beanDefinition

源码注解：

1.ClassPathBeanDefinitionScanner#checkCandidate

2.DefaultListableBeanFactory#registerBeanDefinition

// 返回是否可以被扫描
protected boolean checkCandidate(String beanName, BeanDefinition beanDefinition) throws IllegalStateException {
    // 1.如果注册表中不存在该beanName，返回true
    if (!this.registry.containsBeanDefinition(beanName)) {
        return true;
    }
    // 2.通过beanName找到对应的bean
    BeanDefinition existingDef = this.registry.getBeanDefinition(beanName);
    BeanDefinition originatingDef = existingDef.getOriginatingBeanDefinition();
    if (originatingDef != null) {
    	existingDef = originatingDef;
    }
    // 3.是否兼容，如果存在的beanDefinition兼容要被扫描的beanDefinition，则没必要再次被扫描，然后我们看下isCompatible的逻辑
    if (isCompatible(beanDefinition, existingDef)) {
    	return false;
    }
    throw new ConflictingBeanDefinitionException("Annotation-specified bean name '" + beanName +
            "' for bean class [" + beanDefinition.getBeanClassName() + "] conflicts with existing, " +
                "non-compatible bean definition of same name and class [" + existingDef.getBeanClassName() + "]");
}

// 1. 已经存在的beanDefinition不是ScannedGenericBeanDefinition，显然xml声明的bean不是scanned的
// 2. 已经存在的beanDefinition和new 的beanDefinition是同一source
// 3. 两个beanDefinition equals
// 三者条件满足其一，compatible都是true,那么都不需要再被扫描，所以xml声明过的bean是不会再被扫描的
protected boolean isCompatible(BeanDefinition newDefinition, BeanDefinition existingDefinition) {
    return (!(existingDefinition instanceof ScannedGenericBeanDefinition) ||  // explicitly registered overriding bean
            newDefinition.getSource().equals(existingDefinition.getSource()) ||  // scanned same file twice
            newDefinition.equals(existingDefinition));  // scanned equivalent class twice
}

public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
			throws BeanDefinitionStoreException {
    Assert.hasText(beanName, "Bean name must not be empty");
    Assert.notNull(beanDefinition, "BeanDefinition must not be null");
    if (beanDefinition instanceof AbstractBeanDefinition) {
        try {
            ((AbstractBeanDefinition) beanDefinition).validate();
        }
        catch (BeanDefinitionValidationException ex) {
            throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
                "Validation of bean definition failed", ex);
        }
    }
    synchronized (this.beanDefinitionMap) {
        Object oldBeanDefinition = this.beanDefinitionMap.get(beanName);
        if (oldBeanDefinition != null) {
            // 对于XmlWebApplicationContext使用的DefaultListableBeanFactory，是允许覆盖的，allowBeanDefinitionOverriding为true
            if (!this.allowBeanDefinitionOverriding) {
                throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
                    "Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +
                        "': There is already [" + oldBeanDefinition + "] bound.");
            }
            else {
                if (this.logger.isInfoEnabled()) {
                	this.logger.info("Overriding bean definition for bean '" + beanName +
                        "': replacing [" + oldBeanDefinition + "] with [" + beanDefinition + "]");
                }
            }
        }
    	else {
            this.beanDefinitionNames.add(beanName);
            this.frozenBeanDefinitionNames = null;
    	}
        // 这里xml会覆盖之前声明的同beanName的beanDefinition(当然也包括注解声明的bean)
    	this.beanDefinitionMap.put(beanName, beanDefinition);
    }
    resetBeanDefinition(beanName);
}

contextConfigLocation

首先上一段配置：

<?xml version="1.0" encoding="UTF-8"?>
<web-app xmlns="http://java.sun.com/xml/ns/j2ee" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
     xsi:schemaLocation="http://java.sun.com/xml/ns/j2ee http://java.sun.com/xml/ns/j2ee/web-app_2_4.xsd"
     version="2.4">
<display-name>meilv WebApp</display-name>

<!-- context-params-->
<context-param>
    <param-name>contextConfigLocation</param-name>
    <param-value>classpath:applicationContext.xml</param-value>
    <!--<param-value>classpath*:spring/**/spring-*.xml</param-value>-->
    <!--<param-value>root-context.xml</param-value>-->
</context-param>

...
</web-app>

其中context-param的contextConfigLocation的value可以有三种形式： classpath*、classpath、不带classpath的,然后又可细分为antPath和指明的文件，如下

  1. classpath*: all class path resources，包含依赖的jar包
    1. a class path resource pattern, classpath下满足给定antpath的所有文件（包含jar包中的）
    2. all class path resources with the given name,classpath下文件名为给定的名字的所有文件（包含jar包中的）
  2. classpath: 不会找依赖的jar包里的resource，和classpath*类似，不过不包含jar包中的
    1. a file pattern
    2. a single resource with the given name
  3. 普通文件:会从webApp的/WEB-INF/下找
    1. a file pattern
    2. a single resource with the given name

ps: war包的结构

.
|____index.jsp
|____META-INF
|____WEB-INF
| |____classes
| |____lib
| |____web.xml

源码注解：

1.PathMatchingResourcePatternResolver#getResources

2.PathMatchingResourcePatternResolver#findPathMatchingResources

3.PathMatchingResourcePatternResolver#findAllClassPathResources

4.DefaultResourceLoader#getResource

@Override
public Resource[] getResources(String locationPattern) throws IOException {
    Assert.notNull(locationPattern, "Location pattern must not be null");
    // 如果以classpath*:开头
    if (locationPattern.startsWith(CLASSPATH_ALL_URL_PREFIX)) {
        // a class path resource (multiple resources for same name possible)
        // 默认的getPathMatcher 都是AntPathMatcher，
        // 如果除去classpath*:外还包含有*或者?字符，走findPathMatchingResources
        if (getPathMatcher().isPattern(locationPattern.substring(CLASSPATH_ALL_URL_PREFIX.length()))) {
            // a class path resource pattern
            return findPathMatchingResources(locationPattern);
        }
        else {
            // all class path resources with the given name
            return findAllClassPathResources(locationPattern.substring(CLASSPATH_ALL_URL_PREFIX.length()));
        }
    }
    else {
        // Only look for a pattern after a prefix here
        // (to not get fooled by a pattern symbol in a strange prefix).
        int prefixEnd = locationPattern.indexOf(":") + 1;
        if (getPathMatcher().isPattern(locationPattern.substring(prefixEnd))) {
            // a file pattern
            return findPathMatchingResources(locationPattern);
        }
        else {
            // a single resource with the given name
            return new Resource[] {getResourceLoader().getResource(locationPattern)};
        }
    }
}

protected Resource[] findPathMatchingResources(String locationPattern) throws IOException {
    // 第一个/之前的路径为rootDirPath
    String rootDirPath = determineRootDir(locationPattern);
    // rootDirPath之后的路径为subPattern
    String subPattern = locationPattern.substring(rootDirPath.length());
    // eg: 对于classpath*:spring/**/spring-*.xml，其rootDirPath为classpath*:spring/，subPattern为**/spring-*.xml
    Resource[] rootDirResources = getResources(rootDirPath);
    Set<Resource> result = new LinkedHashSet<Resource>(16);
    for (Resource rootDirResource : rootDirResources) {
    	rootDirResource = resolveRootDirResource(rootDirResource);
    	URL rootDirURL = rootDirResource.getURL();
    	if (equinoxResolveMethod != null) {
    		if (rootDirURL.getProtocol().startsWith("bundle")) {
    			rootDirURL = (URL) ReflectionUtils.invokeMethod(equinoxResolveMethod, null, rootDirURL);
    			rootDirResource = new UrlResource(rootDirURL);
    		}
    	}
    	if (rootDirURL.getProtocol().startsWith(ResourceUtils.URL_PROTOCOL_VFS)) {
    		result.addAll(VfsResourceMatchingDelegate.findMatchingResources(rootDirURL, subPattern, getPathMatcher()));
    	}
    	else if (ResourceUtils.isJarURL(rootDirURL) || isJarResource(rootDirResource)) {
    		result.addAll(doFindPathMatchingJarResources(rootDirResource, rootDirURL, subPattern));
    	}
    	else {
    		result.addAll(doFindPathMatchingFileResources(rootDirResource, subPattern));
    	}
    }
    if (logger.isDebugEnabled()) {
    	logger.debug("Resolved location pattern [" + locationPattern + "] to resources " + result);
    }
    return result.toArray(new Resource[result.size()]);
}

protected Resource[] findAllClassPathResources(String location) throws IOException {
    String path = location;
    if (path.startsWith("/")) {
        path = path.substring(1);
    }
    Set<Resource> result = doFindAllClassPathResources(path);
    if (logger.isDebugEnabled()) {
        logger.debug("Resolved classpath location [" + location + "] to resources " + result);
    }
    return result.toArray(new Resource[result.size()]);
}

beanName、id

  1. 对于xml中声明的bean，如果指定了id属性，beanName为id对应的值，如果没有指定id属性，则spring会自己generate个beanName，规则为:className#数字

  2. 对于注解声明的bean，如果指定了value，则beanName使用指定的value；如果没指定，则spring会自己generate个beanName，规则为：simpleClassName，然后首字母小写（如果前两个字符都是大写的话，就直接返回simpleClassName了）

源码注解：

xml：

public BeanDefinitionHolder parseBeanDefinitionElement(Element ele, BeanDefinition containingBean) {
    // 解析id属性
    String id = ele.getAttribute(ID_ATTRIBUTE);
    // 解析name属性
    String nameAttr = ele.getAttribute(NAME_ATTRIBUTE);

    List<String> aliases = new ArrayList<String>();
    if (StringUtils.hasLength(nameAttr)) {
        String[] nameArr = StringUtils.tokenizeToStringArray(nameAttr, MULTI_VALUE_ATTRIBUTE_DELIMITERS);
        aliases.addAll(Arrays.asList(nameArr));
    }

    // spring容器中会使用id作为该beanDefinition的beanName
    String beanName = id;
    if (!StringUtils.hasText(beanName) && !aliases.isEmpty()) {
        beanName = aliases.remove(0);
        if (logger.isDebugEnabled()) {
            logger.debug("No XML 'id' specified - using '" + beanName +
                    "' as bean name and " + aliases + " as aliases");
        }
    }

    if (containingBean == null) {
        checkNameUniqueness(beanName, aliases, ele);
    }
    // 对标签其他属性的解析过程
    AbstractBeanDefinition beanDefinition = parseBeanDefinitionElement(ele, beanName, containingBean);
    if (beanDefinition != null) {
        if (!StringUtils.hasText(beanName)) {
            try {
                // 如果不存在beanName，spring为当前bean生成对应的beanName
                if (containingBean != null) {
                    beanName = BeanDefinitionReaderUtils.generateBeanName(
                            beanDefinition, this.readerContext.getRegistry(), true);
                }
                else {
                    beanName = this.readerContext.generateBeanName(beanDefinition);
                    // Register an alias for the plain bean class name, if still possible,
                    // if the generator returned the class name plus a suffix.
                    // This is expected for Spring 1.2/2.0 backwards compatibility.
                    String beanClassName = beanDefinition.getBeanClassName();
                    if (beanClassName != null &&
                            beanName.startsWith(beanClassName) && beanName.length() > beanClassName.length() &&
                            !this.readerContext.getRegistry().isBeanNameInUse(beanClassName)) {
                        aliases.add(beanClassName);
                    }
                }
                if (logger.isDebugEnabled()) {
                    logger.debug("Neither XML 'id' nor 'name' specified - " +
                            "using generated bean name [" + beanName + "]");
                }
            }
            catch (Exception ex) {
                error(ex.getMessage(), ele);
                return null;
            }
        }
        String[] aliasesArray = StringUtils.toStringArray(aliases);
        return new BeanDefinitionHolder(beanDefinition, beanName, aliasesArray);
    }

    return null;
}

注解：

protected String buildDefaultBeanName(BeanDefinition definition) {
    String shortClassName = ClassUtils.getShortName(definition.getBeanClassName());
    return Introspector.decapitalize(shortClassName);
}

public static String getShortName(String className) {
	Assert.hasLength(className, "Class name must not be empty");
	int lastDotIndex = className.lastIndexOf(PACKAGE_SEPARATOR);
	int nameEndIndex = className.indexOf(CGLIB_CLASS_SEPARATOR);
	if (nameEndIndex == -1) {
		nameEndIndex = className.length();
	}
	String shortName = className.substring(lastDotIndex + 1, nameEndIndex);
	shortName = shortName.replace(INNER_CLASS_SEPARATOR, PACKAGE_SEPARATOR);
	return shortName;
}

public static String decapitalize(String name) {
    if (name == null || name.length() == 0) {
        return name;
    }
    if (name.length() > 1 && Character.isUpperCase(name.charAt(1)) &&
                    Character.isUpperCase(name.charAt(0))){
        return name;
    }
    char chars[] = name.toCharArray();
    chars[0] = Character.toLowerCase(chars[0]);
    return new String(chars);
}

@Resource和@Autowired的区别

AbstractAutowireCapableBeanFactory#applyBeanPostProcessorsBeforeInstantiation

InstantiationAwareBeanPostProcessor

spring中Autowired和Resource的区别

动态代理的几个问题

"RMI TCP Connection(2)-127.0.0.1@1974" daemon prio=5 tid=0x15 nid=NA runnable
  java.lang.Thread.State: RUNNABLE
	  at org.springframework.cglib.core.DefaultNamingPolicy.getClassName(DefaultNamingPolicy.java:39)
	  at org.springframework.cglib.core.AbstractClassGenerator.generateClassName(AbstractClassGenerator.java:154)
	  at org.springframework.cglib.core.AbstractClassGenerator.generate(AbstractClassGenerator.java:317)
	  - locked <0xdf7> (a org.apache.catalina.loader.WebappClassLoader)
	  at org.springframework.cglib.proxy.Enhancer.generate(Enhancer.java:492)
	  at org.springframework.cglib.core.AbstractClassGenerator$ClassLoaderData$3.apply(AbstractClassGenerator.java:93)
	  at org.springframework.cglib.core.AbstractClassGenerator$ClassLoaderData$3.apply(AbstractClassGenerator.java:91)
	  at org.springframework.cglib.core.internal.LoadingCache$2.call(LoadingCache.java:54)
	  at java.util.concurrent.FutureTask.run(FutureTask.java:266)
	  at org.springframework.cglib.core.internal.LoadingCache.createEntry(LoadingCache.java:61)
	  at org.springframework.cglib.core.internal.LoadingCache.get(LoadingCache.java:34)
	  at org.springframework.cglib.core.AbstractClassGenerator$ClassLoaderData.get(AbstractClassGenerator.java:116)
	  at org.springframework.cglib.core.AbstractClassGenerator.create(AbstractClassGenerator.java:291)
	  at org.springframework.cglib.proxy.Enhancer.createHelper(Enhancer.java:480)
	  at org.springframework.cglib.proxy.Enhancer.createClass(Enhancer.java:337)
	  at org.springframework.aop.framework.ObjenesisCglibAopProxy.createProxyClassAndInstance(ObjenesisCglibAopProxy.java:55)
	  at org.springframework.aop.framework.CglibAopProxy.getProxy(CglibAopProxy.java:203)
	  at org.springframework.aop.framework.ProxyFactory.getProxy(ProxyFactory.java:109)
	  at org.springframework.aop.framework.autoproxy.AbstractAutoProxyCreator.createProxy(AbstractAutoProxyCreator.java:470)
	  at org.springframework.aop.framework.autoproxy.AbstractAutoProxyCreator.wrapIfNecessary(AbstractAutoProxyCreator.java:350)
	  at org.springframework.aop.framework.autoproxy.AbstractAutoProxyCreator.postProcessAfterInitialization(AbstractAutoProxyCreator.java:299)
	  at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyBeanPostProcessorsAfterInitialization(AbstractAutowireCapableBeanFactory.java:422)
	  at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.initializeBean(AbstractAutowireCapableBeanFactory.java:1583)
	  at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:545)
	  at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:482)
	  at org.springframework.beans.factory.support.AbstractBeanFactory$1.getObject(AbstractBeanFactory.java:306)
	  at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:230)
	  - locked <0xeaf> (a java.util.concurrent.ConcurrentHashMap)
	  at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:302)
	  at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:197)
	  at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:775)
	  at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:861)
	  at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:541)
	  - locked <0x1270> (a java.lang.Object)
代理的时机
代理的种类

spring的回调接口

参考：

spring4.0 源码分析 bean标签的解析(三)

spring-alias 标签的解析