這種模式與 byType 非常相似，但它應用于構(gòu)造器參數(shù)。Spring 容器看作 beans，在 XML 配置文件中 beans 的 autowire 屬性設置為 constructor。然后，它嘗試把它的構(gòu)造函數(shù)的參數(shù)與配置文件中 beans 名稱中的一個進行匹配和連線。如果找到匹配項，它會注入這些 bean，否則，它會拋出異常。

例如，在配置文件中，如果一個 bean 定義設置為通過構(gòu)造函數(shù)自動裝配，而且它有一個帶有 SpellChecker 類型的參數(shù)之一的構(gòu)造函數(shù)，那么 Spring 就會查找定義名為 SpellChecker 的 bean，并用它來設置構(gòu)造函數(shù)的參數(shù)。

也就是說，將構(gòu)造函數(shù)的參數(shù)類型同candidate bean的ID進行匹配。

你仍然可以使用 標簽連接其余屬性。

TextEditor：

public class TextEditor {private SpellChecker spellChecker;private String name;public TextEditor( SpellChecker spellChecker, String name ) {this.spellChecker = spellChecker;this.name = name;}public SpellChecker getSpellChecker() {return spellChecker;}public String getName() {return name;}public void spellCheck() {spellChecker.checkSpelling();} }

by constructor裝配模式下的beans.xml:

<?xml version="1.0" encoding="UTF-8"?><beans xmlns="http://www.springframework.org/schema/beans"xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"xsi:schemaLocation="http://www.springframework.org/schema/beanshttp://www.springframework.org/schema/beans/spring-beans-3.0.xsd"><!-- Definition for textEditor bean --><bean id="textEditor" class="com.sap.TextEditor" autowire="constructor"><constructor-arg value="Generic Text Editor"/></bean><!-- Definition for spellChecker bean --><bean id="SpellChecker" class="com.sap.SpellChecker"></bean></beans>

注意，將com.sap.SpellChecker這個bean的ID改成任意值均可。

但是如果將這個bean注釋掉，就會報異常：

Jul 25, 2020 10:42:12 AM org.springframework.context.support.ClassPathXmlApplicationContext prepareRefresh INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@4ee285c6: startup date [Sat Jul 25 10:42:12 CST 2020]; root of context hierarchy Jul 25, 2020 10:42:12 AM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions INFO: Loading XML bean definitions from class path resource [Beans.xml] Jul 25, 2020 10:42:12 AM org.springframework.context.support.ClassPathXmlApplicationContext refresh WARNING: Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'textEditor' defined in class path resource [Beans.xml]: Unsatisfied dependency expressed through constructor parameter 0; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'com.sap.SpellChecker' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {} Exception in thread "main" org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'textEditor' defined in class path resource [Beans.xml]: Unsatisfied dependency expressed through constructor parameter 0; nested exception is org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'com.sap.SpellChecker' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {}at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:729)at org.springframework.beans.factory.support.ConstructorResolver.autowireConstructor(ConstructorResolver.java:192)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.autowireConstructor(AbstractAutowireCapableBeanFactory.java:1270)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBeanInstance(AbstractAutowireCapableBeanFactory.java:1127)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:545)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.createBean(AbstractAutowireCapableBeanFactory.java:502)at org.springframework.beans.factory.support.AbstractBeanFactory.lambda$doGetBean$0(AbstractBeanFactory.java:312)at org.springframework.beans.factory.support.DefaultSingletonBeanRegistry.getSingleton(DefaultSingletonBeanRegistry.java:228)at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:310)at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:200)at org.springframework.beans.factory.support.DefaultListableBeanFactory.preInstantiateSingletons(DefaultListableBeanFactory.java:758)at org.springframework.context.support.AbstractApplicationContext.finishBeanFactoryInitialization(AbstractApplicationContext.java:868)at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:549)at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:144)at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:85)at com.sap.MainApp.main(MainApp.java:8) Caused by: org.springframework.beans.factory.NoSuchBeanDefinitionException: No qualifying bean of type 'com.sap.SpellChecker' available: expected at least 1 bean which qualifies as autowire candidate. Dependency annotations: {}at org.springframework.beans.factory.support.DefaultListableBeanFactory.raiseNoMatchingBeanFound(DefaultListableBeanFactory.java:1504)at org.springframework.beans.factory.support.DefaultListableBeanFactory.doResolveDependency(DefaultListableBeanFactory.java:1101)at org.springframework.beans.factory.support.DefaultListableBeanFactory.resolveDependency(DefaultListableBeanFactory.java:1062)at org.springframework.beans.factory.support.ConstructorResolver.resolveAutowiredArgument(ConstructorResolver.java:815)at org.springframework.beans.factory.support.ConstructorResolver.createArgumentArray(ConstructorResolver.java:721)... 15 more

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