java构造函数_Spring 自动装配模式之构造函数装配方式

这种模式与 byType 非常相似，但它应用于构造器参数。Spring 容器看作 beans，在 XML 配置文件中 beans 的 autowire 属性设置为 constructor。然后，它尝试把它的构造函数的参数与配置文件中 beans 名称中的一个进行匹配和连线。如果找到匹配项，它会注入这些 bean，否则，它会抛出异常。

例如，在配置文件中，如果一个 bean 定义设置为通过构造函数自动装配，而且它有一个带有 SpellChecker 类型的参数之一的构造函数，那么 Spring 就会查找定义名为 SpellChecker 的 bean，并用它来设置构造函数的参数。

也就是说，将构造函数的参数类型同candidate bean的ID进行匹配。

你仍然可以使用 标签连接其余属性。

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装配模式下的beans.xml:

<?xml version="1.0" encoding="UTF-8"?>

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

但是如果将这个bean注释掉，就会报异常：

Jul 25, 2020 10:42:12 AM org.springframework.context.support.ClassPathXmlApplicationContext prepareRefreshINFO: Refreshing [e-mail]4ee285c6: startup date [Sat Jul 25 10:42:12 CST 2020]; root of context hierarchyJul 25, 2020 10:42:12 AM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitionsINFO: Loading XML bean definitions from class path resource [Beans.xml]Jul 25, 2020 10:42:12 AM org.springframework.context.support.ClassPathXmlApplicationContext refreshWARNING: 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.(ClassPathXmlApplicationContext.java:144)at org.springframework.context.support.ClassPathXmlApplicationContext.(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