看個例子：

public class TextEditor {private SpellChecker spellChecker;// a setter method to inject the dependency.public void setSpellChecker(SpellChecker spellChecker) {System.out.println("Inside setSpellChecker." );this.spellChecker = spellChecker;}// a getter method to return spellCheckerpublic SpellChecker getSpellChecker() {return spellChecker;}public void spellCheck() {spellChecker.checkSpelling();} }

另一個依賴類文件 SpellChecker.java 的內容：

public class SpellChecker {public SpellChecker(){System.out.println("Inside SpellChecker constructor." );}public void checkSpelling() {System.out.println("Inside checkSpelling." );} }

Beans.xml的內容：

<?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"><property name="spellChecker" ref="spellChecker"/></bean><!-- Definition for spellChecker bean --><bean id="spellChecker" class="com.sap.SpellChecker"></bean></beans>

執行結果：

為什么Spring IOC容器，如此智能地知道調用TextEditor的setSpellChecker方法呢？

利用Java反射機制調用set方法：

SpellChecker的writeMethod是如何解析出來的呢？

此處可以看到，GenericTypeAwarePropertyDescriptor試圖解析Bean類的set方法作為writeMethod：

將setSpellChecker改成SetSpellChecker，就會遇到異常：org.springframework.beans.NotWritablePropertyException: Invalid property ‘spellChecker’ of bean class [com.sap.TextEditor]: Bean property ‘spellChecker’ is not writable or has an invalid setter method. Does the parameter type of the setter match the return type of the getter?

完整的調用棧：

Jul 24, 2020 4:13:04 PM org.springframework.context.support.ClassPathXmlApplicationContext prepareRefresh INFO: Refreshing org.springframework.context.support.ClassPathXmlApplicationContext@4ee285c6: startup date [Fri Jul 24 16:13:04 CST 2020]; root of context hierarchy Jul 24, 2020 4:13:05 PM org.springframework.beans.factory.xml.XmlBeanDefinitionReader loadBeanDefinitions INFO: Loading XML bean definitions from class path resource [Beans.xml] Inside SpellChecker constructor. Jul 24, 2020 4:13:05 PM org.springframework.context.support.ClassPathXmlApplicationContext refresh WARNING: Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'textEditor' defined in class path resource [Beans.xml]: Error setting property values; nested exception is org.springframework.beans.NotWritablePropertyException: Invalid property 'spellChecker' of bean class [com.sap.TextEditor]: Bean property 'spellChecker' is not writable or has an invalid setter method. Does the parameter type of the setter match the return type of the getter? Exception in thread "main" org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'textEditor' defined in class path resource [Beans.xml]: Error setting property values; nested exception is org.springframework.beans.NotWritablePropertyException: Invalid property 'spellChecker' of bean class [com.sap.TextEditor]: Bean property 'spellChecker' is not writable or has an invalid setter method. Does the parameter type of the setter match the return type of the getter?at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyPropertyValues(AbstractAutowireCapableBeanFactory.java:1650)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.populateBean(AbstractAutowireCapableBeanFactory.java:1357)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.doCreateBean(AbstractAutowireCapableBeanFactory.java:582)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:11) Caused by: org.springframework.beans.NotWritablePropertyException: Invalid property 'spellChecker' of bean class [com.sap.TextEditor]: Bean property 'spellChecker' is not writable or has an invalid setter method. Does the parameter type of the setter match the return type of the getter?at org.springframework.beans.BeanWrapperImpl.createNotWritablePropertyException(BeanWrapperImpl.java:247)at org.springframework.beans.AbstractNestablePropertyAccessor.processLocalProperty(AbstractNestablePropertyAccessor.java:426)at org.springframework.beans.AbstractNestablePropertyAccessor.setPropertyValue(AbstractNestablePropertyAccessor.java:278)at org.springframework.beans.AbstractNestablePropertyAccessor.setPropertyValue(AbstractNestablePropertyAccessor.java:266)at org.springframework.beans.AbstractPropertyAccessor.setPropertyValues(AbstractPropertyAccessor.java:97)at org.springframework.beans.AbstractPropertyAccessor.setPropertyValues(AbstractPropertyAccessor.java:77)at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.applyPropertyValues(AbstractAutowireCapableBeanFactory.java:1646)... 13 more

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