当 Spring遇到一个在 setter 方法中使用的 @Autowired 注解,它会试图执行 byType 自动连接。换言之,加了@Autowired的Setter方法等同于byType自动装配模式。
看个例子:
import org.springframework.beans.factory.annotation.Autowired;
public class TextEditor {
private SpellChecker spellChecker;
@Autowired
public void setSpellChecker( SpellChecker spellChecker ){
this.spellChecker = spellChecker;
}
public SpellChecker getSpellChecker( ) {
return spellChecker;
}
public void spellCheck() {
spellChecker.checkSpelling();
}
}
Main.app:
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
public class MainApp {
public static void main(String[] args) {
ApplicationContext context = new ClassPathXmlApplicationContext("Beans.xml");
TextEditor te = (TextEditor) context.getBean("textEditor");
te.spellCheck();
}
}
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"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context-3.0.xsd">
<context:annotation-config/>
<!-- Definition for textEditor bean without constructor-arg -->
<bean id="textEditor" class="com.sap.TextEditor">
</bean>
<!-- Definition for spellChecker bean -->
<bean id="spellChecker" class="com.sap.SpellChecker">
</bean>
</beans>
因为是采用by Type装配,所以Beans.xml里的com.sap.SpellChecker的id可以任意指定:
删除了Setter方法的@Autowired注释后,TextEditor的spellChecker注入不会发生,因此应用会发生nullpointer异常。
这样做可以不必显式地书写依赖属性的setter方法,可以少敲击几次键盘,让源代码更精简。
textEditor:
import org.springframework.beans.factory.annotation.Autowired;
public class TextEditor {
@Autowired
private SpellChecker spellChecker;
public TextEditor() {
System.out.println("Inside TextEditor constructor." );
}
public SpellChecker getSpellChecker( ){
return spellChecker;
}
public void spellCheck(){
spellChecker.checkSpelling();
}
}
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"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-3.0.xsd
http://www.springframework.org/schema/context
http://www.springframework.org/schema/context/spring-context-3.0.xsd">
<context:annotation-config/>
<!-- Definition for textEditor bean -->
<bean id="textEditor" class="com.sap.TextEditor">
</bean>
<!-- Definition for spellChecker bean -->
<bean id="spellChecker" class="com.sap.SpellChecker">
</bean>
</beans>
同样,如果@Autowired注解被删除,SpellChecker成员变量的依赖注入将不会发生,此时应用会产生nullPointer异常。