ByType is the default, but it is not unique. When a bean has more than one beanId, an error will be reported. In this case, we need to specify the beanId to implement the @autowire class. You’re going to use byName.

Let’s look at a project example

  1. Define MailConfig class

     public class MailConfig {
     private String subject;
     private String from;
     private String text;
     private String replyTo;
     private boolean html;
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  2. Register MailConfig with the Spring Bean through the configuration file and specify the Bean ID

    <bean id="CommonRegBillMail" class="com.sunisco.par.service.listener.MailConfig"> <property name="from" value="[email protected] " /> <property name="replyTo" value="[email protected] " /> <property name="html" value="true" /> <property name="text"> <value><! [CDATA] Dear customer,  #context# #url# #customer_code# #orgpwd# #footer# ]]></value> </property> </bean> <bean id="paymentVerificationImportEmailConfig" class="com.sunisco.par.service.listener.MailConfig"> <property name="from" value="[email protected] " /> <property name="replyTo" value="[email protected] " /> <property name="html" value="true" /> <property name="text"> <value><! [CDATA[dear customer #userCode# : #context#]] </value> </property> </bean>Copy the code

    Here we see MailConfig class corresponding to the two beans id is a CommonRegBillMail, one is paymentVerificationImportEmailConfig

  3. @autowired Inject dependencies

    If you use this

    @Autowired
private MailConfig config;
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    Will be an error, because the program does not know is to use CommonRegBillMail or paymentVerificationImportEmailConfig corresponding configuration, byName in this way

    @Autowired
@Qualifier("CommonRegBillMail")
private MailConfig config;
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    The byName approach uses the @qualifier annotation, which implements dependency lookup based on parameters. There will be no errors.

  4. Depend on the use

    MimeMessageHelper helper = new MimeMessageHelper(message, true, Charsets.UTF_8.name());
helper.setFrom(config.getFrom());
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The above is the flow and scenario of @Autowired via ByName and ByType injection. If you want to understand the underlying implementation principle, you are recommended to see the source code implementation of the two injection methods in “Spring Source In-depth Analysis” page 117.