Spring依赖注入方式有Setter注入，构造函数注入。项目中如何选择？

先说结论：推荐用构造函数注入并在注入时判空；依赖服务是optional或者比较多时部分依赖可以考虑用setter（生命周期比被依赖服务短的）

setter依赖注入

早期Srping推荐用setter注入方式，spring参考例子中也大量使用。缺点是可能用到依赖服务时还未setter注入或者注入了空对象。解决空对象的一种方式是加入Spring 2.0 annotation @Required

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public class Service {

  private Collaborator collaborator;

  @Required
  public void setCollaborator(Collaborator c) {
    this.collaborator = c;
  }
}

或者

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public class Service implements InitializingBean {

  private Collaborator collaborator;

  public void setCollaborator(Collaborator c) {
    this.collaborator = c;
  }

  // from the InitializingBean interface
  public void afterPropertiesSet() {
    if (collaborator == null) {
      throw new IllegalStateException("Collaborator must be set in order for service to work");
    }
  }
}

用setter注入的另外一个原因是构造函数注入不能直观指定依赖名字。

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<bean id="authenticator" class="com.mycompany.service.AuthenticatorImpl"/>

<bean id="accountService" class="com.mycompany.service.AccountService">
  <property name="authenticator" ref="authenticator"/>
</bean>
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<bean id="authenticator" class="com.mycompany.service.AuthenticatorImpl"/>

<bean id="accountService" class="com.mycompany.service.AccountService">
  <constructor-arg ref="authenticator"/>
</bean>

构造函数依赖注入

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public Service(Collaborator collaborator) {
  if (collaborator == null) {
    throw new IllegalArgumentException("Collaborator cannot be null");
  }
  this.collaborator = collaborator;
}