在Spring中依赖注入可以使用@Autowired、@Resource和@Inject来完成,并且在一般的使用中是可以相互替换的(注意是一般),不过三者还是有区别,今天来介绍一下他们的区别:
如果你想更加深入的了解@Autowired、@Resource的区别,请移步:Spring源码分析@Autowired、@Resource注解的区别,这里面可能是你见过最详细的关于@Autowired、@Resource的区别,看完之后,面试你懂得。
@Autowired注解:
1.Spring本身替换的注解(org.springframework.beans.factory.annotation.Autowired),需要导入Spring相应的jar包才能使用
2.可以标注的位置:构造器、方法、方法参数、变量域和注解上面
@Target({ElementType.CONSTRUCTOR, ElementType.METHOD,
ElementType.PARAMETER, ElementType.FIELD, ElementType.ANNOTATION_TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Autowired {
boolean required() default true;
}
3.在Spring容器解析@Autowired注解时,使用的后置处理器为AutowiredAnnotationBeanPostProcessor,在这个后置处理的注释中有这么一段:
{@link org.springframework.beans.factory.config.BeanPostProcessor BeanPostProcessor}
* implementation that autowires annotated fields, setter methods, and arbitrary
* config methods. Such members to be injected are detected through annotations:
* by default, Spring's {@link Autowired @Autowired} and {@link Value @Value}
* annotations.
*
* <p>Also supports JSR-330's {@link javax.inject.Inject @Inject} annotation,
* if available, as a direct alternative to Spring's own {@code @Autowired}.
4. @Autowired注解有一个required属性,当指定required属性为false时,意味着在容器中找相应类型的bean,如果找不到则忽略,而不报错(这一条是两个注解所没有的功能)。
5. 默认优先按照类型去容器中找对应的组件,找到就赋值,如果找到多个相同类型的组件,再将属性的名称作为组件的id去容器中查找,如果组件id对象的bean不存在,而且required属性为true,就报错。
6. 支持@Primary注解,让Spring进行自动装配的时候,默认使用首选的bean;
@Resource
1.JSR250规范提供的注解(javax.annotation.Resource),不需要导入格外的包,这个注解在JDK的rt.jar包中
2.可以标注的位置:TYPE(表示可以标注在接口、类、枚举),FIELD(变量域)和METHOD(方法)上面。
@Target({TYPE, FIELD, METHOD})
@Retention(RUNTIME)
public @interface Resource {
String name() default "";
String lookup() default "";
Class<?> type() default java.lang.Object.class;
enum AuthenticationType {
CONTAINER,
APPLICATION
}
AuthenticationType authenticationType() default AuthenticationType.CONTAINER;
boolean shareable() default true;
String mappedName() default "";
String description() default "";
}
3.在Spring容器解析@Resource注解时,使用的后置处理器为CommonAnnotationBeanPostProcessor,在这个后置处理的注释中有这么一段:
* {@link org.springframework.beans.factory.config.BeanPostProcessor} implementation
* that supports common Java annotations out of the box, in particular the JSR-250
* annotations in the {@code javax.annotation} package. These common Java
* annotations are supported in many Java EE 5 technologies (e.g. JSF 1.2),
* as well as in Java 6's JAX-WS.
*
* <p>This post-processor includes support for the {@link javax.annotation.PostConstruct}
* and {@link javax.annotation.PreDestroy} annotations - as init annotation
* and destroy annotation, respectively - through inheriting from
* {@link InitDestroyAnnotationBeanPostProcessor} with pre-configured annotation types.
*
* <p>The central element is the {@link javax.annotation.Resource} annotation
* for annotation-driven injection of named beans, by default from the containing
* Spring BeanFactory, with only {@code mappedName} references resolved in JNDI.
* The {@link #setAlwaysUseJndiLookup "alwaysUseJndiLookup" flag} enforces JNDI lookups
* equivalent to standard Java EE 5 resource injection for {@code name} references
* and default names as well. The target beans can be simple POJOs, with no special
* requirements other than the type having to match.
4. 默认是按照组件名称进行装配的
这样说大家可能没有很深的印象,为啥@Autowired是根据类型,@Resource是根据组件名称,下面使用代码来进行解释:
@Component
public class Student {
private String num = "1";
public String getNum() {
return num;
}
public void setNum(String num) {
this.num = num;
}
}
@Configuration
@ComponentScan(basePackages = {"it.cast.autowired"})
public class Config {
@Bean
public Student student1(){
Student student = new Student();
student.setNum("2");
return student;
}
}
@Component
public class Teacher {
@Resource //注意这里使用的@Resource注解,Spring支持注入Map、Conllent类型的属性变量
private Map<String,Student> student;
public Map<String, Student> getStudent() {
return student;
}
public void setStudent(Map<String, Student> student) {
this.student = student;
}
}
public class Test01 {
public static void main(String[] args) {
AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(Config.class);
Teacher teacher = context.getBean(Teacher.class);
System.out.println(teacher.getStudent());
}
}
//打印结果:
//Caused by: org.springframework.beans.factory.BeanNotOfRequiredTypeException: Bean named 'student' is expected to be of type 'java.util.Map' but was actually of type 'it.cast.autowired.Student'
// at org.springframework.beans.factory.support.AbstractBeanFactory.doGetBean(AbstractBeanFactory.java:392)
// at org.springframework.beans.factory.support.AbstractBeanFactory.getBean(AbstractBeanFactory.java:204)
// at org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory.resolveBeanByName(AbstractAutowireCapableBeanFactory.java:452)
// at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor.autowireResource(CommonAnnotationBeanPostProcessor.java:527)
// at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor.getResource(CommonAnnotationBeanPostProcessor.java:497)
// at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor$ResourceElement.getResourceToInject(CommonAnnotationBeanPostProcessor.java:637)
// at org.springframework.beans.factory.annotation.InjectionMetadata$InjectedElement.inject(InjectionMetadata.java:180)
// at org.springframework.beans.factory.annotation.InjectionMetadata.inject(InjectionMetadata.java:90)
// at org.springframework.context.annotation.CommonAnnotationBeanPostProcessor.postProcessProperties(CommonAnnotationBeanPostProcessor.java:322)
// ... 12 more
可以看到使用@Resource标注的Map<String,Student>类型的时报错,为什么报错?因为Map类型的变量名为student,容器中已经存在了名称为student的bean,其类型为Student,所以会报错,大致步骤为:根据组件名称student去Spring容器中获取相应的Bean,在获取之后,会将获取到的Bean赋值给相应的属性。
如果此时我们将@Resource换成@Autowired时,其打印结果又会如何?
@Component
public class Student {
private String num = "1";
public String getNum() {
return num;
}
public void setNum(String num) {
this.num = num;
}
}
@Configuration
@ComponentScan(basePackages = {"it.cast.autowired"})
public class Config {
@Bean
public Student student1(){
Student student = new Student();
student.setNum("2");
return student;
}
}
@Component
public class Teacher {
@Autowired //注意这里使用的@Autowired注解,Spring支持注入Map、Conllent类型的属性变量
private Map<String,Student> student;
public Map<String, Student> getStudent() {
return student;
}
public void setStudent(Map<String, Student> student) {
this.student = student;
}
}
public class Test01 {
public static void main(String[] args) {
AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(Config.class);
Teacher teacher = context.getBean(Teacher.class);
System.out.println(teacher.getStudent());
}
}
//其打印结果:
//{student=it.cast.autowired.Student@61009542, student1=it.cast.autowired.Student@77e9807f}
可以看到系统并没有报错,根据上面两组代码的对比,可以得出以下结论:当使用@Resource注解时,是根据组件名称进行查找,当使用@Autowired注解时,是根据类型进行查找的。
5. 支持@Primary注解,不过首先按照会按照名称进行注入bean,如果Spring IOC容器中没有该Bean,则按照@Primary注解标注的bean进行装配(这条是我自己总结的,别人都说不支持,但是代码是不会骗人的,给出验证代码,如有错误,请多指教,这个代码的逻辑其实可以看一下CommonAnnotationBeanPostProcessor是怎么处理的,有时间我来看看源码)
下面验证@Resource默认是按照组件名称进行装配的和持支@Primary注解的:
@Configuration
@ComponentScan({"it.cast.resouce"})
public class ResourceConfig {
@Primary //标有Primary注解,使用@Autowired@Inject注解注解时,优先被加载
@Bean
public Y y1(){
Y y = new Y();
y.setI(0);
return y;
}
}
@Component
public class X {
@Resource //这里使用的是@Resource注解,该注解默认按照组件名称进行装配的,所以会优先加载id为y的bean
private Y y;
public Y getY() {
return y;
}
public void setY(Y y) {
this.y = y;
}
}
@Component
public class Y {
private int i = 2;
public int getI() {
return i;
}
public void setI(int i) {
this.i = i;
}
}
测试一下使用,使用@Resource注解的打印结果:
@Test
public void ResourceConfigTest(){
AnnotationConfigApplicationContext context =
new AnnotationConfigApplicationContext(ResourceConfig.class);
X bean = context.getBean(X.class);
System.out.println(bean.getY().getI());
}
//输出结果为:
// 2
//从而验证了@Resource默认按照名称进行加载
此时,将@Resource注解的属性名称换成y12,这个bean在容器里面没有的
@Configuration
@ComponentScan({"it.cast.resouce"})
public class ResourceConfig {
@Primary //标有Primary注解,使用@Autowired@Inject注解注解时,优先被加载
@Bean
public Y y1(){
Y y = new Y();
y.setI(0);
return y;
}
}
@Component
public class X {
@Resource //这里使用的是@Resource注解,该注解默认按照组件名称进行装配的,所以会优先加载id为y12的bean,
private Y y12; //如果找不到则按Primary注解标注的bean进行注入
public Y getY() {
return y12;
}
public void setY(Y y) {
this.y12 = y;
}
}
@Component
public class Y {
private int i = 2;
public int getI() {
return i;
}
public void setI(int i) {
this.i = i;
}
}
测试一下使用,使用@Resource注解的打印结果:
@Test
public void ResourceConfigTest(){
AnnotationConfigApplicationContext context =
new AnnotationConfigApplicationContext(ResourceConfig.class);
X bean = context.getBean(X.class);
System.out.println(bean.getY().getI());
}
//输出结果为:
// 0
//由于没有找到id为y12的bean,所以注入了使用@Primary标注的bean,
//而且整个程序没有报错,所以验证了@Resource支持@Primary注解
此时,将@Resource注解换成@Autowired注解的打印结果:
@Configuration
@ComponentScan({"it.cast.resouce"})
public class ResourceConfig {
@Primary //标有Primary注解,使用@Autowired@Inject注解注解时,优先被加载
@Bean
public Y y1(){
Y y = new Y();
y.setI(0);
return y;
}
}
@Component
public class X {
@Autowired
private Y y; //此时不管名称是y还是y12,都会使用标有Primary注解的bean
public Y getY() {
return y;
}
public void setY(Y y) {
this.y = y;
}
}
@Component
public class Y {
private int i = 2;
public int getI() {
return i;
}
public void setI(int i) {
this.i = i;
}
}
测试一下使用,使用@Autowired注解的打印结果:
@Test
public void ResourceConfigTest(){
AnnotationConfigApplicationContext context =
new AnnotationConfigApplicationContext(ResourceConfig.class);
X bean = context.getBean(X.class);
System.out.println(bean.getY().getI());
}
//输出结果为:
// 0
//从而验证了@Autowired支持@Primary注解
@Inject
1.JSR330规范提供的注解(javax.inject.Inject),主要导入javax.inject包才能使用
2.可以标注的位置:方法、构造器和变量域中
@Target({ METHOD, CONSTRUCTOR, FIELD })
@Retention(RUNTIME)
@Documented
public @interface Inject {} //该注解中没有任何属性
3.在Spring容器解析@Inject注解时,使用的后置处理器和@Autowired是一样的,都是AutowiredAnnotationBeanPostProcessor。
4.由于@Inject注解没有属性,在加载所需bean失败时,会报错
除了上面的不同点之后,@Inject和@Autowired完全等价。
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