Spring创建Bean的顺序

Posted 2021-01-15 柚子苹果果

一直对Spring创建bean的顺序很好奇，现在总算有时间写个代码测试一下。不想看过程的小伙伴可以直接看结论

目录结构：

其中：bean4、bean5包下的class没有注解@Component，测试过程中，这两个包的class会直接通过<bean class="XXXXX"/>的方式创建。bean1、bean2、bean3包下的class注解了@Component，以便component-scan扫描。另外，bean创建之间没有依赖关系，例如bean1的创建不依赖于其他bean。

applicationContext1.xml

<bean class="com.luych.test.springBeanCreateOrderTest.bean5.Bean5_2"/>
<context:component-scan base-package="com.luych.test.springBeanCreateOrderTest.bean2" />
<bean class="com.luych.test.springBeanCreateOrderTest.bean5.Bean5_1"/>

applicationContext2.xml

<bean class="com.luych.test.springBeanCreateOrderTest.bean4.Bean4_1"/>
<context:component-scan base-package="com.luych.test.springBeanCreateOrderTest.bean1" />
<bean class="com.luych.test.springBeanCreateOrderTest.bean4.Bean4_2"/>

springMVC-servlet.xml

<context:component-scan base-package="com.luych.test.springBeanCreateOrderTest.bean3" />

web.xml

<servlet>
    <servlet-name>springMVC</servlet-name>
    <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>
    <init-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>classpath*:springMVC-servlet.xml</param-value>
    </init-param>
    <load-on-startup>1</load-on-startup>
</servlet>
<servlet-mapping>
    <servlet-name>springMVC</servlet-name>
    <url-pattern>/</url-pattern>
</servlet-mapping>

<listener>
    <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
</listener>
<context-param>
    <param-name>contextConfigLocation</param-name>
    <param-value>
        classpath*:applicationContext*.xml
    </param-value>
</context-param>

 运行结果

class com.luych.test.springBeanCreateOrderTest.bean5.Bean5_2 has been created
class com.luych.test.springBeanCreateOrderTest.bean2.Bean2_1 has been created
class com.luych.test.springBeanCreateOrderTest.bean2.Bean2_2 has been created
class com.luych.test.springBeanCreateOrderTest.bean5.Bean5_1 has been created
class com.luych.test.springBeanCreateOrderTest.bean4.Bean4_1 has been created
class com.luych.test.springBeanCreateOrderTest.bean1.Bean1_1 has been created
class com.luych.test.springBeanCreateOrderTest.bean1.Bean1_2 has been created
class com.luych.test.springBeanCreateOrderTest.bean4.Bean4_2 has been created
class com.luych.test.springBeanCreateOrderTest.bean3.Bean3_1 has been created
class com.luych.test.springBeanCreateOrderTest.bean3.Bean3_2 has been created

结论一：

1. 在web.xml中，ContextLoaderListener和DispatcherServlet的书写顺序不会影响相应的xml文件加载顺序。ContextLoaderListener中的xml先加载，DispatcherServlet中的xml后加载。

2. ContextLoaderListener中如果contextConfigLocation通过模糊匹配到多个xml文件时，xml按照文件命名顺序加载。但是如果contextConfigLocation逐个指定了具体加载某个xml，则会按照其指定顺序加载。

本例中可以改为（注意下面代码中蓝色部分）：

<listener>
        <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>
    </listener>
    <context-param>
        <param-name>contextConfigLocation</param-name>
        <param-value>
            classpath*:applicationContext2.xml,classpath*:applicationContext1.xml
        </param-value>
    </context-param>

则其加载顺序为：

class com.luych.test.springBeanCreateOrderTest.bean4.Bean4_1 has been created
class com.luych.test.springBeanCreateOrderTest.bean1.Bean1_1 has been created
class com.luych.test.springBeanCreateOrderTest.bean1.Bean1_2 has been created
class com.luych.test.springBeanCreateOrderTest.bean4.Bean4_2 has been created
class com.luych.test.springBeanCreateOrderTest.bean5.Bean5_2 has been created
class com.luych.test.springBeanCreateOrderTest.bean2.Bean2_1 has been created
class com.luych.test.springBeanCreateOrderTest.bean2.Bean2_2 has been created
class com.luych.test.springBeanCreateOrderTest.bean5.Bean5_1 has been created
class com.luych.test.springBeanCreateOrderTest.bean3.Bean3_1 has been created
class com.luych.test.springBeanCreateOrderTest.bean3.Bean3_2 has been created

3. 同一个spring的xml文件中，bean的加载顺序按照书写顺序加载

4. 通过component-scan扫描的方式加载bean，在扫描范围内按照class的命名顺序加载

 

以上的测试过程中，bean的创建是没有依赖关系的，那么如果bean之间的创建存在依赖关系，则被依赖的bean会被优先创建。但是如果存在相互依赖的情况，则会发生异常。

例如：

@Component
public class Bean2_1 {

    private final Bean2_2 bean2_2;

    @Autowired
    public Bean2_1(Bean2_2 bean2_2) {
        this.bean2_2 = bean2_2;
        System.out.println(Bean2_1.class + " has been created");
    }
}

@Component
public class Bean2_2 {

    private final Bean2_1 bean2_1;

    @Autowired
    public Bean2_2(Bean2_1 bean2_1) {
        this.bean2_1 = bean2_1;
        System.out.println(Bean2_2.class + " has been created");
    }
}

异常信息：

Error creating bean with name \'bean2_1\' defined in file......