SpringBoot自動(dòng)裝配原理小結(jié)

更新時(shí)間：2021年05月28日 08:29:31 作者：XPacific

Spring Boot主要作用就是簡(jiǎn)化Spring應(yīng)用的開(kāi)發(fā)，開(kāi)發(fā)者只需要通過(guò)少量代碼就可以創(chuàng)建一個(gè)Spring應(yīng)用，而達(dá)到這一目的最核心的思想就是約定優(yōu)于配置。

springboot autoconfig的一些實(shí)驗(yàn)

一個(gè)springboot工程，springbootautoconfig.test.config這個(gè)包和啟動(dòng)類的包不再同一個(gè)路徑下，那么這個(gè)包下的注解等應(yīng)該不會(huì)生效，bean也無(wú)法托管給spring管理

@Configuration//開(kāi)啟配置
@EnableConfigurationProperties(HelloProperties.class)//開(kāi)啟使用映射實(shí)體對(duì)象
@ConditionalOnClass(TestHello.class)//存在TestHello時(shí)初始化該配置類
@ConditionalOnProperty//存在對(duì)應(yīng)配置信息時(shí)初始化該配置類
(
        prefix = "zxp.hello",//存在配置前綴zxp.hello
        value = "flag"
)
public class HelloAutoConfiguration {
    @Autowired
    private HelloProperties helloProperties;

    @Bean//創(chuàng)建HelloService實(shí)體bean
    @ConditionalOnMissingBean(TestHello.class)//缺失HelloService實(shí)體bean時(shí)，初始化HelloService并添加到SpringIoc
    public TestHello helloService()
    {
        System.out.println(">>>The TestHello Not Found，Execute Create New Bean.");
        TestHello testHello = new TestHello(helloProperties.getName(),helloProperties.getFlag());
        return testHello;
    }
}

@ConfigurationProperties(prefix = "zxp.hello")
@Data
public class HelloProperties {
    private String name;
    private String flag;
}

public class TestHello {
    String name;
    String flag;

    public TestHello(String name, String flag) {
        this.name = name;
        this.flag = flag;
    }

    public String print(){
        String msg = "name is "+name + "  " + "flag is "+flag;
        System.out.println(msg);
        return msg;
    }
}

在resources下創(chuàng)建META-INF路徑，并創(chuàng)建spring.factories文件

#配置自定義Starter的自動(dòng)化配置
org.springframework.boot.autoconfigure.EnableAutoConfiguration=springbootautoconfig.test.config.HelloAutoConfiguration

再試啟動(dòng)又報(bào)錯(cuò)了

- Bean method 'helloService' not loaded because @ConditionalOnProperty (zxp.hello) did not find property 'flag'

原因是，如果沒(méi)有配置zxp.hello.flag，怎會(huì)報(bào)錯(cuò)

@ConditionalOnProperty//存在對(duì)應(yīng)配置信息時(shí)初始化該配置類
(
        prefix = "zxp.hello",//存在配置前綴hello  
        value = "flag"http://開(kāi)啟
)

在application.properties中添加

zxp.hello.flag=2

成功了，訪問(wèn)controller

name is null flag is 1

SpringBoot autoconfig部分注解說(shuō)明

@ConditionalOnXxx 可以根據(jù)條件來(lái)決定是否執(zhí)行自動(dòng)配置

@ConditionalOnBean：當(dāng)SpringIoc容器內(nèi)存在指定Bean的條件
@ConditionalOnSingleCandidate：當(dāng)指定Bean在SpringIoc容器內(nèi)只有一個(gè)，或者雖然有多個(gè)但是指定首選的Bean
@ConditionalOnMissingBean：當(dāng)SpringIoc容器內(nèi)不存在指定Bean的條件

@ConditionalOnClass：當(dāng)SpringIoc容器內(nèi)存在指定Class的條件
@ConditionalOnMissingClass：當(dāng)SpringIoc容器內(nèi)不存在指定Class的條件
@ConditionalOnExpression：基于SpEL表達(dá)式作為判斷條件
@ConditionalOnJava：基于JVM版本作為判斷條件
@ConditionalOnJndi：在JNDI存在時(shí)查找指定的位置
@ConditionalOnResource：類路徑是否有指定的值
@ConditionalOnProperty：指定的屬性是否有指定的值

@ConditionalOnNotWebApplication：當(dāng)前項(xiàng)目不是Web項(xiàng)目的條件
@ConditionalOnWebApplication：當(dāng)前項(xiàng)目是Web項(xiàng)目的條件

@AutoConfigureBefore
@AutoConfigureAfter
@AutoConfigureOrder

public @interface ConditionalOnProperty {
    String[] value() default {}; //數(shù)組，獲取對(duì)應(yīng)property名稱的值，與name不可同時(shí)使用  
    String prefix() default "";//property名稱的前綴，可有可無(wú)  
    String[] name() default {};//數(shù)組，property完整名稱或部分名稱（可與prefix組合使用，組成完整的property名稱），與value不可同時(shí)使用  
    String havingValue() default "";//可與name組合使用，比較獲取到的屬性值與havingValue給定的值是否相同，相同才加載配置  
    boolean matchIfMissing() default false;//缺少該property時(shí)是否可以加載。如果為true，沒(méi)有該property也會(huì)正常加載；反之報(bào)錯(cuò)  
    boolean relaxedNames() default true;//是否可以松散匹配，至今不知道怎么使用的  
}

SpringBoot autoconfig原理

springboot啟動(dòng)類注解@SpringBootApplication引入@EnableAutoConfiguration又引入@Import(AutoConfigurationImportSelector.class)

AutoConfigurationImportSelector類中調(diào)用SpringFactoriesLoader.loadFactoryNames 方法掃描了所有JAR包的META-INF/spring.factories，如下代碼：

protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
   List<String> configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(),
         getBeanClassLoader());
   Assert.notEmpty(configurations, "No auto configuration classes found in META-INF/spring.factories. If you "
         + "are using a custom packaging, make sure that file is correct.");
   return configurations;
}
public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";
public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
    ……
    Enumeration<URL> urls = (classLoader != null ?
      classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
      ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
    result = new LinkedMultiValueMap<>();
    while (urls.hasMoreElements()) {

spring-boot-autoconfigure包內(nèi)的spring.factories文件內(nèi)容

……
work.boot.autoconfigure.EnableAutoConfiguration=\
org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\
org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\
org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\
……

包含了所有spring為其增加的自動(dòng)配置的bean配置,這些bean在滿足條件后會(huì)被加載到spring上下文中，從而實(shí)現(xiàn)了自動(dòng)配置

eg:

@Configuration(proxyBeanMethods = false)
@ConditionalOnClass({ RabbitTemplate.class, Channel.class })
@EnableConfigurationProperties(RabbitProperties.class)
@Import(RabbitAnnotationDrivenConfiguration.class)
public class RabbitAutoConfiguration {

會(huì)發(fā)現(xiàn)RabbitTemplate會(huì)報(bào)錯(cuò)，為什么這里不存在的類卻不報(bào)錯(cuò)呢？ 1、這個(gè)jar編譯時(shí)這個(gè)類是有的，保證編譯能過(guò) 2、看下ConditionalOnClass注解的注釋

The classes that must be present. Since this annotation is parsed by loading class bytecode, > it is safe to specify classes here that may ultimately not be on the classpath, only if this annotation is directly on the affected component and not if this annotation is used as a composed, meta-annotation. In order to use this annotation as a meta-annotation, only use the name attribute. Returns: the classes that must be present