SpringBoot——starter解析

作者: 小波同学 | 来源:发表于2020-03-02 18:52 被阅读0次

Conditional注解解析

含义：基于条件的注解
作用：根据是否满足某个特定条件来决定是否创建某个特定的bean
意义：是SpringBoot实现自动配置的关键基础能力

@Conditional表示仅当所有指定条件都匹配时，组件才有资格注册。
该@Conditional注释可以在以下任一方式使用：

作为任何@Bean方法的方法级注释
作为任何类的直接或间接注释的类型级别注释 @Component，包括@Configuration类
作为元注释，目的是组成自定义构造型注释

该注解主要源码之一，通过match匹配，符合条件才装载到Spring容器

@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Conditional {

    /**
     * All {@link Condition Conditions} that must {@linkplain Condition#matches match}
     * in order for the component to be registered.
     */
    Class<? extends Condition>[] value();

}

@FunctionalInterface
public interface Condition {

    /**
     * Determine if the condition matches.
     * @param context the condition context
     * @param metadata metadata of the {@link org.springframework.core.type.AnnotationMetadata class}
     * or {@link org.springframework.core.type.MethodMetadata method} being checked
     * @return {@code true} if the condition matches and the component can be registered,
     * or {@code false} to veto the annotated component's registration
     */
    boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata);

}

class ProfileCondition implements Condition {

    @Override
    public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
        MultiValueMap<String, Object> attrs = metadata.getAllAnnotationAttributes(Profile.class.getName());
        if (attrs != null) {
            for (Object value : attrs.get("value")) {
                if (context.getEnvironment().acceptsProfiles(Profiles.of((String[]) value))) {
                    return true;
                }
            }
            return false;
        }
        return true;
    }

}

作用：总而言之，只有@Conditional指定的条件成立，才给容器添加组件

@Conditional派生注解：@Conditional派生了很多注解，下面给个表格列举一下派生注解的用法

@Conditional派生注解	作用(都是判断是否符合指定的条件)
@ConditionalOnJava	系统的java版本是否符合要求
@ConditionalOnBean	有指定的Bean类
@ConditionalOnMissingBean	没有指定的bean类
@ConditionalOnExpression	符合指定的SpEL表达式
@ConditionalOnClass	有指定的类
@ConditionalOnMissingClass	没有指定的类
@ConditionalOnSingleCandidate	容器只有一个指定的bean，或者这个bean是首选bean
@ConditionalOnProperty	指定的property属性有指定的值
@ConditionalOnResource	路径下存在指定的资源
@ConditionalOnWebApplication	系统环境是web环境
@ConditionalOnNotWebApplication	系统环境不是web环境
@ConditionalOnjndi	JNDI存在指定的项

自定义Conditional注解

新建MyConditionAnnotation注解，引入Condition实现类

@Target({ ElementType.TYPE, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional(MyCondition.class)
public @interface MyConditionAnnotation {

    String[] value() default {};
}

新建MyCondition实现Condition，重写matches方法，符合条件返回true

public class MyCondition implements Condition {
    @Override
    public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
        String[] properties = (String[])metadata
                .getAnnotationAttributes("com.yibo.source.code.condi.MyConditionAnnotation")
                .get("value");
        for (String property : properties) {
            if(StringUtils.isEmpty(context.getEnvironment().getProperty(property))){
                return false;
            }
        }
        return true;
    }
}

新建A类使用@MyConditionAnnotation注解

@Component
@MyConditionAnnotation({"com.yibo.condition1","com.yibo.condition2"})
public class A {
}

在application.properties中添加属性

com.yibo.condition1=test1
com.yibo.condition2=test2

在测试类中测试

@RunWith(SpringRunner.class)
@SpringBootTest(classes = Application.class)
public class ApplicationTest implements ApplicationContextAware {

    @Autowired
    private ApplicationContext applicationContext;

    @Override
    public void setApplicationContext(ApplicationContext applicationContext) throws BeansException {
        this.applicationContext = applicationContext;
    }

    @Test
    public void test2(){
        System.out.println(applicationContext.getBean(A.class));
    }
}

动手搭建starter

starter简介

SpringBoot可以省略SpringFramework众多的繁琐配置，它的众多starter可以说是功不可没。

springboot非常的流行，就是因为starter的存在，starter是springboot的核心，可以理解成可插拔的插件，你想要什么插件配置什么插件就可以，比如我想要使用mybatis，那么配置starter-mybatis就可以。但是有人会说我用mybatis自己导入jar不行吗？？？实际上starter和jar区别在于，它能够自己实现配置，这样大大提高了开发效率，使得使用spring开发变得非常简单方便。

starter类似于SpringBoot的可插拔插件
与jar包区别：starter能实现自动配置
作用：大幅提升开发效率

常用starter

名称	描述
spring-boot-starter-thymeleaf	使MVC Web applications 支持Thymeleaf
spring-boot-starter-mail	使用Java Mail、Spring email发送支持
spring-boot-starter-data-redis	通过Spring Data Redis 、Jedis client使用Redis键值存储数据库
spring-boot-starter-web	构建Web，包含RESTful风格框架SpringMVC和默认的嵌入式容器Tomcat
spring-boot-starter-activemq	为JMS使用Apache ActiveMQ
spring-boot-starter-data-elasticsearch	使用Elasticsearch、analytics engine、Spring Data Elasticsearch
spring-boot-starter-aop	通过Spring AOP、AspectJ面向切面编程
spring-boot-starter-security	使用 Spring Security
spring-boot-starter-data-jpa	通过 Hibernate 使用 Spring Data JPA
spring-boot-starter	Core starter,包括自动配置支持、 logging and YAML
spring-boot-starter-freemarker	使MVC Web applications 支持 FreeMarker
spring-boot-starter-batch	使用Spring Batch
spring-boot-starter-data-solr	通过 Spring Data Solr 使用 Apache Solr
spring-boot-starter-data-mongodb	使用 MongoDB 文件存储数据库、Spring Data MongoDB

手写starter

新建一个starter

首先我们在IDEA中通过Spring Initializer新建一个weather-starter的SpringBoot项目,然后我们可以在其他SpringBoot项目中作为依赖引入.

1、修改weather-starter的pom文件,命名合理一点

<!--修改weather-starter的pom文件,命名合理一点-->
<groupId>com.yibo</groupId>
<artifactId>weather-spring-boot-starter</artifactId>
<version>0.0.1-SNAPSHOT</version>

<properties>
    <java.version>1.8</java.version>
</properties>

<dependencies>
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter</artifactId>
    </dependency>

    <!--pom文件要加入-->
    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-autoconfigure</artifactId>
    </dependency>

    <dependency>
        <groupId>org.springframework.boot</groupId>
        <artifactId>spring-boot-starter-test</artifactId>
        <scope>test</scope>
    </dependency>
</dependencies>

2、新建一个类作为属性源,读取配置

@ConfigurationProperties(prefix = "weather")
public class WeatherSource {

    private String type;

    private String rate;

    public String getType() {
        return type;
    }

    public void setType(String type) {
        this.type = type;
    }

    public String getRate() {
        return rate;
    }

    public void setRate(String rate) {
        this.rate = rate;
    }
}

3、新建一个AutoConfiguration自动配置类,用来让SpringBoot依据它自动配置

@Configuration
@EnableConfigurationProperties(WeatherSource.class)
//自动注入的条件
@ConditionalOnProperty(name="weather.enable",havingValue = "enable")
public class WeatherAutoConfiguration {

    @Autowired
    private WeatherSource weatherSource;

    @Bean
    @ConditionalOnMissingBean(WeatherService.class)
    public WeatherService weatherService(){
        return new WeatherService(weatherSource);
    }
}

4、写一个类用来对外提供服务

public class WeatherService {

    private WeatherSource weatherSource;


    public WeatherService(WeatherSource weatherSource) {
        this.weatherSource = weatherSource;
    }

    public String getType(){
        return weatherSource.getType();
    }

    public String getRate(){
        return weatherSource.getRate();
    }
}

5、利用SPI机制，在resources目录下新建一个META-INF目录，新建一个spring.factories文件,内容如下：

org.springframework.boot.autoconfigure.EnableAutoConfiguration=com.example.weather.WeatherAutoConfiguration

6、一切完毕之后，需要将我们的starter打包发布，让别人使用，本地使用的话可以通过maven helper插件install到本地仓库，也可以直接使用命令行打包

使用Starter

1、在pom文件中引入依赖

<dependency>
    <groupId>com.example</groupId>
    <artifactId>weather-spring-boot-starter</artifactId>
    <version>0.0.1-SNAPSHOT</version>
</dependency>

2、在application.properties中编写配置,用来配置自动配置类

weather.type=rain  
weather.rate=serious  
weather.enable=enable

3、可以直接注入自动配置类中提供的服务

@RestController
@RequestMapping("/demo")
public class DemoController {

    @Autowired
    private WeatherService weatherService;

    @GetMapping("/weather")
    public String test(){
        return weatherService.getType() + ", " + weatherService.getRate();
    }
}

starter原理解析

spring-boot启动的时候会找到starter jar包中的resources/META-INF/spring.factories文件，根据spring.factories文件中的配置，找到需要自动配置的类

字段配置的原理就是这边，在之前的SpringBoot--配置类解析已经分析过了，它会将容器中配置的自动配置类加载进来,见下图,
// 这就引入了我们自己的WeatherAutoConfigration

public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware,
        ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered {
    protected AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata,
            AnnotationMetadata annotationMetadata) {
        if (!isEnabled(annotationMetadata)) {
            return EMPTY_ENTRY;
        }
        AnnotationAttributes attributes = getAttributes(annotationMetadata);
        List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
        configurations = removeDuplicates(configurations);
        Set<String> exclusions = getExclusions(annotationMetadata, attributes);
        checkExcludedClasses(configurations, exclusions);
        configurations.removeAll(exclusions);
        configurations = filter(configurations, autoConfigurationMetadata);
        fireAutoConfigurationImportEvents(configurations, exclusions);
        return new AutoConfigurationEntry(configurations, exclusions);
    }
}

starter自动配置类导入

starter过滤

我们发现WeatherAutoConfiguration自动配置类上面使用的是@ConditionalOnProperty注解，点进去看它用来判断的类是OnPropertyCondition,而OnPropertyCondition实现了SpringBootCondition接口，那么我们先看SpringBootCondition中的matches方法

@Configuration
@EnableConfigurationProperties({WeatherSource.class})
@ConditionalOnProperty(
    name = {"weather.enable"},
    havingValue = "enable"
)
public class WeatherAutoConfiguration {}

@Retention(RetentionPolicy.RUNTIME)
@Target({ ElementType.TYPE, ElementType.METHOD })
@Documented
@Conditional(OnPropertyCondition.class)
public @interface ConditionalOnProperty {

@Order(Ordered.HIGHEST_PRECEDENCE + 40)
class OnPropertyCondition extends SpringBootCondition {}

public abstract class SpringBootCondition implements Condition {

    private final Log logger = LogFactory.getLog(getClass());

    @Override
    public final boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
        String classOrMethodName = getClassOrMethodName(metadata);
        try {
            // 这边调用子类实现的getMatchOutcome方法得到ConditionOutcome
            // 看了下ConditionOutcome,它只是封装了判断结果和分析,所以我们直接看OnPropertyCondition的getMatchOutCome方法
            ConditionOutcome outcome = getMatchOutcome(context, metadata);
            logOutcome(classOrMethodName, outcome);
            recordEvaluation(context, classOrMethodName, outcome);
            return outcome.isMatch();
        }
        catch (NoClassDefFoundError ex) {
            throw new IllegalStateException("Could not evaluate condition on " + classOrMethodName + " due to "
                    + ex.getMessage() + " not " + "found. Make sure your own configuration does not rely on "
                    + "that class. This can also happen if you are "
                    + "@ComponentScanning a springframework package (e.g. if you "
                    + "put a @ComponentScan in the default package by mistake)", ex);
        }
        catch (RuntimeException ex) {
            throw new IllegalStateException("Error processing condition on " + getName(metadata), ex);
        }
    }
}

public class ConditionOutcome {

    private final boolean match;

    private final ConditionMessage message;
}

接下来我们看OnPropertyCondition的getMatchOutcome方法

@Override
public ConditionOutcome getMatchOutcome(ConditionContext context, AnnotatedTypeMetadata metadata) {
    // 首先获取@ConditionalOnProperty注解上的属性,如下图
    List<AnnotationAttributes> allAnnotationAttributes = annotationAttributesFromMultiValueMap(
            metadata.getAllAnnotationAttributes(ConditionalOnProperty.class.getName()));
    // 构造noMatch、match集合,noMatch表示那个注解的属性没有满足
    List<ConditionMessage> noMatch = new ArrayList<>();
    List<ConditionMessage> match = new ArrayList<>();
    // 依次遍历这些注解
    for (AnnotationAttributes annotationAttributes : allAnnotationAttributes) {
        // 对每个注解,遍历其属性判断符不符合,返回outcome结果,下面我们也是重点看这个方法
        ConditionOutcome outcome = determineOutcome(annotationAttributes, context.getEnvironment());
        (outcome.isMatch() ? match : noMatch).add(outcome.getConditionMessage());
    }
    if (!noMatch.isEmpty()) {
        return ConditionOutcome.noMatch(ConditionMessage.of(noMatch));
    }
    return ConditionOutcome.match(ConditionMessage.of(match));
}

继续跟进determineOutcome(annotationAttributes, context.getEnvironment())方法

private ConditionOutcome determineOutcome(AnnotationAttributes annotationAttributes, PropertyResolver resolver) {
    // 获取Spec,里面就是获取注解的属性封装一下,见下图
    Spec spec = new Spec(annotationAttributes);
    // 两个集合,一个存放缺失的属性,一个存放不匹配的属性
    List<String> missingProperties = new ArrayList<>();
    List<String> nonMatchingProperties = new ArrayList<>();
    // 遍历这些属性,判断符不符合,放入集合中
    spec.collectProperties(resolver, missingProperties, nonMatchingProperties);
    // 根据结果返回
    if (!missingProperties.isEmpty()) {
        return ConditionOutcome.noMatch(ConditionMessage.forCondition(ConditionalOnProperty.class, spec)
                .didNotFind("property", "properties").items(Style.QUOTE, missingProperties));
    }
    if (!nonMatchingProperties.isEmpty()) {
        return ConditionOutcome.noMatch(ConditionMessage.forCondition(ConditionalOnProperty.class, spec)
                .found("different value in property", "different value in properties")
                .items(Style.QUOTE, nonMatchingProperties));
    }
    return ConditionOutcome
            .match(ConditionMessage.forCondition(ConditionalOnProperty.class, spec).because("matched"));
}

Spec(AnnotationAttributes annotationAttributes) {
    String prefix = annotationAttributes.getString("prefix").trim();
    if (StringUtils.hasText(prefix) && !prefix.endsWith(".")) {
        prefix = prefix + ".";
    }
    this.prefix = prefix;
    this.havingValue = annotationAttributes.getString("havingValue");
    this.names = getNames(annotationAttributes);
    this.matchIfMissing = annotationAttributes.getBoolean("matchIfMissing");
}

private void collectProperties(PropertyResolver resolver, List<String> missing, List<String> nonMatching) {
    // 依次遍历属性
    for (String name : this.names) {
        String key = this.prefix + name;
        // 从上图中可以看出,resovler其实就是环境
        if (resolver.containsProperty(key)) {
            if (!isMatch(resolver.getProperty(key), this.havingValue)) {
                nonMatching.add(name);
            }
        }
        else {
            if (!this.matchIfMissing) {
                missing.add(name);
            }
        }
    }
}

private boolean isMatch(String value, String requiredValue) {
    if (StringUtils.hasLength(requiredValue)) {
        return requiredValue.equalsIgnoreCase(value);
    }
    return !"false".equalsIgnoreCase(value);
}

starter自动配置类过滤

参考：
https://www.cnblogs.com/mzq123/p/11874128.html

https://my.oschina.net/liwanghong/blog/3168503

SpringBoot——starter解析

Conditional注解解析

自定义Conditional注解

动手搭建starter

starter简介

常用starter

手写starter

新建一个starter

使用Starter

starter原理解析

starter自动配置类导入

starter过滤

starter自动配置类过滤

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