springboot自动装配原理分析

偏执的太偏执、 2021-06-10 20:40 636阅读 0赞

### springboot自动装配 ###

*  @SpringBootApplication
 *   *  4个元注解
     *   *  @Target(ElementType.TYPE)
         *  @Retention(RetentionPolicy.RUNTIME)
         *  @Documented
         *  @Inherited
     *  @SpringBootConfiguration
     *  @EnableAutoConfiguration
     *   *  @AutoConfigurationPackage
         *  @Import(\{AutoConfigurationImportSelector.class\})来加载配置类
     *  案例
     *   *  步骤
     *  @ComponentScan
 *  Condition
 *   *  源码
     *  springboot提供的常用条件注解
     *  案例
     *   *  实体类
         *  UserConfig
         *  UserCondition
         *  启动类
     *  案例升级
     *   *  自定义一个注解
         *  UserConfig
     *  测试自带的注解
     *  小结

# @SpringBootApplication #

@Target(ElementType.TYPE)
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Inherited
    @SpringBootConfiguration
    @EnableAutoConfiguration
    @ComponentScan(excludeFilters = {  @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
    		@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
    public @interface SpringBootApplication {

## 4个元注解 ##

### @Target(ElementType.TYPE) ###

设置当前注解可以标记在哪里，而SpringBootApplication只能用在类上面  
还有一些其他的设置

public enum ElementType { 
        /** Class, interface (including annotation type), or enum declaration */
        TYPE,
    
        /** Field declaration (includes enum constants) */
        FIELD,
    
        /** Method declaration */
        METHOD,
    
        /** Formal parameter declaration */
        PARAMETER,
    
        /** Constructor declaration */
        CONSTRUCTOR,
    
        /** Local variable declaration */
        LOCAL_VARIABLE,
    
        /** Annotation type declaration */
        ANNOTATION_TYPE,
    
        /** Package declaration */
        PACKAGE,
    
        /** * Type parameter declaration * * @since 1.8 */
        TYPE_PARAMETER,
    
        /** * Use of a type * * @since 1.8 */
        TYPE_USE,
    
        /** * Module declaration. * * @since 9 */
        MODULE
    }

### @Retention(RetentionPolicy.RUNTIME) ###

public enum RetentionPolicy { 
        /** * Annotations are to be discarded by the compiler. */
        SOURCE,
    
        /** * Annotations are to be recorded in the class file by the compiler * but need not be retained by the VM at run time. This is the default * behavior. */
        CLASS,
    
        /** * Annotations are to be recorded in the class file by the compiler and * retained by the VM at run time, so they may be read reflectively. * * @see java.lang.reflect.AnnotatedElement */
        RUNTIME
    }

SOURCE 当编译时，注解将不会出现在class源文件中

CLASS 注解将会保留在class源文件中，但是不会被jvm加载，也就意味着不能通过反射去找到该注解，因为没有加载到java虚拟机中

RUNTIME是既会保留在源文件中，也会被虚拟机加载

### @Documented ###

java doc 会生成注解信息

### @Inherited ###

是否会被继承，就是如果一个子类继承了使用了该注解的类，那么子类也能继承该注解

## @SpringBootConfiguration ##

标注在某个类上，表示这是一个Spring Boot的配置类，本质上也是使用了@Configuration注解

@Target({ ElementType.TYPE})
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Configuration
    public @interface SpringBootConfiguration {

## @EnableAutoConfiguration ##

@EnableAutoConfiguration告诉SpringBoot开启自动配置，会帮我们自动去加载 自动配置类

@Target({ ElementType.TYPE})
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    @Inherited
    @AutoConfigurationPackage
    @Import({ AutoConfigurationImportSelector.class})
    public @interface EnableAutoConfiguration {

### @AutoConfigurationPackage ###

将当前配置类所在包保存在BasePackages的Bean中。供Spring内部使用

### @Import(\{AutoConfigurationImportSelector.class\})来加载配置类 ###

配置文件的位置：META-INF/spring.factories,该配置文件中定义了大量的配置类，当springboot启动时，会自动加载这些配置类，初始化Bean

并不是所有Bean都会被初始化，在配置类中使用Condition来加载满足条件的Bean

## 案例 ##

自定义redis-starter，要求当导入redis坐标时，spirngboot自动创建jedis的Bean

### 步骤 ###

1.创建redis-spring-boot-autoconfigure模块  
2.创建redis-spring-boot-starter模块，依赖redis-spring-boot-autoconfigure的模块  
3.在redis-spring-boot-autoconfigure模块中初始化jedis的bean，并定义META-INF/spring.factories文件  
4.在测试模块中引入自定义的redis-starter依赖，测试获取jedis的bean，操作redis  
1.首先新建两个模块  
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删除一些没有用的东西，和启动类否则会报错  
2.redis-spring-boot-starter模块的pom.xml里面引入redis-spring-boot-autoconfigure的模块的坐标  
3.RedisAutoConfiguration配置类

package com.blb;
    
    import org.springframework.boot.context.properties.EnableConfigurationProperties;
    import org.springframework.context.annotation.Bean;
    import org.springframework.context.annotation.Configuration;
    import redis.clients.jedis.Jedis;
    
    @Configuration
    @EnableConfigurationProperties(RedisProperties.class)
    public class RedisAutoConfiguration { 
    // 提供Jedis的bean
        @Bean
        public Jedis jedis(RedisProperties redisProperties){ 
            return new Jedis(redisProperties.getHost(),redisProperties.getPort());
        }
    }

RedisProperties

package com.blb;
    
    import org.springframework.boot.context.properties.ConfigurationProperties;
    
    @ConfigurationProperties(prefix = "redis")
    public class RedisProperties { 
        private String host="localhost";
        private int port=6379;
    
        public String getHost() { 
            return host;
        }
    
        public void setHost(String host) { 
            this.host = host;
        }
    
        public int getPort() { 
            return port;
        }
    
        public void setPort(int port) { 
            this.port = port;
        }
    }

## @ComponentScan ##

扫描包 相当于在spring.xml 配置中context:comonent-scan 但是并没有指定basepackage，如果没有指定spring底层会自动扫描当前配置类所有在的包

排除的类型

public enum FilterType { 
    
    	/** * Filter candidates marked with a given annotation. * @see org.springframework.core.type.filter.AnnotationTypeFilter */
    	ANNOTATION,
    
    	/** * Filter candidates assignable to a given type. * @see org.springframework.core.type.filter.AssignableTypeFilter */
    	ASSIGNABLE_TYPE,
    
    	/** * Filter candidates matching a given AspectJ type pattern expression. * @see org.springframework.core.type.filter.AspectJTypeFilter */
    	ASPECTJ,
    
    	/** * Filter candidates matching a given regex pattern. * @see org.springframework.core.type.filter.RegexPatternTypeFilter */
    	REGEX,
    
    	/** Filter candidates using a given custom * {@link org.springframework.core.type.filter.TypeFilter} implementation. */
    	CUSTOM
    
    }

ANNOTATION 默认根据注解的完整限定名设置排除  
ASSIGNABLE\_TYPE 根据类的完整限定名排除  
ASPECTJ 根据切面表达式设置排除  
REGEX 根据正则表达式设置排除  
CUSTOM 自定义设置排除

@ComponentScan(excludeFilters = {  @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
    		@Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })

按照自定义的方式来排除需要指定一个类，要实现TypeFilter接口，重写match方法

public class TypeExcludeFilter implements TypeFilter, BeanFactoryAware { 
    
    
    public boolean match(MetadataReader metadataReader, MetadataReaderFactory metadataReaderFactory) throws IOException { 
            if (this.beanFactory instanceof ListableBeanFactory && this.getClass() == TypeExcludeFilter.class) { 
                Iterator var3 = this.getDelegates().iterator();
    
                while(var3.hasNext()) { 
                    TypeExcludeFilter delegate = (TypeExcludeFilter)var3.next();
                    if (delegate.match(metadataReader, metadataReaderFactory)) { 
                        return true;
                    }
                }
            }
    
            return false;
        }
    }

TypeExcludeFilter ：springboot对外提供的扩展类， 可以供我们去按照我们的方式进行排除

AutoConfigurationExcludeFilter ：排除所有配置类并且是自动配置类中里面的其中一个  
示例

package com.blb.springbootyuanli.config;
    
    import org.springframework.boot.context.TypeExcludeFilter;
    import org.springframework.core.type.classreading.MetadataReader;
    import org.springframework.core.type.classreading.MetadataReaderFactory;
    
    import java.io.IOException;
    
    public class MyTypeExcludeFilter extends TypeExcludeFilter { 
        @Override
        public boolean match(MetadataReader metadataReader, MetadataReaderFactory metadataReaderFactory) throws IOException { 
            if(metadataReader.getClassMetadata().getClass()==UserConfig.class){ 
                return true;
            }
            return false;
        }
    }

# Condition #

@Conditional是Spring4新提供的注解，它的作用是按照一定的条件进行判断，满足条件给容器注册bean，实现选择性的创建bean的操作，该注解为条件装配注解

## 源码 ##

@Target({ ElementType.TYPE, ElementType.METHOD})
    @Retention(RetentionPolicy.RUNTIME)
    @Documented
    public @interface Conditional { 
    
    	/** * All {@link Condition} classes 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 the 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);
    
    }

重写matches方法如果返回true spring则会帮你创建该对象，否则则不会

## springboot提供的常用条件注解 ##

@ConditionalOnProperty：判断文件中是否有对应属性和值才实例化Bean
    @ConditionalOnClass 检查类在加载器中是否存在对应的类，如果有则被注解修饰的类就有资格被 Spring 容器所注册，否则会被跳过。
    @ConditionalOnBean 仅仅在当前上下文中存在某个对象时，才会实例化一个 Bean
    @ConditionalOnClass 某个 CLASS 位于类路径上，才会实例化一个 Bean
    @ConditionalOnExpression 当表达式为 true 的时候，才会实例化一个 Bean
    @ConditionalOnMissingBean 仅仅在当前上下文中不存在某个对象时，才会实例化一个 Bean
    @ConditionalOnMissingClass 某个 CLASS 类路径上不存在的时候，才会实例化一个 Bean

## 案例 ##

在springIOC容器中有一个User的bean，现要求：  
引入jedis坐标后，加载该bean，没导入则不加载

### 实体类 ###

package com.blb.springbootyuanli.entity;
    
    public class User { 
        private String name;
        private int age;
    
    	get/set

### UserConfig ###

配置类

package com.blb.springbootyuanli.config;
    
    import com.blb.springbootyuanli.condition.UserCondition;
    import com.blb.springbootyuanli.entity.User;
    import org.springframework.context.annotation.Bean;
    import org.springframework.context.annotation.Conditional;
    import org.springframework.context.annotation.Configuration;
    
    @Configuration
    public class UserConfig { 
        @Bean
        @Conditional(UserCondition.class)
        public User user(){ 
            return new User();
        }
    }

### UserCondition ###

实现Condition接口，重写matches方法

### 启动类 ###

package com.blb.springbootyuanli;
    
    import org.springframework.boot.SpringApplication;
    import org.springframework.boot.autoconfigure.SpringBootApplication;
    import org.springframework.context.ConfigurableApplicationContext;
    
    @SpringBootApplication
    public class SpringbootYuanliApplication { 
    
        public static void main(String[] args) { 
    
            ConfigurableApplicationContext app = SpringApplication.run(SpringbootYuanliApplication.class, args);
            Object user = app.getBean("user");
            System.out.println(user);
    
        }
    
    }

当我们在pom.xml引入jedis的坐标时，就可以打印user对象，当删除jedis的坐标时，运行就会报错 No bean named ‘user’ available

## 案例升级 ##

将类的判断定义为动态的，判断那个字节码文件可以动态指定

### 自定义一个注解 ###

添加上元注解

package com.blb.springbootyuanli.condition;
    
    import org.springframework.context.annotation.Conditional;
    
    import java.lang.annotation.*;
    
    @Target({ ElementType.TYPE, ElementType.METHOD}) //该注解的添加范围
    @Retention(RetentionPolicy.RUNTIME) //该注解的生效时机
    @Documented //生成javadoc的文档
    
    @Conditional(UserCondition.class)
    public @interface UserClassCondition { 
        String[] value();
    }

### UserConfig ###

package com.blb.springbootyuanli.config;
    
    import com.blb.springbootyuanli.condition.UserClassCondition;
    import com.blb.springbootyuanli.entity.User;
    import org.springframework.context.annotation.Bean;
    import org.springframework.context.annotation.Configuration;
    
    @Configuration
    public class UserConfig { 
        @Bean
        //@Conditional(UserCondition.class)
        @UserClassCondition("redis.clients.jedis.Jedis")
        public User user(){ 
            return new User();
        }
    }

package com.blb.springbootyuanli.condition;
    
    import org.springframework.context.annotation.Condition;
    import org.springframework.context.annotation.ConditionContext;
    import org.springframework.core.type.AnnotatedTypeMetadata;
    
    import java.util.Map;
    
    public class UserCondition implements Condition { 
        /** * * @param context 上下文对象，用于获取环境，ioc容器，classloader对象 * @param metadata 注解元对象。可以获取注解定义的属性值 * @return */
        @Override
        public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) { 
            //思路判断指定属性的class文件是否存在
    
            //获取注解属性值 value
            Map<String,Object> map=metadata.getAnnotationAttributes(UserClassCondition.class.getName());
            String[] values= (String[])map.get("value");
    
            boolean flag=true;
            try { 
                for(String classname:values){ 
                    Class<?> aClass = Class.forName(classname);
                }
                
            } catch (ClassNotFoundException e) { 
                flag=false;
            }
            return flag;
        }
    }

## 测试自带的注解 ##

package com.blb.springbootyuanli.config;
    
    import com.blb.springbootyuanli.entity.User;
    import org.springframework.boot.autoconfigure.condition.ConditionalOnProperty;
    import org.springframework.context.annotation.Bean;
    import org.springframework.context.annotation.Configuration;
    
    @Configuration
    public class UserConfig { 
        @Bean
        //@Conditional(UserCondition.class)
    
        //@UserClassCondition("redis.clients.jedis.Jedis")
        @ConditionalOnProperty(name="age",havingValue = "18")
        //只有在配置文件中有age并且值为18spring在能注册该bean
        public User user(){ 
            return new User();
        }
    }

## 小结 ##

自定义条件：  
1.定义条件类：自定义类实现Condition接口，重写重写matches方法，在matches方法中进行逻辑判断，返回boolean值  
2.matches方法的两个参数：  
context：上下文对象，可以获取属性值，获取类加载器，获取BeanFactory  
metadata：元数据对象，用于获取注解属性  
3.判断条件：在初始化Bean时，使用@Conditional（条件类.class） 注解

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