浅谈springboot自动装配原理

一、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 {

@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

二、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

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;
    }
}

三、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

package com.blb.springbootyuanli.condition;

import org.springframework.context.annotation.Condition;
import org.springframework.context.annotation.ConditionContext;
import org.springframework.core.type.AnnotatedTypeMetadata;

public class UserCondition implements Condition {
    @Override
    public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
        //思路判断jedis的class文件是否存在
        boolean flag=true;
        try {

            Class<?> aClass = Class.forName("redis.clients.jedis.Jedis");

        } catch (ClassNotFoundException e) {
            flag=false;
        }
        return flag;
    }
}

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

四、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();
    }
}

五、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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