浅谈springboot自动装配原理
作者：向天再借500年
作为Spring Boot的精髓,自动配置原理首当其冲.今天就带大家了解一下springboot自动装配的原理,文中有非常详细的代码示例,对正在学习java的小伙伴们很有帮助,需要的朋友可以参考下
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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