前言:
Spring Boot的自动配置让我们可以快速的开始业务开发,相比于之前使用Spring时需要深刻理解AOP,IOC,在XML里搞一堆繁琐的配置,Spring Boot让事情变得简单多了,它基于"约定大于配置"的理念,再也没有繁琐的配置和难以集成的内容(大多数流行的第三方技术都被集成在内).但是,这种简答背后的原理是什么呢?其实是Spring4.X提供的基于条件配置Bean的能力.
核心包
Spring Boot关于自动配置的源码在spring-boot-autoconfigure-x.x.x.x.jar中,主要包含了如下图所示的配置(并未截全):
Spring Boot为我们做的自动配置都在这个包里.通过在application.properties中设置属性:debug=true,可以通过控制台的输出观察自动配置启动的情况:
运行原理
使用Spring Boot,我们的应用入口一般会象下面这样:
import java.util.ArrayList;
import java.util.List;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration;
import org.springframework.boot.web.servlet.FilterRegistrationBean;
import org.springframework.boot.web.support.SpringBootServletInitializer;
import org.springframework.context.annotation.Bean;
import org.springframework.scheduling.annotation.EnableAsync;
import org.springframework.scheduling.annotation.EnableScheduling;
import com.giveu.cashloan.filter.AuthenticationFilter;
/**
* 描述: spring-boot启动主方法
**/
@EnableAsync
@EnableScheduling
@SpringBootApplication(exclude = { DataSourceAutoConfiguration.class })
public class Application extends SpringBootServletInitializer {
@Autowired
private AuthenticationFilter authenticationFilter;
public static void main(String[] args) throws Exception {
SpringApplication.run(Application.class, args);
}
/**
* @methodName: filterRegistrationBean<br>
* @Describe:注册过滤器
*/
@Bean
public FilterRegistrationBean filterRegistrationBean() {
FilterRegistrationBean registrationBean = new FilterRegistrationBean();
registrationBean.setFilter(authenticationFilter);
List<String> urlPatterns = new ArrayList<>();
urlPatterns.add("/*");
registrationBean.setUrlPatterns(urlPatterns);
return registrationBean;
}
}
这里SpringBoot的应用,程序启动时会加载带有@SpringBootApplicatuiion注解的入口类,并自动扫描该类所在的包及其子包下的类.因此一般会把入口类放在项目包路径下(GroupId + ArtifactId).
@SpringBootApplication是一个组合注解,包含@SpringBootConfiguration、@EnableAutoConfiguration、@ComponentScan这三个注解,源码如下:
package org.springframework.boot.autoconfigure;
import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.springframework.boot.SpringBootConfiguration;
import org.springframework.boot.context.TypeExcludeFilter;
import org.springframework.context.annotation.ComponentScan;
import org.springframework.context.annotation.FilterType;
import org.springframework.context.annotation.ComponentScan.Filter;
import org.springframework.core.annotation.AliasFor;
@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 {
@AliasFor(
annotation = EnableAutoConfiguration.class,
attribute = "exclude"
)
Class<?>[] exclude() default {};
@AliasFor(
annotation = EnableAutoConfiguration.class,
attribute = "excludeName"
)
String[] excludeName() default {};
@AliasFor(
annotation = ComponentScan.class,
attribute = "basePackages"
)
String[] scanBasePackages() default {};
@AliasFor(
annotation = ComponentScan.class,
attribute = "basePackageClasses"
)
Class<?>[] scanBasePackageClasses() default {};
}
它的核心功能是由@EnableAutoConfiguration这个注解提供的,我们来看看@EnableAutoConfiguration的源代码:
package org.springframework.boot.autoconfigure;
import java.lang.annotation.Documented;
import java.lang.annotation.ElementType;
import java.lang.annotation.Inherited;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import org.springframework.context.annotation.Import;
@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import({EnableAutoConfigurationImportSelector.class})
public @interface EnableAutoConfiguration {
String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";
Class<?>[] exclude() default {};
String[] excludeName() default {};
}
这里的关键功能是@Import注解导入的配置功能 :EnableAutoConfigurationImportSelector使用SpringFactoriesLoader.loadFactoryNames方法来扫描具有META-INF/spring.factories文件的jar包,spring-boot-autoconfigure-x.x.x.x.jar里就有一个spring.factories文件,这个文件中声明了有哪些要自动配置。
org.springframework.core.io.support.SpringFactoriesLoader.java的loadFactoryNames()方法:
public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) {
String factoryClassName = factoryClass.getName();
try {
Enumeration<URL> urls = classLoader != null ? classLoader.getResources("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories");
ArrayList result = new ArrayList();
while(urls.hasMoreElements()) {
URL url = (URL)urls.nextElement();
Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url));
String factoryClassNames = properties.getProperty(factoryClassName);
result.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(factoryClassNames)));
}
return result;
} catch (IOException var8) {
throw new IllegalArgumentException("Unable to load [" + factoryClass.getName() + "] factories from location [" + "META-INF/spring.factories" + "]", var8);
}
}
下面我们来分析一下spring boot autoconfigure里面的MongoAutoConfiguration(mongodb的自动配置),相信你就会明白这套自动配置机制到底是怎么一回事儿:
package org.springframework.boot.autoconfigure.mongo;
import com.mongodb.MongoClient;
import com.mongodb.MongoClientOptions;
import java.net.UnknownHostException;
import javax.annotation.PreDestroy;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.autoconfigure.condition.ConditionalOnClass;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.boot.autoconfigure.mongo.MongoProperties;
import org.springframework.boot.context.properties.EnableConfigurationProperties;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.core.env.Environment;
@Configuration
@ConditionalOnClass({MongoClient.class})
@EnableConfigurationProperties({MongoProperties.class}) //开启属性注入。
@ConditionalOnMissingBean(
type = {"org.springframework.data.mongodb.MongoDbFactory"}
)
public class MongoAutoConfiguration {
@Autowired
private MongoProperties properties;
@Autowired(
required = false
)
private MongoClientOptions options;
@Autowired
private Environment environment;
private MongoClient mongo;
public MongoAutoConfiguration() {
}
@PreDestroy
public void close() {
if(this.mongo != null) {
this.mongo.close();
}
}
@Bean //使用java配置,当容器中没有这个bean的时候执行初始化
@ConditionalOnMissingBean
public MongoClient mongo() throws UnknownHostException {
this.mongo = this.properties.createMongoClient(this.options, this.environment);
return this.mongo;
}
}
首先这被@Configuration注解了,是一个配置类,当满足以下条件这个bean被装配:
(1)@ConditionalOnClass(MongoClient.class) 当MongoClient在类路径下
(2)@ConditionalOnMissingBean(type = “org.springframework.data.mongodb.MongoDbFactory”) 当容器中没有org.springframework.data.mongodb.MongoDbFactory这类bean的时候
此外,我们可以看一下通过@EnableConfigurationProperties({MongoProperties.class}) 自动注入的属性(这是习惯优于配置的最终落地点):
package org.springframework.boot.autoconfigure.mongo;
import com.mongodb.MongoClientURI;
import org.springframework.boot.context.properties.ConfigurationProperties;
/**
* Configuration properties for Mongo.
*
* @author Dave Syer
* @author Phillip Webb
* @author Josh Long
* @author Andy Wilkinson
* @author Eddú Meléndez
* @author Stephane Nicoll
* @author Nasko Vasilev
* @author Mark Paluch
*/
@ConfigurationProperties(prefix = "spring.data.mongodb")
public class MongoProperties {
/**
* Default port used when the configured port is {@code null}.
*/
public static final int DEFAULT_PORT = 27017;
/**
* Default URI used when the configured URI is {@code null}.
*/
public static final String DEFAULT_URI = "mongodb://localhost/test";
/**
* Mongo server host. Cannot be set with URI.
*/
private String host;
/**
* Mongo server port. Cannot be set with URI.
*/
private Integer port = null;
/**
* Mongo database URI. Cannot be set with host, port and credentials.
*/
private String uri;
/**
* Database name.
*/
private String database;
/**
* Authentication database name.
*/
private String authenticationDatabase;
/**
* GridFS database name.
*/
private String gridFsDatabase;
/**
* Login user of the mongo server. Cannot be set with URI.
*/
private String username;
/**
* Login password of the mongo server. Cannot be set with URI.
*/
private char[] password;
/**
* Fully qualified name of the FieldNamingStrategy to use.
*/
private Class<?> fieldNamingStrategy;
getter/setter()...
}
所以在我们什么都不干的情况下,只需要引入spring-data-mongodb这个依赖再加上默认的MongoDB server我们就能够快速集成MongoDB,用MongodbTemplate访问数据库。
同时我们可以通过在application.yaml中修改spring.data.mongodb相关的参数就能够修改连接配置,如:
spring:
data:
mongodb:
host: localhost
port: 27017
username: chingzhu
password: test123
database: icekredit
利用这套原理,我们也可以轻松地把目前spring boot还未集成的、我们自己要使用的第三方技术自动集成起来。
附:常见org.springframework.boot.autoconfigure.condition包下的条件注解意思
- @ConditionalOnBean:当容器里有指定的bean的条件下。
- @ConditionalOnMissingBean:当容器里不存在指定bean的条件下。
- @ConditionalOnClass:当类路径下有指定类的条件下。
- @ConditionalOnMissingClass:当类路径下不存在指定类的条件下。
- @ConditionalOnProperty:指定的属性是否有指定的值,比如@ConditionalOnProperties(prefix=”xxx.xxx”,value=”enable”, matchIfMissing=true),代表当xxx.xxx为enable时条件的布尔值为true,如果没有设置的情况下也为true。