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Spring Boot自动注入的原理分析
来源:jb51  时间:2019/10/21 8:38:03  对本文有异议

前言

我们经常会被问到这么一个问题:SpringBoot相对于spring有哪些优势呢?其中有一条答案就是SpringBoot自动注入。那么自动注入的原理是什么呢?

我们进行如下分析。

1:首先我们分析项目的启动类时,发现都会加上@SpringBootApplication这个注解,我们分析这个继续进入这个注解会发现,它是由多个注解组成的,如下

@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 {

2:服务启动会扫描 org.springframework.boot.autoconfigure下的 META-INF/spring.factories 这个文件,这个文件中保存着springboot 启动时默认会自动注入的类,部分如下

# Auto Configure
org.springframework.boot.autoconfigure.EnableAutoConfiguration=org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,org.springframework.boot.autoconfigure.cloud.CloudAutoConfiguration,org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,org.springframework.boot.autoconfigure.context.MessageSourceAutoConfiguration,org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration,org.springframework.boot.autoconfigure.couchbase.CouchbaseAutoConfiguration,org.springframework.boot.autoconfigure.dao.PersistenceExceptionTranslationAutoConfiguration,org.springframework.boot.autoconfigure.data.cassandra.CassandraDataAutoConfiguration,org.springframework.boot.autoconfigure.data.cassandra.CassandraRepositoriesAutoConfiguration,org.springframework.boot.autoconfigure.data.couchbase.CouchbaseDataAutoConfiguration,org.springframework.boot.autoconfigure.data.couchbase.CouchbaseRepositoriesAutoConfiguration,org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchAutoConfiguration,org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchDataAutoConfiguration,org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchRepositoriesAutoConfiguration,org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration,org.springframework.boot.autoconfigure.data.ldap.LdapDataAutoConfiguration,org.springframework.boot.autoconfigure.data.ldap.LdapRepositoriesAutoConfiguration,org.springframework.boot.autoconfigure.data.mongo.MongoDataAutoConfiguration,org.springframework.boot.autoconfigure.data.mongo.MongoRepositoriesAutoConfiguration,org.springframework.boot.autoconfigure.data.neo4j.Neo4jDataAutoConfiguration,org.springframework.boot.autoconfigure.data.neo4j.Neo4jRepositoriesAutoConfiguration,org.springframework.boot.autoconfigure.data.solr.SolrRepositoriesAutoConfiguration,org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration,org.springframework.boot.autoconfigure.data.redis.RedisRepositoriesAutoConfiguration,org.springframework.boot.autoconfigure.data.rest.RepositoryRestMvcAutoConfiguration,org.springframework.boot.autoconfigure.data.web.SpringDataWebAutoConfiguration,org.springframework.boot.autoconfigure.elasticsearch.jest.JestAutoConfiguration,org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration,org.springframework.boot.autoconfigure.gson.GsonAutoConfiguration,org.springframework.boot.autoconfigure.h2.H2ConsoleAutoConfiguration,org.springframework.boot.autoconfigure.hateoas.HypermediaAutoConfiguration,org.springframework.boot.autoconfigure.hazelcast.HazelcastAutoConfiguration,org.springframework.boot.autoconfigure.hazelcast.HazelcastJpaDependencyAutoConfiguration,org.springframework.boot.autoconfigure.info.ProjectInfoAutoConfiguration,org.springframework.boot.autoconfigure.integration.IntegrationAutoConfiguration,org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration,org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration,org.springframework.boot.autoconfigure.jdbc.JdbcTemplateAutoConfiguration,org.springframework.boot.autoconfigure.jdbc.JndiDataSourceAutoConfiguration,org.springframework.boot.autoconfigure.jdbc.XADataSourceAutoConfiguration,org.springframework.boot.autoconfigure.jdbc.DataSourceTransactionManagerAutoConfiguration,org.springframework.boot.autoconfigure.jms.JmsAutoConfiguration,org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration,org.springframework.boot.autoconfigure.jms.JndiConnectionFactoryAutoConfiguration,org.springframework.boot.autoconfigure.jms.activemq.ActiveMQAutoConfiguration,org.springframework.boot.autoconfigure.jms.artemis.ArtemisAutoConfiguration,org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration,org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAutoConfiguration,org.springframework.boot.autoconfigure.jersey.JerseyAutoConfiguration,org.springframework.boot.autoconfigure.jooq.JooqAutoConfiguration,org.springframework.boot.autoconfigure.kafka.KafkaAutoConfiguration,org.springframework.boot.autoconfigure.ldap.embedded.EmbeddedLdapAutoConfiguration,org.springframework.boot.autoconfigure.ldap.LdapAutoConfiguration,org.springframework.boot.autoconfigure.liquibase.LiquibaseAutoConfiguration,org.springframework.boot.autoconfigure.mail.MailSenderAutoConfiguration,org.springframework.boot.autoconfigure.mail.MailSenderValidatorAutoConfiguration,org.springframework.boot.autoconfigure.mobile.DeviceResolverAutoConfiguration,org.springframework.boot.autoconfigure.mobile.DeviceDelegatingViewResolverAutoConfiguration,org.springframework.boot.autoconfigure.mobile.SitePreferenceAutoConfiguration,org.springframework.boot.autoconfigure.mongo.embedded.EmbeddedMongoAutoConfiguration,org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration,org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,org.springframework.boot.autoconfigure.reactor.ReactorAutoConfiguration,org.springframework.boot.autoconfigure.security.SecurityAutoConfiguration,org.springframework.boot.autoconfigure.security.SecurityFilterAutoConfiguration,org.springframework.boot.autoconfigure.security.FallbackWebSecurityAutoConfiguration,org.springframework.boot.autoconfigure.security.oauth2.OAuth2AutoConfiguration,org.springframework.boot.autoconfigure.sendgrid.SendGridAutoConfiguration,org.springframework.boot.autoconfigure.session.SessionAutoConfiguration,org.springframework.boot.autoconfigure.social.SocialWebAutoConfiguration,org.springframework.boot.autoconfigure.social.FacebookAutoConfiguration,org.springframework.boot.autoconfigure.social.LinkedInAutoConfiguration,org.springframework.boot.autoconfigure.social.TwitterAutoConfiguration,org.springframework.boot.autoconfigure.solr.SolrAutoConfiguration,org.springframework.boot.autoconfigure.thymeleaf.ThymeleafAutoConfiguration,org.springframework.boot.autoconfigure.transaction.TransactionAutoConfiguration,org.springframework.boot.autoconfigure.transaction.jta.JtaAutoConfiguration,org.springframework.boot.autoconfigure.validation.ValidationAutoConfiguration,org.springframework.boot.autoconfigure.web.DispatcherServletAutoConfiguration,org.springframework.boot.autoconfigure.web.EmbeddedServletContainerAutoConfiguration,org.springframework.boot.autoconfigure.web.ErrorMvcAutoConfiguration,org.springframework.boot.autoconfigure.web.HttpEncodingAutoConfiguration,org.springframework.boot.autoconfigure.web.HttpMessageConvertersAutoConfiguration,org.springframework.boot.autoconfigure.web.MultipartAutoConfiguration,org.springframework.boot.autoconfigure.web.ServerPropertiesAutoConfiguration,org.springframework.boot.autoconfigure.web.WebClientAutoConfiguration,org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration,org.springframework.boot.autoconfigure.websocket.WebSocketAutoConfiguration,org.springframework.boot.autoconfigure.websocket.WebSocketMessagingAutoConfiguration,org.springframework.boot.autoconfigure.webservices.WebServicesAutoConfiguration

3:你是不是在其中发现了自己常用的redis,mysql等相关的类?没错,springboot会尝试加载这些类,我们以 org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration 这个类为例,进去看一下它的源码,部分示例如下

@Configuration
@ConditionalOnClass({ JedisConnection.class, RedisOperations.class, Jedis.class })
@EnableConfigurationProperties(RedisProperties.class)
public class RedisAutoConfiguration {

 /**
  * Redis connection configuration.
  */
 @Configuration
 @ConditionalOnClass(GenericObjectPool.class)
 protected static class RedisConnectionConfiguration {

  private final RedisProperties properties;

  private final RedisSentinelConfiguration sentinelConfiguration;

  private final RedisClusterConfiguration clusterConfiguration;

  public RedisConnectionConfiguration(RedisProperties properties,
    ObjectProvider<RedisSentinelConfiguration> sentinelConfiguration,
    ObjectProvider<RedisClusterConfiguration> clusterConfiguration) {
   this.properties = properties;
   this.sentinelConfiguration = sentinelConfiguration.getIfAvailable();
   this.clusterConfiguration = clusterConfiguration.getIfAvailable();
  }

  @Bean
  @ConditionalOnMissingBean(RedisConnectionFactory.class)
  public JedisConnectionFactory redisConnectionFactory()
    throws UnknownHostException {
   return applyProperties(createJedisConnectionFactory());
  }

我们能看到这个类上加了这个注解 @ConditionalOnClass({ JedisConnection.class, RedisOperations.class, Jedis.class }) 意思就是如果你的classpath中没有这些类的话,那么这个类就不能被加载,那么这些被依赖的类在哪出现呢?没错,就在我们在pom.xml中引入的依赖所对应的包里。

看到这里你因该就明白了,META-INF/spring.factories 文件中被列出来的那些类都会被springboot去尝试加载,但是有些模块我们没引入相关的依赖,那么这个类就会加载失败。即这个模块没有被成功加载。

4:我们通过上面的redis的自动加载类时,看到上面还有个 @EnableConfigurationProperties(RedisProperties.class) 注解,这个注解来注入关于redis的配置信息,这个信息都在 RedisProperties.class 中保存,我们看下 RedisProperties的源码

@ConfigurationProperties(prefix = "spring.redis")
public class RedisProperties {

  /**
   * Database index used by the connection factory.
   */
  private int database = 0;

  /**
   * Redis url, which will overrule host, port and password if set.
   */
  private String url;

  /**
   * Redis server host.
   */
  private String host = "localhost";

  /**
   * Login password of the redis server.
   */
  private String password;

  /**
   * Redis server port.
   */
  private int port = 6379;

  /**
   * Enable SSL.
   */
  private boolean ssl;

  /**
   * Connection timeout in milliseconds.
   */
  private int timeout;

  private Pool pool;

  private Sentinel sentinel;

  private Cluster cluster;

  public int getDatabase() {
    return this.database;
  }

  public void setDatabase(int database) {
    this.database = database;
  }

  public String getUrl() {
    return this.url;
  }

  public void setUrl(String url) {
    this.url = url;
  }

  public String getHost() {
    return this.host;
  }

  public void setHost(String host) {
    this.host = host;
  }

  public String getPassword() {
    return this.password;
  }

  public void setPassword(String password) {
    this.password = password;
  }

  public int getPort() {
    return this.port;
  }

  public void setPort(int port) {
    this.port = port;
  }

  public boolean isSsl() {
    return this.ssl;
  }

  public void setSsl(boolean ssl) {
    this.ssl = ssl;
  }

  public void setTimeout(int timeout) {
    this.timeout = timeout;
  }

  public int getTimeout() {
    return this.timeout;
  }

  public Sentinel getSentinel() {
    return this.sentinel;
  }

  public void setSentinel(Sentinel sentinel) {
    this.sentinel = sentinel;
  }

  public Pool getPool() {
    return this.pool;
  }

  public void setPool(Pool pool) {
    this.pool = pool;
  }

  public Cluster getCluster() {
    return this.cluster;
  }

  public void setCluster(Cluster cluster) {
    this.cluster = cluster;
  }

  /**
   * Pool properties.
   */
  public static class Pool {

    /**
     * Max number of "idle" connections in the pool. Use a negative value to indicate
     * an unlimited number of idle connections.
     */
    private int maxIdle = 8;

    /**
     * Target for the minimum number of idle connections to maintain in the pool. This
     * setting only has an effect if it is positive.
     */
    private int minIdle = 0;

    /**
     * Max number of connections that can be allocated by the pool at a given time.
     * Use a negative value for no limit.
     */
    private int maxActive = 8;

    /**
     * Maximum amount of time (in milliseconds) a connection allocation should block
     * before throwing an exception when the pool is exhausted. Use a negative value
     * to block indefinitely.
     */
    private int maxWait = -1;

    public int getMaxIdle() {
      return this.maxIdle;
    }

    public void setMaxIdle(int maxIdle) {
      this.maxIdle = maxIdle;
    }

    public int getMinIdle() {
      return this.minIdle;
    }

    public void setMinIdle(int minIdle) {
      this.minIdle = minIdle;
    }

    public int getMaxActive() {
      return this.maxActive;
    }

    public void setMaxActive(int maxActive) {
      this.maxActive = maxActive;
    }

    public int getMaxWait() {
      return this.maxWait;
    }

    public void setMaxWait(int maxWait) {
      this.maxWait = maxWait;
    }

  }

  /**
   * Cluster properties.
   */
  public static class Cluster {

    /**
     * Comma-separated list of "host:port" pairs to bootstrap from. This represents an
     * "initial" list of cluster nodes and is required to have at least one entry.
     */
    private List<String> nodes;

    /**
     * Maximum number of redirects to follow when executing commands across the
     * cluster.
     */
    private Integer maxRedirects;

    public List<String> getNodes() {
      return this.nodes;
    }

    public void setNodes(List<String> nodes) {
      this.nodes = nodes;
    }

    public Integer getMaxRedirects() {
      return this.maxRedirects;
    }

    public void setMaxRedirects(Integer maxRedirects) {
      this.maxRedirects = maxRedirects;
    }

  }

  /**
   * Redis sentinel properties.
   */
  public static class Sentinel {

    /**
     * Name of Redis server.
     */
    private String master;

    /**
     * Comma-separated list of host:port pairs.
     */
    private String nodes;

    public String getMaster() {
      return this.master;
    }

    public void setMaster(String master) {
      this.master = master;
    }

    public String getNodes() {
      return this.nodes;
    }

    public void setNodes(String nodes) {
      this.nodes = nodes;
    }

  }

}

发现里面的配置项基本都是有默认值的,通过上面的注解可以明白,如果配置文件中存在 spring.redis 开头的配置项,则使用配置文件中的,如果没有的话则使用文件中默认写死的配置。你是不是想到了springboot的另外一个优势:约定大于配置。

这里我们大概了解了SpringBoot自动配置的原理和流程,里面的那些细节我们不在分析。同理,对于那些第三方或者我们自己写的starter,springboot启动的时候会扫描starter的jar包下 META-INF/spring.factories 这个文件,解析内容并加载里面对用的类。

由此,我们总结出以下几点

  1:SpringBoot的自动配置是如何实现的

  2:有关的那些注解,如@EnableAutoConfiguration, @ConditionalOnClass, @Configuration等也是SpringBoot的核心注解,也值得我们了解其用法和原理。

总结

以上就是这篇文章的全部内容了,希望本文的内容对大家的学习或者工作具有一定的参考学习价值,谢谢大家对w3xue的支持。

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