SpringBoot自动注入分析

　　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

 1 @Target(ElementType.TYPE)
 2 @Retention(RetentionPolicy.RUNTIME)
 3 @Documented
 4 @Inherited
 5 @SpringBootConfiguration
 6 @EnableAutoConfiguration
 7 @ComponentScan(excludeFilters = {
 8         @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class),
 9         @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })
10 public @interface SpringBootApplication {

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　　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

 1 # Auto Configure
 2 org.springframework.boot.autoconfigure.EnableAutoConfiguration=
 3 org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,
 4 org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,
 5 org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,
 6 org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,
 7 org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,
 8 org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,
 9 org.springframework.boot.autoconfigure.cloud.CloudAutoConfiguration,
10 org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,
11 org.springframework.boot.autoconfigure.context.MessageSourceAutoConfiguration,
12 org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration,
13 org.springframework.boot.autoconfigure.couchbase.CouchbaseAutoConfiguration,
14 org.springframework.boot.autoconfigure.dao.PersistenceExceptionTranslationAutoConfiguration,
15 org.springframework.boot.autoconfigure.data.cassandra.CassandraDataAutoConfiguration,
16 org.springframework.boot.autoconfigure.data.cassandra.CassandraRepositoriesAutoConfiguration,
17 org.springframework.boot.autoconfigure.data.couchbase.CouchbaseDataAutoConfiguration,
18 org.springframework.boot.autoconfigure.data.couchbase.CouchbaseRepositoriesAutoConfiguration,
19 org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchAutoConfiguration,
20 org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchDataAutoConfiguration,
21 org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchRepositoriesAutoConfiguration,
22 org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration,
23 org.springframework.boot.autoconfigure.data.ldap.LdapDataAutoConfiguration,
24 org.springframework.boot.autoconfigure.data.ldap.LdapRepositoriesAutoConfiguration,
25 org.springframework.boot.autoconfigure.data.mongo.MongoDataAutoConfiguration,
26 org.springframework.boot.autoconfigure.data.mongo.MongoRepositoriesAutoConfiguration,
27 org.springframework.boot.autoconfigure.data.neo4j.Neo4jDataAutoConfiguration,
28 org.springframework.boot.autoconfigure.data.neo4j.Neo4jRepositoriesAutoConfiguration,
29 org.springframework.boot.autoconfigure.data.solr.SolrRepositoriesAutoConfiguration,
30 org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration,
31 org.springframework.boot.autoconfigure.data.redis.RedisRepositoriesAutoConfiguration,
32 org.springframework.boot.autoconfigure.data.rest.RepositoryRestMvcAutoConfiguration,
33 org.springframework.boot.autoconfigure.data.web.SpringDataWebAutoConfiguration,
34 org.springframework.boot.autoconfigure.elasticsearch.jest.JestAutoConfiguration,
35 org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration,
36 org.springframework.boot.autoconfigure.gson.GsonAutoConfiguration,
37 org.springframework.boot.autoconfigure.h2.H2ConsoleAutoConfiguration,
38 org.springframework.boot.autoconfigure.hateoas.HypermediaAutoConfiguration,
39 org.springframework.boot.autoconfigure.hazelcast.HazelcastAutoConfiguration,
40 org.springframework.boot.autoconfigure.hazelcast.HazelcastJpaDependencyAutoConfiguration,
41 org.springframework.boot.autoconfigure.info.ProjectInfoAutoConfiguration,
42 org.springframework.boot.autoconfigure.integration.IntegrationAutoConfiguration,
43 org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration,
44 org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration,
45 org.springframework.boot.autoconfigure.jdbc.JdbcTemplateAutoConfiguration,
46 org.springframework.boot.autoconfigure.jdbc.JndiDataSourceAutoConfiguration,
47 org.springframework.boot.autoconfigure.jdbc.XADataSourceAutoConfiguration,
48 org.springframework.boot.autoconfigure.jdbc.DataSourceTransactionManagerAutoConfiguration,
49 org.springframework.boot.autoconfigure.jms.JmsAutoConfiguration,
50 org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration,
51 org.springframework.boot.autoconfigure.jms.JndiConnectionFactoryAutoConfiguration,
52 org.springframework.boot.autoconfigure.jms.activemq.ActiveMQAutoConfiguration,
53 org.springframework.boot.autoconfigure.jms.artemis.ArtemisAutoConfiguration,
54 org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration,
55 org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAutoConfiguration,
56 org.springframework.boot.autoconfigure.jersey.JerseyAutoConfiguration,
57 org.springframework.boot.autoconfigure.jooq.JooqAutoConfiguration,
58 org.springframework.boot.autoconfigure.kafka.KafkaAutoConfiguration,
59 org.springframework.boot.autoconfigure.ldap.embedded.EmbeddedLdapAutoConfiguration,
60 org.springframework.boot.autoconfigure.ldap.LdapAutoConfiguration,
61 org.springframework.boot.autoconfigure.liquibase.LiquibaseAutoConfiguration,
62 org.springframework.boot.autoconfigure.mail.MailSenderAutoConfiguration,
63 org.springframework.boot.autoconfigure.mail.MailSenderValidatorAutoConfiguration,
64 org.springframework.boot.autoconfigure.mobile.DeviceResolverAutoConfiguration,
65 org.springframework.boot.autoconfigure.mobile.DeviceDelegatingViewResolverAutoConfiguration,
66 org.springframework.boot.autoconfigure.mobile.SitePreferenceAutoConfiguration,
67 org.springframework.boot.autoconfigure.mongo.embedded.EmbeddedMongoAutoConfiguration,
68 org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,
69 org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration,
70 org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,
71 org.springframework.boot.autoconfigure.reactor.ReactorAutoConfiguration,
72 org.springframework.boot.autoconfigure.security.SecurityAutoConfiguration,
73 org.springframework.boot.autoconfigure.security.SecurityFilterAutoConfiguration,
74 org.springframework.boot.autoconfigure.security.FallbackWebSecurityAutoConfiguration,
75 org.springframework.boot.autoconfigure.security.oauth2.OAuth2AutoConfiguration,
76 org.springframework.boot.autoconfigure.sendgrid.SendGridAutoConfiguration,
77 org.springframework.boot.autoconfigure.session.SessionAutoConfiguration,
78 org.springframework.boot.autoconfigure.social.SocialWebAutoConfiguration,
79 org.springframework.boot.autoconfigure.social.FacebookAutoConfiguration,
80 org.springframework.boot.autoconfigure.social.LinkedInAutoConfiguration,
81 org.springframework.boot.autoconfigure.social.TwitterAutoConfiguration,
82 org.springframework.boot.autoconfigure.solr.SolrAutoConfiguration,
83 org.springframework.boot.autoconfigure.thymeleaf.ThymeleafAutoConfiguration,
84 org.springframework.boot.autoconfigure.transaction.TransactionAutoConfiguration,
85 org.springframework.boot.autoconfigure.transaction.jta.JtaAutoConfiguration,
86 org.springframework.boot.autoconfigure.validation.ValidationAutoConfiguration,
87 org.springframework.boot.autoconfigure.web.DispatcherServletAutoConfiguration,
88 org.springframework.boot.autoconfigure.web.EmbeddedServletContainerAutoConfiguration,
89 org.springframework.boot.autoconfigure.web.ErrorMvcAutoConfiguration,
90 org.springframework.boot.autoconfigure.web.HttpEncodingAutoConfiguration,
91 org.springframework.boot.autoconfigure.web.HttpMessageConvertersAutoConfiguration,
92 org.springframework.boot.autoconfigure.web.MultipartAutoConfiguration,
93 org.springframework.boot.autoconfigure.web.ServerPropertiesAutoConfiguration,
94 org.springframework.boot.autoconfigure.web.WebClientAutoConfiguration,
95 org.springframework.boot.autoconfigure.web.WebMvcAutoConfiguration,
96 org.springframework.boot.autoconfigure.websocket.WebSocketAutoConfiguration,
97 org.springframework.boot.autoconfigure.websocket.WebSocketMessagingAutoConfiguration,
98 org.springframework.boot.autoconfigure.webservices.WebServicesAutoConfiguration

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　　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

 1 @Configuration
 2 @ConditionalOnClass({ JedisConnection.class, RedisOperations.class, Jedis.class })
 3 @EnableConfigurationProperties(RedisProperties.class)
 4 public class RedisAutoConfiguration {
 5 
 6     /**
 7      * Redis connection configuration.
 8      */
 9     @Configuration
10     @ConditionalOnClass(GenericObjectPool.class)
11     protected static class RedisConnectionConfiguration {
12 
13         private final RedisProperties properties;
14 
15         private final RedisSentinelConfiguration sentinelConfiguration;
16 
17         private final RedisClusterConfiguration clusterConfiguration;
18 
19         public RedisConnectionConfiguration(RedisProperties properties,
20                 ObjectProvider<RedisSentinelConfiguration> sentinelConfiguration,
21                 ObjectProvider<RedisClusterConfiguration> clusterConfiguration) {
22             this.properties = properties;
23             this.sentinelConfiguration = sentinelConfiguration.getIfAvailable();
24             this.clusterConfiguration = clusterConfiguration.getIfAvailable();
25         }
26 
27         @Bean
28         @ConditionalOnMissingBean(RedisConnectionFactory.class)
29         public JedisConnectionFactory redisConnectionFactory()
30                 throws UnknownHostException {
31             return applyProperties(createJedisConnectionFactory());
32         }

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　　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

  1 @ConfigurationProperties(prefix = "spring.redis")
  2 public class RedisProperties {
  3 
  4     /**
  5      * Database index used by the connection factory.
  6      */
  7     private int database = 0;
  8 
  9     /**
 10      * Redis url, which will overrule host, port and password if set.
 11      */
 12     private String url;
 13 
 14     /**
 15      * Redis server host.
 16      */
 17     private String host = "localhost";
 18 
 19     /**
 20      * Login password of the redis server.
 21      */
 22     private String password;
 23 
 24     /**
 25      * Redis server port.
 26      */
 27     private int port = 6379;
 28 
 29     /**
 30      * Enable SSL.
 31      */
 32     private boolean ssl;
 33 
 34     /**
 35      * Connection timeout in milliseconds.
 36      */
 37     private int timeout;
 38 
 39     private Pool pool;
 40 
 41     private Sentinel sentinel;
 42 
 43     private Cluster cluster;
 44 
 45     public int getDatabase() {
 46         return this.database;
 47     }
 48 
 49     public void setDatabase(int database) {
 50         this.database = database;
 51     }
 52 
 53     public String getUrl() {
 54         return this.url;
 55     }
 56 
 57     public void setUrl(String url) {
 58         this.url = url;
 59     }
 60 
 61     public String getHost() {
 62         return this.host;
 63     }
 64 
 65     public void setHost(String host) {
 66         this.host = host;
 67     }
 68 
 69     public String getPassword() {
 70         return this.password;
 71     }
 72 
 73     public void setPassword(String password) {
 74         this.password = password;
 75     }
 76 
 77     public int getPort() {
 78         return this.port;
 79     }
 80 
 81     public void setPort(int port) {
 82         this.port = port;
 83     }
 84 
 85     public boolean isSsl() {
 86         return this.ssl;
 87     }
 88 
 89     public void setSsl(boolean ssl) {
 90         this.ssl = ssl;
 91     }
 92 
 93     public void setTimeout(int timeout) {
 94         this.timeout = timeout;
 95     }
 96 
 97     public int getTimeout() {
 98         return this.timeout;
 99     }
100 
101     public Sentinel getSentinel() {
102         return this.sentinel;
103     }
104 
105     public void setSentinel(Sentinel sentinel) {
106         this.sentinel = sentinel;
107     }
108 
109     public Pool getPool() {
110         return this.pool;
111     }
112 
113     public void setPool(Pool pool) {
114         this.pool = pool;
115     }
116 
117     public Cluster getCluster() {
118         return this.cluster;
119     }
120 
121     public void setCluster(Cluster cluster) {
122         this.cluster = cluster;
123     }
124 
125     /**
126      * Pool properties.
127      */
128     public static class Pool {
129 
130         /**
131          * Max number of "idle" connections in the pool. Use a negative value to indicate
132          * an unlimited number of idle connections.
133          */
134         private int maxIdle = 8;
135 
136         /**
137          * Target for the minimum number of idle connections to maintain in the pool. This
138          * setting only has an effect if it is positive.
139          */
140         private int minIdle = 0;
141 
142         /**
143          * Max number of connections that can be allocated by the pool at a given time.
144          * Use a negative value for no limit.
145          */
146         private int maxActive = 8;
147 
148         /**
149          * Maximum amount of time (in milliseconds) a connection allocation should block
150          * before throwing an exception when the pool is exhausted. Use a negative value
151          * to block indefinitely.
152          */
153         private int maxWait = -1;
154 
155         public int getMaxIdle() {
156             return this.maxIdle;
157         }
158 
159         public void setMaxIdle(int maxIdle) {
160             this.maxIdle = maxIdle;
161         }
162 
163         public int getMinIdle() {
164             return this.minIdle;
165         }
166 
167         public void setMinIdle(int minIdle) {
168             this.minIdle = minIdle;
169         }
170 
171         public int getMaxActive() {
172             return this.maxActive;
173         }
174 
175         public void setMaxActive(int maxActive) {
176             this.maxActive = maxActive;
177         }
178 
179         public int getMaxWait() {
180             return this.maxWait;
181         }
182 
183         public void setMaxWait(int maxWait) {
184             this.maxWait = maxWait;
185         }
186 
187     }
188 
189     /**
190      * Cluster properties.
191      */
192     public static class Cluster {
193 
194         /**
195          * Comma-separated list of "host:port" pairs to bootstrap from. This represents an
196          * "initial" list of cluster nodes and is required to have at least one entry.
197          */
198         private List<String> nodes;
199 
200         /**
201          * Maximum number of redirects to follow when executing commands across the
202          * cluster.
203          */
204         private Integer maxRedirects;
205 
206         public List<String> getNodes() {
207             return this.nodes;
208         }
209 
210         public void setNodes(List<String> nodes) {
211             this.nodes = nodes;
212         }
213 
214         public Integer getMaxRedirects() {
215             return this.maxRedirects;
216         }
217 
218         public void setMaxRedirects(Integer maxRedirects) {
219             this.maxRedirects = maxRedirects;
220         }
221 
222     }
223 
224     /**
225      * Redis sentinel properties.
226      */
227     public static class Sentinel {
228 
229         /**
230          * Name of Redis server.
231          */
232         private String master;
233 
234         /**
235          * Comma-separated list of host:port pairs.
236          */
237         private String nodes;
238 
239         public String getMaster() {
240             return this.master;
241         }
242 
243         public void setMaster(String master) {
244             this.master = master;
245         }
246 
247         public String getNodes() {
248             return this.nodes;
249         }
250 
251         public void setNodes(String nodes) {
252             this.nodes = nodes;
253         }
254 
255     }
256 
257 }

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　　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。

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原文地址：https://www.cnblogs.com/wanghaoyang/p/11693973.html