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.WebServicesAutoConfigurationView Code
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。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。