Spring Boot 自动配置之条件注解浅析

@Target({ElementType.TYPE, ElementType.METHOD})
@Retention(RetentionPolicy.RUNTIME)
@Documented
public @interface Conditional {

 /**
 * All {@link Condition Conditions} that must {@linkplain Condition#matches match}
 * in order for the component to be registered.
 */
 Class<? extends Condition>[] value();

}

@FunctionalInterface
public interface Condition {

 boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata);
}

db-type=oracle

public class MySqlCondition implements Condition {

  @Override
  public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
    return "mysql".equals(context.getEnvironment().getProperty("db-type"));
  }
}

public class OracleCondition implements Condition {

  @Override
  public boolean matches(ConditionContext context, AnnotatedTypeMetadata metadata) {
    return "oracle".equals(context.getEnvironment().getProperty("db-type"));
  }
}

public interface JdbcFactory {

  void create();
}

@ConditionalOnMissingBean(value = JdbcFactory.class, ignored = MySqlDefaultFactory.class)
@Conditional(MySqlCondition.class)
@Component
public class MySqlDefaultFactory implements JdbcFactory {

  @Override
  public void create() {
    System.out.println("Default MySql create ..");
  }

}

@ConditionalOnMissingBean(value = JdbcFactory.class, ignored = OracleDefaultFactory.class)
@Conditional(OracleCondition.class)
@Component
public class OracleDefaultFactory implements JdbcFactory {

  @Override
  public void create() {
    System.out.println("Default oracle create..");
  }
}

@Resource
private JdbcFactory jdbcFactory;

@Test
public void conditionOnMissBean() {
  jdbcFactory.create();
}

@Component
public class MysqlFactory implements JdbcFactory {

  @Override
  public void create() {
    System.out.println("mysql 。。 create");
  }
}

@Resource
private JdbcFactory jdbcFactory;

@Test
public void conditionOnMissBean() {
  jdbcFactory.create();
}