[Java] Spring Boot 自动配置之条件注解浅析

• @conditional 依赖的条件
  
• @conditionalonbean  在某个bean存在的条件下
  
• @conditionalonmissingbean 在某个bean不存在的条件下
  
• @conditionalonclass  在某个class存在的条件下
  
• @conditionalonmissingclass  在某个class不存在的条件下
  

@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();
}