Spring提供了豐富的標簽和注解來進行bean的定義，除此之外框架來提供了擴展機制讓使用可以通過properties來定義bean，與強大的標簽式和注解式的bean定義相比，properties提供的規則要簡單許多。

key部分用.分隔即通過A.B來進行相關的屬性定義，其中A表示bean名稱，B通過不同的值還表達不同的含義：

?

• (class)，bean的類型
• (parent)，bean的父bean
• name，bean的name屬性，name是一個普通屬性
• childBean(ref)，bean的childBean屬性，childBean是一個引用屬性
• (singleton)，是否單例
• (lazy-init)，是否懶加載
• $0，第一個構造子參數
• (scope)，作用域
• (abstract)，是否是抽象bean

?

看一個例子：

bean.properties文件

?

[java]?view plain?copy

1. #bean1??
2. propBean.(class)?=?spring.beans.properties.PropBean??
3. propBean.(parent)?=?commonBean??
4. propBean.name?=?name1??
5. propBean.childBean(ref)?=?childBean??
6. propBean.(singleton)?=?true??
7. propBean.(lazy-init)?=?true??
8. ??
9. #bean2??
10. childBean.(class)?=?spring.beans.properties.ChildBean??
11. childBean.$0?=?cid1??
12. chlldBean.(scope)?=?singleton??
13. ??
14. #abstract?bean??
15. commonBean.(class)?=?spring.beans.properties.CommonBean??
16. commonBean.id?=?1??
17. commonBean.(abstract)?=?true??

上面的properties文件定義了三個bean：

?

?

• commonBean，類型是spring.beans.properties.CommonBean，注入值1到id屬性，這是一個抽象bean
• childBean，類型spring.beans.properties.ChildBean，構造器注入cid1，作用域是singleton
• propBean，類型是spring.beans.properties.PropBean，父bean是commonBean，注入一個普通屬性name，和引用屬性childBean，引用的bean是childBean，bean是單例并且懶加載。

?

bean定義文件寫好之后，通過PropertiesBeanDefinitionReader來加載解析bean定義，這個解析器的原理很簡單，在此不做詳細分析，下面是實例代碼。

?

[java]?view plain?copy

1. public?void?test()?{??
2. ????GenericApplicationContext?ctx?=?new?GenericApplicationContext();??
3. ????Resource?res?=?new?ClassPathResource(??
4. ????????????"spring/beans/properties/bean.properties");??
5. ????PropertiesBeanDefinitionReader?propReader?=?new?PropertiesBeanDefinitionReader(??
6. ????????????ctx);??
7. ????propReader.loadBeanDefinitions(res);??
8. ????PropBean?propBean?=?(PropBean)?ctx.getBean("propBean");??
9. ????assertNotNull(propBean);??
10. ????assertNotNull(propBean.getId());??
11. ??
12. ????assertNotNull(propBean.getChildBean());??
13. ????assertNotNull(propBean.getChildBean().getCid());??
14. }??

也可以完全不用單獨定義個properties文件，只需要把相關的key-value放到一個Map中，再通過PropertiesBeanDefinitionReader加載這個map中的key-value。

?

通過這種方式，使用者可以根據需要自定義些bean的定義規則，比如可以把bean定義放在數據庫中，把數據庫中的信息讀取出來拼接成滿足properties規則的bean定義，在Spring中就定義了一個org.springframework.jdbc.core.support.JdbcBeanDefinitionReader來完成這種需求，看下這個類的代碼。

?

[java]?view plain?copy

1. public?class?JdbcBeanDefinitionReader?{??
2. ??
3. ????private?final?PropertiesBeanDefinitionReader?propReader;??
4. ??
5. ????private?JdbcTemplate?jdbcTemplate;??
6. ??
7. ??
8. ????/**?
9. ?????*?Create?a?new?JdbcBeanDefinitionReader?for?the?given?bean?factory,?
10. ?????*?using?a?default?PropertiesBeanDefinitionReader?underneath.?
11. ?????*?<p>DataSource?or?JdbcTemplate?still?need?to?be?set.?
12. ?????*?@see?#setDataSource?
13. ?????*?@see?#setJdbcTemplate?
14. ?????*/??
15. ????public?JdbcBeanDefinitionReader(BeanDefinitionRegistry?beanFactory)?{??
16. ????????this.propReader?=?new?PropertiesBeanDefinitionReader(beanFactory);??
17. ????}??
18. ??
19. ????/**?
20. ?????*?Create?a?new?JdbcBeanDefinitionReader?that?delegates?to?the?
21. ?????*?given?PropertiesBeanDefinitionReader?underneath.?
22. ?????*?<p>DataSource?or?JdbcTemplate?still?need?to?be?set.?
23. ?????*?@see?#setDataSource?
24. ?????*?@see?#setJdbcTemplate?
25. ?????*/??
26. ????public?JdbcBeanDefinitionReader(PropertiesBeanDefinitionReader?beanDefinitionReader)?{??
27. ????????Assert.notNull(beanDefinitionReader,?"Bean?definition?reader?must?not?be?null");??
28. ????????this.propReader?=?beanDefinitionReader;??
29. ????}??
30. ??
31. ??
32. ????/**?
33. ?????*?Set?the?DataSource?to?use?to?obtain?database?connections.?
34. ?????*?Will?implicitly?create?a?new?JdbcTemplate?with?the?given?DataSource.?
35. ?????*/??
36. ????public?void?setDataSource(DataSource?dataSource)?{??
37. ????????this.jdbcTemplate?=?new?JdbcTemplate(dataSource);??
38. ????}??
39. ??
40. ????/**?
41. ?????*?Set?the?JdbcTemplate?to?be?used?by?this?bean?factory.?
42. ?????*?Contains?settings?for?DataSource,?SQLExceptionTranslator,?NativeJdbcExtractor,?etc.?
43. ?????*/??
44. ????public?void?setJdbcTemplate(JdbcTemplate?jdbcTemplate)?{??
45. ????????Assert.notNull(jdbcTemplate,?"JdbcTemplate?must?not?be?null");??
46. ????????this.jdbcTemplate?=?jdbcTemplate;??
47. ????}??
48. ??
49. ??
50. ????/**?
51. ?????*?Load?bean?definitions?from?the?database?via?the?given?SQL?string.?
52. ?????*?@param?sql?SQL?query?to?use?for?loading?bean?definitions.?
53. ?????*?The?first?three?columns?must?be?bean?name,?property?name?and?value.?
54. ?????*?Any?join?and?any?other?columns?are?permitted:?e.g.?
55. ?????*?{@code?SELECT?BEAN_NAME,?PROPERTY,?VALUE?FROM?CONFIG?WHERE?CONFIG.APP_ID?=?1}?
56. ?????*?It's?also?possible?to?perform?a?join.?Column?names?are?not?significant?--?
57. ?????*?only?the?ordering?of?these?first?three?columns.?
58. ?????*/??
59. ????public?void?loadBeanDefinitions(String?sql)?{??
60. ????????Assert.notNull(this.jdbcTemplate,?"Not?fully?configured?-?specify?DataSource?or?JdbcTemplate");??
61. ????????final?Properties?props?=?new?Properties();??
62. ????????this.jdbcTemplate.query(sql,?new?RowCallbackHandler()?{??
63. ????????????public?void?processRow(ResultSet?rs)?throws?SQLException?{??
64. ????????????????String?beanName?=?rs.getString(1);??
65. ????????????????String?property?=?rs.getString(2);??
66. ????????????????String?value?=?rs.getString(3);??
67. ????????????????//?Make?a?properties?entry?by?combining?bean?name?and?property.??
68. ????????????????props.setProperty(beanName?+?"."?+?property,?value);??
69. ????????????}??
70. ????????});??
71. ????????this.propReader.registerBeanDefinitions(props);??
72. ????}??
73. ??
74. }??

此外，通過理解PropertiesBeanDefinitionReader的實現方式，發現也可以通過擴展BeanDefinitionReader來擴展bean定義，我們可以通過繼承AbstractBeanDefinitionReader來完成這種擴展。