Spring3.1完全基于注解配置@Configuration类中@Autowire无法注入问题解决

在上回介绍Spring3.1+Hibernate4.1.7基于注解配置的时候(《SpringMVC3.1+Hibernate4.1.7完全基于注解配置(零配置文件)》)说过，在修改配置方式的时候遇到过不少问题。这里介绍一下。

1/**
2 * 数据源配置类
3 * 
4 * @author lihzh
5 * @alia OneCoder
6 * @blog http://www.coderli.com
7 */
8@Configuration
9@PropertySource("/conf/jdbc.properties")
10public class DataSourceConfig {
11	
12	@Value("${jdbc.driverClass}") String driverClass;
13	@Value("${jdbc.url}") String url;
14	@Value("${jdbc.user}") String user;
15	@Value("${jdbc.password}") String password;
16
17	@Bean(autowire=Autowire.BY_TYPE)
18	public DataSource dataSource() throws PropertyVetoException {
19		ComboPooledDataSource dataSource = new ComboPooledDataSource();
20		dataSource.setDriverClass(driverClass);
21		dataSource.setJdbcUrl(url);
22		dataSource.setUser(user);
23		dataSource.setPassword(password);
24		return dataSource;
25	}
26}
27

1/**
2 * Spring3.1基于注解的配置类， 用于代替原来的<b>applicationContext.xml</b>配置文件
3 * 
4 * @author lihzh
5 * @date 2012-10-12 下午4:23:13
6 */
7@ComponentScan(basePackages = "com.coderli.shurnim.*.biz")
8@Import(DataSourceConfig.class)
9@Configuration
10@EnableTransactionManagement
11public class DefaultAppConfig {
12	
13	@Autowired
14	DataSourceConfig config;
15
16	@Bean
17	public PropertySourcesPlaceholderConfigurer placehodlerConfigurer() {
18		return new PropertySourcesPlaceholderConfigurer();
19	}
20
21	@Bean
22	public LocalSessionFactoryBean sessionFactory()
23			throws PropertyVetoException {
24		LocalSessionFactoryBean sessionFactoryBean = new LocalSessionFactoryBean();
25		sessionFactoryBean.setDataSource(config.dataSource());
26		Properties hibernateProperties = new Properties();
27		hibernateProperties.setProperty("hibernate.dialect",
28				"org.hibernate.dialect.MySQLDialect");
29		sessionFactoryBean.setHibernateProperties(hibernateProperties);
30		sessionFactoryBean.setPackagesToScan("com.coderli.shurnim.*.model");
31		return sessionFactoryBean;
32	}
33
34	@Bean
35	public HibernateTransactionManager txManager() throws PropertyVetoException {
36		HibernateTransactionManager txManager = new HibernateTransactionManager();
37		txManager.setSessionFactory(sessionFactory().getObject());
38		return txManager;
39	}
40}
41

这里，这样配置的依据是参考Spring中@Configuration注解的注释说明：

With the @Import annotation

@Configuration classes may be composed using the @Import annotation, not unlike the way that works in Spring XML. Because @Configuration objects are managed as Spring beans within the container, imported configurations may be injected using @Autowired or @Inject:

@Configuration public class DatabaseConfig { @Bean public DataSource dataSource() { // instantiate, configure and return DataSource } }

@Configuration @Import(DatabaseConfig.class) public class AppConfig { @Inject DatabaseConfig dataConfig;

Code 5 行

行号 复制

1 @Bean
2 public MyBean myBean() {
3     // reference the dataSource() bean method
4     return new MyBean(dataConfig.dataSource());
5 }
6

}

Now both AppConfig and the imported DatabaseConfig can be bootstrapped by registering only AppConfig against the Spring context: new AnnotationConfigApplicationContext(AppConfig.class);

启动Tomcat，报错：

空指针异常，显然是DataSourceConfig没有注入进来，更换@Inject注解，问题依旧。

既然注入DataSourceCOnfig无效，OneCoder考虑，可否直接注入DataSource Bean。修改代码如下：

1/**
2 * Spring3.1基于注解的配置类， 用于代替原来的<b>applicationContext.xml</b>配置文件
3 * 
4 * @author lihzh
5 * @date 2012-10-12 下午4:23:13
6 */
7@ComponentScan(basePackages = "com.coderli.shurnim.config")
8@Import(DataSourceConfig.class)
9@Configuration
10@EnableTransactionManagement
11public class DefaultAppConfig {
12	
13	@Autowired
14	DataSource dataSource;
15
16	@Bean
17	public PropertySourcesPlaceholderConfigurer placehodlerConfigurer() {
18		return new PropertySourcesPlaceholderConfigurer();
19	}
20
21	@Bean
22	public LocalSessionFactoryBean sessionFactory()
23			throws PropertyVetoException {
24		LocalSessionFactoryBean sessionFactoryBean = new LocalSessionFactoryBean();
25		sessionFactoryBean.setDataSource(dataSource);
26		Properties hibernateProperties = new Properties();
27		hibernateProperties.setProperty("hibernate.dialect",
28				"org.hibernate.dialect.MySQLDialect");
29		sessionFactoryBean.setHibernateProperties(hibernateProperties);
30		sessionFactoryBean.setPackagesToScan("com.coderli.shurnim.*.model");
31		return sessionFactoryBean;
32	}
33

启动Tomcat，报错：

OneCoder在各个Bean的方法上加断点调试，发现dataSource也是null，看来还是没注入进来导致的。也许这两个场景是一个问题，先解决没注入的问题看看吧。

说实话，这个问题困扰了OneCoder近一天，搜索也没什么好的头绪，搜出来的东西差距都比较大。OneCoder只能一遍一遍阅读自己的代码分析可能的原因同时仔细查看Spring官方的注释文档。OneCoder在Debug中似乎意识到一个问题，就是OneCoder这里使用了

1@Bean
2public PropertySourcesPlaceholderConfigurer placehodlerConfigurer() {
3	return new PropertySourcesPlaceholderConfigurer();
4}
5

这个bean用来使用替换变量符"${}"。这个替换符正式在DataSourceConfig中配置数据源使用的，这样的花，是否会造成Spring初始化Bean的生命周期的混乱呢。因为初始化dataSource Bean的时候需要上述bean，而初始化上述bean的时候，已经错过了DataSource bean的注入时机。那么如何解决呢，OneCoder还是得求助官方文档。又是一遍又一遍的阅读，OneCoder在@Bean注解的注释中，发现这样的一段话。

A note on BeanFactoryPostProcessor-returning @Bean methods

Special consideration must be taken for @Bean methods that return Spring BeanFactoryPostProcessor (BFPP) types. Because BFPP objects must be instantiated very early in the container lifecycle, they can interfere with processing of annotations such as @Autowired, @Value, and @PostConstruct within @Configuration classes. To avoid these lifecycle issues, mark BFPP-returning @Bean methods as static. For example:

     @Bean
     public static PropertyPlaceholderConfigurer ppc() {
         // instantiate, configure and return ppc ...
     }
By marking this method as static, it can be invoked without causing instantiation of its declaring @Configuration class, thus avoiding the above-mentioned lifecycle conflicts. Note however that static @Bean methods will not be enhanced for scoping and AOP semantics as mentioned above. This works out in BFPP cases, as they are not typically referenced by other @Bean methods. As a reminder, a WARN-level log message will be issued for any non-static @Bean methods having a return type assignable to BeanFactoryPostProcessor.

果然被我猜中了，这段意思大概是说，类似PropertyPlaceholderConfigurer这种的Bean是需要在其他Bean初始化之前完成的，这会影响到Spring Bean生命周期的控制，所以如果你用到了这样的Bean，需要把他们声明成Static的，这样就会不需要@Configuration的实例而调用，从而提前完成Bean的构造。并且，这里还提到，如果你没有把实现 BeanFactoryPostProcessor接口的Bean声明为static的，他会给出警告。

OneCoder赶紧去检查自己的控制台，果然发现了这样一句话：

WARNING: @Bean method DefaultAppConfig.placehodlerConfigurer is non-static and returns an object assignable to Spring's BeanFactoryPostProcessor interface. This will result in a failure to process annotations such as @Autowired, @Resource and @PostConstruct within the method's declaring @Configuration class. Add the 'static' modifier to this method to avoid these container lifecycle issues; see @Bean Javadoc for complete details

唉，本来OneCoder是很重视警告的，这次怎么就没注意到呢。赶紧改成static的。重新启动。终于，一切OK了！