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

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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。

方式一
OneCoder考虑在DataSourceConfig中，仅保留数据源配置，其他的诸如SessiionFactory的配置都移到AppConfig中。代码如下：
/**
 * 数据源配置类
 * 
 * @author lihzh
 * @alia OneCoder
 * @blog http://www.coderli.com
 */
@Configuration
@PropertySource("/conf/jdbc.properties")
public class DataSourceConfig {
	
	@Value("${jdbc.driverClass}") String driverClass;
	@Value("${jdbc.url}") String url;
	@Value("${jdbc.user}") String user;
	@Value("${jdbc.password}") String password;

	@Bean(autowire=Autowire.BY_TYPE)
	public DataSource dataSource() throws PropertyVetoException {
		ComboPooledDataSource dataSource = new ComboPooledDataSource();
		dataSource.setDriverClass(driverClass);
		dataSource.setJdbcUrl(url);
		dataSource.setUser(user);
		dataSource.setPassword(password);
		return dataSource;
	}
}

/**
 * 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
 * 
 * @author lihzh
 * @date 2012-10-12 下午4:23:13
 */
@ComponentScan(basePackages = "com.coderli.shurnim.*.biz")
@Import(DataSourceConfig.class)
@Configuration
@EnableTransactionManagement
public class DefaultAppConfig {
	
	@Autowired
	DataSourceConfig config;

	@Bean
	public PropertySourcesPlaceholderConfigurer placehodlerConfigurer() {
		return new PropertySourcesPlaceholderConfigurer();
	}

	@Bean
	public LocalSessionFactoryBean sessionFactory()
			throws PropertyVetoException {
		LocalSessionFactoryBean sessionFactoryBean = new LocalSessionFactoryBean();
		sessionFactoryBean.setDataSource(config.dataSource());
		Properties hibernateProperties = new Properties();
		hibernateProperties.setProperty("hibernate.dialect",
				"org.hibernate.dialect.MySQLDialect");
		sessionFactoryBean.setHibernateProperties(hibernateProperties);
		sessionFactoryBean.setPackagesToScan("com.coderli.shurnim.*.model");
		return sessionFactoryBean;
	}

	@Bean
	public HibernateTransactionManager txManager() throws PropertyVetoException {
		HibernateTransactionManager txManager = new HibernateTransactionManager();
		txManager.setSessionFactory(sessionFactory().getObject());
		return txManager;
	}
}

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:</p>

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

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

 @Bean
 public MyBean myBean() {
     // reference the dataSource() bean method
     return new MyBean(dataConfig.dataSource());
 } }

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。

方式二

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

/**
 * 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
 * 
 * @author lihzh
 * @date 2012-10-12 下午4:23:13
 */
@ComponentScan(basePackages = "com.coderli.shurnim.config")
@Import(DataSourceConfig.class)
@Configuration
@EnableTransactionManagement
public class DefaultAppConfig {
	
	@Autowired
	DataSource dataSource;

	@Bean
	public PropertySourcesPlaceholderConfigurer placehodlerConfigurer() {
		return new PropertySourcesPlaceholderConfigurer();
	}

	@Bean
	public LocalSessionFactoryBean sessionFactory()
			throws PropertyVetoException {
		LocalSessionFactoryBean sessionFactoryBean = new LocalSessionFactoryBean();
		sessionFactoryBean.setDataSource(dataSource);
		Properties hibernateProperties = new Properties();
		hibernateProperties.setProperty("hibernate.dialect",
				"org.hibernate.dialect.MySQLDialect");
		sessionFactoryBean.setHibernateProperties(hibernateProperties);
		sessionFactoryBean.setPackagesToScan("com.coderli.shurnim.*.model");
		return sessionFactoryBean;
	}

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

@Bean
public PropertySourcesPlaceholderConfigurer placehodlerConfigurer() {
	return new PropertySourcesPlaceholderConfigurer();
}

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 <p> 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:</p> <p>      @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.</p>

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！

Thanks a lot.