深入Spring Boot：那些注入不了的Spring占位符（${}表达式）

<bean id="dataSource" destroy-method="close" class="org.apache.commons.dbcp.BasicDataSource">
    <property name="url" value="${jdbc.url}"/>
</bean>

@Configuration
@ImportResource("classpath:/com/acme/properties-config.xml")
public class AppConfig {
    @Value("${jdbc.url}")
    private String url;
}

user: ${db.user}

environment.resolvePlaceholders("${db.user}")

package org.mybatis.spring.mapper;

import static org.springframework.util.Assert.notNull;

import java.lang.annotation.Annotation;
import java.util.Map;

import org.apache.ibatis.session.SqlSessionFactory;
import org.mybatis.spring.SqlSessionTemplate;
import org.springframework.beans.PropertyValue;
import org.springframework.beans.PropertyValues;
import org.springframework.beans.factory.BeanNameAware;
import org.springframework.beans.factory.InitializingBean;
import org.springframework.beans.factory.config.BeanDefinition;
import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
import org.springframework.beans.factory.config.PropertyResourceConfigurer;
import org.springframework.beans.factory.config.TypedStringValue;
import org.springframework.beans.factory.support.BeanDefinitionRegistry;
import org.springframework.beans.factory.support.BeanDefinitionRegistryPostProcessor;
import org.springframework.beans.factory.support.BeanNameGenerator;
import org.springframework.beans.factory.support.DefaultListableBeanFactory;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationContextAware;
import org.springframework.context.ConfigurableApplicationContext;
import org.springframework.util.StringUtils;

/**
 * BeanDefinitionRegistryPostProcessor that searches recursively starting from a base package for
 * interfaces and registers them as {@code MapperFactoryBean}. Note that only interfaces with at
 * least one method will be registered; concrete classes will be ignored.
 * <p>
 * This class was a {code BeanFactoryPostProcessor} until 1.0.1 version. It changed to  
 * {@code BeanDefinitionRegistryPostProcessor} in 1.0.2. See https://jira.springsource.org/browse/SPR-8269
 * for the details.
 * <p>
 * The {@code basePackage} property can contain more than one package name, separated by either
 * commas or semicolons.
 * <p>
 * This class supports filtering the mappers created by either specifying a marker interface or an
 * annotation. The {@code annotationClass} property specifies an annotation to search for. The
 * {@code markerInterface} property specifies a parent interface to search for. If both properties
 * are specified, mappers are added for interfaces that match <em>either</em> criteria. By default,
 * these two properties are null, so all interfaces in the given {@code basePackage} are added as
 * mappers.
 * <p>
 * This configurer enables autowire for all the beans that it creates so that they are
 * automatically autowired with the proper {@code SqlSessionFactory} or {@code SqlSessionTemplate}.
 * If there is more than one {@code SqlSessionFactory} in the application, however, autowiring
 * cannot be used. In this case you must explicitly specify either an {@code SqlSessionFactory} or
 * an {@code SqlSessionTemplate} to use via the <em>bean name</em> properties. Bean names are used
 * rather than actual objects because Spring does not initialize property placeholders until after
 * this class is processed. 
 * <p>
 * Passing in an actual object which may require placeholders (i.e. DB user password) will fail. 
 * Using bean names defers actual object creation until later in the startup
 * process, after all placeholder substituation is completed. However, note that this configurer
 * does support property placeholders of its <em>own</em> properties. The <code>basePackage</code>
 * and bean name properties all support <code>${property}</code> style substitution.
 * <p>
 * Configuration sample:
 * <p>
 *
 * <pre class="code">
 * {@code
 *   <bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
 *       <property name="basePackage" value="org.mybatis.spring.sample.mapper" />
 *       <!-- optional unless there are multiple session factories defined -->
 *       <property name="sqlSessionFactoryBeanName" value="sqlSessionFactory" />
 *   </bean>
 * }
 * </pre>
 *
 * @author Hunter Presnall
 * @author Eduardo Macarron
 *
 * @see MapperFactoryBean
 * @see ClassPathMapperScanner
 */
public class MapperScannerConfigurer implements BeanDefinitionRegistryPostProcessor, InitializingBean, ApplicationContextAware, BeanNameAware {

  private String basePackage;

  private boolean addToConfig = true;

  private SqlSessionFactory sqlSessionFactory;

  private SqlSessionTemplate sqlSessionTemplate;

  private String sqlSessionFactoryBeanName;

  private String sqlSessionTemplateBeanName;

  private Class<? extends Annotation> annotationClass;

  private Class<?> markerInterface;

  private ApplicationContext applicationContext;

  private String beanName;

  private boolean processPropertyPlaceHolders;

  private BeanNameGenerator nameGenerator;

@Target({ ElementType.TYPE, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional(OnBeanCondition.class)
public @interface ConditionalOnBean {
    /**
     * The annotation type decorating a bean that should be checked. The condition matches
     * when any of the annotations specified is defined on a bean in the
     * {@link ApplicationContext}.
     * @return the class-level annotation types to check
     */
    Class<? extends Annotation>[] annotation() default {};

@Target({ ElementType.TYPE })
@Retention(RetentionPolicy.RUNTIME)
public @interface MyAnnotation {
}

@Configuration
public class MyAutoConfiguration {
    @Bean
    // if comment this line, it will be fine.
    @ConditionalOnBean(annotation = { MyAnnotation.class })
    public String abc() {
        return "abc";
    }
}

<bean id="sysProps" class="org.springframework.beans.factory.config.MethodInvokingFactoryBean">
  <property name="targetClass" value="java.lang.System" />
  <property name="targetMethod" value="getProperties" />
</bean>

<bean id="javaVersion" class="org.springframework.beans.factory.config.MethodInvokingFactoryBean">
  <property name="targetObject" ref="sysProps" />
  <property name="targetMethod" value="getProperty" />
  <property name="arguments" value="${java.version.key}" />
</bean>

@SpringBootApplication
@ImportResource("classpath:/demo.xml")
public class DemoApplication {

    public static void main(String[] args) {
        ConfigurableApplicationContext context = SpringApplication.run(DemoApplication.class, args);
        System.err.println(context.getBean("javaVersion"));
    }
}