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

Spring里的占位符

spring里的占位符通常表现的形式是：

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

或者

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

Spring应用在有时会出现占位符配置没有注入，原因可能是多样的。

本文介绍两种比较复杂的情况。

占位符是在Spring生命周期的什么时候处理的

Spirng在生命周期里关于Bean的处理大概可以分为下面几步：

1. 加载Bean定义（从xml或者从@Import等）
2. 处理BeanFactoryPostProcessor
3. 实例化Bean
4. 处理Bean的property注入
5. 处理BeanPostProcessor

当然这只是比较理想的状态，实际上因为Spring Context在构造时，也需要创建很多内部的Bean，应用在接口实现里也会做自己的各种逻辑，整个流程会非常复杂。

那么占位符（${}表达式）是在什么时候被处理的？

• 实际上是在org.springframework.context.support.PropertySourcesPlaceholderConfigurer里处理的，它会访问了每一个bean的BeanDefinition，然后做占位符的处理
• PropertySourcesPlaceholderConfigurer实现了BeanFactoryPostProcessor接口
• PropertySourcesPlaceholderConfigurer的 order是Ordered.LOWEST_PRECEDENCE，也就是最低优先级的

结合上面的Spring的生命周期，如果Bean的创建和使用在PropertySourcesPlaceholderConfigurer之前，那么就有可能出现占位符没有被处理的情况。

例子1：Mybatis 的 MapperScannerConfigurer引起的占位符没有处理

例子代码：mybatis-demo.zip

• 首先应用自己在代码里创建了一个DataSource，其中${db.user}是希望从application.properties里注入的。代码在运行时会打印出user的实际值。

  @Configuration
  public class MyDataSourceConfig {
      @Bean(name = "dataSource1")
      public DataSource dataSource1(@Value("${db.user}") String user) {
          System.err.println("user: " + user);
          JdbcDataSource ds = new JdbcDataSource();
          ds.setURL("jdbc:h2:˜/test");
          ds.setUser(user);
          return ds;
      }
  }

• 然后应用用代码的方式来初始化mybatis相关的配置，依赖上面创建的DataSource对象

  @Configuration
  public class MybatisConfig1 {
  
      @Bean(name = "sqlSessionFactory1")
      public SqlSessionFactory sqlSessionFactory1(DataSource dataSource1) throws Exception {
          SqlSessionFactoryBean sqlSessionFactoryBean = new SqlSessionFactoryBean();
          org.apache.ibatis.session.Configuration ibatisConfiguration = new org.apache.ibatis.session.Configuration();
          sqlSessionFactoryBean.setConfiguration(ibatisConfiguration);
  
          sqlSessionFactoryBean.setDataSource(dataSource1);
          sqlSessionFactoryBean.setTypeAliasesPackage("sample.mybatis.domain");
          return sqlSessionFactoryBean.getObject();
      }
  
      @Bean
      MapperScannerConfigurer mapperScannerConfigurer(SqlSessionFactory sqlSessionFactory1) {
          MapperScannerConfigurer mapperScannerConfigurer = new MapperScannerConfigurer();
          mapperScannerConfigurer.setSqlSessionFactoryBeanName("sqlSessionFactory1");
          mapperScannerConfigurer.setBasePackage("sample.mybatis.mapper");
          return mapperScannerConfigurer;
      }
  }

当代码运行时，输出结果是：

user: ${db.user}

为什么会user这个变量没有被注入？

分析下Bean定义，可以发现MapperScannerConfigurer它实现了BeanDefinitionRegistryPostProcessor。这个接口在是Spring扫描Bean定义时会回调的，远早于BeanFactoryPostProcessor。

所以原因是：

• MapperScannerConfigurer它实现了BeanDefinitionRegistryPostProcessor，所以它会Spring的早期会被创建
• 从bean的依赖关系来看，mapperScannerConfigurer依赖了sqlSessionFactory1，sqlSessionFactory1依赖了dataSource1
• MyDataSourceConfig里的dataSource1被提前初始化，没有经过PropertySourcesPlaceholderConfigurer的处理，所以@Value("${db.user}") String user 里的占位符没有被处理

要解决这个问题，可以在代码里，显式来处理占位符：

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;

例子2：Spring boot自身实现问题，导致Bean被提前初始化

例子代码：demo.zip

Spring Boot里提供了@ConditionalOnBean，这个方便用户在不同条件下来创建bean。里面提供了判断是否存在bean上有某个注解的功能。

@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 {};

比如用户自己定义了一个Annotation：

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

然后用下面的写法来创建abc这个bean，意思是当用户显式使用了@MyAnnotation（比如放在main class上），才会创建这个bean。

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

这个功能很好，但是在spring boot 1.4.5 版本之前都有问题，会导致FactoryBean提前初始化。

在例子里，通过xml创建了javaVersion这个bean，想获取到Java的版本号。这里使用的是spring提供的一个调用static函数创建bean的技巧。

<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>

我们在代码里获取到这个javaVersion，然后打印出来：

@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"));
    }
}

在实际运行时，发现javaVersion的值是null。

这个其实是spring boot的锅，要搞清楚这个问题，先要看@ConditionalOnBean的实现。

• @ConditionalOnBean实际上是在ConfigurationClassPostProcessor里被处理的，它实现了BeanDefinitionRegistryPostProcessor
• BeanDefinitionRegistryPostProcessor是在spring早期被处理的
• @ConditionalOnBean的具体处理代码在org.springframework.boot.autoconfigure.condition.OnBeanCondition里

• OnBeanCondition在获取bean的Annotation时，调用了beanFactory.getBeanNamesForAnnotation

  private String[] getBeanNamesForAnnotation(
      ConfigurableListableBeanFactory beanFactory, String type,
      ClassLoader classLoader, boolean considerHierarchy) throws LinkageError {
    String[] result = NO_BEANS;
    try {
      @SuppressWarnings("unchecked")
      Class<? extends Annotation> typeClass = (Class<? extends Annotation>) ClassUtils
          .forName(type, classLoader);
      result = beanFactory.getBeanNamesForAnnotation(typeClass);

• beanFactory.getBeanNamesForAnnotation 会导致FactoryBean提前初始化，创建出javaVersion里，传入的${java.version.key}没有被处理，值为null。

• spring boot 1.4.5 修复了这个问题：https://github.com/spring-projects/spring-boot/issues/8269

实现spring boot starter要注意不能导致bean提前初始化

用户在实现spring boot starter时，通常会实现Spring的一些接口，比如BeanFactoryPostProcessor接口，在处理时，要注意不能调用类似beanFactory.getBeansOfType，beanFactory.getBeanNamesForAnnotation 这些函数，因为会导致一些bean提前初始化。

而上面有提到PropertySourcesPlaceholderConfigurer的order是最低优先级的，所以用户自己实现的BeanFactoryPostProcessor接口在被回调时很有可能占位符还没有被处理。

对于用户自己定义的@ConfigurationProperties对象的注入，可以用类似下面的代码：

@ConfigurationProperties(prefix = "spring.my")
public class MyProperties {
    String key;
}

　　

public static MyProperties buildMyProperties(ConfigurableEnvironment environment) {
  MyProperties myProperties = new MyProperties();

  if (environment != null) {
    MutablePropertySources propertySources = environment.getPropertySources();
    new RelaxedDataBinder(myProperties, "spring.my").bind(new PropertySourcesPropertyValues(propertySources));
  }

  return myProperties;
}

　

总结

• 占位符（${}表达式）是在PropertySourcesPlaceholderConfigurer里处理的，也就是BeanFactoryPostProcessor接口
• spring的生命周期是比较复杂的事情，在实现了一些早期的接口时要小心，不能导致spring bean提前初始化
• 在早期的接口实现里，如果想要处理占位符，可以利用spring自身的api，比如 environment.resolvePlaceholders("${db.user}")

 

 http://blog.csdn.net/hengyunabc/article/details/75453307